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Realtime accelerated interactive MR imaging with adaptive TSENSE and UNFOLD
Magnetic Resonance in Medicine
Abstract
Reduced field-of-view (FOV) acceleration using time-adaptive sensitivity encoding (TSENSE) or unaliasing by Fourier encoding the overlaps using the temporal dimension (UNFOLD) can improve the depiction of motion in real-time MRI. However, increased computational resources are required to maintain a high frame rate and low latency in image reconstruction and display. A high-performance software system has been implemented to perform TSENSE and UNFOLD reconstructions for real-time MRI with interactive, on-line display. Images were displayed in the scanner room to investigate image-guided procedures. Examples are shown for normal volunteers and cardiac interventional experiments in animals using a steady-state free precession (SSFP) sequence. In order to maintain adequate image quality for interventional procedures, the imaging rate was limited to seven frames per second after an acceleration factor of 2 with a voxel size of 1.8 x 3.5 x 8 mm. Initial experiences suggest that TSENSE and UNFOLD can each improve the compromise between spatial and temporal resolution in real-time imaging, and can function well in interactive imaging. UNFOLD places no additional constraints on receiver coils, and is therefore more flexible than SENSE methods; however, the temporal image filtering can blur motion and reduce the effective acceleration. Methods are proposed to overcome the challenges presented by the use of TSENSE in interactive imaging. TSENSE may be temporarily disabled after changing the imaging plane to avoid transient artifacts as the sensitivity coefficients adapt. For imaging with a combination of surface and interventional coils, a hybrid reconstruction approach is proposed whereby UNFOLD is used for the interventional coils, and TSENSE with or without UNFOLD is used for the surface coils.
... We have previously described the use of magnetic resonance (MR) guidance for interventions in which highquality images are obtained at 5 frames per second with low latency. 5,6 With a new generation of imaging systems (1.5-T Magnetom Espree, Siemens Medical Solutions, Munich, Germany) that include a wider bore (70 cm) and shorter cylinder (120 cm), surgical access to the patient within the magnet becomes quite feasible. Additionally, imaging planes and other parameters can be adjusted interactively to provide multiple simultaneous viewpoints and highlight selected features, making real-time MR ideal for guiding cardiac surgical interventions. ...
... A fully interactive, real-time MRI system as previously described has been developed for intravascular and intracardiac interventions. 6,7 This system comprises an interactive user interface, an operating room large-screen display, specialized pulse sequences, and specialized image reconstruction software. Using this system, multiple oblique planes can be imaged and displayed simultaneously at their respective 3D locations. ...
... The software allows quick adjustment of the imaging planes to allow for real-time device tracking. [21][22][23][24][25] ...
... A necropsy was performed on all animals following valve placement to grossly assess valve placement. 17,21,23,31 ...
Objectives:
To demonstrate the feasibility of Real-time magnetic resonance imaging (rtMRI) guided transcatheter aortic valve replacement (TAVR) with an active guidewire and an MRI compatible valve delivery catheter system in a swine model.
Methods:
The CoreValve system was minimally modified to be MRI-compatible by replacing the stainless steel components with fluoroplastic resin and high-density polyethylene components. Eight swine weighing 60-90 kg underwent rtMRI-guided TAVR with an active guidewire through a left subclavian approach.
Results:
Two imaging planes (long-axis view and short-axis view) were used simultaneously for real-time imaging during implantation. Successful deployment was performed without rapid ventricular pacing or cardiopulmonary bypass. Postdeployment images were acquired to evaluate the final valve position in addition to valvular and cardiac function.
Conclusions:
Our results show that the CoreValve can be easily and effectively deployed through a left subclavian approach using rtMRI guidance, a minimally modified valve delivery catheter system, and an active guidewire. This method allows superior visualization before deployment, thereby allowing placement of the valve with pinpoint accuracy. rtMRI has the added benefit of the ability to perform immediate postprocedural functional assessment, while eliminating the morbidity associated with radiation exposure, rapid ventricular pacing, contrast media renal toxicity, and a more invasive procedure. Use of a commercially available device brings this rtMRI-guided approach closer to clinical reality.
... Selfcalibration is conveniently incorporated in the time-domain approaches. Self-calibrated SENSE has been demonstrated for accelerated spatio-temporal hybrid techniques which rely on dynamic data [25][26][27][28][29][30][31]; a requirement which is not met by standardized CMR protocols used for single cardiac phase black blood FSE imaging of the heart [32,33]. ...
... SCSE-FSE helps to address some of the limitations of previous self-calibrated approaches using SENSE reconstruction tech-niques. These approaches are commonly based upon spatiotemporal hybrid algorithms -a concept behind techniques such as UNFOLD-SENSE, TSENSE, auto-SENSE, and k-t SENSEwhich are applied on a frame-by-frame basis [25][26][27][28][29][30][31]. These techniques share the need of a time series of data (for example CINE imaging or first pass bolus perfusion imaging) and hence do not support single cardiac phase black blood FSE imaging of the heart per se. ...
Design, validation and application of an accelerated fast spin-echo (FSE) variant that uses a split-echo approach for self-calibrated parallel imaging.
For self-calibrated, split-echo FSE (SCSE-FSE), extra displacement gradients were incorporated into FSE to decompose odd and even echo groups which were independently phase encoded to derive coil sensitivity maps, and to generate undersampled data (reduction factor up to R = 3). Reference and undersampled data were acquired simultaneously. SENSE reconstruction was employed.
The feasibility of SCSE-FSE was demonstrated in phantom studies. Point spread function performance of SCSE-FSE was found to be competitive with traditional FSE variants. The immunity of SCSE-FSE for motion induced mis-registration between reference and undersampled data was shown using a dynamic left ventricular model and cardiac imaging. The applicability of black blood prepared SCSE-FSE for cardiac imaging was demonstrated in healthy volunteers including accelerated multi-slice per breath-hold imaging and accelerated high spatial resolution imaging.
SCSE-FSE obviates the need of external reference scans for SENSE reconstructed parallel imaging with FSE. SCSE-FSE reduces the risk for mis-registration between reference scans and accelerated acquisitions. SCSE-FSE is feasible for imaging of the heart and of large cardiac vessels but also meets the needs of brain, abdominal and liver imaging.
... The imaging system provides MR sequences for surgical planning and system registration, as well as rtMRI for intervention . The rtMRI interactive system [22] consists of an interactive user interface, and operating room large screen display, specialized pulse sequences, and customized image reconstruction software. With this system, multiple oblique planes can be imaged and displayed simultaneously at their respective 3-D locations. ...
... The imaging system provides MR sequences for surgical planning and system registration, as well as rtMRI for intervention. The rtMRI interactive system[22]consists of an interactive user interface, and operating room large screen display, specialized pulse sequences, and customized image reconstruction software. With this system, multiple oblique planes can be imaged and displayed simultaneously at their respective 3-D locations. ...
We present a pneumatic actuated robotic assistant system for transapical aortic valve replacement under MRI guidance in a beating heart. This is a minimally invasive procedure that is currently performed manually inside the MRI bore. A robotic assistance system that integrates an interactive real-time MRI system, a robotic arm with a newly developed robotic valve delivery module, as well as user interfaces for the physician to plan the procedure and manipulate the robot, would be advantageous for the procedure. An Innomotion arm with hands-on cooperative interface was used as a device holder. A compact MRI compatible robotic delivery module was developed for delivering both balloon-expandable and self-expanding prostheses. A compact fiducial that can be placed close to the volume of interest and requires a single image plane was used for image-based robot registration. The system provides different user interfaces at various stages of the procedure. We present the development and evaluation of the components and the system in ex-vivo experiments.
... Vascular as well as soft tissue visualization can easily be performed simultaneously. The development of real-time magnetic resonance imaging (rtMRI) allows this imaging modality to provide intraoperative guidance for delivery of prosthetic aortic valves [13]. MRI also provides the ability to assess results, such as ventricular and valvular function, and myocardial perfusion, immediately after intervention. ...
... 1.5-T Magnetom Espree (Siemens Medical Solutions, Munich, Germany) was used for the intervention and post intervention assessment. A real-time Interactive MRI system connected to the MRI scanner provides visualization of the progress of the procedure [13]. This system is comprised of an interactive user interface, an operating room large-screen display, gated pulse sequences, and image reconstruction software. ...
Aortic valves have been implanted on self-expanding (SE) and balloon-expandable (BE) stents minimally invasively. We have demonstrated the advantages of transapical aortic valve implantation (tAVI) under real-time magnetic resonance imaging (rtMRI) guidance. Whether there are different advantages to SE or BE stents is unknown. We report rtMRI-guided tAVI in a porcine model using both SE and BE stents, and compare the differences between the stents.
A total of 22 Yucatan pigs (45-57 kg) underwent tAVI. Commercially available stentless bioprostheses (21-25 mm) were mounted on either BE platinum-iridium stents or SE-nitinol stents. rtMRI guidance was employed as the intraoperative imaging. Markers on both types of stents were used to enhance visualization in rtMRI. Pigs were allowed to survive and had follow-up MRI scans and echocardiography at 1, 3, and 6 months postoperatively.
rtMRI provided excellent visualization of the aortic valve implantation mounted on both stent types. The implantation times were shorter with the SE stents (60 ± 14s) than with the BE stents (74 ± 18s), (p=0.027). The total procedure time was 31 and 37 min, respectively (p=0.12). It was considerably easier to manipulate the SE stent during deployment, without hemodynamic compromise. This was not always the case with the BE stent, and its placement occasionally resulted in coronary obstruction and death. Long-term results demonstrated stability of the implants with preservation of myocardial perfusion and function over time for both stents.
SE stents were easier to position and deploy, thus leading to fewer complications during tAVI. Future optimization of SE stent design should improve clinical results.
... Congenital cardiologists are targeting increasingly complex pathologies for minimally invasive catheterbased therapies [8][9][10]. By improving cardiovascular magnetic resonance (CMR)-guidance, the eventual is to advance CMR-guided cardiac interventions not previously possible with x-ray fluoroscopy alone [11][12][13][14][15]. ...
Background:
Today's standard of care, in the congenital heart disease (CHD) population, involves performing cardiac catheterization under x-ray fluoroscopy and cardiac magnetic resonance (CMR) imaging separately. The unique ability of CMR to provide real-time functional imaging in multiple views without ionizing radiation exposure has the potential to be a powerful tool for diagnostic and interventional procedures. Limiting fluoroscopic radiation exposure remains a challenge for pediatric interventional cardiologists. This pilot study's objective is to establish feasibility of right (RHC) and left heart catheterization (LHC) during invasive CMR (iCMR) procedures at our institution in the CHD population. Furthermore, we aim to improve simultaneous visualization of the catheter balloon tip, MR-conditional guidewire, and cardiac/vessel anatomy during iCMR procedures.
Methods:
Subjects with CHD were enrolled in a pilot study for iCMR procedures at 1.5 T with an MR-conditional guidewire. The CMR area is located adjacent to a standard catheterization laboratory. Using the interactive scanning mode for real-time control of the imaging location, a dilute gadolinium-filled balloon-tip catheter was used in combination with an MR-conditional guidewire to obtain cardiac saturations and hemodynamics. A recently developed catheter tracking technique using a real-time single-shot balanced steady-state free precession (bSSFP), flip angle (FA) 35-45°, echo time (TE) 1.3 ms, repetition time (TR) 2.7 ms, 40° partial saturation (pSAT) pre-pulse was used to visualize the gadolinium-filled balloon, MR-conditional guidewire, and cardiac structures simultaneously. MR-conditional guidewire visualization was enabled due to susceptibility artifact created by distal markers. Pre-clinical phantom testing was performed to determine the optimum imaging FA-pSAT combination.
