Holden H. Wu

Simple method for MR gradient system characterization and k-space trajectory estimation.

Fast imaging trajectories are used in MRI to speed up the acquisition process, but imperfections in the gradient system create artifacts in the reconstructed images. Artifacts result from the deviation between k‐space trajectories achieved on the scanner and their original prescription. Measuring or approximating actual k‐space trajectories with predetermined gradient timing delays reduces the artifacts, but are generally based on a specific trajectory and scan orientation. A single linear time‐invariant characterization of the gradient system provides a method to predict k‐space trajectories scanned in arbitrary orientations through convolution. This is done efficiently, by comparing the Fourier transforms of the input and measured waveforms of a single high‐bandwidth test gradient waveform. This new method is tested for spiral, interleaved echo‐planar, and three‐dimensional cones imaging, demonstrating its ability to reduce reconstructed image artifacts for various k‐space trajectories. Magn Reson Med, 2012.

Free‐breathing multiphase whole‐heart coronary MR angiography using image‐based navigators and three‐dimensional cones imaging

Noninvasive visualization of the coronary arteries in vivo is one of the most important goals in cardiovascular imaging. Compared to other paradigms for coronary MR angiography, a free‐breathing three‐dimensional whole‐heart iso‐resolution approach simplifies prescription effort, requires less patient cooperation, reduces overall exam time, and supports retrospective reformats at arbitrary planes. However, this approach requires a long continuous acquisition and must account for respiratory and cardiac motion throughout the scan. In this work, a new free‐breathing coronary MR angiography technique that reduces scan time and improves robustness to motion is developed. Data acquisition is accomplished using a three‐dimensional cones non‐Cartesian trajectory, which can reduce the number of readouts 3‐fold or more compared to conventional three‐dimensional Cartesian encoding and provides greater robustness to motion/flow effects. To further enhance robustness to motion, two‐dimensional navigator images are acquired to directly track respiration‐induced displacement of the heart and enable retrospective compensation of all acquired data (none discarded) for image reconstruction. In addition, multiple cardiac phases are imaged to support retrospective selection of the best phase(s) for visualizing each coronary segment. Experimental results demonstrate that whole‐heart coronary angiograms can be obtained rapidly and robustly with this proposed technique. Magn Reson Med 69:1083–1093, 2013.

Nonrigid Autofocus Motion Correction for Coronary MR Angiography with a 3D Cones Trajectory

To implement a nonrigid autofocus motion correction technique to improve respiratory motion correction of free‐breathing whole‐heart coronary magnetic resonance angiography acquisitions using an image‐navigated 3D cones sequence.

In-Bore 3-T MR-guided Transrectal Targeted Prostate Biopsy: Prostate Imaging Reporting and Data System Version 2–based Diagnostic Performance for Detection of Prostate Cancer

Purpose To determine the diagnostic yield of in-bore 3-T magnetic resonance (MR) imaging-guided prostate biopsy and stratify performance according to Prostate Imaging Reporting and Data System (PI-RADS) versions 1 and 2. Materials and Methods This study was HIPAA compliant and institution review board approved. In-bore 3-T MR-guided prostate biopsy was performed in 134 targets in 106 men who (a) had not previously undergone prostate biopsy, (b) had prior negative biopsy findings with increased prostate-specific antigen (PSA) level, or (c) had a prior history of prostate cancer with increasing PSA level. Clinical, diagnostic 3-T MR imaging was performed with in-bore guided prostate biopsy, and pathology data were collected. The diagnostic yields of MR-guided biopsy per patient and target were analyzed, and differences between biopsy targets with negative and positive findings were determined. Results of logistic regression and areas under the curve were compared between PI-RADS versions 1 and 2. Results Prostate cancer was detected in 63 of 106 patients (59.4%) and in 72 of 134 targets (53.7%) with 3-T MR imaging. Forty-nine of 72 targets (68.0%) had clinically significant cancer (Gleason score ≥ 7). One complication occurred (urosepsis, 0.9%). Patients who had positive target findings had lower apparent diffusion coefficient values (875 × 10-6 mm2/sec vs 1111 × 10-6 mm2/sec, respectively; P < .01), smaller prostate volume (47.2 cm3 vs 75.4 cm3, respectively; P < .01), higher PSA density (0.16 vs 0.10, respectively; P < .01), and higher proportion of PI-RADS version 2 category 3-5 scores when compared with patients with negative target findings. MR targets with PI-RADS version 2 category 2, 3, 4, and 5 scores had a positive diagnostic yield of three of 23 (13.0%), six of 31 (19.4%), 39 of 50 (78.0%), and 24 of 29 (82.8%) targets, respectively. No differences were detected in areas under the curve for PI-RADS version 2 versus 1. Conclusion In-bore 3-T MR-guided biopsy is safe and effective for prostate cancer diagnosis when stratified according to PI-RADS versions 1 and 2. ©RSNA, 2016.

