No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Flow Measurement by Magnetic Resonance: A Unique Asset Worth Optimising
Abstract
Users and manufacturers of cardiovascular magnetic resonance (CMR) systems have, potentially, an unrivalled asset. Phase contrast mapping of velocities through planes transecting the great arteries should provide the most accurate measurements available of cardiac output, shunt flow, aortic or pulmonary regurgitation and, indirectly, of mitral regurgitation. But the reality is that phase contrast velocity mapping remains under-used, and may have become discredited in the eyes of some CMR users and referring clinicians. Even when appropriate methods of acquisition have been used, there can be inaccuracies of flow measurement on some CMR systems caused by background phase errors due to eddy currents or uncorrected concomitant gradients. Measurements of regurgitant or shunt flow can be seriously affected by these errors which should be minimised or corrected by appropriate hardware and software design. If they have not been, inaccuracies can be detected and corrected by repeating identical velocity acquisitions on a static phantom, and subtracting the corresponding apparent phantom velocities from those of the clinical acquisition. For accurate measurements of aortic regurgitation or mitral inflow, motion tracking and velocity correction with respect to the cyclic displacements of the valves are needed, but few if any commercial systems provide this facility. Measurements of jet velocity pose different challenges, mainly related to the size and placement of voxels relative to a narrow jet. Awareness of the potential problems and concerted efforts towards optimisation are needed from manufacturers and users to make appropriate use of phase contrast flow measurement - a unique strength of cardiovascular magnetic resonance.
... Even though PC-MRI is considered to be accurate and the standard MRI method for the assessment of AR severity, it has been questioned under certain conditions [12]. For example, it has been shown that the quantification of the aortic regurgitant volume (RVol) and fraction (RF) depends on the position of the image plane, with systematically lower regurgitation values at more distal positions [13][14][15]. ...
... For example, it has been shown that the quantification of the aortic regurgitant volume (RVol) and fraction (RF) depends on the position of the image plane, with systematically lower regurgitation values at more distal positions [13][14][15]. This observation has been attributed to the effect of aortic wall compliance, coronary flow, as well as through plane motion of the aortic root [7,[12][13][14][15]. Complex flow may be an additional source of error as the PC method only registers blood flow through an orthogonal image plane that is fixed in space. ...
... Special care was taken to improve the temporal resolution by shortening the repetition time and turbo factor of the measurement. The measurements were acquired at the isocenter of the magnet to minimize magnetic field inhomogeneities [12]. Finally, effective compensation for background velocity offsets was performed by the scanner and post-acquisition by means of adaptive image filtering. ...
This study aimed to investigate if and how complex flow influences the assessment of aortic regurgitation (AR) using phase contrast MRI in patients with chronic AR. Patients with moderate (n = 15) and severe (n = 28) chronic AR were categorized into non-complex flow (NCF) or complex flow (CF) based on the presence of systolic backward flow volume. Phase contrast MRI was performed repeatedly at the level of the sinotubular junction (Ao1) and 1 cm distal to the sinotubular junction (Ao2). All AR patients were assessed to have non-severe AR or severe AR (cut-off values: regurgitation volume (RVol) ≥ 60 ml and regurgitation fraction (RF) ≥ 50%) in both measurement positions. The repeatability was significantly lower, i.e. variation was larger, for patients with CF than for NCF (≥ 12 ± 12% versus ≥ 6 ± 4%, P ≤ 0.03). For patients with CF, the repeatability was significantly lower at Ao2 compared to Ao1 (≥ 21 ± 20% versus ≥ 12 ± 12%, P ≤ 0.02), as well as the assessment of regurgitation (RVol: 42 ± 34 ml versus 54 ± 42 ml, P < 0.001; RF: 30 ± 18% versus 34 ± 16%, P = 0.01). This was not the case for patients with NCF. The frequency of patients that changed in AR grade from severe to non-severe when the position of the measurement changed from Ao1 to Ao2 was higher for patients with CF compared to NCF (RVol: 5/26 (19%) versus 1/17 (6%), P = 0.2; RF: 4/26 (15%) versus 0/17 (0%), P = 0.09). Our study shows that complex flow influences the quantification of chronic AR, which can lead to underestimation of AR severity when using PC-MRI.
... Phase-contrast magnetic resonance imaging (MRI) can provide velocity field. but it has a lower temporal resolution than Doppler echocardiography (DE) resolution 15,16 . It is important to note that MRI cannot be used for patients with most implanted medical devices except safely for MRI-conditional devices. ...
... The relationship between the rates of change of the cell volume and the mesh motion flux was governed by conservation law 42 . Equation 16 indicates that the rates of change of the volume and the velocity of the surface are in equilibrium 42 . ...
... In addition, several recent studies have combined LPM with MRI data to obtain anisotropic material properties of the LV for electro-mechanical models [55][56][57]67,68 . However, MRI is costly, lengthy and not possible for many patients with implanted devices like TAVR 15,16 . ...
One of the most common acute and chronic cardiovascular disease conditions is aortic stenosis, a disease in which the aortic valve is damaged and can no longer function properly. Moreover, aortic stenosis commonly exists in combination with other conditions causing so many patients suffer from the most general and fundamentally challenging condition: complex valvular, ventricular and vascular disease (C3VD). Transcatheter aortic valve replacement (TAVR) is a new less invasive intervention and is a growing alternative for patients with aortic stenosis. Although blood flow quantification is critical for accurate and early diagnosis of C3VD in both pre and post-TAVR, proper diagnostic methods are still lacking because the fluid-dynamics methods that can be used as engines of new diagnostic tools are not well developed yet. Despite remarkable advances in medical imaging, imaging on its own is not enough to quantify the blood flow effectively. Moreover, understanding of C3VD in both pre and post-TAVR and its progression has been hindered by the absence of a proper non-invasive tool for the assessment of the cardiovascular function. To enable the development of new non-invasive diagnostic methods, we developed an innovative image-based patient-specific computational fluid dynamics framework for patients with C3VD who undergo TAVR to quantify metrics of: (1) global circulatory function; (2) global cardiac function as well as (3) local cardiac fluid dynamics. This framework is based on an innovative non-invasive Doppler-based patient-specific lumped-parameter algorithm and a 3-D strongly-coupled fluid-solid interaction. We validated the framework against clinical cardiac catheterization and Doppler echocardiographic measurements and demonstrated its diagnostic utility by providing novel analyses and interpretations of clinical data in eleven C3VD patients in pre and post-TAVR status. Our findings position this framework as a promising new non-invasive diagnostic tool that can provide blood flow metrics while posing no risk to the patient. The diagnostic information, that the framework can provide, is vitally needed to improve clinical outcomes, to assess patient risk and to plan treatment.
... Measurement of blood flow is potentially an unrivalled asset of cardiovascular magnetic resonance (CMR), and able to measure the volume flow in large vessels by pixel wise mapping of the velocities through planes transecting the vessels. This should provide the most accurate measurements available of aortic or pulmonary regurgitation, cardiac output, shunt flow and, indirectly, of mitral and tricuspid regurgitation [1,2]. The technique applied clinically for the flow measurements is 2-dimensional (2D) cine phase contrast velocity quantification, using a flow sensitivity perpendicular to the image plane. ...
... The technique applied clinically for the flow measurements is 2-dimensional (2D) cine phase contrast velocity quantification, using a flow sensitivity perpendicular to the image plane. However, phase contrast velocity mapping remains under-used, and may have become discredited in the eyes of some CMR users, because even when appropriate methods of acquisition have been used, inaccurate flow measurements can be caused by background phase errors [1,3]. ...
... In this study we have shown that without velocity offset correction, significant errors in cardiac output can occur in 2D phase contrast velocity quantification in the aorta and pulmonary artery, as reported before [1,3,4] and which occurred in 60% of the included scans in this study. The multi-vendor, multi-center setup allowed a broader evaluation of performance of the cited method. ...
Background
A velocity offset error in phase contrast cardiovascular magnetic resonance (CMR) imaging is a known problem in clinical assessment of flow volumes in vessels around the heart. Earlier studies have shown that this offset error is clinically relevant over different systems, and cannot be removed by protocol optimization. Correction methods using phantom measurements are time consuming, and assume reproducibility of the offsets which is not the case for all systems. An alternative previously published solution is to correct the in-vivo data in post-processing, interpolating the velocity offset from stationary tissue within the field-of-view. This study aims to validate this interpolation-based offset correction in-vivo in a multi-vendor, multi-center setup.
Methods
Data from six 1.5 T CMR systems were evaluated, with two systems from each of the three main vendors. At each system aortic and main pulmonary artery 2D flow studies were acquired during routine clinical or research examinations, with an additional phantom measurement using identical acquisition parameters. To verify the phantom acquisition, a region-of-interest (ROI) at stationary tissue in the thorax wall was placed and compared between in-vivo and phantom measurements. Interpolation-based offset correction was performed on the in-vivo data, after manually excluding regions of spatial wraparound. Correction performance of different spatial orders of interpolation planes was evaluated.
Results
A total of 126 flow measurements in 82 subjects were included. At the thorax wall the agreement between in-vivo and phantom was − 0.2 ± 0.6 cm/s. Twenty-eight studies were excluded because of a difference at the thorax wall exceeding 0.6 cm/s from the phantom scan, leaving 98. Before correction, the offset at the vessel as assessed in the phantom was − 0.4 ± 1.5 cm/s, which resulted in a − 5 ± 16% error in cardiac output. The optimal order of the interpolation correction plane was 1st order, except for one system at which a 2nd order plane was required. Application of the interpolation-based correction revealed a remaining offset velocity of 0.1 ± 0.5 cm/s and 0 ± 5% error in cardiac output.
Conclusions
This study shows that interpolation-based offset correction reduces the offset with comparable efficacy as phantom measurement phase offset correction, without the time penalty imposed by phantom scans.
Trial registration
The study was registered in The Netherlands National Trial Register (NTR) under TC 4865. Registered 19 September 2014. Retrospectively registered.
... Some factors, such as Maxwell terms and gradient field non-linearity, can be corrected automatically 8,9 . However, other factors, including induced electromagnetic eddy currents are difficult to model, and may impact flow measurements [10][11][12] ( Fig. 1), though some investigators have questioned their significance in certain neurovascular applications 13 . To address eddy currents, investigators have suggested using active gradient shielding and non-conducting structural components 11 in addition to adjusting sequence parameters such as slew rate 14 ; these design practices are now commonplace for modern scanners. ...
... However, other factors, including induced electromagnetic eddy currents are difficult to model, and may impact flow measurements [10][11][12] ( Fig. 1), though some investigators have questioned their significance in certain neurovascular applications 13 . To address eddy currents, investigators have suggested using active gradient shielding and non-conducting structural components 11 in addition to adjusting sequence parameters such as slew rate 14 ; these design practices are now commonplace for modern scanners. Alternative methods include the use of stationary phantoms 7,15 , but these solutions can be complicated to implement clinically and may not effectively address the phase errors present at the time of the patient flow acquisition 16 . ...
Background phase errors in 4D Flow MRI may negatively impact blood flow quantification. In this study, we assessed their impact on cerebrovascular flow volume measurements, evaluated the benefit of manual image-based correction, and assessed the potential of a convolutional neural network (CNN), a form of deep learning, to directly infer the correction vector field. With IRB waiver of informed consent, we retrospectively identified 96 MRI exams from 48 patients who underwent cerebrovascular 4D Flow MRI from October 2015 to 2020. Flow measurements of the anterior, posterior, and venous circulation were performed to assess inflow-outflow error and the benefit of manual image-based phase error correction. A CNN was then trained to directly infer the phase-error correction field, without segmentation, from 4D Flow volumes to automate correction, reserving from 23 exams for testing. Statistical analyses included Spearman correlation, Bland–Altman, Wilcoxon-signed rank (WSR) and F-tests. Prior to correction, there was strong correlation between inflow and outflow (ρ = 0.833–0.947) measurements with the largest discrepancy in the venous circulation. Manual phase error correction improved inflow-outflow correlation (ρ = 0.945–0.981) and decreased variance (p < 0.001, F-test). Fully automated CNN correction was non-inferior to manual correction with no significant differences in correlation (ρ = 0.971 vs ρ = 0.982) or bias (p = 0.82, Wilcoxon-Signed Rank test) of inflow and outflow measurements. Residual background phase error can impair inflow-outflow consistency of cerebrovascular flow volume measurements. A CNN can be used to directly infer the phase-error vector field to fully automate phase error correction.
... An elevation in velocity is a reflection of increased pulmonary artery pressure (PAP) [59]. Current CMR techniques are not able to measure the tricuspid regurgitant jet velocity due to its dispersed nature [60]. Instead, it estimates PAP by its effect on the heart, including RV mass and septal displacement [61,62]. ...
