Jaeseok Park

MRI of the Knee at 3T: First Clinical Results With an Isotropic PDfs-Weighted 3D-TSE-Sequence

Purpose:To clinically evaluate MRI of the knee using a highly resolved isotropic fat-saturated (fs) proton-density weighted 3D-TSE-sequence (SPACE) at 3T. Materials and Methods:Imaging was performed on a 3T-scanner (Magnetom TRIO). For technical evaluation, sagittally orientated SPACE-datasets (repetition-time [TR], 1200 milliseconds/[TE], 30 milliseconds/voxel-size, 0.5 mm3/acquisition time, 10:35 minutes) were acquired from the dominant knee of 10 healthy volunteers. In the 3 major anatomic planes, 0.5, 1, and 2 mm thick reconstructions were performed. Signal-to-noise (SNR), SNR-efficiency, contrast-to-noise (CNR) ratios, and anatomic detail visualization were compared with a state-of-the-art 2D-TSE-sequence in 3 imaging planes (TR, 3200 milliseconds/TE, 30 milliseconds/acquisition time, 12:34 minutes). Sixty patients with cartilage and meniscus pathologies were examined with these techniques. Patient SPACE-datasets were assessed in 1-mm thick reconstructions. Arthroscopical correlation was available for 18 patients. Lesion detection and diagnostic confidence were assessed by 2 radiologists independently. Statistical analysis was performed using 95% confidence intervals, Wilcoxon signed rank tests, and Weighted-&kgr;. Results:SNR-efficiency of SPACE was 4 to 5 times higher than for 2D-TSE-sequences. SNR and CNR of 1-mm thick SPACE-reconstructions were comparable to 2D-TSE-sequences and provided superior visualization of small structures such as meniscal roots.Correlation with arthroscopy did not show significant differences between 2D- and 3D-sequences. One reader detected significantly more cartilage abnormalities with the 2D-TSE-sequence (131 vs. 151, P = 0.04), probably because of an unfamiliar fluid/cartilage contrast. Diagnostic confidence was significantly higher for meniscus abnormalities for SPACE for 1 reader. Intersequence-correlation was excellent (&kgr; = 0.82–0.92). Interreader-correlation was good to excellent (&kgr; = 0.71–0.80), intrareader-correlation was excellent (&kgr; = 0.90–0.92) for both sequences. Conclusions:Time-efficient 3D-TSE-imaging of the knee at 3T is feasible with adequate SNR and CNR and excellent anatomic detail visualization. Detection and visualization of meniscus and cartilage pathologies is comparable to standard 2D-TSE-sequences. 3D-TSE-sequences with consecutive multiplanar reconstruction may become a valuable component of future knee-MRI protocols.

Artifact and Noise Suppression in GRAPPA Imaging Using Improved k-Space Coil Calibration and Variable Density Sampling

A parallel imaging technique, GRAPPA (GeneRalized Auto‐calibrating Partially Parallel Acquisitions), has been used to improve temporal or spatial resolution. Coil calibration in GRAPPA is performed in central k‐space by fitting a target signal using its adjacent signals. Missing signals in outer k‐space are reconstructed. However, coil calibration operates with signals that exhibit large amplitude variation while reconstruction is performed using signals with small amplitude variation. Different signal variations in coil calibration and reconstruction may result in residual image artifact and noise. The purpose of this work was to improve GRAPPA coil calibration and variable density (VD) sampling for suppressing residual artifact and noise. The proposed coil calibration was performed in local k‐space along both the phase and frequency encoding directions. Outer k‐space was acquired with two different reduction factors. Phantom data were reconstructed by both the conventional GRAPPA and the improved technique for comparison at an acceleration of two. Under the same acceleration, optimal sampling and calibration parameters were determined. An in vivo image was reconstructed in the same way using the predetermined optimal parameters. The performance of GRAPPA was improved by the localized coil calibration and VD sampling scheme. Magn Reson Med 53:186–193, 2005.

