[Show abstract][Hide abstract] ABSTRACT: Magnetic resonance elastography (MRE) can noninvasively measure the stiffness of liver tissue and display this information in anatomic maps. Magnetic resonance imaging (MRI) guidance has not previously been used to biopsy segments of heterogeneous stiffness identified on MRE. Dedicated study of MRE in post-liver transplant patients is also limited. In this study, the ability of real-time MRI to guide biopsies of segments of the liver with different MRE stiffness values in the same post-transplant patient was assessed.
MRE was performed in 9 consecutive posttransplant patients with history of hepatitis C. Segments of highest and lower stiffness on MRE served as targets for subsequent real-time MRI-guided biopsy using T2-weighted imaging. The ability of MRI-guided biopsy to successfully obtain tissue specimens was assessed. The Wilcoxon signed-rank test was used to compare mean stiffness differences for highest and lower MRE stiffness segments, with α = 0.05.
MRI guidance allowed successful sampling of liver tissue for all (18/18) biopsies. There was a statistically significant difference in mean MRE stiffness values between highest (4.61 ± 1.99 kPa) and lower stiffness (3.03 ± 1.75 kPa) (P = .0039) segments biopsied in the 9 posttransplant patients.
Real-time MRI can guide biopsy in patients after liver transplantation based on MRE stiffness values. This study supports the use of MRI guidance to sample tissue based on functional information.
[Show abstract][Hide abstract] ABSTRACT: Example of improved T1 mapping after motion correction. Left column: T1 maps of original modified Look-Locker inversion recovery (MOLLI) sequence images indicating smearing on the myocardium due to imperfect breath-holding. Right column: Sharp myocardial boundary is recovered after motion correction using proposed technique. These images were from three different patients from the article by Xue et al (pp 1644-1655).
Magnetic Resonance in Medicine 06/2012; 67(6):spcone. · 3.27 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Three-dimensional black-blood MRI is a promising noninvasive imaging technique for the assessment of atherosclerotic carotid artery disease. However, this technique is inherently susceptible to motion. In particular, swallowing can result in considerable wall motion at the carotid bifurcations, which may induce drastic image degradation or substantial overestimation of wall thickness. Self-gating techniques have previously been shown to be capable of resolving and compensating for cardiac or respiratory motion during MRI. This work presents a self-gating-based prospective motion gating scheme that is combined with a three-dimensional variable-flip-angle turbo spin-echo sequence (SPACE) for detecting swallowing motion. Self-gating signal readouts along the superior-inferior direction during each repetition time period are used to derive the projection profiles of the imaging volume. Based on cross-correlation analysis between the projection profiles and the corresponding reference profiles, swallowing motion can be detected and the motion-contaminated data will subsequently be discarded and reacquired in the next repetition time. The self-gated SPACE sequence was validated on eight healthy volunteers and two patients and, when compared with the conventional SPACE sequence, proved to be more resistant to swallowing motion and significantly improved image quality as well as the sharpness of carotid artery wall boundaries.
Magnetic Resonance in Medicine 12/2011; 67(2):490-8. · 3.27 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To compare different state-of-the-art T2-weighted (T2w) imaging sequences combined with late gadolinium enhancement (LGE) for myocardial salvage area (MSA) assessment by cardiac magnetic resonance (CMR). T2w imaging has been used to assess the myocardial area at risk (AAR) in acute myocardial infarction (AMI) patients, but its clinical application is challenging due to technical and physical limitations.
Thirty patients with reperfused AMI underwent complete CMR imaging 2-5 days after hospital admission. Myocardial AAR and MSA were quantified on four different T2w sequences: (a) free-breathing T2-prepared single-shot balanced steady-state free precession (T2p_ssbSSFP); (b) breathhold T2-weighted acquisition for cardiac unified T2 edema (ACUTE); (c) breathhold T2w dark-blood inversion recovery turbo-spin echo (IR-TSE) (short-term inversion recovery: STIR); and (d) free-breathing high-resolution T2 dark-blood navigated BLADE. The diagnostic performance of each technique was also assessed.
Quantitative analysis showed significant differences in myocardial AAR extent as quantified by the four T2w sequences (P < 0.05). There were also significant differences in sensitivity, specificity and overall diagnostic performance.
Detection and quantification of AAR, and thus of MSA, by T2wCMR in reperfused AMI patients varied significantly between different T2w sequences in the same clinical setting.
