Tzu-Chao Chuang

PROPELLER EPI: An MRI technique suitable for diffusion tensor imaging at high field strength with reduced geometric distortions†

A technique suitable for diffusion tensor imaging (DTI) at high field strengths is presented in this work. The method is based on a periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) k‐space trajectory using EPI as the signal readout module, and hence is dubbed PROPELLER EPI. The implementation of PROPELLER EPI included a series of correction schemes to reduce possible errors associated with the intrinsically higher sensitivity of EPI to off‐resonance effects. Experimental results on a 3.0 Tesla MR system showed that the PROPELLER EPI images exhibit substantially reduced geometric distortions compared with single‐shot EPI, at a much lower RF specific absorption rate (SAR) than the original version of the PROPELLER fast spin‐echo (FSE) technique. For DTI, the self‐navigated phase‐correction capability of the PROPELLER EPI sequence was shown to be effective for in vivo imaging. A higher signal‐to‐noise ratio (SNR) compared to single‐shot EPI at an identical total scan time was achieved, which is advantageous for routine DTI applications in clinical practice. Magn Reson Med, 2005.

PROPELLER-EPI with parallel imaging using a circularly symmetric phased-array RF coil at 3.0 T : Application to high-resolution diffusion tensor imaging

A technique integrating multishot periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) and parallel imaging is presented for diffusion echo‐planar imaging (EPI) at high spatial resolution. The method combines the advantages of parallel imaging to achieve accelerated sampling along the phase‐encoding direction, and PROPELLER acquisition to further decrease the echo train length (ETL) in EPI. With an eight‐element circularly symmetric RF coil, a parallel acceleration factor of 4 was applied such that, when combined with PROPELLER acquisition, a reduction of geometric distortions by a factor substantially greater than 4 was achieved. The resulting phantom and human brain images acquired with a 256 × 256 matrix and an ETL of only 16 were visually identical in shape to those acquired using the fast spin‐echo (FSE) technique, even without field‐map corrections. It is concluded that parallel PROPELLER‐EPI is an effective technique that can substantially reduce susceptibility‐induced geometric distortions at high field strength. Magn Reson Med, 2006.

Diffusion tensor imaging study of white matter fiber tracts in adolescent attention-deficit/hyperactivity disorder

A diffusion tensor imaging (DTI) study was conducted in 12 adolescents with attention deficit/hyperactivity disorder and 14 age- and IQ-matched healthy controls. Inter-subject comparison of fractional anisotropy (FA) of the whole brain between the groups was obtained using the tract-based spatial statistics method. Results revealed significantly lower FA in widespread white matter tracts in cases relative to controls. Also, the FA measure of identified regions was associated with cognitive performance.

Susceptibility-weighted imaging in patients with pyogenic brain abscesses at 1.5T: characteristics of the abscess capsule.

The rim of cerebral abscesses is dark on T2 presumably due to the accumulation of oxygen radicals in inflammatory cells. Because DWI is more sensitive to susceptibility effects than other MRI sequences, these authors used it to evaluate 14 abscesses. DWI agreed with previous observations detecting mild hypointensity in most abscess rims, compatible with the presence of paramagnetic substances due to oxygen free radicals from phagocytosis. SWI provides another means of establishing the diagnosis and helping us to understand the pathophysiology of abscesses. BACKGROUND AND PURPOSE: SWI is a high-resolution 3D, fully velocity-compensated gradient-echo sequence that uses both magnitude and phase data. The purpose of this study was to investigate the phase behavior of the capsule of pyogenic brain abscesses with noncontrast SWI. MATERIALS AND METHODS: Fourteen patients with pyogenic brain abscesses were studied at 1.5T. In all of the patients, SWI images were obtained and reviewed in addition to conventional MR images. Phase values within the abscess capsule were measured and compared with those from the abscess cavities and contralateral normal white matter using 1-way repeated measures ANOVA with post hoc Bonferroni analysis. RESULTS: SWI phase images showed mild hypointesity in 6 patients, isointensity in 3 patients, and mixed iso- to mild hypointensity in 5 patients. The means of phase in the cavity, rim of abscesses, and contralateral normal white matter were –7.552 × 10−3 ± 0.024, –0.105 ± 0.080, and +0.029 ± 0.011 radians, respectively. Post hoc comparisons showed significant differences between any pair of the 3 regions (abscess cavity, rim capsule, and normal white matter) in SWI (all Ps < .005). CONCLUSIONS: SWI phase imaging shows evidence of paramagnetic substances in agreement with the presence of free radicals from phagocytosis. SWI may provide additional information valuable in the characterization of pyogenic brain abscesses.

Correction of geometric distortion in Propeller echo planar imaging using a modified reversed gradient approach.

OBJECTIVE This study investigates the application of a modified reversed gradient algorithm to the Propeller-EPI imaging method (periodically rotated overlapping parallel lines with enhanced reconstruction based on echo-planar imaging readout) for corrections of geometric distortions due to the EPI readout. MATERIALS AND METHODS Propeller-EPI acquisition was executed with 360-degree rotational coverage of the k-space, from which the image pairs with opposite phase-encoding gradient polarities were extracted for reversed gradient geometric and intensity corrections. The spatial displacements obtained on a pixel-by-pixel basis were fitted using a two-dimensional polynomial followed by low-pass filtering to assure correction reliability in low-signal regions. Single-shot EPI images were obtained on a phantom, whereas high spatial resolution T2-weighted and diffusion tensor Propeller-EPI data were acquired in vivo from healthy subjects at 3.0 Tesla, to demonstrate the effectiveness of the proposed algorithm. RESULTS Phantom images show success of the smoothed displacement map concept in providing improvements of the geometric corrections at low-signal regions. Human brain images demonstrate prominently superior reconstruction quality of Propeller-EPI images with modified reversed gradient corrections as compared with those obtained without corrections, as evidenced from verification against the distortion-free fast spin-echo images at the same level. CONCLUSIONS The modified reversed gradient method is an effective approach to obtain high-resolution Propeller-EPI images with substantially reduced artifacts.