... Given the substantial mobility of the mitral valve plane, the use of 2D phase contrast imaging can come with the cost of significant errors [22]. Many difficulties could also arise in those MR cases with eccentric jets, as accurately positioning the imaging plane perpendicular to the predominant flow direction, rather than the mitral valve plane itself, is crucial to avoid inaccurate measurements [24,25]. These challenges seem to be addressed using the promising 4D flow analysis, which is similar to classic phase contrast imaging but with flow velocity encoding in all three spatial directions and, additionally, that is relative to the dimension of time [26]. ...
Mitral regurgitation (MR), a primary cause of valvular disease in adults, affects millions and is growing due to an ageing population. Cardiovascular magnetic resonance (CMR) has emerged as an essential tool, offering insights into valvular and myocardial pathology when compared to the primary imaging modality, echocardiography. This review highlights CMR’s superiority in high-resolution volumetric assessment and tissue characterization, including also advanced techniques like late gadolinium enhancement imaging, parametric mapping, feature tracking and 4D flow analysis. These techniques provide a deeper understanding of MR’s pathophysiology and its effect on cardiac chambers, enabling CMR to surpass echocardiography in predicting hard clinical outcomes and left ventricular (LV) remodelling post mitral valve surgery. Despite its advantages, CMR’s application faces limitations like cost, lack of standardization, and susceptibility to arrhythmia artifacts. Nonetheless, as technological advancements continue and new evidence emerges, CMR’s role in MR assessment is set to expand, offering a more nuanced and personalized approach to cardiac care. This review emphasizes the need for further research and standardized protocols to maximize CMR’s potential in MR management.
... The development of MRI over recent decades has given rise to the emergence of time-resolved two-or three-dimensional (2D or 3D) cine phasecontrast magnetic resonance imaging (PC-MRI). 7,8 Hemodynamic patterns through vascular areas of interest may be quantified using PC-MRI. Its availability has expanded to encompass most modern MR systems, making it increasingly preferred for the evaluation of blood flow, cardiac function, and valve performance in both the heart and large vessels. ...
For numerically studying blood flow in a pathological vessel under the influence of a magnetic field, it is necessary to develop an approach that tracks the moving tissue and accounts for interactions between the fluid, the arterial wall, and the magnetic field. The current study discusses a mathematical approach of the fluid's motion under the influence of a magnetic field using fluid mechanics principles. A mixed Euler–Lagrange formulation is introduced to mathematically describe the blood flow in the aneurysm during the entire cardiac cycle. Blood is considered a Newtonian, incompressible, and electrically conducting fluid, subjected to a static and uniform magnetic field. Generalized curvilinear coordinates are used to transform the transport equations into body-fitted geometries and provide a manageable form of equations. The system of equations related to motion consists of a coupled and nonlinear system of partial differential equations (PDEs). The discretization of the PDEs is performed using the finite volume method. The addition of the Lorentz force in the momentum PDEs describes the applied uniform magnetic field in the blood flow. Due to strong coupling and nonlinear terms, a simultaneous solution approach is applied. The results show that the magnetic field strongly influences blood flow, reducing the velocity field q¯ and increasing the pressure drop, Δp.
... In the study by Hong et al., to build the VP loop in the ascending aorta, the aortic velocity was measured immediately after the acquisition of invasive pressures at the center of the LVOT with close attention paid to obtaining an angle of the Doppler signal to aortic blood flow close to 0° (11). Noteworthy, in studies aimed to measure the valvuloarterial impedance in patients with AS using magnetic resonance imaging, flow measurement is routinely performed at the LVOT or just above the valve (15,16). The flow measurement in the LVOT, where complex flow is less prominent, is thought to provide a more accurate measurement of forward flow (17). ...
Background
Up to one-fifth of patients continue to have poor quality of life after transcatheter aortic valve implantation (TAVI), with an additional similar proportion not surviving 1 year after the procedure. We aimed to assess the value of a new method based on an integrated analysis of left ventricular outflow tract flow velocity and aortic pressure to predict objective functional improvement and prognosis after TAVI.
Methods
In a cohort of consecutive patients undergoing TAVI, flow velocity–pressure integrated analysis was obtained from simultaneous pressure recordings in the ascending aorta and flow velocity recordings in the left ventricular outflow tract by echocardiography. Objective functional improvement 6 months after TAVI was assessed through changes in a 6-min walk test and NT-proBNP levels. A clinical follow-up was conducted at 2 years.
Results
Of the 102 patients studied, 82 (80.4%) showed objective functional improvement. The 2-year mortality of these patients was significantly lower (9% vs. 44%, p = 0.001). In multivariate analysis, parameter “(Pressure at Vmax − Pressure at Vo)/Vmax” was found to be an independent predictor for objective improvement. The C-statistic was 0.70 in the overall population and 0.78 in the low-gradient subgroup. All echocardiographic parameters and the valvuloarterial impedance showed a C-statistic of <0.6 for the overall and low-gradient patients. In a validation cohort of 119 patients, the C-statistic was 0.67 for the total cohort and 0.76 for the low-gradient subgroup.
Conclusion
This new method allows predicting objective functional improvement after TAVI more precisely than the conventional parameters used to assess the severity of aortic stenosis, particularly in low-gradient patients.
... Velocity encoding was set to 150 cm/s and increased for repeat imaging if aliased. All PC sequences were planned with region of interest in the CMR scanner isocenter to reduce potential background phase-offset errors [26]. Other PC parameters: typical FOV 350 × 280 mm, TR 5.1 ms, TE 3.2 ms, flip angle 15°, temporal resolution 28 ms, number of signal averages 1, SENSE factor 2, turbo field echo factor 3, turbo field echo acquisition duration 30.8 ms, slice thickness 8 mm, 30 phases, phase percentage 100%, in-plane spatial resolution 2.5 × 2.5 mm, matrix 140 × 112, Cartesian sampling, and typical acquisition times 12-15 s for breath-held sequences. ...
Background
When feasible, guidelines recommend mitral valve repair (MVr) over mitral valve replacement (MVR) to treat primary mitral regurgitation (MR), based upon historic outcome studies and transthoracic echocardiography (TTE) reverse remodeling studies. Cardiovascular magnetic resonance (CMR) offers reference standard biventricular assessment with superior MR quantification compared to TTE. Using serial CMR in primary MR patients, we aimed to investigate cardiac reverse remodeling and residual MR post-MVr vs MVR with chordal preservation.
Methods
83 patients with ≥ moderate-severe MR on TTE were prospectively recruited. 6-min walk tests (6MWT) and CMR imaging including cine imaging, aortic/pulmonary through-plane phase contrast imaging, T1 maps and late-gadolinium-enhanced (LGE) imaging were performed at baseline and 6 months after mitral surgery or watchful waiting (control group).
Results
72 patients completed follow-up (Controls = 20, MVr = 30 and MVR = 22). Surgical groups demonstrated comparable baseline cardiac indices and co-morbidities. At 6-months, MVr and MVR groups demonstrated comparable improvements in 6MWT distances (+ 57 ± 54 m vs + 64 ± 76 m respectively, p = 1), reduced indexed left ventricular end-diastolic volumes (LVEDVi; − 29 ± 21 ml/m ² vs − 37 ± 22 ml/m ² respectively, p = 0.584) and left atrial volumes (− 23 ± 30 ml/m ² and − 39 ± 26 ml/m ² respectively, p = 0.545). At 6-months, compared with controls, right ventricular ejection fraction was poorer post-MVr (47 ± 6.1% vs 53 ± 8.0% respectively, p = 0.01) compared to post-MVR (50 ± 5.7% vs 53 ± 8.0% respectively, p = 0.698). MVR resulted in lower residual MR-regurgitant fraction (RF) than MVr (12 ± 8.0% vs 21 ± 11% respectively, p = 0.022). Baseline and follow-up indices of diffuse and focal myocardial fibrosis (Native T1 relaxation times, extra-cellular volume and quantified LGE respectively) were comparable between groups. Stepwise multiple linear regression of indexed variables in the surgical groups demonstrated baseline indexed mitral regurgitant volume as the sole multivariate predictor of left ventricular (LV) end-diastolic reverse remodelling, baseline LVEDVi as the most significant independent multivariate predictor of follow-up LVEDVi, baseline indexed LV end-systolic volume as the sole multivariate predictor of follow-up LV ejection fraction and undergoing MVR (vs MVr) as the most significant (p < 0.001) baseline multivariate predictor of lower residual MR.
Conclusion
In primary MR, MVR with chordal preservation may offer comparable cardiac reverse remodeling and functional benefits at 6-months when compared to MVr. Larger, multicenter CMR studies are required, which if the findings are confirmed could impact future surgical practice.
... Furthermore, phase-contrast (PC) CMR provides a tool to non-invasively quantify blood flow and thus assess valvular function [16]. Cardiac valves, especially the aortic and mitral valves, are visualized by dynamic b-SSFP imaging in order to identify the morphology and function. ...
Marfan syndrome (MFS) is an inherited autosomal-dominant connective tissue disorder with multiorgan involvement including musculoskeletal, respiratory, cardiovascular, ocular, and skin manifestations. Life expectancy in patients with MFS is primarily determined by the degree of cardiovascular involvement. Aortic disease is the major cardiovascular manifestation of MFS. However, non-aortic cardiac diseases, such as impaired myocardial function and arrhythmia, have been increasingly acknowledged as additional causes of morbidity and mortality. We present two cases demonstrating the phenotypical variation in patients with MFS and how CMR (Cardiovascular Magnetic Resonance) could serve as a “one stop shop” to retrieveS all the necessary information regarding aortic/vascular pathology as well as any potential underlying arrhythmogenic substrate or cardiomyopathic process.
... In patients with AS, especially in bicuspid aortic valve patients, measurement is routinely performed at the LVOT or just above the valve. [21,22] Flow measurement in the LVOT or directly above the AV, where complex flow is less prominent is thought to provide a more accurate Summation of these resistances (as in the case of Z VA and Z VA-INS ) may lead to an overestimate. [24] Secondly, inconsistencies in the measurement of AV gradient on TTE, and more recently CMR, are well described. ...
... The out-of-plane motion may pose an issue in the segmentation of scans and give rise to inaccurate area estimates, particularly if this analysis was repeated on the right side. Previous studies have quantified this movement on the aortic side to be less than 1 cm [33,34], and so area measurements should remain largely unaffected, especially as regional wall properties remain constant. Finally, from a methodological standpoint, this study used retrospective data, and the CMR data are of Cartesian type (~30 ms temporal resolution). ...
This study aimed to investigate the effect of pulmonary regurgitation (PR) on left ventricular ventriculo–arterial (VA) coupling in patients with repaired tetralogy of Fallot (ToF). It was hypothesised that increasing PR severity results in a smaller forward compression wave (FCW) peak in the aortic wave intensity, because of right-to-left ventricular interactions. The use of cardiovascular magnetic resonance (CMR)-derived wave-intensity analysis provided a non-invasive comparison between patients with varying PR degrees. A total of n = 201 patients were studied and both hemodynamic and wave-intensity data were compared. Wave-intensity peaks and areas of the forward compression and forward expansion waves were calculated as surrogates of ventricular function. Any extent of PR resulted in a significant reduction in the FCW peak. A correlation was found between aortic distensibility and the FCW peak, suggesting unfavourable (VA) coupling in patients that also present stiffer ascending aortas. Data suggest that VA coupling is affected by increased impedance.
... However, comprehensive hemodynamics of the cardiovascular system can be difficult to measure due to the limitation of existing clinical tools. For instance, magnetic resonance imaging (MRI) and computed tomography (CT) has a lower temporal resolution [6][7][8]. Cardiac catheterization can evaluate cardiac function, but it is invasive and can only achieve the information within a limited region instead of comprehensive hemodynamics [9]. Doppler echocardiography has a high temporal resolution but cannot evaluate hemodynamics precisely. ...
Hemodynamics in the aorta from computational fluid dynamics (CFD) simulations can provide a comprehensive analysis of relevant cardiovascular diseases. Coupling the three-element Windkessel model with the patient-specific CFD simulation to form a multi-scale model is a trending approach to capture more realistic flow fields. However, a set of parameters (e.g., R_c, R_p, and C) for the Windkessel model need to be tuned case by case to reflect patient-specific flow conditions. In this study, we propose a fast approach to estimating these parameters under both physiological and pathological conditions. The approach consists of the following steps: (1) finding geometric resistances for each branch using a steady CFD simulation; (2) using the pattern search algorithm to search the parameter spaces by solving the flow circuit system with the consideration of geometric resistances; (3) performing the multi-scale modeling of aortic flow with the optimized Windkessel model parameters. The method was validated through a series of numerical experiments to show its flexibility and robustness, including physiological and pathological flow distributions at each downstream branch from a healthy aortic geometry or a stenosed geometry. This study demonstrates a flexible and computationally efficient way to capture patient-specific hemodynamics in the aorta, facilitating the personalized biomechanical analysis of aortic flow.