Journal of Magnetic Resonance Imaging 09/2011; 35(2):328-39. · 2.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Quantification of myocardial T1 relaxation has potential value in the diagnosis of both ischemic and nonischemic cardiomyopathies. Image acquisition using the modified Look-Locker inversion recovery technique is clinically feasible for T1 mapping. However, respiratory motion limits its applicability and degrades the accuracy of T1 estimation. The robust registration of acquired inversion recovery images is particularly challenging due to the large changes in image contrast, especially for those images acquired near the signal null point of the inversion recovery and other inversion times for which there is little tissue contrast. In this article, we propose a novel motion correction algorithm. This approach is based on estimating synthetic images presenting contrast changes similar to the acquired images. The estimation of synthetic images is formulated as a variational energy minimization problem. Validation on a consecutive patient data cohort shows that this strategy can perform robust nonrigid registration to align inversion recovery images experiencing significant motion and lead to suppression of motion induced artifacts in the T1 map.
Magnetic Resonance in Medicine 08/2011; 67(6):1644-55. · 3.27 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The aim of this study was to develop a targeted volumetric radiofrequency field (B(1)(+)) mapping technique to provide region-of-interest B(1)(+) information.
Targeted B(1)(+) maps were acquired using three-dimensional (3D) reduced field-of-view (FOV) inner-volume turbo spin echo-catalyzed double-angle method (DAM). Targeted B(1)(+) maps were compared with full-FOV B(1)(+) maps acquired using 3D catalyzed DAM in a phantom and in the brain of a healthy volunteer. In addition, targeted volumetric abdomeninal B(1)(+) mapping was demonstrated in the abdomen of another healthy volunteer.
The targeted reduced-FOV images demonstrated no aliasing artifacts in all experiments. Close match between targeted B(1)(+) map and reference full-FOV B(1)(+) map in the same region was observed, with percentage root-mean-squared error <0.4% in the phantom and <0.8% in the healthy volunteer brain. The abdominal B(1)(+) maps showed small B(1)(+) variation in the kidneys and liver from the healthy volunteer.
The proposed 3D reduced-FOV catalyzed DAM provides a rapid, simple and accurate method for targeted volumetric B(1)(+) mapping and can be easily implemented for applications related to radiofrequency field mapping in small targeted regions.
Magnetic Resonance Imaging 06/2011; 29(8):1131-7. · 2.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The navigator gating and slice tracking approach currently used for respiratory motion compensation during free-breathing coronary magnetic resonance angiography (MRA) has low imaging efficiency (typically 30-50%), resulting in long imaging times. In this work, a novel respiratory motion correction technique with 100% scan efficiency was developed for free-breathing whole-heart coronary MRA. The navigator signal was used as a reference respiratory signal to segment the data into six bins. 3D projection reconstruction k-space sampling was used for data acquisition and enabled reconstruction of low resolution images within each respiratory bin. The motion between bins was estimated by image registration with a 3D affine transform. The data from the different respiratory bins was retrospectively combined after motion correction to produce the final image. The proposed method was compared with a traditional navigator gating approach in nine healthy subjects. The proposed technique acquired whole-heart coronary MRA with 1.0 mm(3) isotropic spatial resolution in a scan time of 6.8 ± 0.9 min, compared with 16.2 ± 2.8 min for the navigator gating approach. The image quality scores, and length, diameter and sharpness of the right coronary artery (RCA), left anterior descending coronary artery (LAD), and left circumflex coronary artery (LCX) were similar for both approaches (P > 0.05 for all), but the proposed technique reduced scan time by a factor of 2.5.
Magnetic Resonance in Medicine 05/2011; 65(5):1269-77. · 3.27 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To investigate the contribution of proton density (PD) in T(2) -STIR based edema imaging in the setting of acute myocardial infarction (AMI).
Canines (n = 5), subjected to full occlusion of the left anterior descending artery for 3 hours, underwent serial magnetic resonance imaging (MRI) studies 2 hours postreperfusion (day 0) and on day 2. During each study, T(1) and T(2) maps, STIR (TE = 7.1 msec and 64 msec) and late gadolinium enhancement (LGE) images were acquired. Using T(1) and T(2) maps, relaxation and PD contributions to myocardial edema contrast (EC) in STIR images at both TEs were calculated.
Edematous territories showed significant increase in PD (20.3 ± 14.3%, P < 0.05) relative to healthy territories. The contributions of T(1) changes and T(2) or PD changes toward EC were in opposite directions. One-tailed t-test confirmed that the mean T(2) and PD-based EC at both TEs were greater than zero. EC from STIR images at TE = 7.1 msec was dominated by PD than T(2) effects (94.3 ± 11.3% vs. 17.6 ± 2.5%, P < 0.05), while at TE = 64 msec, T(2) effects were significantly greater than PD effects (90.8 ± 20.3% vs. 12.5 ± 11.9%, P < 0.05). The contribution from PD in standard STIR acquisitions (TE = 64 msec) was significantly higher than 0 (P < 0.05).
In addition to T(2) -weighting, edema detection in the setting of AMI with T(2) -weighted STIR imaging has a substantial contribution from PD changes, likely stemming from increased free-water content within the affected tissue. This suggests that imaging approaches that take advantage of both PD as well as T(2) effects may provide the optimal sensitivity for detecting myocardial edema.