Results:
The iCMR procedure was successfully performed to completion in 31/34 (91%) subjects between August 1st, 2017 to December 13th, 2018. Median age and weight were 7.7 years and 25.2 kg (range: 3 months - 33 years and 8 - 80 kg). Twenty-one subjects had single ventricle (SV) anatomy: one subject was referred for pre-Glenn evaluation, 11 were pre-Fontan evaluations and 9 post-Fontan evaluations for protein losing enteropathy (PLE) and/or cyanosis. Thirteen subjects had bi-ventricular (BiV) anatomy, 4 were referred for coarctation of the aorta (CoA) evaluations, 3 underwent vaso-reactivity testing with inhaled nitric oxide, 3 investigated RV volume dimensions, two underwent branch PA stenosis evaluation, and the remaining subject was status post heart transplant. No catheter related complications were encountered. Average time taken for first pass RHC, LHC/aortic pull back, and to cross the Fontan fenestration was 5.2, 3.0, and 6.5 min, respectively. Total success rate to obtain required data points to complete Fick principle calculations for all patients was 331/337 (98%). Subjects were transferred to the x-ray fluoroscopy lab if further intervention was required including Fontan fenestration device closure, balloon angioplasty of pulmonary arteries/conduits, CoA stenting, and/or coiling of aortopulmonary (AP) collaterals. Starting with subject #10, an MR-conditional guidewire was used in all subsequent subjects (15 SV and 10 BiV) with a success rate of 96% (24/25). Real-time CMR-guided RHC (25/25 subjects, 100%), retrograde and prograde LHC/aortic pull back (24/25 subjects, 96%), CoA crossing (3/4 subjects, 75%) and Fontan fenestration test occlusion (2/3 subjects, 67%) were successfully performed in the majority of subjects when an MR-conditional guidewire was utilized.
Conclusion:
Feasibility for detailed diagnostic RHC, LHC, and Fontan fenestration test occlusion iCMR procedures in SV and BiV pediatric subjects with complex CHD is demonstrated with the aid of an MR-conditional guidewire. A novel real-time pSAT GRE sequence with optimized FA-pSAT angle has facilitated simultaneous visualization of the catheter balloon tip, MR-conditional guidewire, and cardiac/vessel anatomy during iCMR procedures.
... These conditions might not always be given, as for example, in interactive real-time MRI the imaging plane is repeatedly being rotated and shifted. [16][17][18][19] Recently, Jiang et al. 20 introduced an framework which simultaneously estimates gradient delays and coil sensitivities using an alternating minimization approach. This method inspired by SAKE 21 uses a computationally rather demanding low-rank constraint in conjunction with the Gauss-Newton method to solve a nonlinear optimization problem. ...
Purpose
To develop a simple and robust tool for the estimation of gradient delays from highly undersampled radial k‐space data.
Theory
In radial imaging gradient delays induce parallel and orthogonal trajectory shifts, which can be described using an ellipse model. The intersection points of the radial spokes, which can be estimated by spoke‐by‐spoke comparison of k‐space samples, distinctly determine the parameters of the ellipse. Using the proposed method (RING), these parameters can be obtained using a least‐squares fit and utilized for the correction of gradient delays.
Methods
The functionality and accuracy of the proposed RING method is validated and compared to correlation‐based gradient‐delay estimation from opposing spokes using numerical simulations, phantom and in vivo heart measurements.
Results
In all experiments, RING robustly provides accurate gradient delay estimations even for as few as three radial spokes.
Conclusions
The simple and straightforward to implement RING method provides accurate gradient delay estimation for highly undersampled radial imaging.
... These methods require good estimates for the gridding operators, which in turn demands sufficient in-plane coil sensitivity variation and a certain number of spokes for auto-calibration. These conditions might not always be given, as e.g. in interactive real-time MRI the imaging plane is repeatedly being rotated and shifted [16,17,18,19]. Recently, Jiang et al. [20] introduced an framework which simultaneously estimates gradient delays and coil sensitivities using an alternating minimization approach. ...
Purpose: To develop a simple and robust tool for the estimation of gradient delays from highly undersampled radial k-space data. Theory: In radial imaging gradient delays induce parallel and orthogonal trajectory shifts, which can be described using an ellipse model. The intersection points of the radial spokes, which can be estimated by spoke-by-spoke comparison of k-space samples, distinctly determine the parameters of the ellipse. Using the proposed method (RING), these parameters can be obtained using a least-squares fit and utilized for the correction of gradient delays. Methods: The functionality and accuracy of the proposed RING method is validated and compared to correlation-based gradient-delay estimation from opposing spokes using numerical simulations, phantom and in vivo heart measurements. Results: In all experiments, RING robustly provides accurate gradient delay estimations even for as few as three radial spokes. Conclusion: The simple and straightforward to implement RING method provides accurate gradient delay estimation for highly undersampled radial imaging. Keywords: trajectory correction, radial imaging, gradient delay, artifacts, system imperfections, RING}
... Under the umbrella of k-t, a number of acceleration techniques have been developed. For example, for image-based recovery TSENSE (temporal SENSE) [15], k-t BLAST (broad-use linear acquisition speed-up technique)/SENSE [16] and k-t PCA (phasecontrast angiography) [17] have been proposed; for kspace-based recovery TGRAPPA (temporal GRAPPA) [18], k-t GRAPPA [19] and parallel MRI with extended and averaged GRAPPA kernels (PEAK-GRAPPA) [20] have been proposed. A number of view-sharing methods, which also qualify as k-t methods, have also been proposed, including keyhole [21], TWIST (timeresolved angiography with stochastic trajectories) [22] and TRICKS (time-resolved imaging of contrast kinetics) [23]. ...
Magnetic resonance imaging (MRI) is an established diagnostic imaging tool for investigating pediatric disease. MRI allows assessment of structure, function, and morphology in cardiovascular imaging, as well as tissue characterization in body imaging, without the use of ionizing radiation. For MRI in children, sedation and general anesthesia (GA) are often utilized to suppress patient motion, which can otherwise compromise image quality and diagnostic efficacy. However, evidence is emerging that use of sedation and GA in children might have long-term neurocognitive side effects, in addition to the short-term procedure-related risks. These concerns make risk–benefit assessment of sedation and GA more challenging. Therefore, reducing or eliminating the need for sedation and GA is an important goal of imaging innovation and research in pediatric MRI. In this review, the authors focus on technical and clinical approaches to reducing and eliminating the use of sedation in the pediatric population based on image acquisition acceleration and imaging protocols abbreviation. This paper covers important physiological and technical considerations for pediatric body MR imaging and discusses MRI techniques that offer the potential of recovering diagnostic-quality images from accelerated scans. In this review, the authors also introduce the concept of reporting elements for important indications for pediatric body MRI and use this as a basis for abbreviating the MR protocols. By employing appropriate accelerated and abbreviated approaches based on an understanding of the imaging needs and reporting elements for a given clinical indication, it is possible to reduce sedation and GA for pediatric chest, cardiovascular and abdominal MRI.
... further advances including interactive MRI,[19][20][21] manipulator-driven MRI13 and visuo-haptic interfacing with the operator.17 The current work describes linking the manipulator control module and marker control module for tracking of the MR-compatible manipulator. ...
Background:
A method for the identification of semi-active fiducial magnetic resonance (MR) markers is presented based on selectively optically tuning and detuning them.
Methods:
Four inductively coupled solenoid coils with photoresistors were connected to light sources. A microcontroller timed the optical tuning/detuning of coils and image collection. The markers were tested on an MR manipulator linking the microcontroller to the manipulator control to visibly select the marker subset according to the actuated joint.
Results:
In closed-loop control, the average and maximum were 0.76° ± 0.41° and 1.18° errors for a rotational joint, and 0.87 mm ± 0.26 mm and 1.13 mm for the prismatic joint.
Conclusions:
This technique is suitable for MR-compatible actuated devices that use semi-active MR-compatible markers.
... 35 A real-time display and reconstruction system was also developed at the National Institutes of Health and is widely used for interventional procedures. 36 Recently, a flexible stand-alone MRI image reconstruction framework 37,38 has been developed that provides additional flexibility for reconstruction and use of cloud computing to facilitate image reconstruction. These reconstruction platforms are designed mainly for situations where real-time or immediate reconstruction of MRI data is needed, such as image guidance or real-time imaging. ...
Purpose:
To evaluate diagnostic image quality of 3D late gadolinium enhancement (LGE) with high isotropic spatial resolution (∼1.4 mm(3) ) images reconstructed from randomly undersampled k-space using LOw-dimensional-structure Self-learning and Thresholding (LOST).
Materials and methods:
We prospectively enrolled 270 patients (181 men; 55 ± 14 years) referred for myocardial viability assessment. 3D LGE with isotropic spatial resolution of 1.4 ± 0.1 mm(3) was acquired at 1.5T using a LOST acceleration rate of 3 to 5. In a subset of 121 patients, 3D LGE or phase-sensitive LGE were acquired with parallel imaging with an acceleration rate of 2 for comparison. Two readers evaluated image quality using a scale of 1 (poor) to 4 (excellent) and assessed for scar presence. The McNemar test statistic was used to compare the proportion of detected scar between the two sequences. We assessed the association between image quality and characteristics (age, gender, torso dimension, weight, heart rate), using generalized linear models.
Results:
Overall, LGE detection proportions for 3D LGE with LOST were similar between readers 1 and 2 (16.30% vs. 18.15%). For image quality, readers gave 85.9% and 80.0%, respectively, for images categorized as good or excellent. Overall proportion of scar presence was not statistically different from conventional 3D LGE (28% vs. 33% [P = 0.17] for reader 1 and 26% vs. 31% [P = 0.37] for reader 2). Increasing subject heart rate was associated with lower image quality (estimated slope = -0.009 (P = 0.001)).
Conclusion:
High-resolution 3D LGE with LOST yields good to excellent image quality in >80% of patients and identifies patients with LV scar at the same rate as conventional 3D LGE.
Level of evidence:
2 J. Magn. Reson. Imaging 2017.
... Recently, it was suggested that the TSENSE (Kellman et al., 2001) parallel imaging technique may be well-suited for real-time cardiac imaging (Guttman et al., 2003), as it provides adaptive coil sensitivity profile calculations. With a TSENSE acceleration factor of 5, images with true (non-interpolated) temporal resolution of 29 ms and sampling matrices of 64 Â 64 points can be obtained. ...
Real-time cardiac MRI appears as a promising technique to evaluate the mechanical function of the heart. However, ultra-fast MRI acquisitions come with an important signal-to-noise ratio (SNR) penalty, which drastically reduces the image quality. Hence, a real-time denoising approach would be desirable for SNR amelioration. In the clinical context of cardiac dysfunction assessment, long acquisitions are required and for most patients the acquisition takes place with free breathing. Hence, it is necessary to compensate respiratory motion in real-time. In this article, a real-time and interactive method for sequential registration and denoising of real-time MR cardiac images is presented. The method has been experimented on 60 fast MRI acquisitions in five healthy volunteers and five patients. These experiments assessed the feasibility of the method in a real-time context.
... [1][2][3][4], to real-time imaging, e.g. [5][6][7][8], and model-based reconstructions of parametric maps, e.g. [9][10][11]. ...
Purpose
To evaluate the temporal accuracy of a self-consistent nonlinear inverse reconstruction method (NLINV) for real-time MRI using highly undersampled radial gradient-echo sequences and to present an open source framework for the motion assessment of real-time MRI methods.
Methods
Serial image reconstructions by NLINV combine a joint estimation of individual frames and corresponding coil sensitivities with temporal regularization to a preceding frame. The temporal fidelity of the method was determined with a phantom consisting of water-filled tubes rotating at defined angular velocity. The conditions tested correspond to real-time cardiac MRI using SSFP contrast at 1.5 T (40 ms resolution) and T1 contrast at 3.0 T (33 ms and 18 ms resolution). In addition, the performance of a post-processing temporal median filter was evaluated.
Results
NLINV reconstructions without temporal filtering yield accurate estimations as long as the speed of a small moving object corresponds to a spatial displacement during the acquisition of a single frame which is smaller than the object itself. Faster movements may lead to geometric distortions. For small objects moving at high velocity, a median filter may severely compromise the spatiotemporal accuracy.
Conclusion
NLINV reconstructions offer excellent temporal fidelity as long as the image acquisition time is short enough to adequately sample (“freeze”) the object movement. Temporal filtering should be applied with caution. The motion framework emerges as a valuable tool for the evaluation of real-time MRI methods.
... Methods for scan time reduction are, therefore, of particular interest to this application. The temporal filtering technique UNFOLD [1] has been demonstrated to provide a reduction by up to a factor of two in interventional imaging [2]. It is, however, limited to such a twofold acceleration in most cases [3]. ...