... Although remarkable advances have been made in medical imaging, offering progressively detailed anatomy and flow information (in some cases), there are no tools or imaging modalities available to invasively or noninvasively quantify local and global hemodynamics. Phase-contrast magnetic resonance imaging (MRI) can provide velocity field but it has a lower temporal resolution than Doppler echocardiography (DE) [29,30]. It is important to note that magnetic resonance imaging (MRI) cannot be used for patients with most implanted medical devices except for MRI-conditional devices. ...
Cardiovascular (CV) disease impacts tens of millions of people annually and carries a massive global economic burden. Continued advances in medical imaging, hardware and computational efficiency are leading to an increased interest in the field of cardiovascular computational modelling to help combat the devastating impact of CV disease. This review article will focus on a computational modelling technique known as lumped parameter modelling (LPM). Due to its rapid computation time, ease of automation and relative simplicity, LPM holds the potential of aiding in the early diagnosis of CV disease, assisting clinicians in determining personalized and optimal treatments and offering a unique in-silico setting to study cardiac and circulatory diseases. In addition, it is one of the many tools that are needed in the eventual development of patient specific cardiovascular “digital twin” frameworks. This review focuses on how the personalization of cardiovascular lumped parameter models are beginning to impact the field of patient specific cardiovascular care. It presents an in-depth examination of the approaches used to develop current predictive LPM hemodynamic frameworks as well as their applications within the realm of cardiovascular disease. The roles of these models in higher order blood flow (1D/3D) simulations are also explored in addition to the different algorithms used to personalize the models. The article outlines the future directions of this field and the current challenges and opportunities related to the translation of this technology into clinical settings.
... All the 4D flow analysis methods used in this study underestimated peak flow velocities by approximately 1 m/s as compared to 2D flow and TTE, the MIP-based approach described by Rose et al. yielding slightly higher values than the commercially available methods. Narrow and oblique jets are a known challenge in optimising CMR phase-contrast flow velocity measurements [28]. Indeed, a possible cause for the observed underestimation of the peak flow velocity is the intravoxel phase averaging due to a decreased spatial resolution in the 4D flow technique. ...
Background
Aortic valve stenosis (AS) is the most prevalent valvular disease in the developed countries. Four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) is an emerging imaging technique, which has been suggested to improve the evaluation of AS severity compared to two-dimensional (2D) flow and transthoracic echocardiography (TTE). We investigated the reliability of CMR 2D flow and 4D flow techniques in measuring aortic transvalvular peak systolic flow in patients with severe AS.
Methods
We prospectively recruited 90 patients referred for aortic valve replacement due to severe AS (73.3 ± 11.3 years, aortic valve area 0.7 ± 0.1 cm ² , and 54/36 tricuspid/bicuspid), and 10 non-valvular disease controls. All the patients underwent echocardiography and 2D flow and 4D flow CMR. Peak flow velocity measurements were compared using Wilcoxon signed rank sum test and Bland–Altman analysis.
Results
4D flow underestimated peak flow velocity in the AS group when compared with TTE (bias − 1.1 m/s, limits of agreement ± 1.4 m/s) and 2D flow (bias − 1.2 m/s, limits of agreement ± 1.6 m/s). The differences between values obtained by TTE (median 4.3 m/s, range 2.7–6.1 m/s) and 2D flow (median 4.5 m/s, range 2.9–6.5 m/s) compared to 4D flow (median 3.1 m/s, range 1.7–5.1 m/s) were significant (p < 0.001). The difference between 2D flow and TTE were insignificant (bias 0.07 m/s, limits of agreement ± 1.5 m/s). In non-valvular disease controls, peak flow velocity was measured higher by 4D flow than 2D flow (1.4 m/s, 1.1–1.7 m/s and 1.3 m/s, 1.1–1.5 m/s, respectively; bias 0.2 m/s, limits of agreement ± 0.16 m/s).
Conclusions
CMR 4D flow significantly underestimates systolic peak flow velocity in patients with severe AS. 2D flow, in turn, estimated the AS velocity accurately, with measured peak flow velocities comparable to TTE.
... In our hospital, we have used the QFlow technique to measure pelvic and lower limb venous blood flow. Although this application is rarely encountered, it should be applicable because, even though QFlow usually underestimates the blood flow velocity when blood flow is turbulent and when a vessel diameter is small [30][31][32][33][34], the iliac veins and FVs are usually large, and their blood flow usually presents as laminar flow; therefore, related blood flow measurements are generally accurate. Combining TRANCE MRI and QFlow provides a safe and convenient diagnostic tool for surveying and following up on lower limb venous disease. ...
Background and Objectives: Compression of the common iliac veins (CIV) is not always associated with lower extremity symptoms. This study analyzed this issue from the perspective of patient venous blood flow changes using quantitative flow magnetic resonance imaging. Materials and Methods: After we excluded patients with active deep vein thrombosis, the mean flux (MF) and mean velocity (MV) of the popliteal vein, femoral vein, and external iliac vein (EIV) were compared between the left and right sides. Results: Overall, 26 of the patients had unilateral CIV compression, of which 16 patients had symptoms. No significant differences were noted in the MF or MV of the veins between the two sides. However, for the 10 patients without symptoms, the EIV MF of the compression side was significantly lower than the EIV MF of the non-compression side (p = 0.04). The receiver operating characteristic curve and chi-squared analyses showed that when the percentage difference of EIV MF between the compression and non-compression sides was ≤−18.5%, the relative risk of associated lower extremity symptoms was 0.44 (p = 0.016). Conclusions: If a person has compression of the CIV, a decrease in EIV blood flow rate on the compression side reduces the rate of symptom occurrence.
... Furthermore, measurement of aortic flow velocity by CMR poses different challenges, mainly related to the size and placement of voxels relative to a narrow jet. In patients with AS, especially in bicuspid aortic valve patients, measurement is routinely performed at the LV outflow tract (LVOT) or just above the valve [49,50]. Flow measurement in the LVOT or directly above the AV, where complex flow is less prominent is thought to provide a more accurate measurement of forward flow. ...
Aortic valve stenosis (AS) is no longer considered to be a disease of fixed left ventricular (LV) afterload (due to an obstructive valve), but rather, functions as a series circuit with important contributions from both the valve and ageing vasculature. Patients with AS are frequently elderly, with hypertension and a markedly remodelled aorta. The arterial component is sizable, and yet, the contribution of ventricular afterload has been difficult to determine. Arterial stiffening increases the speed of propagation of the blood pressure wave along the central arteries (estimated as the pulse wave velocity), which results in an earlier return of reflected waves. The effect is to augment blood pressure in the proximal aorta during systole, increasing the central pulse pressure and, in turn, placing even greater afterload on the heart. Elevated global LV afterload is known to have adverse consequences on LV remodelling, function and survival in patients with AS. Consequently, there is renewed focus on methods to estimate the relative contributions of local versus global changes in arterial mechanics and valvular haemodynamics in patients with AS. We present a review on existing and upcoming methods to quantify valvulo-arterial impedance and thereby global LV load in patients with AS.
... Phase-contrast magnetic resonance imaging (MRI) can offer the fluid dynamics. However, MRI has a lower temporal resolution than doppler echocardiography (DE) (Elkins and Alley, 2007;Kilner et al., 2007). It is important to note that, due to the high risk of the magnetic field of the machine for patients with implanted devices, MRI cannot be used for patients with most implanted medical devices except safely for MRI-conditional devices (Orwat et al., 2014). ...
Due to the high individual differences in the anatomy and pathophysiology of patients, planning individualized treatment requires patient-specific diagnosis. Indeed, hemodynamic quantification can be immensely valuable for accurate diagnosis, however, we still lack precise diagnostic methods for numerous cardiovascular diseases including complex (and mixed) valvular, vascular, and ventricular interactions (C3VI) which is a complicated situation made even more challenging in the face of other cardiovascular pathologies. Transcatheter aortic valve replacement (TAVR) is a new less invasive intervention and is a growing alternative for patients with aortic stenosis. In a recent paper, we developed a non-invasive and Doppler-based diagnostic and monitoring computational mechanics framework for C3VI, called C3VI-DE that uses input parameters measured reliably using Doppler echocardiography. In the present work, we have developed another computational-mechanics framework for C3VI (called C3VI-CT). C3VI-CT uses the same lumped-parameter model core as C3VI-DE but its input parameters are measured using computed tomography and a sphygmomanometer. Both frameworks can quantify: (1) global hemodynamics (metrics of cardiac function); (2) local hemodynamics (metrics of circulatory function). We compared accuracy of the results obtained using C3VI-DE and C3VI-CT against catheterization data (gold standard) using a C3VI dataset (N = 49) for patients with C3VI who undergo TAVR in both pre and post-TAVR with a high variability. Because of the dataset variability and the broad range of diseases that it covers, it enables determining which framework can yield the most accurate results. In contrast with C3VI-CT, C3VI-DE tracks both the cardiac and vascular status and is in great agreement with cardiac catheter data.
... To expand anatomical information, clinicians rely mainly on cardiac catheterization data to assess flow and pressure through the circulatory system, instead of sophisticated methods such as fluid dynamics, but this is invasive, high risk, and expensive 56,57 . MRI may provide a 3-D velocity field, but it is costly and impossible for many patients with a low temporal resolution 30,[58][59][60] . In this study, three different CoA degrees have been developed, and an FSI model is applied to evaluate the CoA severity's impact on haemodynamics correctly. ...
Coarctation of the aorta (CoA) is a congenital tightening of the proximal descending aorta. Flow quantification can be immensely valuable for an early and accurate diagnosis. However, there is a lack of appropriate diagnostic approaches for a variety of cardiovascular diseases, such as CoA. An accurate understanding of the disease depends on measurements of the global haemodynamics (criteria for heart function) and also the local haemodynamics (detailed data on the dynamics of blood flow). Playing a significant role in clinical processes, wall shear stress (WSS) cannot be measured clinically; thus, computation tools are needed to give an insight into this crucial haemodynamic parameter. In the present study, in order to enable the progress of non-invasive approaches that quantify global and local haemodynamics for different CoA severities, innovative computational blueprint simulations that include fluid–solid interaction models are developed. Since there is no clear approach for managing the CoA regarding its severity, this study proposes the use of WSS indices and pressure gradient to better establish a framework for treatment procedures in CoA patients with different severities. This provides a platform for improving CoA therapy on a patient-specific level, in which physicians can perform treatment methods based on WSS indices on top of using a mere experience. Results show how severe CoA affects the aorta in comparison to the milder cases, which can give the medical community valuable information before and after any intervention.
... To expand anatomical information, clinicians rely mainly on cardiac catheterization data to assess flow and pressure through the circulatory system, instead of sophisticated methods such as fluid dynamics, but this is invasive, high risk, and expensive [46,47]. MRI may provide a 3-D velocity field, but it is costly and impossible for many patients with a low temporal resolution [20,[48][49][50]. In this study, three different CoA degrees have been developed, and an FSI model is applied to evaluate the CoA severity's impact on haemodynamics correctly. ...
Coarctation of the aorta (CoA) is a congenital tightening of the proximal descending aorta. Flow quantification can be immensely valuable for an early and accurate diagnosis. However, there is a lack of appropriate diagnostic approaches for a variety of cardiovascular diseases, such as CoA. An accurate understanding of the disease depends on measurements of the global haemodynamics (criteria for heart function) and also the local haemodynamics (detailed data on the dynamics of blood flow). Playing a significant role in clinical processes, wall shear stress (WSS) cannot be measured clinically; thus, computation tools are needed to give an insight into this crucial haemodynamic parameter. In the present study, in order to enable the progress of non-invasive approaches that quantify global and local haemodynamics for different CoA severities, innovative computational blueprint simulations that include fluid-solid interaction (FSI) models are developed. Since there is no specific routine for managing the CoA regarding its severity, this study investigates haemodynamics in regions where clinicians do not have any information that would help physicians introduce a framework when and where initiating the intervention.
... Two-dimensional velocity-encoded cine magnetic resonance imaging flow analysis is an established technique for assessing hemodynamics in cardiovascular magnetic resonance (MRI) [1][2][3] in congenital heart diseases (CHD) [2]. ...