Journal of Magnetic Resonance Imaging 04/2011; 33(4):962-7. · 2.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We assessed the hypothesis that black-blood steady-state free precession (SSFP) would provide coronary wall images comparable to images from TSE and have better performance than TSE under conditions of fast heart rate. With IRB approval, thirty participants without a history of coronary artery disease (19 men, 11 women, 26-83 y/o) were scanned with a 1.5 T MR scanner. Cross-sectional black-blood images of the proximal portions of coronary arteries were acquired with a two-dimensional (2D), double inversion recovery (DIR) prepared TSE sequence and a 2D DIR SSFP sequence on the same planes. Image quality (ranked with a 4-point system, scored from 0 to 3), vessel wall area and thickness, signal-to-noise ratio (SNR) of the wall and contrast-to-noise ratio (CNR, wall to lumen) were compared between SSFP and TSE with SPSS software (v 13.0). Totally 28 scans were completed. For SSFP and TSE, there was no difference in image quality. SSFP had a higher SNR (23.7 ± 10.1 vs. 14.4 ± 5.2, P < 0.001) and wall-lumen CNR (8.8 ± 4.5 vs. 6.7 ± 3.2, P = 0.001). Good agreements between measured wall area (r = 0.701, P < 0.001) and thickness (r = 0.560, P < 0.001) were found. For 10 participants with heart rate more than 80 beats/min, the image quality of SSFP was higher than TSE (P = 0.016). SSFP provided image quality and measurement accuracy that was comparable to TSE. With its higher performance under fast heart rate conditions, SSFP may break through the existing thresholds for heart rate and extend clinical applicability of coronary wall MR imaging to a larger population.
The international journal of cardiovascular imaging 04/2011; 28(3):567-75. · 2.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cardiac perfusion magnetic resonance imaging (MRI) has proven clinical significance in diagnosis of heart diseases. However, analysis of perfusion data is time-consuming, where automatic detection of anatomic landmarks and key-frames from perfusion MR sequences is helpful for anchoring structures and functional analysis of the heart, leading toward fully automated perfusion analysis. Learning-based object detection methods have demonstrated their capabilities to handle large variations of the object by exploring a local region, i.e., context. Conventional 2D approaches take into account spatial context only. Temporal signals in perfusion data present a strong cue for anchoring. We propose a joint context model to encode both spatial and temporal evidence. In addition, our spatial context is constructed not only based on the landmark of interest, but also the landmarks that are correlated in the neighboring anatomies. A discriminative model is learned through a probabilistic boosting tree. A marginal space learning strategy is applied to efficiently learn and search in a high dimensional parameter space. A fully automatic system is developed to simultaneously detect anatomic landmarks and key frames in both RV and LV from perfusion sequences. The proposed approach was evaluated on a database of 373 cardiac perfusion MRI sequences from 77 patients. Experimental results of a 4-fold cross validation show superior landmark detection accuracies of the proposed joint spatial-temporal approach to the 2D approach that is based on spatial context only. The key-frame identification results are promising.
Society of Photo-Optical Instrumentation Engineers (SPIE); 03/2011
[Show abstract][Hide abstract] ABSTRACT: The biophysical mechanisms influencing balanced steady-state free precession (bSSFP) based edema imaging in the setting of acute myocardial infarction are not well understood. To assess the various mechanisms that enable the detection of myocardial edema on bSSFP-based imaging approaches (cine bSSFP and T(2)-prepared bSSFP), experiments were conducted in canine models subjected to ischemia-reperfusion injury. Results showed that in addition to relaxation effects, the alteration in thermal equilibrium (M(0)) (including magnetization transfer) has a significant contribution to the image contrast between edematous and healthy myocardium. The relative signal-intensity ratios between edematous and healthy myocardium were: 1.51 ± 0.18 (cine bSSFP) and 1.58 ± 0.20 (T(2)-prepared bSSFP); the theoretically estimated relative relaxation and M(0) effects were: 1.17 ± 0.09 and 1.30 ± 0.19, respectively (cine bSSFP), and 1.49 ± 0.23 and 1.06 ± 0.07, respectively (T(2)-prepared bSSFP). There were no significant difference between cine bSSFP and T(2)-prep bSSFP relative signal-intensity ratios. However, the relative relaxation effect in cine bSSFP was significantly lower than in T(2)-prep bSSFP (P < 0.05), and the M(0) effect in cine bSSFP was significantly higher than in T(2)-prep bSSFP (P < 0.05). Hence the acquisition strategies that wish to maximize myocardial edema contrast in cine bSSFP imaging should take both relaxation and M(0) effects into account.
Magnetic Resonance in Medicine 03/2011; 66(1):187-91. · 3.27 Impact Factor