... The Vurtigo platform has an open-source (modified BSD) license, although it permits proprietary plugins. Frameworks for various image-guided applications such as surgery [4], and specifically for MRI-guided, percutaneous cardiovascular interventions [5] have been previously described. There are a number of 2D and 3D visualization applications for MRI such as the Slicer [6] project. ...
Guidance of electrophysiological (EP) procedures by magnetic resonance imaging (MRI) has significant advantages over x-ray fluoroscopy. Display of electroanatomic mapping (EAM) during an intervention fused with a prior MR volume and DE-MRI derived tissue classification should improve the accuracy of cardiac resynchronization therapy (CRT) for ventricular arrhythmias. Improved accuracy in the spatial localization of recorded EP points will produce an EAM to constrain and customize patient-specific cardiac electroanatomic models being developed for understanding the patterns of arrhythmogenic slow conduction zones causing reentry circuits and treatment planning. The Vurtigo software presented here is a four dimensional (3D+time) real-time visualization application for guiding interventions capable of displaying prior volumes, real-time MRI scan planes, EAM (voltage or activation times), segmented models, and tracked catheters. This paper will describe the architecture and features of Vurtigo followed by the application example of guiding percutaneous cardiac electroanatomic mapping in porcine models.
... TSENSE and other variants of these techniques, such as hybrid-domain temporal GRAPPA (HTGRAPPA), have now been shown with real-time image reconstruction and interactive acquisition. (18)(19)(20)(21). Even though fast reconstruction rates have been reported with Cartesian imaging (e.g. ...
Combination of non-Cartesian trajectories with parallel MRI permits to attain unmatched acceleration rates when compared to traditional Cartesian MRI during real-time imaging. However, computationally demanding reconstructions of such imaging techniques, such as k-space domain radial generalized auto-calibrating partially parallel acquisitions (radial GRAPPA) and image domain conjugate gradient sensitivity encoding (CG-SENSE), lead to longer reconstruction times and unacceptable latency for online real-time MRI on conventional computational hardware. Though CG-SENSE has been shown to work with low-latency using a general purpose graphics processing unit (GPU), to the best of our knowledge, no such effort has been made for radial GRAPPA. Radial GRAPPA reconstruction, which is robust even with highly undersampled acquisitions, is not iterative, requiring only significant computation during initial calibration while achieving good image quality for low-latency imaging applications. In this work, we present a very fast, low-latency, reconstruction framework based on a heterogeneous system using multi-core CPUs and GPUs. We demonstrate an implementation of radial GRAPPA that permits reconstruction times on par with or faster than acquisition of highly accelerated datasets in both cardiac and dynamic musculoskeletal imaging scenarios. Acquisition and reconstruction times are reported.
... Ferner ist die zeitliche Auflçsung limitiert, da für jede Ortsbestimmung ein Bild aufgenommen werden muss. Selbst bei Verwendung von schnellen Akquisitionstechniken wie Schlüssellochtechniken[23,27] oder paralleler Bildgebung[13] werden die Abtastraten (bis zu 10 Bilder/s), die bei manchen vaskulären Intervention notwendig sind, nicht immer erreicht. Ferner ist es nur schwer mçglich, die Position der Instrumente in den Bildern automatisch zu detektieren und zu verfolgen. ...
Interventionen unter MRT-Kontrolle waren in der Vergangenheit aufgrund langer Messzeiten und des eingeschränkten Patientenzugangs nur mit großen Schwierigkeiten durchzuführen. Die Weiterentwicklung der MR-Geräte, die Verbesserungen der MR-Messsequenzen, die schnelle Bildgebung ermöglichen, und die zunehmende Verfügbarkeit von im MRT einsetzbaren Instrumenten haben viele dieser Hindernisse beseitigt. Parallel zur Entwicklung der MRT werden perkutane und endovaskuläre Interventionen unter Bildkontrolle immer komplexer und stellen immer höhere Ansprüche an die Bildgebung, mit denen solche Eingriffe gesteuert und kontrolliert werden. Diese zunächst parallel laufenden Trends zeigen in den letzten Jahren eine gewisse Konvergenz. Das Interesse an der MRT-Kontrolle von perkutanen und endovaskulären Interventionen nimmt zu, nicht zuletzt auch aufgrund der Eigenschaften der MRT, sowohl die Morphologie mit exzellentem Weichteilkontrast als auch funktionelle Informationen darzustellen. Das Ziel dieser Übersicht ist es, die technischen Voraussetzungen MRT-gesteuerter endovaskulärer Interventionen darzustellen, erste experimentelle und klinische Anwendungen zu diskutieren und Sicherheitsaspekte der Technik zu erörtern.
... In patients with regular heart rhythm or with infrequent ectopy, a standard segmented acquisition is used to acquire a stack of cine images through the heart. In patients with frequent ectopy or very irregular rhythm, real-time approaches may be necessary (47). ...
This article reviews the magnetic resonance imaging (MRI) and angiography (MRA) techniques, imaging findings, and evidence for evaluating patients with acute chest pain due to acute pulmonary embolus (PE), aortic dissection (AD), and myocardial infarction (MI). When computed tomographic angiography (CTA) is contraindicated, MRI and MRA are important alternative imaging modalities for diagnosis and management of patients with acute PE, AD, and MI. Familiarity with the techniques, imaging findings, and evidence is critical to safely and appropriately managing patients presenting with acute chest pain. J. Magn. Reson. Imaging 2013;00:000-000. © 2013 Wiley Periodicals, Inc.
... The acceleration techniques described here and summarised inTable 1 all involve a reduction in the number [ 25,28,31,32] *Self calibrating (auto-calibrating) techniques – reference data acquired as part of acquisition. Summary of acceleration techniques where the data acquisition time is shortened by reducing the number of phase encoding steps. of phase encoding steps, and therefore the acquired number of k-space lines, to achieve the reduction in acquisition time. ...
This is the second of two reviews that is intended to cover the essential aspects of cardiovascular magnetic resonance (CMR) physics in a way that is understandable and relevant to clinicians using CMR in their daily practice. Starting with the basic pulse sequences and contrast mechanisms described in part I, it briefly discusses further approaches to accelerate image acquisition. It then continues by showing in detail how the contrast behaviour of black blood fast spin echo and bright blood cine gradient echo techniques can be modified by adding rf preparation pulses to derive a number of more specialised pulse sequences. The simplest examples described include T2-weighted oedema imaging, fat suppression and myocardial tagging cine pulse sequences. Two further important derivatives of the gradient echo pulse sequence, obtained by adding preparation pulses, are used in combination with the administration of a gadolinium-based contrast agent for myocardial perfusion imaging and the assessment of myocardial tissue viability using a late gadolinium enhancement (LGE) technique. These two imaging techniques are discussed in more detail, outlining the basic principles of each pulse sequence, the practical steps required to achieve the best results in a clinical setting and, in the case of perfusion, explaining some of the factors that influence current approaches to perfusion image analysis. The key principles of contrast-enhanced magnetic resonance angiography (CE-MRA) are also explained in detail, especially focusing on timing of the acquisition following contrast agent bolus administration, and current approaches to achieving time resolved MRA. Alternative MRA techniques that do not require the use of an endogenous contrast agent are summarised, and the specialised pulse sequence used to image the coronary arteries, using respiratory navigator gating, is described in detail. The article concludes by explaining the principle behind phase contrast imaging techniques which create images that represent the phase of the MR signal rather than the magnitude. It is shown how this principle can be used to generate velocity maps by designing gradient waveforms that give rise to a relative phase change that is proportional to velocity. Choice of velocity encoding range and key pitfalls in the use of this technique are discussed.
... Multiple oblique slices can be obtained in rapid succession and can be simultaneously displayed in a 3D rendering to provide optimal 3D anatomic information. Image contrast, image plane orientations , acquisition speed, 3D rendering, and device tracking can be readily adjusted as needed during scanning [28]. ...
Minimally invasive cardiac surgery is less traumatic and therefore leads to quicker recovery. With the assistance of engineering technologies on devices, imaging, and robotics, in conjunction with surgical technique, minimally invasive cardiac surgery will improve clinical outcomes and expand the cohort of patients that can be treated. We used transapical aortic valve implantation as an example to demonstrate that minimally invasive cardiac surgery can be implemented with the integration of surgical techniques and engineering technologies. Feasibility studies and long-term evaluation results prove that transapical aortic valve implantation under MRI guidance is feasible and practical. We are investigating an MRI compatible robotic surgical system to further assist the surgeon to precisely deliver aortic valve prostheses via a transapical approach. Ex vivo experimentation results indicate that a robotic system can also be employed in in vivo models.
... By acquiring the left and the right view within the same TR interval, the stereoscopic DE-FLASH sequence is 30 % faster than a FLASH sequence with separate acquisitions per view. Thus, higher image update rates can be achieved-here, a frame rate of about 3 Hz was used; however, this can be further increased using additional acceleration techniques such as tSENSE [16]. Even with the current implementation higher frame rates could be reached if the coil sensitivity profiles were acquired prior to the real-time imaging procedure. ...
Object
A three-dimensional (3D) visualization of the target region during intravascular interventions in real-time is challenging since the acquisition of a time-consuming 3D dataset is required. In this work, a novel stereoscopic double echo sequence for achieving 3D depth perception by sampling only two oblique projection images is presented.
Materials and methods
A double echo (DE) FLASH pulse sequence was developed to acquire continuously stereoscopic image pairs of the vascular target anatomy. Stereo image data were displayed on a stereoscopic 3D LCD monitor in real time after image reconstruction. Phantom experiments followed by a depth perception test were performed to assess the usability of the stereo image pairs for 3D visualization. In an animal experiment the sequence was tested in vivo and was compared with a slower interleaved (IL) sequence variant.
Results
In the phantom experiments an SNR difference of 6 % between left and right image was found which did not influence the depth perception. The DE acquisition was superior to the IL sequence (SNRDE = 10.3, 2.3 images/s over SNRIL = 7.1, 1.7 images/s), and during contrast enhancement the abdominal arterial vasculature was clearly perceived as a 3D structure.
Conclusion
A novel stereoscopic DE pulse sequence can be utilized for the fast 3D stereoscopic visualization of vascular structures in real-time.
... With novel fast imaging technologies, MRI became an imaging tool for guiding and monitoring various interventions and biopsy procedures on various organs including the brain, breast, prostate, liver and spine (Jolesz, 1998, Melzer & Seibel, 1999). High performance magnetic field gradients, multi-channel receivers, and advanced reconstruction systems improve the clinical applicability of real time MRI procedures (Nayak et al., 2004, Bock et al., 2004, Guttman et al., 2003, Lederman, 2005). Modern scanners designed for the interventional environment can provide real-time images of acceptable quality in excess of 10 frames per second. ...
MRI provides excellent visualization of soft tissue, its sub-structure and surrounding tissues. The unique features of MRI, such as oblique image planes, multi-slice imaging, realtime visualization, and freedom from radiation exposure risk, enable MRI to be a critical tool in the guidance of many interventional procedures. Additionally, a mechatronic system can provide more accurate and smooth access to targeting organs in a confined space. The marriage of a MRI and a robot makes the benefit of minimally invasive interventions substantial.
... We also plan to address the recovery using multichannel data. Several authors have used the spatial diversity of the coil sensitivity profiles to accelerate cardac MRI [45], [46]. We expect that these extensions will enable k-t SLR to provide robust high-resolution perfusion MRI data from 8-12 slices with a temporal resolution of upto 2-3 frames per second. ...
We introduce a novel algorithm to reconstruct dynamic magnetic resonance imaging (MRI) data from under-sampled k-t space data. In contrast to classical model based cine MRI schemes that rely on the sparsity or banded structure in Fourier space, we use the compact representation of the data in the Karhunen Louve transform (KLT) domain to exploit the correlations in the dataset. The use of the data-dependent KL transform makes our approach ideally suited to a range of dynamic imaging problems, even when the motion is not periodic. In comparison to current KLT-based methods that rely on a two-step approach to first estimate the basis functions and then use it for reconstruction, we pose the problem as a spectrally regularized matrix recovery problem. By simultaneously determining the temporal basis functions and its spatial weights from the entire measured data, the proposed scheme is capable of providing high quality reconstructions at a range of accelerations. In addition to using the compact representation in the KLT domain, we also exploit the sparsity of the data to further improve the recovery rate. Validations using numerical phantoms and in vivo cardiac perfusion MRI data demonstrate the significant improvement in performance offered by the proposed scheme over existing methods.
