Sohae Chung

Rapid B1+ mapping using a preconditioning RF pulse with TurboFLASH readout

In MRI, the transmit radiofrequency field (B  1+ ) inhomogeneity can lead to signal intensity variations and quantitative measurement errors. By independently mapping the local B  1+ variation, the radiofrequency‐related signal variations can be corrected for. In this study, we present a new fast B  1+ mapping method using a slice‐selective preconditioning radiofrequency pulse. Immediately after applying a slice‐selective preconditioning pulse, a turbo fast low‐angle‐shot imaging sequence with centric k‐space reordering is performed to capture the residual longitudinal magnetization left behind by the slice‐selective preconditioning pulse due to B  1+ variation. Compared to the reference double‐angle method, this method is considerably faster. Specifically, the total scan time for the double‐angle method is equal to the product of 2 (number of images), the number of phase‐encoding lines, and approximately 5T1, whereas the slice‐selective preconditioning method takes approximately 5T1. This method was validated in vitro and in vivo with a 3‐T whole‐body MRI system. The combined brain and pelvis B  1+ measurements showed excellent agreement and strong correlation with those by the double‐angle method (mean difference = 0.025; upper and lower 95% limits of agreement were −0.07 and 0.12; R = 0.93; P < 0.001). This fast B  1+ mapping method can be used for a variety of applications, including body imaging where fast imaging is desirable. Magn Reson Med, 2010.

Liver Stiffness Assessment by Tagged MRI of Cardiac-induced Liver Motion

Cirrhosis is an important and growing public health problem, affecting millions of Americans and many more people internationally. A pathological hallmark of the progression to cirrhosis is the development of liver fibrosis, so that monitoring the appearance and progression of liver fibrosis can be used to guide therapy. Here, we report a method to use magnetization‐tagged magnetic resonance imaging to measure the cardiac‐induced motion and deformation in the liver, as a means for noninvasively assessing liver stiffness, which is related to fibrosis. The initial results show statistically significant differences between healthy and cirrhotic subjects in the direct comparisons of the maximum displacement (mm), and the maximum (P1) and minimum (P2) two‐dimensional strains, through the cardiac cycle (3.514 ± 0.793, 2.184 ± 0.611; 0.116 ± 0.043, 0.048 ± 0.011; −0.094 ± 0.020, −0.041 ± 0.015; healthy, cirrhosis, respectively; P < 0.005 for all). There are also significant differences in the displacement‐normalized P1 and P2 strains (mm−1) (0.030 ± 0.008, 0.017 ± 0.007; −0.024 ± 0.006, −0.013 ± 0.004; healthy, cirrhosis, respectively; P < 0.005 for all). Therefore, this noninvasive imaging‐based method is a promising means to assess liver stiffness using clinically available imaging tools. Magn Reson Med, 2011.

Evaluation of diastolic function by three-dimensional volume tracking of the mitral annulus with cardiovascular magnetic resonance: comparison with tissue Doppler imaging

BackgroundMeasurement of mitral annulus (MA) dynamics is an important component of the evaluation of left ventricular (LV) diastolic function; MA velocities are commonly measured using tissue Doppler imaging (TDI). This study aimed to examine the clinical potential of a semi-automated cardiovascular magnetic resonance (CMR) technique for quantifying global LV diastolic function, using 3D volume tracking of the MA with conventional cine-CMR images.Methods124 consecutive patients with normal ejection fraction underwent both clinically indicated transthoracic echocardiography (TTE) and CMR within 2 months. Interpolated 3D reconstruction of the MA over time was performed with semi-automated atrioventricular junction (AVJ) tracking in long-axis cine-CMR images, producing an MA sweep volume over the cardiac cycle. CMR-based diastolic function was evaluated, using the following parameters: peak volume sweep rates in early diastole (PSRE) and atrial systole (PSRA), PSRE/PSRA ratio, deceleration time of sweep volume (DTSV), and 50% diastolic sweep volume recovery time (DSVRT50); these were compared with TTE diastolic measurements.ResultsPatients with TTE-based diastolic dysfunction (n = 62) showed significantly different normalized MA sweep volume profiles compared to those with TTE-based normal diastolic function (n = 62), including a lower PSRE (5.25 ± 1.38 s−1 vs. 7.72 ± 1.7 s−1), a higher PSRA (6.56 ± 1.99 s−1 vs. 4.67 ± 1.38 s−1), a lower PSRE/PSRA ratio (0.9 ± 0.44 vs. 1.82 ± 0.69), a longer DTSV (144 ± 55 ms vs. 96 ± 37 ms), and a longer DSVRT50 (25.0 ± 11.0% vs. 15.6 ± 4.0%) (all p < 0.05). CMR diastolic parameters were independent predictors of TTE-based diastolic dysfunction after adjusting for left ventricular hypertrophy, hypertension, and coronary artery disease. Good correlations were observed between CMR PSRE/PSRA and early-to-late diastolic annular velocity ratios (e′/a′) measured by TDI (r = 0.756 to 0.828, p < 0.001).Conclusions3D MA sweep volumes generated by semi-automated AVJ tracking in routinely acquired CMR images yielded diastolic parameters that were effective in identifying patients with diastolic dysfunction when correlated with TTE-based variables.