Abstract
Background
Comparing four-dimensional flow against two-dimensional flow measurements in patients with complex flow pattern is still lacking. This study aimed to compare four-dimensional against the two-dimensional flow measurement in patients with bicuspid aortic valve and to test potentials of four-dimensional operator-dependent sources of error.
Results
The two- and four-dimensional flow data sets of sixteen patients with bicuspid aortic valve and eighteen healthy subjects were studied. Flow analyses were performed by two observers blindly. Patients with bicuspid aortic valve mean differences between the two- and four-dimensional measurements in both observers were − 8 and − 4 ml, respectively. Four-dimensional measurements resulted in systematically higher flow values than the two-dimensional flow in bicuspid aortic valve patients. The upper and lower limits of agreement between the two- and four-dimensional measurements by both observers were + 12/− 28 ml and + 14/− 21 ml, respectively. In the healthy volunteers, mean differences between the two- and four-dimensional measurements in both observers were ± 0 and + 1 ml, respectively. The upper and lower limits of agreement between the two- and four-dimensional measurements by both observers were + 21/− 18 ml and + 12/− 13 ml, respectively. Inter-observer variability in four-dimensional flow measurement was 4% mean net forward flow in bicuspid aortic valve patients and 8% in healthy volunteers.
Conclusion
Inter-observer variability in four-dimensional flow assessment is 8% or less which is acceptable for clinical cardiac MRI routine. There is close agreement of two- and four-dimensional flow tools in normal and complex flow pattern. In complex flow pattern, however, four-dimensional flow measurement picks up 4–9% higher flow values. It seems, therefore, that four-dimensional flow is closer to real flow values than two-dimensional flow, which is however to be proven by further studies.
... Phase-contrast imaging, however, allows accurate measurement of aortic blood velocity and flow and is often considered a noninvasive gold standard for forward LV SV measurement. 10,[28][29][30] Our results also showed that SV and AVA obtained using the CMR volumetric method were systematically higher than those using phase-contrast CMR. It is unlikely that overestimation of SV and AVA by the volumetric method was caused exclusively by mitral regurgitation, as <10% of our cohort had more than trace mitral regurgitation. ...
Background
In aortic stenosis, accurate measurement of left ventricular stroke volume (SV) is essential for the calculation of aortic valve area (AVA) and the assessment of flow status. Current American Society of Echocardiography and European Association of Cardiovascular Imaging guidelines suggest that measurements of left ventricular outflow tract diameter (LVOTd) at different levels (at the annulus vs 5 or 10 mm below) yield similar measures of SV and AVA. The aim of this study was to assess the effect of the location of LVOTd measurement on the accuracy of SV and AVA measured on transthoracic echocardiography (TTE) compared with cardiovascular magnetic resonance (CMR).
Methods
One hundred six patients with aortic stenosis underwent both TTE and CMR. SV was estimated on TTE using the continuity equation with LVOTd measurements at four locations: at the annulus and 2, 5, and 10 mm below annulus. SV was also determined on CMR using phase contrast acquired in the aorta (SVCMR-PC), and a hybrid AVACMR-PC was calculated by diving SVCMR-PC by the transthoracic echocardiographic Doppler aortic velocity-time integral. Comparison between methods was made using Bland-Altman analysis.
Results
Compared with the referent method of phase-contrast CMR for the estimation of SVCMR-PC and AVACMR-PC (SVCMR-PC 83 ± 16 mL, AVACMR-PC 1.27 ± 0.35 cm²), the best agreement was obtained by measuring LVOTd at the annulus or 2 mm below (P = NS), whereas measuring 5 and 10 mm below the annulus resulted in significant underestimation of SV and AVA by up to 15.9 ± 17.3 mL and 0.24 ± 0.28 cm², respectively (P < .01 for all). Accuracy for classification of low flow was best at the annulus (86%) and 2 mm below (82%), whereas measuring 5 and 10 mm below the annulus significantly underperformed (69% and 61%, respectively, P < .001).
Conclusions
Measuring LVOTd at the annulus or very close to it provides the most accurate measures of SV and AVA, whereas measuring LVOTd 5 or 10 mm below significantly underestimates these parameters and leads to significant overestimation of the severity of aortic stenosis and prevalence of low-flow status.
... Most importantly, cardiac catheterization only provides access to the blood pressure in very limited regions rather than details of the physiological pulsatile flow and pressures throughout the heart and the circulatory system. Phase-contrast magnetic resonance imaging can provide 3-D velocity field but it has poor temporal resolution [62][63][64] , is costly, lengthy and not possible for many patients with implanted devices. Doppler echocardiography (DE) is potentially the most versatile tool for hemodynamics diagnosis [65][66][67] . ...
Coarctation of the aorta (COA) is a congenital narrowing of the proximal descending aorta. Although accurate and early diagnosis of COA hinges on blood flow quantification, proper diagnostic methods for COA are still lacking because fluid-dynamics methods that can be used for accurate flow quantification are not well developed yet. Most importantly, COA and the heart interact with each other and because the heart resides in a complex vascular network that imposes boundary conditions on its function, accurate diagnosis relies on quantifications of the global hemodynamics (heart-function metrics) as well as the local hemodynamics (detailed information of the blood flow dynamics in COA). In this study, to enable the development of new non-invasive methods that can quantify local and global hemodynamics for COA diagnosis, we developed an innovative fast computational-mechanics and imaging-based framework that uses Lattice Boltzmann method and lumped-parameter modeling that only need routine non-invasive clinical patient data. We used clinical data of patients with COA to validate the proposed framework and to demonstrate its abilities to provide new diagnostic analyses not possible with conventional diagnostic methods. We validated this framework against clinical cardiac catheterization data, calculations using the conventional finite-volume method and clinical Doppler echocardiographic measurements. The diagnostic information, that the framework can provide, is vitally needed to improve clinical outcomes, to assess patient risk and to plan treatment.
... Most importantly, cardiac catheterization only provides access to the blood pressure in very limited regions rather than details of the physiological pulsatile flow and pressures throughout the heart and the circulatory system. Phase-contrast magnetic resonance imaging can provide flow but it has poor temporal resolution, is costly, lengthy and not possible for many patients with implanted devices 18,19 . Doppler echocardiography (DE) is potentially the most versatile tool for hemodynamics as it is low-cost and risk-free and has a high temporal resolution. ...
Hemodynamics quantification is critically useful for accurate and early diagnosis, but we still lack proper diagnosticmethods for many cardiovascular diseases. Furthermore, as most interventions intend to recover the healthy condition, the ability to monitor and predict hemodynamics following interventions can have significant impacts on saving lives. Predictive methods are rare, enabling prediction of effects of interventions, allowing timely and personalized interventions and helping critical clinical decision making about life-threatening risks based on quantitative data. In this study, an innovative non-invasive imaged-based patient-specific diagnostic, monitoring and predictive tool (called C3VI-CMF) was developed, enabling quantifying (1) details of physiological flow and pressures through the heart and circulatory system; (2) heart function metrics. C3VI-CMF also predicts the breakdown of the effects of each disease constituents on the heart function. Presently, neither of these can be obtained noninvasively in patients and when invasive procedures are undertaken, the collected metrics cannot be by any means as complete as the ones C3VI-CMF provides. C3VI-CMF purposefully uses a limited number of noninvasive input parameters all of which can be measured using Doppler echocardiography and sphygmomanometer. Validation of C3VI-CMF, against cardiac catheterization in forty-nine patients with complex cardiovascular diseases, showed very good agreement with the measurements.
... Forward and reverse flow is derived by integrating the velocity of each pixel and its area over the cardiac cycle. 6,200 This information can then be used to calculate RF (reverse volume/forward volume * 100%). In most instances this direct method is the preferred technique for assessment of AR as it is the most validated and is not affected by coexisting valvular regurgitant lesions. ...
... It is possible that these sequence modifications have minimized overall phase offsets and improved the accuracy of our baseline flow measurements. As several others have reported, different platforms or protocols produce varying degrees of background velocity errors [13][14][15][16]. However, the potential might exist to optimize phase-contrast imaging sequences on other platforms as well. ...
Background
The need for background error correction in phase-contrast flow analysis has historically posed a challenge in cardiac magnetic resonance (MR) imaging. While previous studies have shown that phantom correction improves flow measurements, it impedes scanner workflow.Objective
To evaluate the efficacy of self-calibrated non-linear phase-contrast correction on flows in pediatric and congenital cardiac MR compared to phantom correction as the standard.Materials and methodsWe retrospectively identified children who had great-vessel phase-contrast and static phantom sequences acquired between January 2015 and June 2015. We applied a novel correction method to each phase-contrast sequence post hoc. Uncorrected, non-linear, and phantom-corrected flows were compared using intraclass correlation. We used paired t-tests to compare how closely non-linear and uncorrected flows approximated phantom-corrected flows. In children without intra- or extracardiac shunts or significant semilunar valvular regurgitation, we used paired t-tests to compare how closely the uncorrected pulmonary-to-systemic flow ratio (Qp:Qs) and non-linear Qp:Qs approximated phantom-corrected Qp:Qs.ResultsWe included 211 diagnostic-quality phase-contrast sequences (93 aorta, 74 main pulmonary artery [MPA], 21 left pulmonary artery [LPA], 23 right pulmonary artery [RPA]) from 108 children (median age 15 years, interquartile range 11–18 years). Intraclass correlation showed strong agreement between non-linear and phantom-corrected flow measurements but also between uncorrected and phantom-corrected flow measurements. Non-linear flow measurements did not more closely approximate phantom-corrected measurements than did uncorrected measurements for any vessel. In 39 children without significant shunting or regurgitation, mean non-linear Qp:Qs (1.07; 95% confidence interval [CI] = 1.01, 1.13) was no closer than mean uncorrected Qp:Qs (1.06; 95% CI = 1.00, 1.13) to mean phantom-corrected Qp:Qs (1.02; 95% CI = 0.98, 1.06).Conclusion
Despite strong agreement between self-calibrated non-linear and phantom correction, cardiac flows and shunt calculations with non-linear correction were no closer to phantom-corrected measurements than those without background correction. However, phantom-corrected flows also demonstrated minimal differences from uncorrected flows. These findings suggest that in the current era, more accurate phase-contrast flow measurements might limit the need for background correction. Further investigation of the clinical impact and optimal methods of background correction in the pediatric and congenital cardiac population is needed.
... Previous studies have shown that PC-CMR provides the most accurate measurement of cardiac output and LV volumes in cases with mitral, aortic, or pulmonary valve regurgitation [11][12][13][14]. ...
Background
To establish a more accurate technique for the assessment of the left ventricular function correlated with patients’ clinical condition avoiding the miscalculation of the ejection fraction in valvular regurgitation. A prospective study carried out between July 2018 and June 2019. The studied group included 35 subjects, 25 patients with valvular regurgitation, and 10 healthy control subjects. All subjects underwent cardiovascular magnetic resonance examination to evaluate the ejection fraction by two methods: the volumetric method which assesses stroke volume via subtraction of the end-systolic volume from the end-diastolic volume, and phase-contrast method which assesses the aortic stroke volume via a through-plane phase contrast across the aortic valve. The sensitivity, specificity, P value and the area under the curve of both methods were calculated.
Results
In the healthy group, using the volumetric method, the calculated mean ejection fraction was 62.44 ± 6.61, while that calculated by the phase-contrast method was 64.34 ± 5.33, with a non-significant difference ( P = 0.62) showing the validity of the phase-contrast method. In the patients’ group, by using the volumetric method, the calculated mean ejection fraction was 47.17 ± 14.31%, which was significantly higher than that calculated by the phase-contrast method (29.39 ± 7.98%) ( P = 0.02). According to the results of the calculation of the ejection fraction by the volumetric method, there were 18 patients (72%) having impaired cardiac function and 7 (28%) patients of normal function; while according to the phase-contrast method, all the 25 patients had impaired cardiac function. The current study shows that the phase-contrast cardiac magnetic resonance had 89.29% sensitivity and 85.7% specificity in diagnosing impaired cardiac function with the area under the curve of 0.87 ( P = 0.00).
Conclusion
The phase-contrast cardiac magnetic resonance can provide a better assessment of the ejection fraction in valvular regurgitation.
... Free breathing, respiratory navigator-based signal-averaging techniques can be applied to improve the temporal or spatial resolution if necessary. The potential for background flow offset errors can be reduced by ensuring that phase-contrast sequences are acquired with the region of interest (the ascending aorta) located at the iso-centre of the magnet to minimize any inhomogeneities in the magnetic field 24 . Background phase offset errors can significantly hinder the accuracy of flow measurement 25 , and background flow correction processes should be used, such as the interpolated automatic sequence 26 , where available. ...
