Youngkyoo Jung

Multiphase pseudocontinuous arterial spin labeling (MP-PCASL) for robust quantification of cerebral blood flow.

Pseudocontinuous arterial spin labeling (PCASL) has been demonstrated to provide the sensitivity of the continuous arterial spin labeling method while overcoming many of the limitations of that method. Because the specification of the phases in the radiofrequency pulse train in PCASL defines the tag and control conditions of the flowing arterial blood, its tagging efficiency is sensitive to factors, such as off‐resonance fields, that induce phase mismatches between the radiofrequency pulses and the flowing spins. As a result, the quantitative estimation of cerebral blood flow with PCASL can exhibit a significant amount of error when these factors are not taken into account. In this paper, the sources of the tagging efficiency loss are characterized and a novel PCASL method that utilizes multiple phase offsets is proposed to reduce the tagging efficiency loss in PCASL. Simulations are performed to evaluate the feasibility and the performance of the proposed method. Quantitative estimates of cerebral blood flow obtained with multiple phase offset PCASL are compared to estimates obtained with conventional PCASL and pulsed arterial spin labeling. Our results show that multiple phase offset PCASL provides robust cerebral blood flow quantification while retaining much of the sensitivity advantage of PCASL. Magn Reson Med, 2010.

3D hyperpolarized He-3 MRI of ventilation using a multi-echo projection acquisition

A method is presented for high‐resolution 3D imaging of the whole lung using inhaled hyperpolarized (HP) He‐3 MR with multiple half‐echo radial trajectories that can accelerate imaging through undersampling. A multiple half‐echo radial trajectory can be used to reduce the level of artifact for undersampled 3D projection reconstruction (PR) imaging by increasing the amount of data acquired per unit time for HP He‐3 lung imaging. The point spread functions (PSFs) for breath‐held He‐3 MRI using multiple half‐echo trajectories were evaluated using simulations to predict the effects of T2* and gas diffusion on image quality. Results from PSF simulations were consistent with imaging results in volunteer studies showing improved image quality with increasing number of echoes using up to 8 half‐echoes. The 8‐half‐echo acquisition is shown to accommodate lost breath‐holds as short as 6 sec using a retrospective reconstruction at reduced resolution and also to allow reduced breath‐hold time compared with an equivalent Cartesian trajectory. Furthermore, preliminary results from a 3D dynamic inhalation‐exhalation maneuver are demonstrated using the 8‐half‐echo trajectory. Results demonstrate the first high‐resolution 3D PR imaging of ventilation and respiratory dynamics in humans using HP He‐3 MR. Magn Reson Med 59:1062–1071, 2008.

High efficiency, low distortion 3D diffusion tensor imaging with variable density spiral fast spin echoes (3D DW VDS RARE)

We present an acquisition and reconstruction method designed to acquire high resolution 3D fast spin echo diffusion tensor images while mitigating the major sources of artifacts in DTI-field distortions, eddy currents and motion. The resulting images, being 3D, are of high SNR, and being fast spin echoes, exhibit greatly reduced field distortions. This sequence utilizes variable density spiral acquisition gradients, which allow for the implementation of a self-navigation scheme by which both eddy current and motion artifacts are removed. The result is that high resolution 3D DTI images are produced without the need for eddy current compensating gradients or B(0) field correction. In addition, a novel method for fast and accurate reconstruction of the non-Cartesian data is employed. Results are demonstrated in the brains of normal human volunteers.

Consistent non-cartesian off-axis MRI quality: calibrating and removing multiple sources of demodulation phase errors.

The consistency of off‐axis MRI with non‐Cartesian sequences across a large number of scanners is highly variable. Improper timing alignment of the gradient fields, data acquisition system, and real‐time frequency demodulation reference signal, which are necessary for off‐axis imaging, is an important source of this variability. In addition, eddy currents and anisotropic gradient delays cause deviations in k‐space trajectories that in turn make the demodulation reference signals inaccurate. A method is presented to quickly measure the timing error in the frequency demodulation reference signal and separate it from anisotropic gradient delays. k‐Space deviations, as measured with a previous gradient calibration technique, are shown to be a second source of demodulation phase errors that degrade image quality. Using the timing delay and k‐space deviations, a retrospective phase correction is applied to each k‐space sample before the data are regridded during reconstruction. The timing delays of four MR scanners were measured to be 4.2–7.5 μs below the manufacturers suggested delay. Significant degradation in 3D radial (3D projection reconstruction (PR)) knee and breast images are retrospectively corrected while a partial prospective correction is applied for spiral imaging. The method allows for more consistent performance of non‐Cartesian sequences across multiple scanners without operator intervention. Magn Reson Med 57:206–212, 2007.