Background:
Current functional cardiovascular imaging protocols mostly rely on electrocardiogram (ECG) gating and breathholding. The resulting image quality can substantially suffer from insufficient patient cooperation or severe arrhythmia. Real-time imaging can mitigate these effects but requires highly accelerated techniques, usually relying on non-cartesian trajectories and Compressed Sensing (CS).
Methods:
We investigate a sliding window reduced field of view (FOV) Echo Planar Imaging (EPI) technique for real-time cardiac MRI. Segmented EPI has been combined with a subtraction technique for reducing the FOV in cardiac applications to the region of the beating heart. Residual respiratory motion, potentially impairing the image quality, has been addressed by continuous update of the static image fraction, which is derived from a low-temporal resolution sliding window reconstruction. For further acceleration, the proposed technique was combined with parallel imaging.
Results:
The sliding window reduced FOV technique was proven feasible to reconstruct images of diagnostic image quality at a temporal resolution of 36.5ms per image. Semi-quantitative evaluation of image quality showed significant improvement over the existing rFOV method (p=0.039). Derived functional parameters show comparable results as with the BH-CINE reference. However, a trend to a slight underestimation of the largest and smallest in-plane volumes is observed.
Conclusion:
The proposed technique is feasible of providing real-time cardiac MRI with a temporal resolution better than 40ms without the need of computably complex reconstruction techniques.
This work presents a platform that integrates a customized MRI data acquisition scheme with reconstruction and three-dimensional (3D) visualization modules along with a module for controlling an MRI-compatible robotic device to facilitate the performance of robot-assisted, MRI-guided interventional procedures. Using dynamically-acquired MRI data, the computational framework of the platform generates and updates a 3D model representing the area of the procedure (AoP). To image structures of interest in the AoP that do not reside inside the same or parallel slices, the MRI acquisition scheme was modified to collect a multi-slice set of intraoblique to each other slices; which are termed composing slices. Moreover, this approach interleaves the collection of the composing slices so the same k-space segments of all slices are collected during similar time instances. This time matching of the k-space segments results in spatial matching of the imaged objects in the individual composing slices. The composing slices were used to generate and update the 3D model of the AoP. The MRI acquisition scheme was evaluated with computer simulations and experimental studies. Computer simulations demonstrated that k-space segmentation and time-matched interleaved acquisition of these segments provide spatial matching of the structures imaged with composing slices. Experimental studies used the platform to image the maneuvering of an MRI-compatible manipulator that carried tubing filled with MRI contrast agent. In vivo experimental studies to image the abdomen and contrast enhanced heart on free-breathing subjects without cardiac triggering demonstrated spatial matching of imaged anatomies in the composing planes. The described interventional MRI framework could assist in performing real-time MRI-guided interventions.
321. THE PREVALENCE OF MYOCARDIAL SCAR IN PATIENTS NO PRIOR HISTORY OF CARDIAC DISEASE DETECTED BY DELAYED-ENHANCEMENT CARDIAC MAGNETIC RESONANCE
Simon Greulich, MD,¹ Igor Klem, MD,¹ John F. Heitner, MD,² Holger Vogelsberg, MD,¹ Srivani Ambati, MD,³ Martina Mangin, MD,¹ Udo Sechtem, MD¹. ¹Robert-Bosch-Krankehaus, Stuttgart, Germany, ²New York Methodist Hospital, New York, NY, USA, ³Duke University Medical Center, Durham, NC, USA.
Background: Patients with myocardial scar (scar) are at increased risk for cardiovascular mortality and morbidity. Delayed enhancement cardiac magnetic resonance imaging (DE-CMR) is highly accurate in the detection of scar. The pattern of myocardial scar can be divided into 2 groups: 1. Coronary artery disease (CAD) based on location, ie extending from subendocardium to subepicardium; and 2. Non-CAD based on mid-myocardial or epicardial location. The prevelance of these patterns of scar in patients with no prior history of infarction or CAD is unknown.
Purpose: To assess the prevalence of scar (both CAD and Non-CAD) by DE-CMR in patients with clinically suspected CAD and no previous cardiac disease.
Methods: We prospectively enrolled 42 consecutive patients (pts) without a prior history of cardiac disease including myocardial infarction who were referred for elective coronary angiography (CA) based on clinical suspicion of CAD. DE-CMR was performed in all patients within 24 hrs of CA. DE-CMR images were scored visually, blinded to patient identity, using a 17-segment model. The presence of silent MI by DE-CMR was defined as the presence of hyperenhanced myocardium in a pattern typical of CAD. Non-CAD pattern of hyperenhancement was defined as either midmyocardial or epicardial HE representing primary myocardial disease. Patients were considered positive for CAD if there was ≥ 70% coronary stenosis on CA.
Results: The prevalence of CAD by CA was 38% (16 pts). DE-CMR showed evidence of HE in 9 pts (21%). Four (44%) patients had a HE pattern consistent with CAD, and 5 pts. (56%) had Non-CAD type of HE. The mean silent MI size was 9.4% of total LV mass, the mean size of scar in the Non-CAD group was 1% of total LV mass. Of the 4 pts. with evidence of silent MI by DE-CMR, three had significant stenosis on CA, and one pt. had non-obstructive disease on CA. Among the 5 pts. with evidence of Non-CAD type of scar, 4 had no CAD on CA, one pt. with HE in the midmyocardial basal septum (0.8% of LV mass) had evidence of obstructive CAD (posterior descending artery) on CA.
Conclusion: There is a high prevalence of scar in patients with clinically suspected CAD but without previously known cardiac disease. DE-CMR has additive diagnostic value for the evaluation of patients with clinical suspicion of CAD.
322. CARDIOVASCULAR SAFETY OF EVP 1001-1 (SEEMORE™), AN INTRACELLULAR AGENT FOR MAGNETIC RESONANCE IMAGING OF THE ISCHEMIC HEART
Peter R. Seoane, PhD, Phillip P. Harnish, PhD. Eagle Vision Pharmaceutical Corp., Exton, PA, USA.
Introduction: Manganese (Mn) has demonstrated potential utility for imaging of the heart from the early days of magnetic resonance imaging. Mn has been shown to distribute rapidly from the blood to myocardium, providing a persistent pattern of enhancement that reflects local perfusion at the time of intracellular uptake. Unfortunately, Mn activity at calcium channels depresses the heart and relaxes blood vessels at doses and rates of administration relevant for imaging. This results in acute decreases in blood pressure, electrical disturbances such as prolonged P-R and Q-T intervals and ventricular arrhythmias. One may improve the cardiac safety of Mn via chelation while sacrificing two key advantages of Mn, rapid tissue uptake and high relaxivity. These factors significantly limit the utility of chelated Mn for imaging the ischemic heart. Through formulation with calcium, EVP 1001-1 mitigates the undesired cardiovascular effects associated with Mn while retaining the kinetic and magnetic properties that enable imaging of the ischemic heart.
Purpose: To evaluate the cardiovascular safety of EVP 1001-1 administered to beagle dogs at rest and under peak pharmacologic stress.
Methods: Groups of three anesthetized beagle dogs were dosed with EVP 1001-1 (120 μ mol/kg IV over one minute) at rest, at peak dipyridamole stress (142 μ mol/kg/min IV for 4 minutes) and at peak dobutamine stress (40 μ g/kg/min IV for 20 minutes). Blood pressure, heart rate and electrocardiogram (ECG) were continuously monitored from induction of anesthesia through up to one hour following administration of EVP 1001-1. ECG recordings (12 Lead) were obtained and P-R, R-R and Q-T intervals measured at predetermined timepoints prior to stressor administration, during stress induction and for up to one hour following EVP 1001-1 administration. QTc was derived from Q-T and R-R measurements using Fridericias's correction.
Results: When animals were dosed with EVP 1001-1 at rest, a small increase in mean arterial blood pressure was noted that resolved within minutes of administration. The animal's heart rate was not affected and no significant changes were noted on ECG. In animals that were underwent maximal stress with either dipryridamole or dobutamine, EVP 1001-1 did not exacerbate the hemodynamic stress, alter ECG or result in cardiac rhythm changes. Thus the No Observable Adverse Event Level (NOAEL) for EVP 1001-1 is greater than 120 μ mol/kg, or more than 12 times the anticipated maximum clinical dose.
Conclusions: Cardiac MRI with EVP 1001-1 may be accomplished without the cardiac depression or ECG changes typically associated with Mn, even when given under peak pharmacologic stress. Previous studies have shown that the magnetic and pharmacokinetic properties of EVP 1001-1 are consistent with the requirements for steady state imaging of the ischemic heart. Thus, EVP 1001-1 may be safely administered under exercise or pharmacologic stress away from the magnet, with full cardiac monitoring. Imaging of the stress induced pattern of enhancement, which evolves shortly after administration of EVP 1001-1 and persists for more than 90 minutes, may be performed once the patient has returned to the resting condition. Imaging of the perfusion deficit may be performed as desired during this period without the need for additional stress or doses of EVP 1001-1. Studies in patients are currently underway.
Acknowledgment: This work was supported in part by the National Heart Lung and Blood Institute/NIH, Grant # R44HL63518.
323. CLINICAL SAFETY EVALUATION OF CARDIAC MRI EARLY AFTER CORONARY STENT IMPLANTATION IN ACUTE MYOCARDIAL INFARCTION PATIENTS
Paula Tejedor,¹ Alberto San Román,² Itziar Gómez,² José Sierra,³ Juan Manuel Durán,¹ Francisco Fernández-Avilés.²¹Hospital General Yagüe, Burgos, Spain, ²Hospital Clínico Universitario, Valladolid, Spain, ³Centro Diagnóstico Valladolid, Valladolid, Spain.
Introduction: MRI provides helpful information in patients in the inmediate post-stent PCI (percutaneous coronary intervention) period. However, current “information for use guidelines” recommend to wait at least 8 weeks for the MRI to be safe, because of theoretical concerns of stent dislodgment when exposed under a magnetic field. Most clinicians perform cardiac MRI before 8 weeks, although information on safety is lacking.
Purpose: To determine whether to perform a cardiac MRI in the first 2 weeks after stent-PCI in patients with the diagnosis of AMI is a safe procedure.
Methods: We retrospectively study 409 postAMI patients. Mean age was 62 ± 11, 85% were males. 43% of all the patients were treated by primary PCI (percuataneous coronary intervention) and 76% by facilitated PCI (thrombolysis followed by PCI within 24 hours). Cardiac MRI was performed in 86 patients (group 1, n = 86) according to physician's criteria an average of 14 ± 11 days after the stent implantation. MRI was not performed in group 2(n = 312). All MRI examinations were performed in a 1.5 Tesla scan. Cine-MRI images were obtained using an ultra-fast gradient-echo sequence (FIESTA™, General Electric). Safety outcomes included occurrence of stent thrombosis, myocardial infarction, target vessel revascularization and rehospitalization during index hospitalization and at 6 and 12 months after the AMI.
Results: Baseline and cardiovascular risk factors were not different in both groups. Reperfusion therapy was similar in both groups of patients (). Infarct size determined by quantification of cardiac markers was also quite similar between the two groups. Baseline angiographic mean ejection fraction (EF) was slightly inferior in group 1 (EF = 51 ± 11%) than in group 2 (EF = 55 ± 10, p < 0.008). The vast majority of the implanted stents were 316 L stainless steel in both groups of patients. There was no significant difference in the percentage of patients receiving double antiplatelet therapy (aspirine plus clopidogrel) for at least one month after the stent implantation. No clinical complication was described in the post-inmediate MRI procedure. During index hospitalization, 3 acute stent thrombosis were registered, all of them in group 2 (no CMR). At 12 months, MACE (including death, reinfarction, rehospitalization and revascularization) was 14% in group 1 and 15.6% in group 2 (p = 0.7) ().
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DOI
http://dx.doi.org/10.1080/10976640500479011
Published online
13 July 2009
Table 1
CSVDisplay Table
Conclusions: Cardiac MRI performed within 2 weeks of stent-PCI in the postAMI setting appears to be a safe procedure, with a very low risk of MACE. Delaying MRI in the immediate post stent implantation period does not appear to be necessary.