Quantitative contrast-enhanced first-pass cardiac perfusion MRI at 3 tesla with accurate arterial input function and myocardial wall enhancement

To develop, and validate in vivo, a robust quantitative first‐pass perfusion cardiovascular MR (CMR) method with accurate arterial input function (AIF) and myocardial wall enhancement.

Automated tag tracking using Gabor filter bank, robust point matching, and deformable models

Tagged Magnetic Resonance Imaging (tagged MRI or tMRI) provides a means of directly and noninvasively displaying the motion of the myocardium. Reconstruction of the motion field is needed for quantitative analysis of important clinical information, e.g., the myocardial strain. In this paper, we present a two-step method for this task. First, we use a Gabor filter bank to generate a corresponding phase map of tMRI images. Second, deformable models are initialized at the discontinuities in the wrapped phase map, and are deformed under the influence of the image gradient to track the motion of tags. Unlike previous approaches, a Robust Point Matching (RPM) module has been integrated into the model evolution to avoid false tracking results caused by 1) through-plane motion, and 2) small tag spacing. The method has been tested on a numeric phantom, as well as in vivo heart data. The experimental results show that the new method has a good performance on both synthetic and real data, and has the potential to be used in clinical applications.

New Clinically Feasible 3T MRI Protocol to Discriminate Internal Brain Stem Anatomy

Track density imaging (TDI) is a novel MR imaging postprocessing technique based on high angular-resolution diffusion acquisitions that generate super-resolution images derived by whole-brain probabilistic streamline tractography. TDI and echo modulation curve T2 mapping were combined with simultaneous multisection acquisition to reveal anatomic detail at 7 canonical levels of the brain stem. Compared with conventional MR imaging contrasts, many individual brain stem tracts and nuclear groups were directly visualized for the first time at 3T. SUMMARY: Two new 3T MR imaging contrast methods, track density imaging and echo modulation curve T2 mapping, were combined with simultaneous multisection acquisition to reveal exquisite anatomic detail at 7 canonical levels of the brain stem. Compared with conventional MR imaging contrasts, many individual brain stem tracts and nuclear groups were directly visualized for the first time at 3T. This new approach is clinically practical and feasible (total scan time = 20 minutes), allowing better brain stem anatomic localization and characterization.

Liver stiffness assessment with tagged MRI of cardiac-induced liver motion in cirrhosis patients

To assess liver stiffness using magnetization‐tagged magnetic resonance imaging (MRI) to measure the cardiac‐induced motion in the liver of cirrhosis patients with known Child‐Pugh scores.

Cardiac MRI correlates of diastolic left ventricular function assessment by echocardiography

Background Transthoracic echocardiography (TTE) provides noninvasive measures of diastolic left ventricular (LV) function by assessing mitral inflow and mitral annular motion. Given the excellent spatial and temporal resolution of CMR, we developed a novel method to calculate several correlative measures of diastolic and systolic LV function. The maximum velocity of the atrioventricular junction (AVJ) during early diastole represents a correlate of the maximum velocity of the mitral annulus (e’). To determine the correlation between CMR and TTE indices of diastolic function, we performed a retrospective analysis.

TAGGED MRI ANALYSIS USING GABOR FILTERS

Tagged magnetic resonance imaging (MRI) provides a means of directly and noninvasively displaying the motion of the myocardium. However, efficient means of tagged image analysis are needed for practical quantitative applications. Gabor filters can provide a useful automated approach for such quantitative tagged MRI analysis

2D motion analysis of long axis cardiac tagged MRI

The tracking and reconstruction of myocardial motion is critical to the diagnosis and treatment of heart disease. Currently, little has been done for the analysis of motion in long axis (LA) cardiac images. We propose a new fully automated motion reconstruction method for grid- tagged MRI that combines Gabor filters and deformable models. First, we use a Gabor filter bank to generate the corresponding phase map in the myocardium and estimate the location of grid tag intersections. Second, we use a non-rigid registration module driven by thin plate splines (TPS) to generate a transformation function between tag intersections in two consecutive images. Third, deformable spline models are initialized using Fourier domain analysis and tracked during the cardiac cycle using the TPS generated transformation function. The splines will then locally deform under the influence of gradient flow and image phase information. The final motion is decomposed into tangential and normal components corresponding to the local orientation of the heart wall. The new method has been tested on LA phantoms and in vivo heart data, and its performance has been quantitatively validated. The results show that our method can reconstruct the motion field in LA cardiac tagged MR images accurately and efficiently.