Optimized T1-weighted contrast for single-slab 3D turbo spin-echo imaging with long echo trains: Application to whole-brain imaging

T1‐weighted contrast is conventionally obtained using multislice two‐dimensional (2D) spin‐echo (SE) imaging. Achieving isotropic, high spatial resolution is problematic with conventional methods due to a long acquisition time, imperfect slice profiles, or high‐energy deposition. Single‐slab 3D SE imaging was recently developed employing long echo trains with variable low flip angles to address these problems. However, long echo trains may yield suboptimal T1‐weighted contrast, since T2 weighting of the signals tends to develop along the echo train. Image blurring may also occur if high spatial frequency signals are acquired with low signal intensity. The purpose of this work was to develop an optimized T1‐weighted version of single‐slab 3D SE imaging with long echo trains. Refocusing flip angles were calculated based on a tissue‐specific prescribed signal evolution. Spatially nonselective excitation was used, followed by half‐Fourier acquisition in the in‐plane phase encoding (PE) direction. Restore radio frequency (RF) pulses were applied at the end of the echo train to optimize T1‐weighted contrast. Imaging parameters were optimized by using Bloch equation simulation, and imaging studies of healthy subjects were performed to investigate the feasibility of whole‐brain imaging with isotropic, high spatial resolution. The proposed technique permitted highly‐efficient T1‐weighted 3D SE imaging of the brain. Magn Reson Med 58:982–992, 2007.

Respiratory self-gated four-dimensional coronary MR angiography: a feasibility study.

The four‐dimensional (4D) coronary MR angiography (MRA) approach has been developed to eliminate the need for accurate determination of the acquisition window and trigger delay time. Diaphragm navigator (NAV) has been the conventional respiratory gating method for free‐breathing coronary MRA. However, NAV echo acquisition interrupts the continuous radiofrequency pulse application required for 4D steady‐state free precession coronary MRA. The objective of this work was to investigate the feasibility of a respiratory self‐gating (RSG) technique for 4D coronary MRA and its effectiveness by comparing with retrospective NAV gating. Data were acquired continuously throughout the cardiac cycle and retrospectively remapped to cardiac phases based on the electrocardiogram signal simultaneously recorded. An RSG signal extracted from a direct measurement of the heart position was used for retrospective respiratory gating and motion correction. In seven healthy volunteers, 4D MRA images were reconstructed, allowing retrospective assessment of the cardiac motion of the coronary artery and selection of the images with the best vessel delineation. Statistical analysis shows that 4D RSG provides coronary artery delineation comparable to mid‐diastole images acquired using NAV. Respiratory self‐gating is an effective method for eliminating respiratory motion artifacts and allows 4D coronary MRA during free breathing. Magn Reson Med 59:1378–1385, 2008.

4D radial coronary artery imaging within a single breath-hold : Cine angiography with phase-sensitive fat suppression (CAPS)

Coronary artery data acquisition with steady‐state free precession (SSFP) is typically performed in a single frame in mid‐diastole with a spectrally selective pulse to suppress epicardial fat signal. Data are acquired while the signal approaches steady state, which may lead to artifacts from the SSFP transient response. To avoid sensitivity to cardiac motion, an accurate trigger delay and data acquisition window must be determined. Cine data acquisition is an alternative approach for resolving these limitations. However, it is challenging to use conventional fat saturation with cine imaging because it interrupts the steady‐state condition. The purpose of this study was to develop a 4D coronary artery imaging technique, termed “cine angiography with phase‐sensitive fat suppression” (CAPS), that would result in high temporal and spatial resolution simultaneously. A 3D radial stacked k‐space was acquired over the entire cardiac cycle and then interleaved with a sliding window. Sensitivity‐encoded (SENSE) reconstruction with rescaling was developed to reduce streak artifact and noise. Phase‐sensitive SSFP was employed for fat suppression using phase detection. Experimental studies were performed on volunteers. The proposed technique provides high‐resolution coronary artery imaging for all cardiac phases, and allows multiple images at mid‐diastole to be averaged, thus enhancing the signal‐to‐noise ratio (SNR) and vessel delineation. Magn Reson Med, 2005.