3D diffusion tensor MRI with isotropic resolution using a steady-state radial acquisition

To obtain diffusion tensor images (DTI) over a large image volume rapidly with 3D isotropic spatial resolution, minimal spatial distortions, and reduced motion artifacts, a diffusion‐weighted steady‐state 3D projection (SS 3DPR) pulse sequence was developed.

POCS-ENHANCED CORRECTION OF MOTION ARTIFACTS IN PARALLEL MRI

A new method for correction of MRI motion artifacts induced by corrupted k‐space data, acquired by multiple receiver coils such as phased arrays, is presented. In our approach, a projections onto convex sets (POCS)‐based method for reconstruction of sensitivity encoded MRI data (POCSENSE) is employed to identify corrupted k‐space samples. After the erroneous data are discarded from the dataset, the artifact‐free images are restored from the remaining data using coil sensitivity profiles. The error detection and data restoration are based on informational redundancy of phased‐array data and may be applied to full and reduced datasets. An important advantage of the new POCS‐based method is that, in addition to multicoil data redundancy, it can use a priori known properties about the imaged object for improved MR image artifact correction. The use of such information was shown to improve significantly k‐space error detection and image artifact correction. The method was validated on data corrupted by simulated and real motion such as head motion and pulsatile flow. Magn Reson Med 63:1104–1110, 2010.

Single breathhold cardiac CINE imaging with multi‐echo three‐dimensional hybrid radial SSFP acquisition

To achieve single breathhold whole heart cardiac CINE imaging with improved spatial resolution and temporal resolution by using a multi‐echo three‐dimensional (3D) hybrid radial SSFP acquisition.

A nonlinear regularization strategy for GRAPPA calibration

A regularization strategy is described for generalized autocalibrating partially parallel acquisition (GRAPPA) that allows successful calibration using a small number of autocalibration signals (ACS). The approach requires certain nonlinear relationships between the GRAPPA coefficients to be satisfied, which increases the redundancy so that fewer ACS need to be acquired.

Detecting and classifying life-threatening ECG ventricular arrythmias using wavelet decomposition

In this study, we developed a wavelet-based algorithm for detecting and classifying four types of ventricular arrhythmias. We implemented the algorithm using four different wavelets and compared each result. For extracted arrhythmia episodes from the MIT-BIH arrhythmia and malignant ventricular arrhythmia databases, a Daubechies wavelet of length four gave the best result of the four different wavelets studied. By using wavelet decomposition, we reduced the amount of data necessary to be processed by the algorithm to less than ten percent of the original data.

Dual half-echo phase correction for implementation of 3D radial SSFP at 3.0 T.

Fat/water separation methods such as fluctuating equilibrium magnetic resonance and linear combination steady‐state free precession have not yet been successfully implemented at 3.0 T due to extreme limitations on the time available for spatial encoding with the increase in magnetic field strength. We present a method to utilize a three‐dimensional radial sequence combined with linear combination steady‐state free precession at 3.0 T to take advantage of the increased signal levels over 1.5 T and demonstrate high spatial resolution compared to Cartesian techniques. We exploit information from the two half‐echoes within each pulse repetition time to correct the accumulated phase on a point‐by‐point basis, thereby fully aligning the phase of both half‐echoes. The correction provides reduced sensitivity to static field (B0) inhomogeneity and robust fat/water separation. Resultant images in the knee joint demonstrate the necessity of such a correction, as well as the increased isotropic spatial resolution attainable at 3.0 T. Results of a clinical study comparing this sequence to conventional joint imaging sequences are included. Magn Reson Med, 2010.