Convex optimized diffusion encoding (CODE) gradient waveforms for minimum echo time and bulk motion-compensated diffusion-weighted MRI.

To evaluate convex optimized diffusion encoding (CODE) gradient waveforms for minimum echo time and bulk motion–compensated diffusion‐weighted imaging (DWI).

High‐resolution variable‐density 3D cones coronary MRA

To improve the spatial/temporal resolution of whole‐heart coronary MR angiography by developing a variable‐density (VD) 3D cones acquisition suitable for image reconstruction with parallel imaging and compressed sensing techniques.

Rapid quantitative T2 mapping of the prostate using three-dimensional dual echo steady state MRI at 3T.

To develop and evaluate a rapid three‐dimensional (3D) quantitative T2 mapping method for prostate cancer imaging using dual echo steady state (DESS) MRI at 3T.

Contrast Enhancement Patterns after Irreversible Electroporation: Experimental Study of CT Perfusion Correlated to Histopathology in Normal Porcine Liver

PURPOSE To analyze ablated tissue zones after irreversible electroporation (IRE) of porcine liver using computed tomography (CT) perfusion imaging with histopathologic correlation. MATERIALS AND METHODS Under ultrasound and CT guidance, 10 IRE ablations were performed percutaneously in three Yorkshire pigs using a single bipolar electrode. CT perfusion imaging was performed in all pigs immediately after ablation and on day 2. Pathologic sections were prepared for correlation with histopathology (hematoxylin-eosin and terminal deoxynucleotidyl transferase dUTP nick end labeling stains, 5-mm-thick slices). The short diameter of different enhancing zones on CT was correlated with the gross specimen. RESULTS CT perfusion images showed three differently enhancing zones: zone 1, inner nonenhancing zone; zone 2, middle well-defined progressive internal enhancement zone; and zone 3, outer ill-defined arterial enhancement zone with rapid washout. On histopathology, zone 1 showed a strong correlation with a pale zone, and zone 2 correlated with a red zone, together accounting for the extent of cell death. Zone 3 was outside of the ablation zone and contained inflammatory cells. Each enhancing zone had different perfusion parameters. CONCLUSIONS CT perfusion imaging in the acute setting effectively demonstrates histopathologic tissue zones after IRE ablation. Zone 2 is unique to IRE not seen in thermal ablation, characterized by progressive intra-zonal enhancement, and its outer boundary defines the extent of cell death.

Three-dimensional magnetization-prepared imaging using a concentric cylinders trajectory

To develop new magnetization‐prepared imaging schemes based on a three‐dimensional (3D) concentric cylinders trajectory.

3D Magnetization-Prepared Imaging Using a Stack-of-Rings Trajectory

Efficient acquisition strategies for magnetization‐prepared imaging based on the three‐dimensional (3D) stack‐of‐rings k‐space trajectory are presented in this work. The 3D stack‐of‐rings can be acquired with centric ordering in all three dimensions for greater efficiency in capturing the desired contrast. In addition, the 3D stack‐of‐rings naturally supports spherical coverage in k‐space for shorter scan times while achieving isotropic spatial resolution. While non‐Cartesian trajectories generally suffer from greater sensitivity to system imperfections, the 3D stack‐of‐rings can enhance magnetization‐prepared imaging with a high degree of robustness to timing delays and off‐resonance effects. As demonstrated with phantom scans, timing errors and gradient delays only cause a bulk rotation of the 3D stack‐of‐rings reconstruction. Furthermore, each ring can be acquired with a time‐efficient retracing design to resolve field inhomogeneities and enable fat/water separation. To demonstrate its effectiveness, the 3D stack‐of‐rings are considered for the case of inversion‐recovery‐prepared structural brain imaging. Experimental results show that the 3D stack‐of‐rings can achieve higher signal‐to‐noise ratio and higher contrast‐to‐noise ratio within a shorter scan time when compared to the standard inversion‐recovery‐prepared sequence based on 3D Cartesian encoding. The design principles used for this specific case of inversion‐recovery‐prepared brain imaging can be applied to other magnetization‐prepared imaging applications. Magn Reson Med 63:1210–1218, 2010.