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Echo peak shift as a function of gradient amplitude for each physical axis. Reference measurement is for 3 mT m −1
Source publication
“Ultrashort TE” (UTE) has gained attention in recent years because of its ability to visualize tissues with very short T2 that cannot normally be seen with MRI. In principle, the method for implementing 2-D UTE measurements is quite simple, requiring the use of selective excitation by half RF pulses, rapid readout of a half echo, and summation of p...
Contexts in source publication
Context 1
... the sake of completeness, we did an additional experiment to test whether time delay error varied significantly with gradient amplitude. It did (Figure 9), confirming the presence of another difficulty we had already suspected. It is most practical to view this fact as just one more way in which relative signal timing can be disturbed. ...
Context 2
... time-delay errors vary with position in the scanner ( Figure 8) and with gradient amplitude (Figure 9) it is necessary to devise more elaborate calibration methods in order to improve the reliability of the slice-selection process. One might consider the following possibilities: ...
Context 3
... echo peak shift value varies with gradient amplitude and with physical axis in the actual gradient system (Figure 9), hence the need for measurement at all the true slice positions at the correct angulation. ...
Citations
... However, the k-space value at each time point during readout may deviate from the nominal value due to the effects of time delays and gradient distortions. 19,21,30,34 For quantification, UTE is more sensitive to these distortions compared to full-echo radial schemes because the central k-space region, which is directly impacted by these errors, dominates image contrast. For example, Takizawa et al 21 have shown that even in the absence of timing delays, the residual higher order gradient distortions lead to significant image degradation in UTE images, whereas artifacts are unnoticeable in full-echo radial images. ...
Purpose
The impact of gradient imperfections on UTE images and UTE image‐derived bone water quantification was investigated at 3 T field strength.
Methods
The effects of simple gradient time delays and eddy currents on UTE images, as well as the effects of gradient error corrections, were studied with simulation and phantom experiments. The k‐space trajectory was mapped with a 2D sequence with phase encoding on both spatial axes by measuring the phase of the signal in small time increments during ramp‐up of the read gradient. In vivo 3D UTE images were reconstructed with and without gradient error compensation to determine the bias in bone water quantification. Finally, imaging was performed on 2 equally configured Siemens TIM Trio systems (Siemens Medical Solutions, Erlangen, Germany) to investigate the impact of such gradient imperfections on inter‐scanner measurement bias.
Results
Compared to values derived from UTE images with full gradient error compensation, total bone water was found to deviate substantially with no (up to 17%) or partial (delay‐only) compensation (up to 10.8%). Bound water, obtained with inversion recovery‐prepared UTE, was somewhat less susceptible to gradient errors (up to 2.2% for both correction strategies). Inter‐scanner comparison indicated a statistically significant bias between measurements from the 2 MR systems for both total and bound water, which either vanished or was substantially reduced following gradient error correction.
Conclusion
Gradient imperfections impose spatially dependent artifacts on UTE images, which compromise not only bone water quantification accuracy but also inter‐scanner measurement agreement if left uncompensated.
... Over time, various strategies have been proposed to target slice-profile distortions for 2D-UTE imaging, such as gradient waveform corrections based on transfer function analyses, 11,12 iterative methods to predistort the slice-selection gradient, 13,14 eddy current measurement and compensation strategies, 15 out-of-slice saturation, 16,17 a dedicated prescan to estimate slice and correct slice-profile errors, 18,19 excitation with a double HP combination, 20 or saturation-based slice selection. 21 In this paper, we propose an alternative approach to target the HP slice-distortion problem induced by a nonperfect slice-selective gradient. ...
Purpose
To improve the slice profile quality obtained by RF half‐pulse excitation for 2D‐UTE applications.
Methods
The overall first‐order and zero‐order phase errors along the slice‐selection direction were obtained with the help of an optimization task to minimize the out‐of‐slice signal contamination from the calibration 1‐dimenisonal (1D) profile data. The time‐phase‐error evolution was approximated from the k‐space readout data, which were acquired primarily for correction of the readout trajectories during data regridding to the rectilinear grids. The correction of the slice profile was achieved by rephasing gradient pulses applied immediately after the end of excitation. The total prescan calibration typically took less than 2 minutes.