324. MICROVASCULAR OBSTRUCTION IN AN EXPERIMENTAL REPERFUSED ACUTE MYOCARDIAL INFARCTION AT THE VERY EARLY STAGE: EVALUATION USING A MODIFIED T1 PREP LOOK-LOCKER SEQUENCE
Yuesong Yang, MD, PhD, Warren D. Foltz, PhD, John Graham, MD, Jay S. Detsky, BSc, Alexander J. Dick, MD, Graham A. Wright, PhD. Sunnybrook and Women's College Health Sciences Centre, Toronto, ON, Canada.
Introduction: The concept of microvascular obstruction (MO) or the “no-reflow” phenomenon in the infarcted myocardium was proposed decades ago using a canine model. However, the linkage between the MO and unfavorable clinical prognosis has been established only in recent years using TIMI flow, myocardial contrast echocardiography and MRI. Delayed enhanced MRI (DE-MRI) has been widely used for myocardial viability determination. The MO in acute myocardial infarctions (AMI) has been detected as hypoenhanced regions. A noninvasive MRI technique with positive enhancement of MO would be preferred.
Purpose: To investigate a modified T1 prep Look-Locker sequence before and during a Gd-DTPA infusion for evaluation of MO in a porcine model of reperfused AMI.
Methods: In seven Yorkshire pigs (22–28 kg) a reperfused AMI was produced under X-ray guidance using a 90-minute percutaneous balloon occlusion of the distal LAD, followed by reperfusion. MRI studies were performed on a GE 1.5T Signa Excite system. All pigs underwent a baseline MRI examination including a SSFP functional study and T1 mapping. The T1 map uses a modified Look-Locker sequence acquiring a set of 8 spiral images, corresponding to the differences between signals in a train of 20-deg excitations at intervals of 120 ms, obtained with and without a preceding inversion at the same cardiac phase. The signal difference isolates the T1 contribution from the approach to steady-state in the small-tip train, so that longer T1 values yield bright signal at later points (effectively longer TI). After the intervention, SSFP and T1 mapping sequences were repeated in the same location. First pass myocardial perfusion (FPMP) was obtained immediately after a Gd-DTPA bolus injection (0.2 mmol/kg) followed by a continuous intravenous drip of Gd-DTPA. DE-MRI was performed 30 minutes post-injection and T1 mapping was applied 45 minutes post-injection. Pigs were sacrificed for TTC staining and histology. SSFP LV function, FPMP and DE-MRI analysis were conducted using Mass Plus software (Medis). T1 change was calculated from the MO, DE-MRI hyperenhanced (DHE), control segments and LV with the following formula: [(T1 at baseline—T1 at steady state post Gd-DTPA)/T1 at baseline] using manually drawn regions-of-interest and custom or commercial fitting algorithms (Xcinema, Stanford; Functool 2, GE).
Results: MO was seen in six of seven pigs. MO was defined as the persistent hypoenhanced area in the infarcted myocardium in FPMP and DE-MRI (). Upon the modified Look-Locker technique post-contrast, MO was identified as bright regions in later difference images while the surrounding DHE regions appeared dark (). shows the typical signals from MO, DHE and control regions in difference images across the small tip train. MO areas calculated from the DE-MRI (1.46 ± 0.84 cm²) and T1 images (1.54 ± 0.91 cm²) in the same location were comparable with a trend toward greater area in the T1 images although no statistical significance was reached (p = 0.20, paired t-test). T1 reduction (%) in MO regions (23.5 ± 21.8) was small compared to measurements from the control segments (38.2 ± 7.3, p = 0.13), DHE regions (72.8 ± 14.5, p = 0.0004) and LV (83.5 ± 10.9, p = 0.005) using a paired t-test. Pre-contrast T1 values across the myocardial segments were the same. All pigs had an AMI demonstrated by TTC staining and histology and occluded microvessels were seen in MO ().
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DOI
http://dx.doi.org/10.1080/10976640500479011
Published online
13 July 2009
FIG. 1. a. DE-MRI: MO as hypo-enhanced region (TI = 250 ms). b. One T1 prep difference image (post-Gd, TI = 606 ms): MO was positive enhanced area. c. T1 map: ROI 1: MO, ROI 2: DHE, ROI 3: Control region. d. Normalized signal intensity changes over eight data points. e. TTC staining. f. Histology (HE statining): necerosis and hemorrhage present. A small arteriole totally occluded by disrupted red blood cells, platelet and fibrin was observed in MO region.
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FIG. 1. a. DE-MRI: MO as hypo-enhanced region (TI = 250 ms). b. One T1 prep difference image (post-Gd, TI = 606 ms): MO was positive enhanced area. c. T1 map: ROI 1: MO, ROI 2: DHE, ROI 3: Control region. d. Normalized signal intensity changes over eight data points. e. TTC staining. f. Histology (HE statining): necerosis and hemorrhage present. A small arteriole totally occluded by disrupted red blood cells, platelet and fibrin was observed in MO region.
Friday Posters
All authors
DOI
http://dx.doi.org/10.1080/10976640500479011
Published online
13 July 2009
FIG. 1.
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FIG. 1.
Conclusions: MO at early stages of reperfused AMI can be identified as a bright area on images from a modified T1 prep Look-Locker sequence post-contrast. Observations suggest reduced Gd-DTPA distribution volume in MO relative to both control and DHE regions. Quantitative results yield greater specificity while positive contrast may help identify even partial volumes of MO.
Background: Real-time magnetic resonance imaging (rtMRI)-guided transcatheter aortic valve replacement (TAVR) offers improved visualization, real-time imaging, and pinpoint accuracy with device delivery. Unfortunately, performing a TAVR in a MRI scanner can be a difficult task owing to limited space and an awkward working environment. Our solution was to design a MRI-compatible robot-assisted device to insert and deploy a self-expanding valve from a remote computer console. We present our preliminary results in a swine model. Methods: We used an MRI-compatible robotic arm and developed a valve delivery module. A 12-mm trocar was inserted in the apex of the heart via a subxiphoid incision. The delivery device and nitinol stented prosthesis were mounted on the robot. Two continuous real-time imaging planes provided a virtual real-time 3-dimensional reconstruction. The valve was deployed remotely by the surgeon via a graphic user interface. Results: In this acute nonsurvival study, 8 swine underwent robot-assisted rtMRI TAVR for evaluation of feasibility. Device deployment took a mean of 61 +/- 5 seconds. Postdeployment necropsy was performed to confirm correlations between imaging and actual valve positions. Conclusions: These results demonstrate the feasibility of robotic-assisted TAVR using rtMRI guidance. This approach may eliminate some of the challenges of performing a procedure while working inside of an MRI scanner, and may improve the success of TAVR. It provides superior visualization during the insertion process, pinpoint accuracy of deployment, and, potentially, communication between the imaging device and the robotic module to prevent incorrect or misaligned deployment.
Free of ionizing radiation, unlike x-ray, CT, and PET, MRI is recognized as an ideal tool to monitor and control interventions. Its advantages include a superior spatial resolution and soft tissue contrast to help delineate tissue abnormality and the ability to provide cross-sectional images in any orientation. It is also able to noninvasively monitor temperature changes, making it ideal for monitoring energy deposition during thermal ablation. To leverage these advantages in various types of interventions, a number of groups have made and continue to make efforts to exploit MRI in the operating room. Interventional MRI differs from diagnostic MRI in its interactive and real-time nature whereby the progress of intervention is instantly visualized by the physician as feedback. To this end, imaging parameters may often require adjustment so that position and orientation of the imaging plane track the locations of needles and catheters or movement of a focal lesion during a procedure. Since there are trade-offs to be made in image contrast, signal-to-noise ratio, spatial resolution, and temporal resolution when choosing MRI techniques, they must be carefully selected depending on their purpose. In this chapter, we review a wide range of imaging techniques that enable interactive and real-time MRI for interventional guidance.
Cardiac MR is considered the gold standard in assessing RV function. The purpose of this study is to evaluate the clinical utility of an investigational iterative reconstruction algorithm in the quantitative assessment of RV function. This technique has the potential to improve the clinical utility of CMR in the evaluation of RV pathologies, particularly in patients with dyspnea, by shortening acquisition times without adversely influencing imaging performance. Segmented cine images were acquired on 9 healthy volunteers and 29 patients without documented RV pathologies using conventional GRAPPA acquisition with factor 2 acceleration (GRAPPA 2), a spatio-temporal TSENSE acquisition with factor 4 acceleration (TSENSE 4), and iteratively reconstructed Sparse SENSE acquisition with factor 4 acceleration (IS-SENSE 4). 14 subjects were re-analyzed and intraclass correlation coefficients (ICC) were calculated and Bland-Altman plots generated to assess agreement. Two independent reviewers qualitatively scored images. Comparison of acquisition techniques was performed using univariate analysis of variance (ANOVA). Differences in RV EF, BSA-indexed ESV (ESVi), BSA-indexed EDV (EDVi), and BSA-indexed SV (SVi) were shown to be statistically insignificant via ANOVA testing. R(2) values for linear regression of TSENSE 4 and IS-SENSE 4 versus GRAPPA 2 were 0.34 and 0.72 for RV-EF, and 0.61 and 0.76 for RV-EDVi. ICC values for intraobserver and interobserver quantification yielded excellent agreement, and Bland-Altman plots assessing agreement were generated as well. Qualitative review yielded small, but statistically significant differences in image quality and noise between TSENSE 4 and IS-SENSE 4. All three techniques were rated nearly artifact free. Segmented imaging acquisitions with IS-SENSE reconstruction and an acceleration factor of 4 accurately and reliably quantitates RV systolic function parameters, while maintaining image quality. TSENSE-4 accelerated acquisitions showed poorer correlation to standard imaging, and inferior interobserver and intraobserver agreement. IS-SENSE has the potential to shorten cine acquisition times by 50 %, improving imaging options in patients with intermittent arrhythmias or difficulties with breath holding.
Real-time magnetic resonance imaging (rtMRI) techniques have been described for intravascular guidance and may be immediately applicable to provide vision in a beating heart with circulating blood to guide the surgeon in a minimally invasive cardiac surgery environment. Operative rtMRI guidance has stringent requirements for surgical instruments and devices. Typically, devices that are used during interventions, such as catheters and wires, are not designed to be magnetic resonance visible or compatible as they often contain ferromagnetic materials or long electrical conductors. This chapter illustrates the interactive rtMRI, MRI compatibility of the instruments and devices, and device tracking techniques. It also presents a promising example of MRI-guided cardiac surgery, specifically, transapical aortic valve replacement under rtMRI.
Current methods for MRI of infarcted myocardium require ECG-gating and breath-holding during contrast-enhanced segmented k-space inversion-recovery (IR) imaging. However, ECG-gating can be problematic in MRI, and breath-holding can be difficult for some patients. This work demonstrates that infarcted tissue can be visualized without ECG-gating or breath-holding with the use of intermittent inversion pulses during real-time (RT) interactive imaging with steady-state free precession (SSFP). The sequence generates a RT image stream containing a myocardium-nulled image every few frames, which allows nearly simultaneous observation of both infarcted regions and wall motion. First-pass perfusion and wall motion can be simultaneously observed with minor parameter modifications. This method may reduce diagnostic scan time, expand the target population, improve patient comfort, and facilitate targeted, interventional treatment of infarcted myocardium.
Real-time magnetic resonance imaging (rtMRI) is a compelling modality for guidance of surgical interventions. An effective toolkit for planning and guidance of surgery using rtMRI includes continuously updated images with excellent soft tissue contrast, devices that are visible in the images, interactively adjustable imaging parameters, simultaneous imaging and display of multiple intersecting oblique planes, and the ability to measure blood flow and perfusion. MRI has the benefit of not exposing the patient, physician, or staff to ionizing radiation from X-rays. This chapter describes the initial experience in the development of minimally invasive surgical implantation of an aortic valve in the beating heart, using continuously updated rtMRI. The potential benefits of this approach include reduction of patient trauma from open heart surgery using cardiopulmonary bypass, and the ability to implant a more robust device than can be delivered by catheter-based methods. Since the heart is a moving target, the surgeon is guided by continuously updated images, rather than those previously acquired as in stereotactic procedures.