Contrast-enhanced 4D radial coronary artery imaging at 3.0 T within a single breath-hold

Coronary magnetic resonance angiography data are usually acquired during mid‐diastole of each heartbeat to minimize cardiac motion related artifacts. The proper trigger delay time, which may vary widely among subjects, must be determined individually for each subject before data acquisition to achieve optimal image quality. These complications could be resolved by acquiring contiguous cardiac phase images through the cardiac cycle. In this study, we used a radial sampling technique to acquire 3D cine coronary artery images at 3 T within a single breath‐hold. An extravascular, paramagnetic contrast agent was i.v. administered to improve the blood signal intensity. Relatively high temporal resolution and spatial resolution were achieved simultaneously with radial sampling, parallel data acquisition, and interleaved sliding window image reconstruction. Volunteer studies demonstrate the feasibility of this technique in acquiring 4D coronary artery images and the flexibility in postprocessing of 3D image sets. Magn Reson Med 54:470–475, 2005.

Feasibility and Performance of Breath-Hold 3D TRUE-FISP Coronary MRA Using Self-Calibrating Parallel Acquisition

Spatial resolution in 3D breath‐hold coronary MR angiography (MRA) is limited by imaging time. The purpose of this work was to investigate the feasibility of improving the spatial resolution of coronary MRA using generalized autocalibrating partially parallel acquisition (GRAPPA) and fast imaging with steady state precession (True‐FISP) data acquisition. Coronary data were acquired in 10 healthy volunteers. In five volunteers, the data were fully acquired in k‐space and decimated for GRAPPA with an outer reduction factor (ORF) of 2. The coil calibration in GRAPPA was improved by segmented least‐squares fitting along the frequency‐encoding direction. More than 5% of the total k‐space lines were required for the calibration to achieve acceptable artifact suppression despite slightly lower signal‐to‐noise ratio (SNR). In another five volunteers, coronary data were obtained with both conventional and accelerated data acquisitions in the same imaging time. GRAPPA allowed a submillimeter in‐plane resolution, and improved coronary artery definition with an acceptable loss of SNR. In conclusion, 3D breath‐hold coronary MRA by GRAPPA and True‐FISP is highly feasible. Magn Reson Med 52:7–13, 2004.

High-resolution steady-state free precession coronary magnetic resonance angiography within a breath-hold: parallel imaging with extended cardiac data acquisition.

Coronary artery imaging data are conventionally acquired in a single imaging frame during mid‐diastole. The data acquisition window must be sufficiently short to avoid cardiac motion artifacts. A short data acquisition window results in decreased imaging efficiency and limited spatial resolution. Parallel imaging may lessen these limitations, but requires highly accurate coil sensitivity. The purpose of this work was to increase the imaging efficiency and spatial resolution in coronary artery imaging using parallel imaging with an extended acquisition window. External coil calibration data were acquired before and after a short mid‐diastolic period of accelerated imaging data acquisition. It was assumed that residual cardiac motion in the extended acquisition window would not impede accurate estimation of coil sensitivity since only low spatial frequency signals were acquired for coil calibration. Experimental studies were performed in five healthy volunteers at 3 T using steady‐state free precession sequence. Statistical comparison was made between the proposed method and conventional data acquisition for visual quality of image and vessel sharpness. The proposed technique demonstrated higher visual grading and improved vessel sharpness. The proposed method is a new approach to enhance the imaging efficiency and spatial resolution in coronary artery imaging. Magn Reson Med, 2005.

Use of contrast-enhancement and high-resolution 3D black-blood MR Imaging to identify inflammation in rabbit atherosclerotic plaques

Methods Ten atherosclerotic rabbits and three normal control rabbits underwent high-resolution 3D contrast-enhanced black-blood MRI. MR images and the corresponding histopathological sections were divided into four quadrants. Plaque composition was analyzed for each quadrant according to histopathological (percent of lipid-rich, fibrous, macrophage area and microvessel density) and imaging criteria (enhancement ratio (ER), ER=SIpost/SIpre).