Results
The improved image quality using the proposed calibration method was demonstrated both on phantoms and on ankle images obtained from healthy volunteers. It was demonstrated that calibration can be performed either as a separate water phantom measurement or directly as a prescan procedure.
Conclusion
The slice‐profile distortion from the half‐pulse excitation could substantially affect the overall fidelity of 2D‐UTE images. The presented experiments proved that the image quality could be substantially increased by application of the proposed slice‐correction method.
... Second, the readout is performed under a constant gradient, therefore avoiding potential eddy-current-related image distortions associated with ramp sampling. 21 ZTE imaging also leads to a dead-time gap in the k-space center that may be filled using different strategies, such as algebraic reconstruction (AR), water-and fat-suppressed proton projection MRI (WASPI), or point-wise encoding time radial acquisition (PETRA). In contrast to WASPI or PETRA, AR obviates the need to acquire complementary data to fill the k-space center and, under certain experimental conditions, offers a benign point spread function (PSF) solely driven by radial T 2 decay. ...
Purpose
To perform direct, selective MRI of short‐T2 tissues using zero echo time (ZTE) imaging with weighted echo subtraction (WSUB).
Methods
Radial imaging was performed at 7T, acquiring both ZTE and gradient echo (GRE) signals created by bipolar gradients. Long‐T2 suppression was achieved by weighted subtraction of ZTE and GRE images. Special attention was given to imperfections of gradient dynamics, to which radial GRE imaging is particularly susceptible. To compensate for gradient errors, matching of gradient history was combined with data correction based on trajectory measurement. The proposed approach was first validated in phantom experiments and then demonstrated in musculoskeletal (MSK) imaging.
Results
Trajectory analysis and phantom imaging demonstrated that gradient imperfections were successfully addressed. Gradient history matching enabled consistency between antiparallel projections as required for deriving zeroth‐order eddy current dynamics. Trajectory measurement provided individual echo times per projection that showed considerable variation between gradient directions. In in vivo imaging of knee, ankle, and tibia, the proposed approach enabled high‐resolution 3D depiction of bone, tendons, and ligaments. Distinct contrast of these structures indicates excellent selectivity of long‐T2 suppression. Clarity of depiction also confirmed sufficient SNR of short‐T2 tissues, achieved by high baseline sensitivity at 7T combined with high SNR efficiency of ZTE acquisition.
Conclusion
Weighted subtraction of ZTE and GRE data reconciles robust long‐T2 suppression with fastest k‐space coverage and high SNR efficiency. This approach enables high‐resolution imaging with excellent selectivity to short‐T2 tissues, which are of major interest in MSK and neuroimaging applications.
... The boxes mark the images shown in Figure 8 gradient pulse and inconsistency of RF power output between pairs of half-pulse excitations are the main source of error. 44 Some of these issues are complex and difficult to address, nevertheless, especially in this SMS approach the half-pulse excitation profile has a large effect on the image quality. The high modulation of the SMS half-pulses may further intensify the problem. ...
Purpose
To describe a simultaneous multislice (SMS) ultrashort echo time (UTE) imaging method using radiofrequency phase encoded half‐pulses in combination with power independent of number of slices (PINS) inversion recovery (IR) pulses to generate multiple‐slice images with short T2* contrasts in less than 3 min with close to an eightfold acceleration compared with a standard 2D approach.
Theory and Methods
Radiofrequency phase encoding is applied in an SMS (NSMS = 4) excitation scheme using “sinc” half‐pulses. With the use of coil sensitivity encoding (SENSE) and controlled aliasing in parallel imaging (CAIPI) in combination with a gradient echo 2D spiral readout trajectory and IR PINS pulses for contrast enhancement a fast UTE sequence is developed. Images are obtained using a model‐based reconstruction method. Sequence details and performance tests on phantoms as well as the heads of healthy volunteers at 3T are presented.