Automatic calibration of trigger delay time for cardiac MRI.

This study aimed to automatically identify the cardiac rest period using a rapid free‐breathing (FB) calibration scanning procedure, and to determine the optimal trigger delay for cardiac imaging. A standard deviation (SD) method was used to rapidly identify cardiac quiescent phases employing multiphase cine cardiac images. The accuracy of this method was investigated using 46 datasets acquired from 22 healthy volunteers. The possibility of using a low‐resolution FB method to rapidly acquire cine images was also evaluated. The reproducibility and accuracy of the trigger delay obtained using the rapid calibration scanning process were assessed before its application to a real‐time feedback system. The real‐time trigger delay calibration system was then used to perform T1‐weighted, short‐axis imaging at the end of the cardiac systolic period. Linear regression analysis of the trigger times obtained using the SD method and a reference method indicated that the SD algorithm accurately identified the cardiac rest period (linear regression: slope = 0.94–1, R2 = 0.68–0.84). Group analysis showed that the number of pixels in the left ventricular blood pool in images acquired at the end‐systolic time calculated in real time was significantly lower than in those acquired 50 ms in advance or later (p < 0.01, paired t‐test). The low‐resolution FB imaging method was reproducible for the calibration scanning of an image in a vertical long‐axis slice position (average SD of trigger times, 16–39 ms). Combined with rapid FB calibration scanning, the real‐time feedback system accurately adjusted the trigger delay for T1‐weighted short‐axis imaging. The real‐time feedback method is rapid and reliable for trigger time calibration, and could facilitate cardiac imaging during routine examination. Copyright

Quantitative Comparison of Post-Processing Methods for Reduction of Frequency Modulation Sidebands in Non-water Suppression 1H MRS

Non‐water suppression MRS (NWS MRS) has several advantages. First, the unsuppressed water signal can be used as internal calibration for metabolite quantification and as a reliable frequency/phase reference for retrospective motion correction. Second, it avoids the potential artifacts caused by incomplete water suppression (WS) and extra radiofrequency deposition from WS pulses. However, the frequency modulation (FM) sidebands originating from a large water signal will distort the spectrum. Among the methods proposed to solve the problems caused by FM sidebands, post‐acquisition processing methods are superior in flexibility for general use compared with experimental methods. In this study, we propose two algorithms based on advanced matrix decomposition to remove the FM sidebands. These methods, the simultaneous diagonalization (QZ) algorithm and its subsequent variant, the simultaneously generalized Schur decomposition (SGSD) algorithm, were numerically evaluated using computer simulations. In addition, we quantitatively compared the performance of these methods and the modulus method in an in vitro experiment and in vivo NWS MRS against conventional WS data. Our results show that the proposed SGSD algorithm can reduce the FM sidebands to achieve superior estimation of concentration on three major metabolites. This method can be applied directly to spectra pre‐acquired under various experimental conditions without modifying the acquisition sequences. Copyright

Multilayer appearance on contrast-enhanced susceptibility-weighted images on patients with brain abscesses: Possible origins and effects of postprocessing

To demonstrate the presence of a multilayer appearance of the capsule on contrast‐enhanced (CE) susceptibility‐weighted imaging (SWI) in patients with pyogenic brain abscesses. Possible origins for the appearance and effects of postprocessing settings are discussed.

High spatial resolution brain functional MRI using submillimeter balanced steady-state free precession acquisition

PURPOSE One of the technical advantages of functional magnetic resonance imaging (fMRI) is its precise localization of changes from neuronal activities. While current practice of fMRI acquisition at voxel size around 3 × 3 × 3 mm(3) achieves satisfactory results in studies of basic brain functions, higher spatial resolution is required in order to resolve finer cortical structures. This study investigated spatial resolution effects on brain fMRI experiments using balanced steady-state free precession (bSSFP) imaging with 0.37 mm(3) voxel volume at 3.0 T. METHODS In fMRI experiments, full and unilateral visual field 5 Hz flashing checkerboard stimulations were given to healthy subjects. The bSSFP imaging experiments were performed at three different frequency offsets to widen the coverage, with functional activations in the primary visual cortex analyzed using the general linear model. Variations of the spatial resolution were achieved by removing outer k-space data components. RESULTS Results show that a reduction in voxel volume from 3.44 × 3.44 × 2 mm(3) to 0.43 × 0.43 × 2 mm(3) has resulted in an increase of the functional activation signals from (7.7 ± 1.7)% to (20.9 ± 2.0)% at 3.0 T, despite of the threefold SNR decreases in the original images, leading to nearly invariant functional contrast-to-noise ratios (fCNR) even at high spatial resolution. Activation signals aligning nicely with gray matter sulci at high spatial resolution would, on the other hand, have possibly been mistaken as noise at low spatial resolution. CONCLUSIONS It is concluded that the bSSFP sequence is a plausible technique for fMRI investigations at submillimeter voxel widths without compromising fCNR. The reduction of partial volume averaging with nonactivated brain tissues to retain fCNR is uniquely suitable for high spatial resolution applications such as the resolving of columnar organization in the brain.

Accelerating EPI distortion correction by utilizing a modern GPU-based parallel computation.

The combination of phase demodulation and field mapping is a practical method to correct echo planar imaging (EPI) geometric distortion. However, since phase dispersion accumulates in each phase‐encoding step, the calculation complexity of phase modulation is Ny‐fold higher than conventional image reconstructions. Thus, correcting EPI images via phase demodulation is generally a time‐consuming task.