Walking is a popular recreation activity. In addition, it is a mild and healthy exercise for the elderly. However, people suffering from physical and / or cognitive disorders have a difficulty in going out and walking in real environments. Therefore we have proposed a virtual walking system with a haptic device on feet. This system consists of visual, sound, and haptic devices to present various kinds of ambient information for users. The development of the haptic interface for feet soles is a main topic of this paper. We discuss a use of magnetic field sensitive elastomers (MSEs) as working materials for the device. In the previous report, we developed an electromagnet as a testbed of MSE and evaluated it performance. According to the measurement of planter pressures, this device can perform different pressures on soles by applying magnetic field. The result also implied necessities of the improvement of the magnetic circuit and basic structure for more feasible device. In this paper, we propose a new structure to control a frictional resistance force that is generated on the surface of the MSE and a metal pin. In addition, we propose to use this device as a single unit for the haptic device on feet. According to the compressive test of the unit, the maximum force that can be controlled with the unit is insufficient yet. However, it appears that we can control frictional force with the proposed structure. We also conducted a simulation to estimate required values of the performance for the unit. It is much higher than the current state of the unit. We have to improve the performance of the unit in the future.
Following recent technological revolutions, the investigation of massive biomedical data with growing scale, diversity, and complexity has taken a center stage in modern data analysis. Although complex, the underlying representations of many biomedical data are often sparse. For example, for a certain disease such as leukemia, even though humans have tens of thousands of genes, only a few genes are relevant to the disease; a gene network is sparse since a regulatory pathway involves only a small number of genes; many biomedical signals are sparse or compressible in the sense that they have concise representations when expressed in a proper basis. Therefore, finding sparse representations is fundamentally important for scientific discovery. Sparse methods based on the [Formula: see text] norm have attracted a great amount of research efforts in the past decade due to its sparsity-inducing property, convenient convexity, and strong theoretical guarantees. They have achieved great success in various applications such as biomarker selection, biological network construction, and magnetic resonance imaging. In this paper, we review state-of-the-art sparse methods and their applications to biomedical data.
The clinical utility of myocardial perfusion MR imaging (MPI) is often restricted by the inability of current acquisition schemes to simultaneously achieve high spatio-temporal resolution, good volume coverage, and high signal to noise ratio. Moreover, many subjects often find it difficult to hold their breath for sufficiently long durations making it difficult to obtain reliable MPI data. Accelerated acquisition of free breathing MPI data can overcome some of these challenges. Recently, an algorithm termed as k - t SLR has been proposed to accelerate dynamic MRI by exploiting sparsity and low rank properties of dynamic MRI data. The main focus of this paper is to further improve k - t SLR and demonstrate its utility in considerably accelerating free breathing MPI. We extend its previous implementation to account for multi-coil radial MPI acquisitions. We perform k - t sampling experiments to compare different radial trajectories and determine the best sampling pattern. We also introduce a novel augmented Lagrangian framework to considerably improve the algorithm's convergence rate. The proposed algorithm is validated using free breathing rest and stress radial perfusion data sets from two normal subjects and one patient with ischemia. k - t SLR was observed to provide faithful reconstructions at high acceleration levels with minimal artifacts compared to existing MPI acceleration schemes such as spatio-temporal constrained reconstruction and k - t SPARSE/SENSE.
In this paper we present a surgical assistant system for implanting prosthetic aortic valve transapically under MRI guidance, in a beating heart. The system integrates an MR imaging system, a robotic system, as well as user interfaces for a surgeon to plan the procedure and manipulate the robot. A compact robotic delivery module mounted on a robotic arm is used for delivering both balloon-expandable and self-expanding prosthesis. The system provides different user interfaces at different stages of the procedure. A compact fiducial pattern close to the volume of interest is proposed for robot registration. The image processing and the transformation recovery methods using this fiducial in MRI are presented. The registration accuracy obtained by using this compact fiducial is comparable to the larger multi-spherical marker registration method. The registration accuracy using these two methods is less than 0.62+/-0.50 deg (mean +/- std. dev.) and 0.63+/-0.72 deg (mean +/- std. dev.), respectively. We evaluated each of the components and show that they can work together to form a complete system for transapical aortic valve replacement.
With the rapid progress of minimally invasive medicine in the recent decades, image guidance during interventional procedures
plays an increasingly important role. Since the complexity of minimally invasive procedures is steadily increasing, the technical
requirements grow for the imaging modalities that are used to monitor the procedure. Traditionally, interventions have been
monitored with imaging modalities using either X-ray (fluoroscopy, digital subtraction angiography [DSA], or computed tomography)
or ultrasound. In particular, ultrasound is widely available and provides a cost-effective alternative to other imaging modalities.
Unfortunately, both X-ray and ultrasound have distinct disadvantages: the unwanted ionizing radiation in X-ray techniques,
the lack of soft tissue contrast, the fixed geometry of X-ray source and detector, and the sound reflections in ultrasound.
Introduction: TSENSE [1] based real-time reconstruction software for interventional applications has been previously described [2]. In this work, we present a parallel imaging algorithm based on TGRAPPA [3] for real-time MRI, called HTGRAPPA and its real-time, low latency implementation suitable for interventional MR applications. Our method calculates GRAPPA coefficients in k-space, but applies them in the image domain to avoid time-consuming convolution operations [4] allowing reconstruction fast enough for real-time imaging. In HTGRAPPA, image domain GRAPPA weights were combined into composite unmixing coefficients using adaptive B1-map estimates and optimal noise weighting to eliminate per coil reconstructions. That makes it possible to reconstruct images in the image domain by pixel-by-pixel multiplication and summing, instead of time-consuming convolution operations in k-space. Weight-sets were computed asynchronously to the image reconstruction, and updated quickly to adapt to changes in the image plane and coil sensitivity profiles. Our algorithm provides constant reconstruction performance, independent of the acceleration rate and GRAPPA kernel size. We evaluated our method using 30-coil, rate 4 dataset and compared it to TGRAPPA and TSENSE in performance and image quality. HTGRAPPA reconstruction algorithm was up to 265 times faster than TGRAPPA with no reduction in image quality. A frame rate exceeding 70 was reached on previously acquired data, which is more than sufficient for real-time data rates. Additionally, HTGRAPPA doesn't exhibit pre-folding artifacts when small FOV is used. Methods: Real-time cardiac images from healthy individuals were acquired using a Siemens Magnetom Avanto 1.5T MRI scanner (Siemens Medical Solutions, Erlangen, Germany). Sequence parameters for the experiments were as follows: TR=3.06ms, flip angle=45º, and acquisition matrix of 192x108. Data were acquired using a 32-element array (Invivo Corporation), 16 elements on the chest, 16 elements under the spine The HTGRAPPA imaging technique was employed with acceleration factor R=4. Images were reconstructed and displayed in real-time.Weight-set calculation and reconstruction were computed in parallel in separate processing threads using 8 dual-core AMD Opteron 8220 processor (2.8 GHz) on Linux with SMP configuration, and weights were updated continuously and employed when available.
In recent years, there has been an explosive growth of magnetic resonance imaging (MRI) techniques that allow faster scan speed by exploiting temporal or spatiotemporal redundancy of the images. These techniques improve the performance of dynamic imaging significantly across multiple clinical applications, including cardiac functional examinations, perfusion imaging, blood flow assessment, contrast-enhanced angiography, functional MRI, and interventional imaging, among others. The scan acceleration permits higher spatial resolution, increased temporal resolution, shorter scan duration, or a combination of these benefits. Along with the exciting developments is a dizzying proliferation of acronyms and variations of the techniques. The present review attempts to summarize this rapidly growing topic and presents conceptual frameworks to understand these techniques in terms of their underlying mechanics and connections. Techniques from view sharing, keyhole, k-t, to compressed sensing are covered.
To demonstrate the feasibility of real-time phase contrast magnetic resonance (PCMR) assessment of continuous cardiac output with a heterogeneous (CPU/GPU) system for online image reconstruction.
Twenty healthy volunteers underwent aortic flow examination during exercise using a real-time spiral PCMR sequence. Acquired data were reconstructed in online fashion using an iterative sensitivity encoding (SENSE) algorithm implemented on an external computer equipped with a GPU card. Importantly, data were sent back to the scanner console for viewing. A multithreaded CPU implementation of the real-time PCMR reconstruction was used as a reference point for the online GPU reconstruction assessment and validation. A semiautomated segmentation and registration algorithm was applied for flow data analysis.
There was good agreement between the GPU and CPU reconstruction (−0.4 ± 0.8 mL). There was a significant speed-up compared to the CPU reconstruction (15×). This translated into the flow data being available on the scanner console ≈9 seconds after acquisition finished. This compares to an estimated time using the CPU implementation of 83 minutes.
Our heterogeneous image reconstruction system provides a base for translation of complex MRI algorithms into clinical workflow. We demonstrated its feasibility using real-time PCMR assessment of continuous cardiac output as an example. J. Magn. Reson. Imaging 2012; 36:1477–1482.
PurposeTo test the hypothesis that cardiac and coronary catheterization can be successfully performed under real-time MR guidance using a conventional x-ray angiographic catheter.Materials and Methods
Cardiac and coronary catheterization was conducted on eight farm pigs using a real-time True FISP sequence. A pigtail catheter was used for both left- and right-heart catheterizations performed on all eight animals, while an Amplatz or Judkins catheter was used for the right coronary catheterization that was attempted on five animals. The intravascular devices were visualized by means of their native susceptibility artifacts. For right coronary artery catheterizations, 25% diluted gadolinium (Gd) contrast material was injected to confirm engagement of the right coronary artery.ResultsCardiac catheterization of both the right- and left-heart chambers was successfully performed in all eight pigs. In addition, right coronary catheterization was successfully completed in four of the five pigs in which it was attempted. The procedure time for cardiac catheterization was one minute, while the time range required for coronary catheterization was 32–91 minutes.Conclusion
This work demonstrates that MRI-guided cardiac catheterization using conventional X-ray angiographic catheters is feasible; however, coronary catheterization with this passive-tracking technique is limited. J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc.
The accurate assessment of left ventricular (LV) systolic function is clinically important in nearly all types of cardiac
disease and often directly affects patient management. One example in ischemic heart disease include acute myocardial infarction
(MI) for which ejection fraction and end-systolic volume are important predictors of survival (1) and are used to determine medical therapy. In other examples, a positive functional response to low doses of dobutamine
accurately predicts recovery of function after acute MI and identifies myocardial viability in chronic MI. In the latter case,
the functional response to dobutamine can be used to guide coronary revascularization therapy. In addition, the presence or
absence of inducible wall motion abnormalities during stress testing indicates the presence or absence of significant coronary
artery stenoses and consequently determines whether cardiac catheterization is indicated. Finally, recent research shows that
the degree of LV dyssynchrony may be better than QRS morphology or width for predicting the response to cardiac resynchronization
therapy in patients with heart failure.
: The principal limitations of percutaneous techniques to replace the aortic valve are detailed visualization and durable prostheses. We report the feasibility of using real-time magnetic resonance imaging (MRI) to provide precise anatomic detail and visual feedback to implant a proven bioprosthesis.
: Twelve domestic pigs were anesthetized, and, through a minimally invasive approach using real-time MRI guidance, underwent aortic valve replacement. This was accomplished on the beating heart by using a commercially available bioprosthesis. MRI was used to precisely identify the anatomic landmarks of the aortic annulus, coronary artery ostia, and the mitral valve leaflets. Additional intraoperative perfusion, flow velocity, and functional imaging were used to confirm adequacy of placement and function of the valve.
: Under real-time MRI, multiple oblique planes were prescribed to delineate the anatomy of the native aortic valve and left ventricular outflow track. Enhanced by the use of an active marker wire, this imaging allowed correct placement and orientation of the valve. Through a transapical approach, a series of bioprosthetic aortic valves (21 to 25 mm) were inserted. The time to implantation after the placement of the trocar to deployment of the valve was less than 90 seconds. The average procedure duration was less than 40 minutes
: Real-time MRI provides excellent anatomic detail and intraoperative assessment that permits placement of durable valve prostheses on the beating heart without the limitations of percutaneous approaches.