Results
An SMS UTE sequence with an undersampling factor of 4 is developed using radiofrequency phase encoded half‐pulses and PINS IR pulses which enables the acquisition of 8 slices at 0.86² mm² resolution in less than 3‐min scan time. UTE images of the head are obtained showing highlighted signal of cortical bone. Image quality and T2 contrast are comparable to the one obtained by corresponding single slice acquisitions with only minor deviations.
Conclusions
The method combining radiofrequency phase encoded SMS half‐pulses and PINS IR pulses presents a suitable approach to SMS UTE imaging. The usage of coil sensitivity information and increased sampling density by means of interleaved slice group acquisitions allows to reduce the total scan time by a factor close to 8.
... However, streak artifacts in the images from the UTE sequences become more severe when larger body parts are imaged [20]. Even though the UTE pulse sequences can be simple in theory, its successful implementation needs accurate timing and a detailed understanding of the hardware performance [30]. ...
... However, streak artifacts in the images from the UTE sequences become more severe when larger body parts are imaged (Johansson et al., 2011). Even though the UTE pulse sequences can be simple in theory, its successful implementation needs accurate timing and a detailed understanding of the hardware performance (Margosian et al., 2012). ...
There is increasing interest in computed tomography (CT) image estimations from magnetic resonance (MR) images. This study aims to introduce a novel statistical learning approach for improving CT estimation from MR images. Prior knowledges about tissue-types, roughly speaking non-bone and bone tissue-types from CT images, have been used in collaboration with a Gaussian mixture model (GMM) to explore CT image estimations from MR images. Due to the introduced prior knowledges, GMMs were trained for each of the tissue-type. At the prediction stage, we have no CT image, that is there are no prior knowledges about the tissue-types and thereby we trained RUSBoost algorithm on the training dataset in order to estimate the tissue-types from MR images of the new patient. The estimated RUSBoost algorithm and GMMs were used to predict CT image from MR images of the new patient. We validated the RUSBoost algorithm by applying 10-fold cross-validation while the Gaussian mixture models were validated by using leave-one-out cross-validation of the datasets from the patients. In comparison with the existing model-based CT image estimation methods, the proposed method has improved the estimation, especially in bone tissues. More specifically, our method improved CT image estimation by 23 Hounsfield units (HU) and 6 HU on average for datasets obtained from nine and five patients, respectively. Bone tissue estimations have been improved by 107 HU and 62 HU on average for datasets from nine and five patients, respectively. Evaluation of our method shows that it is a promising method to generate CT image substitutes for the implementation of fully MR-based radiotherapy and PET/MRI applications. Keywords: Computed tomography; magnetic resonance imaging; CT image estimation; supervised learning; Gaussian mixture model
... This approach allows for omission of slice rephasing and phase encoding gradients, however, relies on perfect gradient pulses which make UTE imaging prone to any imperfections caused by eddy currents or a timing mismatch. For example, gradient deviation during the half pulse excitation deforms the slice profile and causes out-of-slice signal contamination [8], while readout gradient imperfections are responsible for in-plane intensity variation and blurring artifacts [9]. Over the last few years a lot of effort has been spent on issues related to imperfect 2D-UTE slice excitation as the source of the systematic errors [8,10,11]. ...
Purpose:
To evaluate the impact of MR gradient system imperfections and limitations for the quantitative mapping of short T2* signals performed by ultrashort echo time (UTE) acquisition approach.
Materials and methods:
The measurement of short T2* signals from a phantom and a healthy volunteer study (8 subjects of average age 28 ± 4 years) were performed on a 3T scanner. The characteristics of the gradient system were obtained using calibration method performed directly on the measured subject or phantom. This information was used to calculate the actual sampling trajectory with the help of a parametric eddy current model. The actual sample positions were used to reconstruct corrected images and compared with uncorrected data.
Results:
Comparison of both approaches, i.e., without and with correction of k-space sampling trajectories revealed substantial improvement when correction was applied. The phantom experiments demonstrate substantial in-plane signal intensity variations for uncorrected sampling trajectories. In the case of the volunteer study, this led to significant differences in relative proton density (RPD) estimation between the uncorrected and corrected data (P = 0.0117 by Wilcoxon matched-pairs test) and provides for about ~15% higher values for short T2* components of white matter (WM) in the case of uncorrected images.