Mitral regurgitation (MR) is a common valvular heart disease and is the second most frequent indication for heart valve surgery in Western countries. Echocardiography is the recommended first-line test for the assessment of valvular heart disease, but cardiovascular magnetic resonance imaging (CMR) provides complementary information, especially for assessing MR severity and to plan the timing of intervention. As new CMR techniques for the assessment of MR have arisen, standardizing CMR protocols for research and clinical studies has become important in order to optimize diagnostic utility and support the wider use of CMR for the clinical assessment of MR. In this Consensus Statement, we provide a detailed description of the current evidence on the use of CMR for MR assessment, highlight its current clinical utility, and recommend a standardized CMR protocol and report for MR assessment. In this Consensus Statement, Garg and colleagues describe the current evidence on the use of cardiovascular magnetic resonance imaging for the assessment of mitral regurgitation, highlight its current clinical utility, and recommend a standardized imaging protocol and report.
... Cardiac MRI was performed as previously described in detail [24], and in accordance with the recommendations from the Society for Cardiovascular Magnetic Resonance [25]. Cardiac output was assessed by phase-contrast imaging with blood velocity encoded through-plane images of the mid-ascending aorta, a technique considered to provide robust and accurate cardiac output measurements [24,26]. The images were analysed using the software Medviso Segment version 2.1 R6005 (http://segment.heiberg.se) ...
Background:
The effectiveness of adrenaline during resuscitation continues to be debated despite being recommended in international guidelines. There is evidence that the β-adrenergic receptor (AR) effects of adrenaline are harmful due to increased myocardial oxygen consumption, post-defibrillation ventricular arrhythmias and increased severity of post-arrest myocardial dysfunction. Esmolol may counteract these unfavourable β-AR effects and thus preserve post-arrest myocardial function. We evaluated whether a single dose of esmolol administered prior to adrenaline preserves post-arrest cardiac output among successfully resuscitated animals in a novel, ischaemic cardiac arrest porcine model.
Methods:
Myocardial infarction was induced in 20 anaesthetized pigs by inflating a percutaneous coronary intervention (PCI) balloon in the circumflex artery 15 min prior to induction of ventricular fibrillation. After 10 min of untreated VF, resuscitation with veno-arterial extracorporeal membrane oxygenation (VA-ECMO) was initiated and the animals were randomized to receive an injection of either 1 mg/kg esmolol or 9 mg/ml NaCl, prior to adrenaline. Investigators were blinded to allocation. Successful defibrillation was followed by a 1-h high-flow VA-ECMO before weaning and an additional 1-h stabilization period. The PCI-balloon was deflated 40 min after inflation. Cardiac function pre- and post-arrest (including cardiac output) was assessed by magnetic resonance imaging (MRI) and invasive pressure measurements. Myocardial injury was estimated with MRI, triphenyl tetrazolium chloride (TTC) staining and serum concentrations of cardiac troponin T.
Results:
Only seven esmolol and five placebo-treated pigs were successfully resuscitated and available for post-arrest measurements (p = 0.7). MRI revealed severe but similar reductions in post-arrest cardiac function with cardiac output 3.5 (3.3, 3.7) and 3.3 (3.2, 3.9) l/min for esmolol and control (placebo) groups, respectively (p = 0.7). The control group had larger left ventricular end-systolic and end-diastolic ventricular volumes compared to the esmolol group (75 (65, 100) vs. 62 (53, 70) ml, p = 0.03 and 103 (86, 124) vs. 87 (72, 91) ml, p = 0.03 for control and esmolol groups, respectively). There were no other significant differences in MRI characteristics, myocardial infarct size or other haemodynamic measurements between the two groups.
Conclusions:
We observed similar post-arrest cardiac output with and without a single dose of esmolol prior to adrenaline administration during low-flow VA-ECMO in an ischaemic cardiac arrest pig model.
... Uniquely, velocity encoding CINE PC-MRI provides the most accurate method available to assess cardiac output and measurements of aortic, pulmonary and mitral valve regurgitations, all non-invasively and without ionizing radiation and contrast material (25)(26)(27)(28). Cardiac-gated time-resolved 3D CINE PC-MRI allows for registration of blood flow in cardiac chambers and in targeted arteries (29,30), however, with limited applicability due to very time-consuming acquisition protocols (31). ...
Background:
Non-invasive computer tomography (CT)- and magnetic resonance (MR)-based cardiac imaging still remains challenging in rodents. To investigate the robustness of non-invasive multimodality cardiac imaging in rabbits using clinical-grade CT and MR scanners.
Methods:
A total of 16 rabbits (2.7-4.0 kg) serially underwent cardiac-gated imaging using a clinical-grade 256-row CT and a 1.5 Tesla MR-scanner at baseline and at 4-month follow-up (16±1 weeks). Image analysis included image quality (5-grade scale), left ventricular (LV) volumes, LV stroke volume, LV diameters, LV wall thickness and ejection fraction (LVEF).
Results:
Cardiac MR (CMR) and CT angiography (CTA) provide images with an overall good image quality (excellent or good quality: CMR 82% vs. CTA 78%, P=0.68). Linear regression analysis demonstrated a good correlation of all diameters (diam.) and volumes (vol.) as assessed by CTA and CMR (diam.: r=0.9, 95% CI: 0.8-0.9; vol.: r=0.8, 95% CI: 0.6-0.9; P<0.0001 for both). CTA-based volumetric analysis revealed slightly higher LVEF values as compared to CMR (CTA: 64%±1%, CMR: 59%±1%, P=0.002). Analysis of inter-/intra-observer agreement demonstrated excellent agreements for diameters (CMR: 98.5%/98.7%; CTA: 98.2%/97.4%) and volumes (CMR: 99.9%/98.8%; CTA 98.7%/98.7%). Finally, serial CMR- and CTA-based assessment of cardiac diameters and volumes delivered excellent intersession agreements of baseline versus follow-up data (diam.: CMR: r=0.89; CTA: r=0.92; vol.: CMR: r=0.87; CTA: r=0.96, P<0.0001 for all).
Conclusions:
Multimodality non-invasive assessment of cardiac function and aortic hemodynamics is feasible and robust in rabbits using clinical-grade and MR and CT scanners. These imaging modalities could improve serial cardiac assessment for disease monitoring in preclinical settings.
... The assessment of cardiovascular haemodynamics is important for the diagnosis and monitoring of many cardiovascular diseases including valvular heart diseases, congenital heart defects and aortopathies. In addition to echocardiography, phase-contrast (PC) cardiovascular magnetic resonance imaging (CMR) provides a tool to non-invasively quantify blood flow [1]. 4D flow CMR allows the assessment of the 3D, threedirectional, time-resolved blood flow [2]. ...
Background:
Three-dimensional time-resolved phase-contrast cardiovascular magnetic resonance (4D flow CMR) enables the quantification and visualisation of blood flow, but its clinical applicability remains hampered by its long scan time. The aim of this study was to evaluate the use of compressed sensing (CS) with on-line reconstruction to accelerate the acquisition and reconstruction of 4D flow CMR of the thoracic aorta.
Methods:
4D flow CMR of the thoracic aorta was acquired in 20 healthy subjects using CS with acceleration factors ranging from 4 to 10. As a reference, conventional parallel imaging (SENSE) with acceleration factor 2 was used. Flow curves, net flows, peak flows and peak velocities were extracted from six contours along the aorta. To measure internal data consistency, a quantitative particle trace analysis was performed. Additionally, scan-rescan, inter- and intraobserver reproducibility were assessed. Subsequently, 4D flow CMR with CS factor 6 was acquired in 3 patients with differing aortopathies. The flow patterns resulting from particle trace visualisation were qualitatively analysed.
Results:
All collected data were successfully acquired and reconstructed on-line. The average acquisition time including respiratory navigator efficiency with CS factor 6 was 5:02 ± 2:23 min while reconstruction took approximately 9 min. For CS factors of 8 or less, mean differences in net flow, peak flow and peak velocity as compared to SENSE were below 2.2 ± 7.8 ml/cycle, 4.6 ± 25.2 ml/s and - 7.9 ± 13.0 cm/s, respectively. For a CS factor of 10 differences reached 5.4 ± 8.0 ml/cycle, 14.4 ± 28.3 ml/s and - 4.0 ± 12.2 cm/s. Scan-rescan analysis yielded mean differences in net flow of - 0.7 ± 4.9 ml/cycle for SENSE and - 0.2 ± 8.5 ml/cycle for CS factor of 6.
Conclusions:
A six- to eightfold acceleration of 4D flow CMR using CS is feasible. Up to a CS acceleration rate of 6, no statistically significant differences in measured flow parameters could be observed with respect to the reference technique. Acquisitions in patients with aortopathies confirm the potential to integrate the proposed method in a clinical routine setting, whereby its main benefits are scan-time savings and direct on-line reconstruction.
... Although transthoracic echocardiography is the first-line cardiovascular imaging modality for adults with CHD, 4D flow MRI is increasingly used as a means of obtaining accurate values for difficult flow measurements (eg, pulmonary regurgitation). Cardiac MRI is the current reference standard for measuring both right and left ventricular volumes (20,21). The more complex the CHD, the greater the usefulness of cardiac 4D flow MRI in understanding the disease and providing guidance for planning surgery or other treatments. ...
In-plane phase-contrast (PC) imaging is now a routine component of MRI of regional blood flow in the heart and great vessels. In-plane PC MRI provides a volumetric, isotropic, time-resolved cine sequence that enables three-directional velocity encoding, a technique known as four-dimensional (4D) flow MRI. Recent advances in 4D flow MRI have shortened imaging times, while progress in big-data processing has improved dataset pre- and postprocessing, thereby increasing the feasibility of 4D flow MRI in clinical practice. Important technical issues include selection of the optimal velocity-encoding sensitivity before acquisition and preprocessing of the raw data for phase-offset corrections. Four-dimensional flow MRI provides unprecedented capabilities for comprehensive analysis of complex blood flow patterns using new visualization tools such as streamlines and velocity vectors. Retrospective multiplanar navigation enables flexible retrospective flow quantification through any plane across the volume with good accuracy. Current flow parameters include forward flow, reverse flow, regurgitation fraction, and peak velocity. Four-dimensional flow MRI also supplies advanced flow parameters of use for research, such as wall shear stress. The vigorous burgeoning of new applications indicates that 4D flow MRI is becoming an important imaging modality for cardiovascular disorders. This article reviews the main technical issues of 4D flow MRI and the different parameters provided by it and describes the main applications in cardiovascular diseases, including congenital heart disease, cardiac valvular disease, aortic disease, and pulmonary hypertension. Online supplemental material is available for this article.
... Our findings are in line with Roes et al. [23], who compared 4D flow and 2D flow volumes over the AV and the other intracardiac valves in both healthy volunteers and in patients with AR. 4D flow CMR showed excellent agreement over all intracardiac valves in both groups in their study. 2D flow in healthy subjects demonstrated only average correlation between valves in a study by Kilner et al. [24]. ...
Background
Aortic regurgitation (AR) and subclinical left ventricular (LV) dysfunction expressed by myocardial deformation imaging are common in patients with transposition of the great arteries after the arterial switch operation (ASO). Echocardiographic evaluation is often hampered by reduced acoustic window settings. Cardiovascular magnetic resonance (CMR) imaging provides a robust alternative as it allows for comprehensive assessment of degree of AR and LV function. The purpose of this study is to validate CMR based 4-dimensional flow quantification (4D flow) for degree of AR and feature tracking strain measurements for LV deformation assessment in ASO patients.
Methods
A total of 81 ASO patients (median 20.6 years, IQR 13.5–28.4) underwent CMR for 4D and 2D flow analysis. CMR global longitudinal strain (GLS) feature tracking was compared to echocardiographic (echo) speckle tracking. Agreements between and within tests were expressed as intra-class correlation coefficients (ICC).
Results
Eleven ASO patients (13.6%) showed AR > 5% by 4D flow, with good correlation to 2D flow assessment (ICC = 0.85). 4D flow stroke volume of the aortic valve demonstrated good agreement to 2D stroke volume over the mitral valve (internal validation, ICC = 0.85) and multi-slice planimetric LV stroke volume (external validation, ICC = 0.95). 2D flow stroke volume showed slightly less, though still good agreement with 4D flow (ICC = 0.78) and planimetric LV stroke volume (ICC = 0.87). GLS by CMR was normal (− 18.8 ± 4.4%) and demonstrated good agreement with GLS and segmental analysis by echocardiographic speckle tracking (GLS = − 17.3 ± 3.1%, ICC of 0.80).