Virtual and augmented reality environments have been adopted in medicine as a means to enhance the clinician's view of the anatomy and facilitate the performance of minimally invasive procedures. Their value is truly appreciated during interventions where the surgeon cannot directly visualize the targets to be treated, such as during cardiac procedures performed on the beating heart. These environments must accurately represent the real surgical field and require seamless integration of pre- and intra-operative imaging, surgical tracking, and visualization technology in a common framework centered around the patient. This review begins with an overview of minimally invasive cardiac interventions, describes the architecture of a typical surgical guidance platform including imaging, tracking, registration and visualization, highlights both clinical and engineering accuracy limitations in cardiac image guidance, and discusses the translation of the work from the laboratory into the operating room together with typically encountered challenges.
TSENSE and TGRAPPA are autocalibrated parallel imaging techniques that can improve the temporal resolution and/or spatial resolution in dynamic magnetic resonance imaging applications. In its original form, TSENSE uses temporal low-pass filtering of the undersampled frames to create the sensitivity map. TGRAPPA uses a sliding-window moving average when finding the autocalibrating signals. Both filtering methods are suboptimal in the least-squares sense and may give rise to mismatches between the undersampled k-space raw data and the corresponding coil sensitivities. Such mismatches may result in aliasing artifacts when imaging patients with heavy breathing, as in real-time imaging of wall motion by MRI following a treadmill exercise stress test. In this study, we demonstrate the use of an optimal linear filter, i.e., the Karhunen-Loeve transform filter, to estimate the channel sensitivity for TSENSE and acquire the autocalibration signals for TGRAPPA. Phantom experiments show that the new reconstruction method has comparable signal-to-noise ratio performance to traditional TSENSE/TGRAPPA reconstruction. In vivo real-time cardiac cine experiments performed in five healthy volunteers post-exercise during rapid respiration show that the new method significantly reduces the chest wall aliasing artifacts caused by respiratory motion (P < 0.001).
There are many excellent specialised texts and articles that describe the physical principles of cardiovascular magnetic resonance (CMR) techniques. There are also many texts written with the clinician in mind that provide an understandable, more general introduction to the basic physical principles of magnetic resonance (MR) techniques and applications. There are however very few texts or articles that attempt to provide a basic MR physics introduction that is tailored for clinicians using CMR in their daily practice. This is the first of two reviews that are intended to cover the essential aspects of CMR physics in a way that is understandable and relevant to this group. It begins by explaining the basic physical principles of MR, including a description of the main components of an MR imaging system and the three types of magnetic field that they generate. The origin and method of production of the MR signal in biological systems are explained, focusing in particular on the two tissue magnetisation relaxation properties (T1 and T2) that give rise to signal differences from tissues, showing how they can be exploited to generate image contrast for tissue characterisation. The method most commonly used to localise and encode MR signal echoes to form a cross sectional image is described, introducing the concept of k-space and showing how the MR signal data stored within it relates to properties within the reconstructed image. Before describing the CMR acquisition methods in detail, the basic spin echo and gradient pulse sequences are introduced, identifying the key parameters that influence image contrast, including appearances in the presence of flowing blood, resolution and image acquisition time. The main derivatives of these two pulse sequences used for cardiac imaging are then described in more detail. Two of the key requirements for CMR are the need for data acquisition first to be to be synchronised with the subject's ECG and to be fast enough for the subject to be able to hold their breath. Methods of ECG synchronisation using both triggering and retrospective gating approaches, and accelerated data acquisition using turbo or fast spin echo and gradient echo pulse sequences are therefore outlined in some detail. It is shown how double inversion black blood preparation combined with turbo or fast spin echo pulse sequences acquisition is used to achieve high quality anatomical imaging. For functional cardiac imaging using cine gradient echo pulse sequences two derivatives of the gradient echo pulse sequence; spoiled gradient echo and balanced steady state free precession (bSSFP) are compared. In each case key relevant imaging parameters and vendor-specific terms are defined and explained.
Background — We tested the feasibility of targeted left ventricular (LV) mural injection using real-time MRI (rtMRI).
Methods and Results — A 1.5T MRI scanner was customized with a fast reconstruction engine, transfemoral guiding catheter–receiver coil (GCC), MRI-compatible needle, and tableside consoles. Commercial real-time imaging software was customized to facilitate catheter navigation and visualization of injections at 4 completely refreshed frames per second. The aorta was traversed and the left ventricular cavity was entered under direct rtMRI guidance. Pigs underwent multiple injections with dilute gadolinium-DTPA. All myocardial segments were readily accessed. The active GCC and the passive Stiletto needle injector were readily visualized. More than 50 endomyocardial injections were performed with the aid of rtMRI; 81% were successful with this first-generation prototype.
Conclusion — Percutaneous endomyocardial drug delivery is feasible with the aid of rtMRI, which permits precise 3-dimensional localization of injection within the LV wall.
The performance of parallel imaging using sensitivity encoding (SENSE) for accelerated acquisition is high dependent on the number and geometry of surface coils. The SNR loss due to the noise amplification of the SENSE method is characterized by the g-factor. Arrays optimized for cardiac SENSE application using 6-elements have been previously described. We present experimental cardiac imaging results comparing SENSE at various rates acquired with several different 8-element surface coil configurations, as well as 4-coil cardiac array. SENSE was used to reduce the brett-hold time from 48s to 12s for acquiring cardiac cine images with spatial and temporal resolution of 256x192 and 30 ms, respectively.
Imaging speed is a key factor in most cardiovascular applications of magnetic resonance imaging. Recently, simultaneous signal acquisition with multiple coils has received increasing attention as a means of enhancing scan speed in MRI. Based on this approach, the sensitivity encoding technique SENSE enables substantial scan time reduction by exploiting the inherent spatial encoding effect of receiver coil sensitivity. This work studies the benefit of sensitivity encoding for cardiovascular MRI. SENSE is applied to accelerate common breath-hold imaging as well as real-time imaging by factors up to 3.2. In the breath-hold mode with ECG triggering, this speed benefit has been used both for reducing the breath-hold interval and for improving spatial resolution. In cardiac real-time imaging without triggering and breath control, the SENSE approach has enabled significantly enhanced temporal resolution, ranging down to 13 ms (77 frames/s). Cardiac real-time SENSE is demonstrated in several modes, including real-time imaging of three parallel slices at a rate of 25 triple frames per second.
Volume renderings from magnetic resonance imaging data can be created and displayed in real-time with user interactivity. This can provide continuous 3D feedback to assist in guiding an interventional procedure. A system is presented which can produce real-time volume renderings from 2D multi-slice or 3D MR pulse sequences. Imaging frame rates up to 30 per second have been demonstrated with a latency of approximately one-third of a second, depending on the image matrix size. Several interactive capabilities have been implemented to enhance visualization such as cut planes, individual channel scaling and color highlighting, view sharing, saturation preparation, complex subtraction, gating control, and choice of alpha blending or MIP rendering. The system is described and some interventional application examples are shown. To view movies of some of the examples, enter the following address into a web browser: http://nhlbi.nih.gov/labs/papers/lce/guttman/rtvolmri/index/htm.
New theoretical and practical concepts are presented for considerably enhancing the performance of magnetic resonance imaging (MRI) by means of arrays of multiple receiver coils. Sensitivity encoding (SENSE) is based on the fact that receiver sensitivity generally has an encoding effect complementary to Fourier preparation by linear field gradients. Thus, by using multiple receiver coils in parallel scan time in Fourier imaging can be considerably reduced. The problem of image reconstruction from sensitivity encoded data is formulated in a general fashion and solved for arbitrary coil configurations and k-space sampling patterns. Special attention is given to the currently most practical case, namely, sampling a common Cartesian grid with reduced density. For this case the feasibility of the proposed methods was verified both in vitro and in vivo. Scan time was reduced to one-half using a two-coil array in brain imaging. With an array of five coils double-oblique heart images were obtained in one-third of conventional scan time. Magn Reson Med 42:952-962, 1999.
A highly interactive MRI scanner interface has been developed that allows, for the first time, real-time graphic control of one-dimensional (1D) and two-dimensional (2D) cardiac MRI exams. The system comprises a Mercury array processor (AP) in a Sun SPARCserver with two connections to the MRI scanner, a data link that passes the NMR data directly to the AP as they are collected, and a control link that passes commands from the Sun to the scanner to redirect the imaging pulse sequence in real time. In the 1D techniques, a cylinder or “pencil” of magnetization is repeatedly excited using gradient-echo or spin-echo line-scan sequences, with the magnetization read out each time along the length of the cylinder, and a scrolling display generated on the Sun monitor. Rubber-band lines drawn on the scout image redirect the pencil or imaging slice to different locations, with the changes immediately visible in the display. M-mode imaging, 1D flow imaging, and 2D fast cardiac imaging have been demonstrated on normal volunteers using this system. This platform represents an operator-“friendly” way of directing real-time imaging of the heart.
The success of x-ray fluoroscopy-guided coronary catheterization depends in part on the ability to obtain simultaneous and real-time visualization of the guidewire, guiding catheter, and anatomy of the chest. The hypothesis explored in this paper is that magnetic resonance imaging (MRI) could provide this ability. This hypothesis was tested with loopless antennas used as the guidewire and a guiding catheter and two surface coils, each connected to four different receiver channels of a GE 1.5-T CV/I MRI scanner. Experiments were conducted on six healthy dogs. Intravascular antennas were inserted in the right carotid artery and maneuvered in the aorta while running a fast gradient-echo sequence (TR/TE 5/1.3 msec, flip angle 7°). Real-time projection images of the chest anatomy, together with the guidewire and guiding catheter, were obtained. Positioning of the MRI guiding catheter either in the descending aorta, ascending aorta, or heart was achieved easily. This study represents a step toward MRI-guided coronary catheterization. J. Magn. Reson. Imaging 2000;12:590–594. © 2000 Wiley-Liss, Inc.
A number of different methods have been demonstrated which increase the speed of MR acquisition by decreasing the number of sequential phase encodes. The UNFOLD technique is based on time interleaving of k-space lines in sequential images and exploits the property that the outer portion of the field-of-view is relatively static. The differences in spatial sensitivity of multiple receiver coils may be exploited using SENSE or SMASH techniques to eliminate the aliased component that results from undersampling k-space. In this article, an adaptive method of sensitivity encoding is presented which incorporates both spatial and temporal filtering. Temporal filtering and spatial encoding may be combined by acquiring phase encodes in an interleaved manner. In this way the aliased components are alternating phase. The SENSE formulation is not altered by the phase of the alias artifact; however, for imperfect estimates of coil sensitivities the residual artifact will have alternating phase using this approach. This is the essence of combining temporal filtering (UNFOLD) with spatial sensitivity encoding (SENSE). Any residual artifact will be temporally frequency-shifted to the band edge and thus may be further suppressed by temporal low-pass filtering. By combining both temporal and spatial filtering a high degree of alias artifact rejection may be achieved with less stringent requirements on accuracy of coil sensitivity estimates and temporal low-pass filter selectivity than would be required using each method individually. Experimental results that demonstrate the adaptive spatiotemporal filtering method (adaptive TSENSE) with acceleration factor R = 2, for real-time nonbreath-held cardiac MR imaging during exercise induced stress are presented. Magn Reson Med 45:846–852, 2001. Published 2001 Wiley-Liss, Inc.
We describe an experimental system for performing high-speed reconstruction of MR image data acquired with a GRASS sequence. System characteristics are an image acquisition time of 627 ms, continuous image reconstruction at a rate of 6 images/s, and an image reconstruction time of 120 ms. The results is a system for performing MR imaging in real time.
SiMultaneous Acquisition of Spatial Harmonics (SMASH) is a new fast-imaging technique that increases MR image acquisition speed by an integer factor over existing fast-imaging methods, without significant sacrifices in spatial resolution or signal-to-noise ratio. Image acquisition time is reduced by exploiting spatial information inherent in the geometry of a surface coil array to substitute for some of the phase encoding usually produced by magnetic field gradients. This allows for partially parallel image acquisitions using many of the existing fast-imaging sequences. Unlike the data combination algorithms of prior proposals for parallel imaging, SMASH reconstruction involves a small set of MR signal combinations prior to Fourier transformation, which can be advantageous for artifact handling and practical implementation. A twofold savings in image acquisition time is demonstrated here using commercial phased array coils on two different MR-imaging systems. Larger time savings factors can be expected for appropriate coil designs.