Conclusion:
The imperfection of the applied gradients could induce errors in k-space data sampling which further propagates into the fidelity of the UTE images and jeopardizes precision of quantification. However, the study proved that measurement of gradient errors together with correction of sample positions can contribute to increased accuracy and unbiased characterization of short T2* signals.
... Ultrashort echo time (UTE) [1] imaging is a valuable technique for imaging short T2 and T2* samples, however, its implementation is challenging and acquisition times can be long. Although the UTE pulse sequence is simple in theory, successful implementation requires accurate timing and a detailed understanding of the hardware performance [2]. This paper outlines a method to implement and optimize UTE to achieve accurate slice selection. ...
... The Bloch equations describe the evolution over time of the magnetization in x, y, and z (Mx, My, and Mz) as a function of the strength of the homogeneous magnetic field (B0), any applied gradients in the magnetic field (G), transverse relaxation (T2), and longitudinal relaxation (T1). [2] [3] [4] The Bloch equations were solved in Matlab using numerical integration [31]. A homogeneous sample of length 5 mm was used and resolved with a spatial resolution of 0.1 mm. ...
Ultrashort echo time (UTE) imaging is a well-known technique used in medical MRI, however, the implementation of the sequence remains non-trivial. This paper introduces UTE for non-medical applications and outlines a method for the implementation of UTE to enable accurate slice selection and short acquisition times. Slice selection in UTE requires fast, accurate switching of the gradient and r.f. pulses. Here a gradient "pre-equalization" technique is used to optimize the gradient switching and achieve an effective echo time of 10 mu s. In order to minimize the echo time, k-space is sampled radially. A compressed sensing approach is used to minimize the total acquisition time. Using the corrections for slice selection and acquisition along with novel image reconstruction techniques, UTE is shown to be a viable method to study samples of cork and rubber with a shorter signal lifetime than can typically be measured. Further, the compressed sensing image reconstruction algorithm is shown to provide accurate images of the samples with as little as 12.5% of the full k-space data set, potentially permitting real time imaging of short T-2* materials. (C) 2014 The Authors. Published by Elsevier Inc.
Global climate change is the most important challenge humankind is facing in the modern era. One of the main scientific concerns is the monitoring of contaminants in the environment, which require the right environmental remediation strategies. In this context, nuclear magnetic resonance (NMR) techniques have a very important role in enabling the discovery of how pollutants are transformed, how they can move and how they can affect human health.
This book discusses the present and the future perspectives of NMR techniques for environmental evaluations. It covers, amongst other topics, the importance of NMR as a contamination discovery tool, how to improve sensitivity in environmental NMR, and multiphase NMR for measurement of samples in their natural state. Samples include lubricant oils, soils and porous media. Due to the direct relationship between the environment and human health, there is information dedicated to the use of magnetic resonance imaging (MRI) to monitor human health as related to environmental pollution. There is also a chapter on how NMR is used in cultural heritage to measure artefacts directly affected by environmental pollution.
Filling a gap in the literature, the book is for researchers explaining how to apply their knowledge of NMR techniques to solve environmental problems, and for students who want to deepen their understanding of this topic.
Magnetic Resonance Imaging (MRI) scanners produce loud acoustic noise originating from vibrational Lorentz forces induced by rapidly changing currents in the magnetic field gradient coils. Using zero echo time (ZTE) MRI pulse sequences, gradient switching can be reduced to a minimum, which enables near silent operation. Besides silent MRI, ZTE offers further interesting characteristics, including a nominal echo time of TE=0 (thus capturing short-lived signals from MR tissues which are otherwise MR-invisible), 3D radial sampling (providing motion robustness), and ultra-short repetition times (providing fast and efficient scanning). In this work we describe the main concepts behind ZTE imaging with a focus on conceptual understanding of the imaging sequences, relevant acquisition parameters, commonly observed image artefacts, and image contrasts. We will further describe a range of methods for anatomical and functional neuroimaging, together with recommendations for successful implementation.