Conclusions
Aortic 4D flow and CMR feature tracking GLS analysis demonstrate good to excellent agreement with 2D flow assessment and echocardiographic speckle tracking, respectively, and can therefore reliably be used for an integrated and comprehensive CMR analysis of aortic valve competence and LV deformation analysis in ASO patients.
... For example, our model coronary flow (1.5-3.0 ml/beat in average [31]) and closing volume of the valve (3.3 ± 1.2 ml per beat [32]) were not taken into account. Another possible difference between the used model and clinical scenario might be aortic compliance and aortic root movement that can lead to underestimation of AR up to 20% and 15%, respectively [22,33]. Nevertheless, by measuring close to the aortic valve and by using a method to compensate for motion of the valve annulus, e.g., a moving slice velocity mapping technique [34], these possible underestimations of AR may be neither present in our model nor in clinical practice. ...
Accuracy of aortic regurgitation (AR) quantification by magnetic resonance (MR) imaging in the presence of a transcatheter heart valve (THV) remains to be established. We evaluated the accuracy of cardiac MR velocity mapping for quantification of antegrade flow (AF) and retrograde flow (RF) across a THV and the optimal slice position to use in cardiac MR imaging. In a systematic and fully controlled laboratory ex vivo setting, two THVs (Edwards SAPIEN XT, Medtronic CoreValve) were tested in a porcine model (n = 1) under steady flow conditions. Results showed a high level of accuracy and precision. For both THVs, AF was best measured at left ventricular outflow tract level, and RF at ascending aorta level. At these levels, MR had an excellent repeatability (ICC > 0.99), with a tendency to overestimate (4.6 ± 2.4% to 9.4 ± 7.0%). Quantification of AR by MR velocity mapping in the presence of a THV was accurate, precise, and repeatable in this pilot study, when corrected for the systematic error and when the best MR slice position was used. Confirmation of these results in future clinical studies would be a step forward in increasing the accuracy of the assessment of paravalvular AR severity.
... Time-resolved 3D phase contrast magnetic resonance imaging (MRI) with three-directional velocity encoding, also known as 4D-flow MRI, is a novel imaging modality capable of measuring blood flow in the three principal directions and as a function of time, which allows for the quantification of blood velocity in both the heart and the great vessels [4]. 4D-flow MRI can be used to calculate hemodynamic parameters in vivo, such as wall shear stress (WSS) [5,6]. ...
Progressive ascending aortic dilatation has been observed after mechanical aortic valve replacement (mAVR), possibly due to altered blood flow and wall shear stress (WSS) patterns induced by their bileaflet design. We examined the effect of mAVR on WSS in the ascending aorta using time-resolved 4D flow MRI. Fifteen patients with mechanical aortic valve prostheses, 10 patients with bicuspid aortic valve disease and 10 healthy individuals underwent thoracic 4D flow MRI. Peak systolic hemodynamic parameters (velocity and WSS) and vessel diameters were assessed in the ascending aorta. In addition, three-dimensional per-voxel analysis was used to compare velocity and WSS between patient groups and healthy controls. Peak aortic diameters were significantly higher in mAVR and BAV patients compared to healthy controls (p = 0.011). Mean aortic diameters were comparable between mAVR and BAV patients. No differences in 4D flow MRI-derived mean blood flow velocity and peak WSS were found between the three groups. Compared to healthy controls, mean WSS was significantly lower in mAVR patients (p = 0.031). Per-voxel analysis revealed no increased WSS in the ascending aortic wall and significantly lower velocity and WSS values in mAVR patients compared to healthy controls. In contrast, regions of significantly increased outer lumen velocities and WSS in BAV patients compared to healthy controls were found. This study shows that there is no increased ascending aortic WSS after mAVR. Our results suggest that, in contrast to BAV patients, there is no indication for intensified follow-up of the ascending aorta after mAVR.
Septal defects are the most common congenital heart defects. Although often being classified as simple congenital cardiac defects, many of these anomalies are of hemodynamic importance. Cardiovascular magnetic resonance (CMR) imaging is able to describe the anatomical details of atrial and ventricular septal defects as well as their associated anomalies. In addition, CMR provides clinically important information about the hemodynamic significance of a particular defect by enabling accurate shunt quantification and precise measurement of atrial and ventricular volumes. In this chapter, the different atrial and ventricular septal defects and the CMR techniques for their evaluation are described.
4D Flow MRI is an advanced imaging technique for comprehensive non-invasive assessment of the cardiovascular system. The capture of the blood velocity vector field throughout the cardiac cycle enables measures of flow, pulse wave velocity, kinetic energy, wall shear stress, and more. Advances in hardware, MRI data acquisition and reconstruction methodology allow for clinically feasible scan times. The availability of 4D Flow analysis packages allows for more widespread use in research and the clinic and will facilitate much needed multi-center, multi-vendor studies in order to establish consistency across scanner platforms and to enable larger scale studies to demonstrate clinical value.
Aims:
To determine the phase-contrast cardiovascular magnetic resonance imaging (PC-CMR) slice-position above aortic leaflet-attachment-plane (LAP) that provides flow-velocity, -volume and aortic valve area (AVA) measurements with best agreement to invasive and echocardiographic measurements in aortic stenosis (AS).
Methods and results:
Fifty-five patients with moderate/severe AS underwent cardiac catheterization, transthoracic echocardiography (TTE) and CMR. Overall, 171 image-planes parallel to LAP were measured via PC-CMR between 22 mm below and 24 mm above LAP. AVA via PC-CMR was calculated as flow-volume divided by peak-velocity during systole. Stroke volume (SV) and AVA were compared to volumetric SV and invasive AVA via the Gorlin-formula, respectively. Above LAP, SV by PC-CMR showed no significant dependence on image-plane-position and correlated strongly with volumetry (rho: 0.633, p < 0.001, marginal-mean-difference (MMD): 1 ml, 95 % confidence-interval (CI): -4 to 6). AVA assessed in image-planes 0-10 mm above LAP differed significantly from invasive measurement (MMD: -0.14 cm2, 95 %CI: 0.08-0.21). In contrast, AVA-values by PC-CMR measured 10-20 mm above LAP showed good agreement with invasive determination without significant MMD (0.003 cm2, 95 %CI: -0.09 to 0.09). Within these measurements, a plane 15 mm above LAP resulted in the lowest bias (MMD: 0.02 cm2, 95 %CI:-0.29 to 0.33). SV and AVA via TTE correlated moderately with volumetry (rho: 0.461, p < 0.001; bias: 15 ml, p < 0.001) and cardiac catheterization (rho: 0.486, p < 0.001, bias: -0.13 cm2, p < 0.001), respectively.
Conclusion:
PC-CMR measurements at 0-10 mm above LAP should be avoided due to significant AVA-overestimation compared to invasive determination. AVA-assessment by PC-CMR between 10 and 20 mm above LAP did not differ from invasive measurements, with the lowest intermethodical bias measured 15 mm above LAP.
A 37-year-old Chinese man was admitted to the department of cardiology of the First Hospital of Jilin University for intermittent palpitation for 9 months, aggravating with chest pain for 3 days. After several examinations, he was diagnosed with giant left ventricular fistula of the diagonal branch of the left coronary artery. After routine treatment, which included improving circulation and administration of dual antiplatelet as well as hypolipidemic drugs among others, the patient’s symptoms did not improve. The fistula was too big for transcatheter occlusion to be performed. A multi-disciplinary suggestion was that the patient be subjected to “surgical closure treatment”; however, for personal reasons, he refused the operation. After discharge, oral beta-blockers were prescribed for the patient. Incidences of congenital coronary arterial fistula in congenital cardiovascular disease are rare, and incidences of the giant fistula being located in the left heart system are even rarer. We report an adult male with a giant left anterior descending diagonal coronary artery left ventricular fistula and show various accessory examination results. Non-invasive ultrasonic cardiography was the first diagnostic option for the disease and pre-admission evaluation. Auxiliary diagnosis and exclusion value of cardiovascular magnetic resonance (CMR) were revealed for the first time. Invasive coronary angiography (ICA) was demonstrated to be the gold standard method again and it was also found that computed tomography angiography (CTA) might be used instead of ICA for determining the exact relationships among anatomic structures. Furthermore, we performed a literature review on the diagnosis and treatment of patients with this condition.
The left ventricle (LV), the aortic valve, and the aorta have tight anatomical and functional interrelations that are essential to ensure adequate cardiac output and arterial pressures, which, in turn, are key for optimal organ perfusion. The size and elastic properties of the aorta have an influence on the aortic valve hemodynamics and global LV afterload. Thus, proximal aortic characteristic impedance and arterial wave reflections have a direct impact on flow-gradient patterns (and discordant grading), LV systolic function, and the degree of myocardial fibrosis. Conversely, the state of LV myocardial function as well as the size and shape of the LV outflow tract and the morphology and function of the aortic valve may alter the cardiac outflow and the arterial hemodynamics, which will have an impact on the morphology of the aorta. These interactions between the LV, aortic valve, and aorta, known as ventricular–valvular–vascular coupling, have important implications with regard to the diagnosis and treatment of aortic valve disease. Advanced noninvasive cardiovascular imaging and hemodynamic assessments with pressure–flow relations have a growing role in unveiling this intricate relationship between valve, vessel, and myocardium.
Purpose:
To elucidate the influence of through-plane heart motion on the assessment of aortic regurgitation (AR) severity using phase contrast magnetic resonance imaging (PC-MRI).
Approach:
A patient cohort with chronic AR (n = 34) was examined with PC-MRI. The regurgitant volume (RVol) and fraction (RFrac) were extracted from the PC-MRI data before and after through-plane heart motion correction and was then used for assessment of AR severity.
Results:
The flow volume errors were strongly correlated to aortic diameter (R = 0.80, p < 0.001) with median (IQR 25%;75%): 16 (14; 17) ml for diameter>40mm, compared with 9 (7; 10) ml for normal aortic size (p < 0.001). RVol and RFrac were underestimated (uncorrected:64 ± 37 ml and 39 ± 17%; corrected:76 ± 37 ml and 44 ± 15%; p < 0.001) and ~ 20% of the patients received lower severity grade without correction.
Conclusion:
Through-plane heart motion introduces relevant flow volume errors, especially in patients with aortic dilatation, that may result in underestimation of the severity grade in patients with chronic AR.
Echocardiographic evaluation of chronic aortic regurgitation (AR) severity can lead to diagnostic ambiguity due to few feasible parameters or incongruent findings. The aim of the present study was to improve the diagnostic usefulness of left ventricular (LV) enlargement and aortic end-diastolic flow velocity (EDFV) using cardiovascular magnetic resonance (CMR) as reference. Patients (n = 120) were recruited either prospectively (n = 45) or retrospectively (n = 75). Severe AR (CMR regurgitant fraction > 33%) was present in 51% and 93% of the patients had LV ejection fraction ≥ 50%. EDFV and LV end-diastolic volume index (EDVI) were assessed by echocardiography using the traditional (excluding trabeculae) and recommended approach (including trabeculae). The patients were randomised to a derivation (n = 60) or a test group (n = 60). EDVI (traditional/recommended) to rule in (>99/118 ml/m²) and rule out severe AR (≤75/87 ml/m²) were identified using ROC analyses in the derivation group. The corresponding thresholds for EDFV were >17 cm/s and ≤10 cm/s. In the test group, the positive/negative likelihood ratios to rule in/rule out severe AR using EDVI were 10.0/0.14 (traditional), 6.2/0.11 (recommended), and using EDFV were 10.2/0.08. To rule in and rule out severe AR using derived cut-off values instead of >2 SD reduced the false positives by 92%, whereas using EDFV ≤10 cm/s instead of ≤20 cm/s reduced the false negatives by 94%. In conclusion, EDVI and EDFV as quantitative parameters are useful to rule in or rule out severe chronic AR. Importantly, other causes of LV enlargement have to be considered.
Multimodality imaging provides important information to guide patient selection and pre-procedural decision making for shunt lesions in CHD. While echocardiography, CT, and CMR are well-established, 3D printing and now virtual reality imaging are beginning to show promise.