A real-time interactive MRI system capable of localizing coronary arteries and imaging arrhythmic hearts in real-time is described. Non-2DFT acquisition strategies such as spiral-interleaf, spiral-ring, and circular echo-planar imaging provide short scan times on a conventional scanner. Real-time gridding reconstruction at 8-20 images/s is achieved by distributing the reconstruction on general-purpose UNIX workstations. An X-windows application provides interactive control. A six-interleaf spiral sequence is used for cardiac imaging and can acquire six images/s. A sliding window reconstruction achieves display rates of 16-20 images/s. This allows cardiac images to be acquired in real-time, with minimal motion and flow artifacts, and without breath holding or cardiac gating. Abdominal images are acquired at over 2.5 images/s with spiral-ring or circular echo-planar sequences. Reconstruction rates are 8-10 images/s. Rapid localization in the abdomen is demonstrated with the spiral-ring acquisition, whereas peristaltic motion in the small bowel is well visualized using the circular echo-planar sequence.
New theoretical and practical concepts are presented for considerably enhancing the performance of magnetic resonance imaging (MRI) by means of arrays of multiple receiver coils. Sensitivity encoding (SENSE) is based on the fact that receiver sensitivity generally has an encoding effect complementary to Fourier preparation by linear field gradients. Thus, by using multiple receiver coils in parallel scan time in Fourier imaging can be considerably reduced. The problem of image reconstruction from sensitivity encoded data is formulated in a general fashion and solved for arbitrary coil configurations and k-space sampling patterns. Special attention is given to the currently most practical case, namely, sampling a common Cartesian grid with reduced density. For this case the feasibility of the proposed methods was verified both in vitro and in vivo. Scan time was reduced to one-half using a two-coil array in brain imaging. With an array of five coils double-oblique heart images were obtained in one-third of conventional scan time. Magn Reson Med 42:952-962, 1999.
In several applications, MRI is used to monitor the time behavior of the signal in an organ of interest; e.g., signal evolution because of physiological motion, activation, or contrast-agent accumulation. Dynamic applications involve acquiring data in a k-t space, which contains both temporal and spatial information. It is shown here that in some dynamic applications, the t axis of k-t space is not densely filled with information. A method is introduced that can transfer information from the k axes to the t axis, allowing a denser, smaller k-t space to be acquired, and leading to significant reductions in the acquisition time of the temporal frames. Results are presented for cardiac-triggered imaging and functional MRI (fMRI), and are compared with data obtained in a conventional way. The temporal resolution was increased by nearly a factor of two in the cardiac-triggered study, and by as much as a factor of eight in the fMRI study. This increase allowed the acquisition of fMRI activation maps, even when the acquisition time for a single full time frame was actually longer than the paradigm cycle period itself. The new method can be used to significantly reduce the acquisition time of the individual temporal frames in certain dynamic studies. This can be used, for example, to increase the temporal or spatial resolution, increase the spatial coverage, decrease the total imaging time, or alter sequence parameters e.g., repetition time (TR) and echo time (TE) and thereby alter contrast. Magn Reson Med 42:813-828, 1999.
To improve real-time control of interventional procedures such as guidance of catheters, monitoring of ablation therapy, or control of dosage during drug delivery, the image acquisition and reconstruction must be high speed and have low latency (small time delay) in processing. A number of different methods have been demonstrated which increase the speed of MR acquisition by decreasing the number of sequential phase-encodes. A design and implementation of the UNFOLD method which achieves the desired low latency with a recursive temporal filter is presented. The recursive filter design is characterized for this application and compared with more commonly used moving average filters. Experimental results demonstrate low-latency UNFOLD for two applications: 1) high-speed, real-time imaging of the heart to be used in conjunction with cardiac interventional procedures; and 2) the injection of drugs into muscle tissue with contrast enhancement, i.e., monitoring needle insertion and injection of a drug with contrast enhancement properties. Proof-of-concept was demonstrated by injecting a contrast agent. In both applications the UNFOLD technique was used to double the frame rate.
Stereoscopic MRI can impart 3D perception with only two image acquisitions. This economy over standard multiplanar 3D volume renderings allows faster frame rates, which are needed for real-time imaging applications. Real-time 3D perception may enhance the appreciation of complex anatomical structures, and may improve hand-eye coordination while manipulating a medical device during an image-guided interventional procedure. To this goal, a system is being developed to acquire and display stereoscopic MR images in real-time. A clinically used, fast gradient-recalled echo-train sequence has been modified to produce stereo image pairs. Features have been added for depth cueing, view sharing, and bulk signal suppression. A workstation was attached to a clinical MR scanner for fast data extraction, image reconstruction and stereoscopic image display.
This work describes a real-time imaging and visualization technique that allows multiple field of view (FOV) imaging. A stream of images from a single receiver channel can be reconstructed at multiple FOVs within each image frame. Alternately, or in addition, when multiple receiver channels are available, image streams from each channel can be independently reconstructed at multiple FOVs. The implementation described here provides for real-time visualization of the placement of guidewires and catheters on a dynamic roadmap during interventional procedures. The loopless catheter antenna, an electrically active intravascular probe, was used for MR signal reception. In 2D projection images, the catheter and surrounding structures within its diameter of sensitivity appear as bright signal. The simplicity of the resulting images allows very-narrow-FOV imaging to decrease imaging time. Very-narrow-FOV images are acquired on MR receiver channels that collect guidewire or catheter data. These very-narrow-FOV images provide very high frame rate continuous, real-time imaging of the interventional devices (25 fps). Large-FOV images are formed from receiver channels that collect anatomical data from standard imaging surface coils, and simultaneously provide a dynamic, frequently updated roadmap. These multiple-FOV images are displayed together, improving visualization of interventional device placement.
The design and application of a two-wire electrophysiology (EP) catheter that simultaneously records the intracardiac electrogram and receives the MR signal for active catheter tracking is described. The catheter acts as a long loop receiver, allowing for visualization of the entire catheter length while simultaneously behaving as a traditional two-wire EP catheter, allowing for intracardiac electrogram recording and ablation. The application of the device is demonstrated by simultaneously tracking the catheter and recording the intracardiac electrogram in canine models using 7 and 10 frame/sec real-time imaging sequences. Using solely MR imaging, the entire catheter was visualized and guided from the jugular vein into the cardiac chambers, where the intracardiac electrogram was recorded. By combining several functions in a single, simple structure, the excellent tissue contrast and functional imaging capabilities of MR can be used to improve the efficacy of EP interventions. This catheter will facilitate MR-guided interventions and demonstrates the design of multifunctional interventional devices for use in MRI.
Calibration of the spatial sensitivity functions of coil arrays is a crucial element in parallel magnetic resonance imaging (PMRI). The most common approach has been to measure coil sensitivities directly using one or more low-resolution images acquired before or after accelerated data acquisition. However, since it is difficult to ensure that the patient and coil array will be in exactly the same positions during both calibration scans and accelerated imaging, this approach can introduce sensitivity miscalibration errors into PMRI reconstructions. This work shows that it is possible to extract sensitivity calibration images directly from a fully sampled central region of a variable-density k-space acquisition. These images have all the features of traditional PMRI sensitivity calibrations and therefore may be used for any PMRI reconstruction technique without modification. Because these calibration data are acquired simultaneously with the data to be reconstructed, errors due to sensitivity miscalibration are eliminated. In vivo implementations of self-calibrating parallel imaging using a flexible coil array are demonstrated in abdominal imaging and in real-time cardiac imaging studies.
We tested the feasibility of targeted left ventricular (LV) mural injection using real-time MRI (rtMRI).
A 1.5T MRI scanner was customized with a fast reconstruction engine, transfemoral guiding catheter-receiver coil (GCC), MRI-compatible needle, and tableside consoles. Commercial real-time imaging software was customized to facilitate catheter navigation and visualization of injections at 4 completely refreshed frames per second. The aorta was traversed and the left ventricular cavity was entered under direct rtMRI guidance. Pigs underwent multiple injections with dilute gadolinium-DTPA. All myocardial segments were readily accessed. The active GCC and the passive Stiletto needle injector were readily visualized. More than 50 endomyocardial injections were performed with the aid of rtMRI; 81% were successful with this first-generation prototype.
Percutaneous endomyocardial drug delivery is feasible with the aid of rtMRI, which permits precise 3-dimensional localization of injection within the LV wall.
In dynamic MRI, it is often difficult to achieve the acquisition speed required to resolve or freeze the temporal variations of the imaged object. Several MRI methods aim at speeding up the image acquisition process. Through assumptions and/or prior knowledge, these dynamic MRI methods allow part of the needed data to be calculated instead of acquired. For example, partial-Fourier imaging assumes that phase varies smoothly within the object, and parallel imaging (e.g., simultaneous acquisition of spatial harmonics (SMASH) and sensitivity encoding (SENSE)) uses prior knowledge about receiver-coil sensitivity. While these methods accelerate acquisition, they can introduce artifacts or amplify noise in doing so. The present work aims at accelerating image acquisition significantly, while introducing almost no artifacts or noise amplification. It is shown here that new, extra information is gained if dynamic MRI methods are modified so that the sampling function changes in specific ways from time-frame to time-frame. In other words, the set of k-space locations that are acquired (instead of calculated) changes with time. The present temporal strategy, based on the UNaliasing by Fourier-encoding the Overlaps in the temporaL Dimension (UNFOLD) method, can be incorporated into common dynamic MRI methods. Results with partial-Fourier, SMASH, and SENSE imaging are presented here, where UNFOLD's contribution is to very significantly reduce the artifact and/or amplified noise content. Used in this way, UNFOLD contributes indirectly, rather than directly to the improvement in image acquisition speed, as it allows companion methods to operate properly at greater acceleration settings than would otherwise be feasible.
An integrated system for performing interventional magnetic resonance angiography (MRA) with actively visualized instruments and real-time image fusion was implemented on a 1.5 T scanner. True fast imaging with steady precession (TrueFISP) imaging provided high acquisition speed paired with high signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) for the simultaneous visualization of active instruments and arterial morphology. The system enabled simultaneous image reconstruction and image postprocessing of multiple receiver channels, with subsequent image fusion display in real time. Optional interleaved image acquisition in two planes provided additional important information for biplanar instrument guidance. Various vascular interventions, including selective catheterization and subsequent selective MRA of the abdominal aorta, renal arteries, superior mesenteric artery (SMA), hepatic artery, and aortic arch, were performed on 10 pigs under MR guidance. In terms of instrument contrast, image acquisition, reconstruction, and fusion speed, the setup represents a powerful platform for performing interventional MRA procedures.
Low-latency temporal filter design for real-time MRI using UNFOLDPubMed: 11108631] Guttman et al. Page 7 Magn Reson Med Author manuscript; available in PMC 2007 October 17. NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript 15. Oppelt A. FISP—a new fast MRI sequence
- P Kellman
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- Fh Epstein
- Mcveigh
Kellman P, Sorger JM, Epstein FH, McVeigh ER. Low-latency temporal filter design for real-time MRI using UNFOLD. Magn Reson Med 2000;44:933–939. [PubMed: 11108631] Guttman et al. Page 7 Magn Reson Med. Author manuscript; available in PMC 2007 October 17. NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript 15. Oppelt A. FISP—a new fast MRI sequence. Electromedica 1986;54:15–18.
Real-time interactive accelerated imaging with on-line adaptive TSENSE
- M A Guttman
- P Kellman
- E R Mcveigh
Guttman, MA.; Kellman, P.; McVeigh, ER. Real-time interactive accelerated imaging with on-line
adaptive TSENSE; Proceedings of the 10th Annual Meeting of ISMRM; Honolulu. 2002. p. 195
- Madore
Real-time reconstruction of sensitivity-encoded magnetic resonance imaging
- Eggersh Weigerm Proksar Pruessmannkp Boesigerp
Real-time interactive accelerated imaging with on-line adaptive TSENSE
- Guttmanma Kellmanp Mcveigher
Comparison of several 8-element surface coil configurations for cardiac imaging using SENSE
- Kellmanp Derbyshireja Morrishd Leddenpj Mcveigher