Yuval Zur

Mapping apparent eccentricity and residual ensemble anisotropy in the gray matter using angular double-pulsed-field-gradient MRI

Conventional diffusion MRI methods are mostly capable of portraying microarchitectural elements such as fiber orientation in white matter from detection of diffusion anisotropy, which arises from the coherent organization of anisotropic compartments. Double‐pulsed‐field‐gradient MR methods provide a means for obtaining microstructural information such as compartment shape and microscopic anisotropies even in scenarios where macroscopic organization is absent. Here, we apply angular double‐pulsed‐gradient‐spin‐echo MRI in the rat brain both ex vivo and in vivo for the first time. Robust angular dependencies are detected in the brain at long mixing time (tm). In many pixels, the oscillations seem to originate from residual directors in randomly oriented media, i.e., from residual ensemble anisotropy, as corroborated by quantitative simulations. We then developed an analysis scheme that enables one to map of structural indices such as apparent eccentricity (aE) and residual phase (φ) that enables characterization of the rat brain in general, and especially the rat gray matter. We conclude that double‐pulsed‐gradient‐spin‐echo MRI may in principle become important in characterizing gray matter morphological features and pathologies in both basic and applied neurosciences. Magn Reson Med, 2012.

Robust 2D phase correction for echo planar imaging under a tight field‐of‐view

Nyquist ghost artifacts are a serious issue in echo planar imaging. These artifacts primarily originate from phase difference between even and odd echo images and can be removed or reduced using phase correction methods. The commonly used 1D phase correction can only correct phase difference along readout axis. 2D correction is, therefore, necessary when phase difference presents along both readout and phase encoding axes. However, existing 2D methods have several unaddressed issues that affect their practicality. These issues include uncharacterized noise behavior, image artifact due to unoptimized phase estimation, Gibbs ringing artifact when directly applying to partial ky data, and most seriously a new image artifact under tight field‐of‐view (i.e., field‐of‐view slightly smaller than object size). All these issues are addressed in this article. Specifically, theoretical analysis of noise amplification and effect of phase estimation error is provided, and tradeoff between noise and ghost is studied. A new 2D phase correction method with improved polynomial fitting, joint homodyne processing and phase correction, compatibility with tight field‐of‐view is then proposed. Various results show that the proposed method can robustly generate images free of Nyquist ghosts and other image artifacts even in oblique scans or when cross‐term eddy current terms are significant. Magn Reson Med, 2010.

Ex Situ Endorectal MRI Probe for Prostate Imaging

A unique ex situ MRI probe, which examines samples external to its geometry, is presented. The probe is intended to be used for imaging the prostate gland via an endorectal pathway. It has a semicylindrical shape with a length of 6 cm and typical diameter of ∼3 cm. The probes imaging field of view spans almost along its entire length and up to a distance of 2 cm away from its surface, with an angular sector of ∼90°. The detailed design of the probe is presented, followed by a set of representative results obtained by the current bench prototype of this system. Magn Reson Med, 2009.

Two‐dimensional phase correction method for single and multi‐shot echo planar imaging

Ghost artifacts are a serious issue in single and multi‐shot echo planar imaging. Because of these coherent artifacts, it is essential to consistently suppress the ghosts. In this article, we present a phase correction algorithm that achieves excellent ghost suppression for single and multi‐shot echo planar imaging. The phase correction is performed along both the x (read) direction and y (phase) direction. To this end, we apply a double field of view prescan and compute the phase required for ghost suppression. This phase is fitted to a 2D polynomial. The fitted phase is used to correct the echo planar imaging images. The correction algorithm can be used with any readout gradient polarities and any number of shots. A flow chart of the correction method is provided to better clarify the full process. Finally, phantom and volunteer images demonstrate the improvement of artifact suppression obtained with this algorithm over conventional phase correction methods. Magn Reson Med, 2011.