Aortic stenosis is an acute and chronic cardiovascular disease that often coexists with other complex valvular, ventricular and vascular diseases (C3VD). Transcatheter aortic valve replacement is an emerging less invasive intervention for patients with aortic stenosis. Although hemodynamics quantification is critical for accurate and early diagnosis of aortic stenosis and C3VD, proper diagnostic methods for these diseases are still lacking because fluid-dynamics methods, that can be used as engines of new diagnostic tools, are not well developed yet. As the heart resides in a sophisticated vascular network which imposes a load on the heart, effective diagnosis requires quantifications of the global hemodynamics (metrics of circulatory function and metrics of cardiac function), and of the local hemodynamics (cardiac fluid dynamics). To enable the development of new non-invasive diagnostic methods that can quantify local and global hemodynamics, we developed an innovative computational-mechanics and imaging-based framework that only needs patient data routinely and non-invasively measured in clinics. We not only validated the framework against clinical cardiac catheterization and Doppler echocardiographic measurements but also, we demonstrated its diagnostic utility in providing novel analyses and interpretations of clinical data.
Phase-contrast magnetic resonance imaging (PC-MRI) also known as velocity-encoded cine magnetic resonance imaging (VENC-MRI) is the primary technique for blood flow measurements, qualitative and quantitative information on both flow volume and flow velocity in the great vessels. In this chapter, we discuss the basics of phase-contrast MRI techniques, describe the pitfalls and limitations associated with phase-contrast imaging, provide guidelines for post processing and data analysis, and present specific clinical applications.
Valvular heart disease (VHD) is common worldwide and accounts for a large volume of cardiac surgeries performed yearly. Cardiovascular evaluation is required to determine the valve morphology, abnormal flow dynamics such as stenosis/regurgitation, the severity of VHD, and associated hemodynamic complications due to pressure or volume overload. Cardiac imaging is used as a surveillance tool to monitor the progression of VHD, to assess the severity, preoperative surgical risk, optimal time for intervention, and postoperative follow-up.
Purpose: To show that adjustment of velocity encoding (VENC) for phase-contrast (PC) flow volume measurements is not necessary in modern MR scanners with effective background velocity offset corrections. Approach: The independence on VENC was demonstrated theoretically, but also experimentally on dedicated phantoms and on patients with chronic aortic regurgitation (
n
=
17
) and one healthy volunteer. All PC measurements were performed using a modern MR scanner, where the pre-emphasis circuit but also a subsequent post-processing filter were used for effective correction of background velocity offset errors. Results: The VENC level strongly affected the velocity noise level in the PC images and, hence, the estimated peak flow velocity. However, neither the regurgitant blood flow volume nor the mean flow velocity displayed any clinically relevant dependency on the VENC level. Also, the background velocity offset was shown to be close to zero (
<
0.6
cm
/
s
) for a VENC range of 150 to
500
cm
/
s
, adding no significant errors to the PC flow volume measurement. Conclusions: Our study shows that reliable PC flow volume measurements are feasible without adjustment of the VENC parameter. Without the need for VENC adjustments, the scan time can be reduced for the benefit of the patient.
Objective
Cardiac MRI is quickly emerging as the gold standard for assessment of mitral regurgitation, most commonly with the indirect method subtracting forward flow in aorta from volumetric segmentation of the left ventricle. We aimed to investigate how aortic flow measurements with increasing distance from the aortic valve affect calculated mitral regurgitations and whether measurements were influenced by breath-hold regimen.
Methods
Free-breathing and breath-hold phase contrast flows were measured in aorta at valve level, sinotubular (ST) junction, mid-ascending aorta and in the pulmonary trunk. Flow measurements were pairwise compared, and subsequently, after exclusion of patients with visible mitral and tricuspid regurgitations for left-sided and right-sided comparisons, respectively, flow-measured stroke volumes were compared with ventricular volumetric segmentations.
Results
Thirty-nine participants without arrhythmias or structural abnormalities of the large vessels were included. Stroke volumes measured with free-breathing and breath-hold flow decreased equally with increasing distance to the aortic valves (breath-hold flow: aortic valve 105.6±20.8 mL, ST junction 101.5±20.7 mL, mid-ascending aorta 98.1±21.5 mL). After exclusion of atrioventricular regurgitations, stroke volumes determined by volumetric measurements were higher compared with values determined by flow measurements, corresponding to ‘false’ atrioventricular regurgitations of 8.0%±5.8% with flow measured at valve level, 11.6%±5.2% at the ST junction and 15.3%±5.0% at the mid-ascending aorta.
Conclusions
Stroke volumes determined by flow decrease throughout the proximal aorta and are systematically lower than volumetrically measured stroke volumes. The indirect method systematically overestimates mitral regurgitations, especially with increasing distance from the aortic valves.
Background
Phase contrast velocity mapping sequences utilising ultrashort echo time (UTE) radial k-space sequences have been used to reduce intravoxel dephasing at high velocities. We evaluated the accuracy of the UTE flow sequence for mitral regurgitation (MR) quantification, including patients with atrial fibrillation.
Methods
Forty patients underwent cardiac MRI for indirect MR quantification by assessment of aortic flow using a UTE phase contrast sequence (TE 0.65 ms) combined with left ventricular stroke volume. Retrospective ECG-gating was used in sinus rhythm (30 patients), prospective ECG-triggering in atrial fibrillation (10). MR was also quantified by a standard phase contrast sequence (TE 2.85 ms, standard flow method) and by comparing stroke volumes (volumetric method).
Results
UTE flow-derived MR measurement showed modest agreement in sinus rhythm (95% limits of agreement: ±38.2 ml; ±29.8%) and atrial fibrillation (±33.7 ml; ±30.3%) compared to standard flow assessment. There was little systematic bias in sinus rhythm (mean offset −4.4 ml /−3.5% compared to standard flow assessment), but a slight bias towards greater regurgitation in atrial fibrillation (+15.2 ml /+14.0%). There were wider limits of agreement between the UTE flow method and volumetric method than between the regular flow method and the volumetric method in sinus rhythm (±48.4 ml; ±36.4%; mean offset: −12.2 ml /−9.0%) and similar limits of agreement in atrial fibrillation (±29.6 ml; 25.8%; +12.0 ml /+10.3%).
Conclusions
UTE flow imaging is inferior to conventional flow techniques for MR assessment in patients with sinus rhythm as well as atrial fibrillation. However, the number of atrial fibrillation patients in this initial study is small.
A review of cardiovascular clinical and research applications of MRI phase-contrast velocity imaging, also known as velocity mapping or flow imaging. Phase-contrast basic principles, advantages, limitations, common pitfalls and artefacts are described. It can measure many different aspects of the complicated blood flow in the heart and vessels: volume flow (cardiac output, shunt, valve regurgitation), peak blood velocity (for stenosis), patterns and timings of velocity waveforms and flow distributions within heart chambers (abnormal ventricular function) and vessels (pulse-wave velocity, vessel wall disease). The review includes phase-contrast applications in cardiac function, heart valves, congenital heart diseases, major blood vessels, coronary arteries and myocardial wall velocity.
Flow assessment is an integral part of the comprehensive evaluation of the cardiovascular system. Cardiovascular magnetic resonance is well suited for flow assessment due to its non-invasive, multi-plane imaging capability unrestricted by windows of access and its ability to measure blood flow and velocity. Phase-contrast velocity mapping for flow assessment has been incorporated in all commercial scanners. It is versatile, and with appropriate hardware, software and expertise, it should be accurate and reproducible. In this article, we briefly describe the technique and indications for its use in current clinical practice. We suggest some practical tips in using the technique and describe some of the potential sources of errors and ways to overcome them. Finally, we provide several clinical examples demonstrating how to use phase-contrast velocity mapping in a number of acquired and congenital cardiovascular conditions.
Comprehensive assessment of the severity of valvular insufficiency includes quantification of regurgitant volumes. Previous methods lack reliable slice positioning with respect to the valve and are prone to velocity offsets due to through-plane motion of the valvular plane of the heart. Recently, the moving slice velocity mapping technique was proposed. In this study, the technique was applied for quantification of mitral and aortic regurgitation. Time-efficient navigator-based respiratory artifact suppression was achieved by implementing a prospective k-space reordering scheme in conjunction with slice position correction. Twelve patients with aortic insufficiency and three patients with mitral insufficiency were studied. Aortic regurgitant volumes were calculated from diastolic velocities mapped with a moving slice 5 mm distal to the aortic valve annulus. Mitral regurgitant flow was indirectly assessed by measuring mitral inflow at the level of the mitral annulus and net aortic outflow. Regurgitant fractions, derived from velocity data corrected for through-plane motion, were compared to data without correction for through-plane motion. In patients with mild and moderate aortic regurgitation, regurgitant fractions differed by 60% and 15%, on average, when comparing corrected and uncorrected data, respectively. Differences in severe aortic regurgitation were less (7%). Due to the large orifice area of the mitral valve, differences were still substantial in moderate-to-severe mitral regurgitation (19%). The moving slice velocity mapping technique was successfully applied in patients with aortic and mitral regurgitation. The importance of correction for valvular through-plane motion is demonstrated. J. Magn. Reson. Imaging 2001;14:106–112. © 2001 Wiley-Liss, Inc.
Whenever a linear gradient is activated, concomitant magnetic fields with non-linear spatial dependence result. This is a consequence of Maxwell's equations, i.e., within the imaging volume the magnetic field must have zero divergence, and has negligible curl. The concomitant, or Maxwell field has been described in the MRI literature for over 10 years. In this paper, we theoretically and experimentally show the existence of two additional lowest-order terms in the concomitant field, which we call cross-terms. The concomitant gradient cross-terms only arise when the longitudinal gradient Gz is simultaneously active with a transverse gradient (Gx or Gy). The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail. Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described. The feasibility of a joint pulse sequence-reconstruction method, which requires no increase in minimum TE, is demonstrated. Since the lowest-order terms of the concomitant field are proportional to G2/B0, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields.
A brief overview of the history of the application of phase shifts in NMR, and in particular NMR imaging, is presented. The imaging methods include direct phase mapping, Fourier flow imaging (where the flow data are Fourier transformed into one dimension of an image), and alternative methods, where flow-related phase shifts are utilized for flow measurement from the magnitude of the signal. A discussion then follows of the principal errors that can affect the accuracy of the various flow imaging techniques, with particular reference to the phase mapping methods that have been used extensively in our institution. The results from a number of experiments are included to illustrate the extent of the errors and methods of removing or minimizing these effects are suggested.
A technique for measuring blood flow by whole body nuclear magnetic resonance is described. This method uses imaging gradient profiles that combine even echo rephasing with a field echo sequence to overcome the problem of signal loss from flowing blood. The flow velocity component in any desired direction may be measured by appropriate gradient profile modifications, producing velocity dependent phase shifts that can be displayed by phase mapping. The sequence allows for fast repetition so that flow information may be acquired rapidly from many points in the cardiac cycle and has been used in this mode to observe and measure blood flow in the heart chambers and great vessels. Flow measurements in the femoral artery were also carried out using the same technique; these were compared with similar measurements obtained by Doppler ultrasound. The technique can readily be applied using standard imaging equipment and should prove useful in the clinical assessment of many diseases of the cardiovascular system.
In the patient with mitral regurgitation who is being considered for valvular surgery, cardiac catheterization is usually performed to quantify the severity of regurgitation and to determine its influence on left ventricular volumes and systolic function. Magnetic resonance imaging (MRI) potentially provides a rapid, noninvasive method of acquiring these data. Thus, this study was done to determine whether MRI can reliably measure the magnitude of mitral regurgitation and evaluate the effect of regurgitation on left ventricular volumes and systolic function.
Twenty-three subjects (14 women and 9 men 15 to 72 years of age) with (n = 17) or without (n = 6) mitral regurgitation underwent MRI scanning followed immediately by cardiac catheterization. The presence (or absence) of valvular regurgitation was determined, and left ventricular volumes and regurgitant fraction were quantified during each procedure. There was excellent correlation between invasive and MRI assessments of left ventricular end-diastolic (r = .95) and end-systolic (r = .95) volumes and regurgitant fraction (r = .96). All MRI examinations were completed in < 28 minutes.
In the patient with mitral regurgitation, MRI compares favorably with cardiac catheterization for assessment of the magnitude of regurgitation and its influence on left ventricular volumes and systolic function.
Pulmonary regurgitation frequently occurs after surgical correction of tetralogy of Fallot. To date, reliable quantitation of pulmonary regurgitation has not been possible, and therefore the clinical significance of pulmonary regurgitation is controversial. Nuclear magnetic resonance (NMR) velocity mapping allows accurate measurement of volumetric flow. The feasibility and accuracy of NMR velocity mapping to quantify pulmonary regurgitation volumes are studied in patients after Fallot repair.
In 18 patients (mean age, 16.5 +/- 6.5 years), late (12.6 +/- 5.2 years) after Fallot surgery, forward and regurgitant volume flow was measured in the main pulmonary artery with NMR velocity mapping. To validate the measurements of pulmonary forward flow, right ventricular stroke volume was used as an internal reference standard. Pulmonary regurgitation volumes were compared with the differences between the corresponding right and left ventricular stroke volumes. Ventricular volumes were measured with a multisection gradient echo NMR method. In addition, the relation between pulmonary regurgitation and right ventricular volumes was studied. Measurements of pulmonary regurgitation volume with NMR velocity mapping closely corresponded with the tomographically determined volumes (r = .93). Forward pulmonary volume flow was nearly identical to right ventricular stroke volume (r = .98). Pulmonary regurgitation volume was significantly correlated with end-diastolic volume (r = .82, P < .0005), end-systolic volume (r = .63, P < .01), and stroke volume (r = .89, P < .0005) of the right ventricular but not with right ventricular ejection fraction (r = .41, P = NS).
NMR velocity mapping is an accurate method for the noninvasive, volumetric quantification of pulmonary regurgitation after surgical correction of tetralogy of Fallot.
Although several methods have been used clinically to assess aortic regurgitation (AR), there is no "gold standard" for regurgitant volume measurement. Magnetic resonance phase velocity mapping (PVM) can be used for noninvasive blood flow measurements. To evaluate the accuracy of PVM in quantifying AR with a single imaging slice in the ascending aorta, in vitro experiments were performed by using a compliant aortic model. Attention was focused on determining the slice location that provided the best results. The most accurate measurements were taken between the aortic valve annulus and the coronary ostia where the measured (Y) and actual (X) flow rate had close agreement (Y = 0.954 x + 0.126, r2 = 0.995, standard deviation of error = 0.139 L/min). Beyond the coronary ostia, coronary flow and aortic compliance negatively affected the accuracy of the measurements. In vivo measurements taken on patients with AR showed the same tendency with the in vitro results. In making decisions regarding patient treatment, diagnostic accuracy is very important. The results from this study suggest that higher accuracy is achieved by placing the slice between the aortic valve and the coronary ostia and that this is the region where attention should be focused for further clinical investigation.
Whenever a linear gradient is activated, concomitant magnetic fields with non-linear spatial dependence result. This is a consequence of Maxwell's equations, i.e., within the imaging volume the magnetic field must have zero divergence, and has negligible curl. The concomitant, or Maxwell field has been described in the MRI literature for over 10 years. In this paper, we theoretically and experimentally show the existence of two additional lowest-order terms in the concomitant field, which we call cross-terms. The concomitant gradient cross-terms only arise when the longitudinal gradient Gz is simultaneously active with a transverse gradient (Gx or Gy). The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail. Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described. The feasibility of a joint pulse sequence-reconstruction method, which requires no increase in minimum TE, is demonstrated. Since the lowest-order terms of the concomitant field are proportional to G2/B0, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields.
A method for magnetic resonance cine velocity mapping through heart valves with adaptation of both slice offset and angulation according to the motion of the valvular plane of the heart is presented. By means of a subtractive labeling technique, basal myocardial markers are obtained and automatically extracted for quantification of heart motion at the valvular level. The captured excursion of the basal plane is used to calculate the slice offset and angulation of each required time frame for cine velocity mapping. Through-plane velocity offsets are corrected by subtracting velocities introduced by basal plane motion from the measured velocities. For evaluation of the method, flow measurements downstream from the aortic valve were performed both with and without slice adaptation in 11 healthy volunteers and in four patients with aortic regurgitation. Maximum through-plane motion at the aortic root level as calculated from the labeled markers averaged 8.9 mm in the volunteers and 6.5 mm in the patients. The left coronary root was visible in 2-4 (mean: 2.2) time frames during early diastole when imaging with a spatially fixed slice. Time frames obtained with slice adaptation did not contain the coronary roots. Motion correction increased the apparent regurgitant volume by 5.7 +/- 0.4 ml for patients with clinical aortic regurgitation, for an increase of approximately 50%. The proposed method provides flow measurements with correction for through-plane motion perpendicular to the aortic root between the valvular annulus and the coronary ostia throughout the cardiac cycle. Magn Reson Med 42:970-978, 1999.
Comparison of breath-hold MR phase contrast technique in the estimation of cardiac shunt volumes with the invasive oximetric technique.
Seventeen patients with various cardiac shunts (10 ASD, 3 VSD, 1 PDA, 3 PFO) and five healthy volunteers were investigated using a 1.5 Tesla system. The mean flow velocity, the mean volume flow and the transverse area in the ascending aorta and the left and right pulmonary artery were measured using the MR phase contrast breath-hold technique (through plane, FLASH 2D-sequence, TR/TE 11/5 ms, phase length 106 ms, VENC 250 cm/s). The ratio of mean flow in the pulmonary (Qp: sum of mean flows in the left and right pulmonary arteries) and the systemic circulation (Qs: mean flow in the ascending aorta) was calculated and compared with invasively measured Qp:Qs ratios. Oximetry was performed within 24 h of the MR investigation. The non-invasive shunt measurement in the 17 patients showed a mean Qp:Qs ratio of 2.00 +/- 0.86. Comparing the MR data with the invasively measured Qp:Qs showed a correlation coefficient of r = 0.91 (p < 0.001).
Cardiac shunt volumes can be measured reliably using a shorter acquisition time with breath-hold MR phase contrast technique.
Pulmonary vascular resistance (PVR) quantification is important in the treatment of children with pulmonary hypertension. The Fick principle, used to quantify pulmonary artery flow, may be a flawed technique. We describe a novel method of PVR quantification by the use of magnetic resonance (MR) flow data and invasive pressure measurements.
In 24 patients with either suspected pulmonary hypertension or congenital heart disease requiring preoperative assessment, PVR was calculated by the use of simultaneously acquired MR flow and invasive pressure measurements (condition 1). In 19 of the 24 patients, PVR was also calculated at 20 ppm nitric oxide +30% (condition 2) and at 20 ppm nitric oxide +100% oxygen (condition 3), with the use of the MR method. This method proved safe and feasible in all patients. In 15 of 19 patients, PVR calculated by Fick flow was compared with the MR method. At condition 1, Bland-Altman analysis revealed a bias of 2.3% (MR > Fick) and limits of agreement of 50.2% to -45.5%. At condition 2, there was poorer agreement (bias was 28%, and the limits of agreement were 151.3% to 95.2%). At condition 3, there was very poor agreement (bias was 54.2%, and the limits of agreement were 174.4% to -66.0%).
We have demonstrated the feasibility of using simultaneous invasive pressure measurements and MR flow data to measure PVR in humans.
Cardiovascular magnetic resonance (CMR) assessment of mitral regurgitant volume from the subtraction of the right ventricular stroke volume (RVSV) from left ventricular stroke volume (LVSV) has commonly been performed using volumetric techniques. This is sensitive to errors in RVSV visualization and regurgitation of other heart valves, and therefore subtracting aortic flow volume from LVSV may be preferable. The study aim was to compare both techniques in a single CMR examination.
Twenty-eight patients with isolated mitral regurgitation underwent left ventricular (LV) and right ventricular (RV) volumetry and aortic flow volume measurements. Mitral regurgitant fraction (RF) was calculated as either RF(VOL) = [LVSV - RVSV] or RF(FLOW) = [LVSV - aortic flow volume], both expressed as a fraction of LVSV. The agreement of the measurements was assessed as a measure of robustness in clinical practice.
There was good agreement between aortic and pulmonary flow (mean +/- SD difference -0.8 +/- 8.1 ml), and aortic flow volume and RVSV by volumetry (mean difference -2.6 +/- 11.8 ml). Intra- and interobserver variability (SD) of aortic flow volume (+/-6.6 ml and +/-5.3 ml) was superior to that of the RVSV (+/-8.5 ml and +/-12 ml). The intra- and inter-observer variability (SD) of RF(FLOW) was lower (+/-4.8% and +/-7.7%) than by RF(VOL) (+/-6.7% and +/-8.8%).
The RF(FLOW) technique maximized intra- and inter-observer agreement, and is the optimal CMR technique to quantify mitral regurgitation. RF(FLOW) also has the advantage of allowing correction for aortic regurgitation when it is present, and is potentially independent of the effects of tricuspid and pulmonary regurgitation.
Intracardiac shunts including atrial septal defect, ventricular septal defect, endocardial cushion defects, and surgical baffles may be identified, localized, and quantified using cardiac MRI methods. Both dark-blood and bright-blood techniques are helpful to identify anatomy. Contrast enhancement is especially useful for identifying associated vascular anomalies. Dynamic first-pass contrast agent signal-time studies may demonstrate rapid recirculation and shunting. Volumetric and phase contrast cine methods are useful to quantify flow. Pulmonary to systemic (Qp/Qs) flow ratios may be calculated noninvasively by comparing the pulmonary artery flow to the aortic flow measurement.
Cardiovascular magnetic resonance (CMR) is widely recognized as a non-invasive gold standard for quantification of ventricular volumes. In addition, it is an emerging diagnostic modality for clinical evaluation of mitral regurgitation (MR) and aortic regurgitation (AR). CMR facilitates accurate quantitation of regurgitation volumes and regurgitant fraction, but referring physicians are often more comfortable with qualitative measures, and few data exist for correlation of qualitative CMR regurgitation severity with that obtained by more conventional qualitative Doppler echocardiography. Because patients with AR and MR may commonly be assessed by both echocardiography and CMR modalities, consistency between qualitative gradient of regurgitation severity is important for follow-up. Therefore, we sought to define the CMR regurgitant fractions that best correlate with qualitative mild, moderate, and severe regurgitation by color Doppler echocardiography.
Data from 141 consecutive patients (age 53 +/- 15 yr; 43% female) with contemporary (median, 31 days) CMR and echocardiographic data, including 107 regurgitant valves and 70 normal valves, were compared. Thresholds were developed on an initial cohort of patients with 55 regurgitant valves, and subsequently tested on a later cohort of patients with 52 regurgitant valves. Regurgitation fraction (RF) limits that optimized concordance of CMR and echo severity grades were similar for MR and AR and were: mild < or = 15%, moderate 16-25%, moderate-severe 26-48%, severe > 48%.
The current study provides simple qualititative threshold grades for MR and AR severity that allows for standardized reporting of regurgitation severity by CMR and excellent correlation with clinical echocardiography.
Pulmonary incompetence is the single most important factor in the midterm outcome of patients after repair of tetralogy of Fallot. The pulmonary regurgitant volume and, hence, the burden on the right ventricle, is dependent on a subtle balance of orifice size, pulmonary afterload, and right ventricular diastolic physiology. Although the ability to measure pulmonary incompetence has improved significantly, there remain important pitfalls to its assessment in the individual patient, and our interpretation of its implications, regardless of which method is used, remains unsophisticated.
Phase-contrast Cardiovascular Magnetic Resonance Imaging (CMR) generally requires the analysis of stationary tissue adjacent to a blood vessel to serve as a baseline reference for zero velocity. However, for the heart and great vessels, there is often no stationary tissue immediately adjacent to the vessel. Consequently, uncorrected velocity offsets may introduce substantial errors in flow quantification. The purpose of this study was to assess the magnitude of these flow errors and to validate a clinically applicable method for their correction.
In 10 normal volunteers, phase-contrast CMR was used to quantify blood flow in the main pulmonary artery (Qp) and the aorta (Qs). Following image acquisition, phase contrast CMR was performed on a stationary phantom using identical acquisition parameters so as to provide a baseline reference for zero velocity. Aortic and pulmonary blood flow was then corrected using the offset values from the phantom.
The mean difference between pulmonary and aortic flow was 26 +/- 21 mL before correction and 7.1 +/- 6.6 mL after correction (p = 0.002). The measured Qp/Qs was 1.25 +/- 0.20 before correction and 1.05 +/- 0.07 after correction (p = 0.001).
Phase-contrast CMR can have substantial errors in great vessel flow quantification if there is no correction for velocity offset errors. The proposed method of correction is clinically applicable and provides a more accurate measurement of blood flow.
Magnetic resonance imaging assessment of the severity of mitral regurgitation
- Wg Hundley
- Willard Hf Li
- Je
Hundley WG, Li HF, Willard JE, et al. Magnetic resonance imaging assessment of the severity of mitral regurgitation. Circulation 1995;92:1151–1158.
Petti-grewRI,YoganathanAP.Slicelocationdependenceofaorticregur-gitation measurements with CMR phase velocity mapping
- Gp Chatzimavroudis
- Walker
- Pg
- Jn Oshinski
- Franch
Chatzimavroudis GP, Walker PG, Oshinski JN, Franch RH, Petti-grewRI,YoganathanAP.Slicelocationdependenceofaorticregur-gitation measurements with CMR phase velocity mapping. Magn Reson Med 1997;37:545–551