Nicholas R. Zwart

Turboprop: Improved PROPELLER imaging

A variant of periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) MRI, called turboprop, is introduced. This method employs an oscillating readout gradient during each spin echo of the echo train to collect more lines of data per echo train, which reduces the minimum scan time, motion‐related artifact, and specific absorption rate (SAR) while increasing sampling efficiency. It can be applied to conventional fast spin‐echo (FSE) imaging; however, this article emphasizes its application in diffusion‐weighted imaging (DWI). The method is described and compared with conventional PROPELLER imaging, and clinical images collected with this PROPELLER variant are shown. Magn Reson Med, 2006.

A new design and rationale for 3D orthogonally oversampled k-space trajectories.

A novel center‐out 3D trajectory for sampling magnetic resonance data is presented. The trajectory set is based on a single Fermat spiral waveform, which is substantially undersampled in the center of k‐space. Multiple trajectories are combined in a “stacked cone” configuration to give very uniform sampling throughout a “hub,” which is very efficient in terms of gradient performance and uniform trajectory spacing. The fermat looped, orthogonally encoded trajectories (FLORET) design produces less gradient‐efficient trajectories near the poles, so multiple orthogonal hub designs are shown. These multihub designs oversample k‐space twice with orthogonal trajectories, which gives unique properties but also doubles the minimum scan time for critical sampling of k‐space. The trajectory is shown to be much more efficient than the conventional stack of cones trajectory, and has nearly the same signal‐to‐noise ratio efficiency (but twice the minimum scan time) as a stack of spirals trajectory. As a center‐out trajectory, it provides a shorter minimum echo time than stack of spirals, and its spherical k‐space coverage can dramatically reduce Gibbs ringing. Magn Reson Med, 2011.

Efficient sample density estimation by combining gridding and an optimized kernel

The reconstruction of non‐Cartesian k‐space trajectories often requires the estimation of nonuniform sampling density. Particularly for 3D, this calculation can be computationally expensive. The method proposed in this work combines an iterative algorithm previously proposed by Pipe and Menon (Magn Reson Med 1999;41:179–186) with the optimal kernel design previously proposed by Johnson and Pipe (Magn Reson Med 2009;61:439–447). The proposed method shows substantial time reductions in estimating the densities of center‐out trajectories, when compared with that of Johnson. It is demonstrated that, depending on the trajectory, the proposed method can provide reductions in execution time by factors of 12 to 85. The method is also shown to be robust in areas of high trajectory overlap, when compared with two analytical density estimation methods, producing a 10‐fold increase in accuracy in one case. Initial conditions allow the proposed method to converge in fewer iterations and are shown to be flexible in terms of the accuracy of information supplied. The proposed method is not only one of the fastest and most accurate algorithms, it is also completely generic, allowing any arbitrary trajectory to be density compensated extemporaneously. The proposed method is also simple and can be implemented on parallel computing platforms in a straightforward manner. Magn Reson Med, 2012.

Spiral trajectory design: A flexible numerical algorithm and base analytical equations

Spiral‐based trajectories for magnetic resonance imaging can be advantageous, but are often cumbersome to design or create. This work presents a flexible numerical algorithm for designing trajectories based on explicit definition of radial undersampling, and also gives several analytical expressions for charactering the base (critically sampled) class of these trajectories.

Revised motion estimation algorithm for PROPELLER MRI.

To introduce a new algorithm for estimating data shifts (used for both rotation and translation estimates) for motion‐corrected PROPELLER MRI. The method estimates shifts for all blades jointly, emphasizing blade‐pair correlations that are both strong and more robust to noise.

Graphical programming interface: A development environment for MRI methods

To introduce a multiplatform, Python language‐based, development environment called graphical programming interface for prototyping MRI techniques.

Dixon water‐fat separation in PROPELLER MRI acquired with two interleaved echoes

To propose a novel combination of robust Dixon fat suppression and motion insensitive PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction) MRI.

Multidirectional high‐moment encoding in phase contrast MRI

The use of phase contrast MRI to measure vascular flow provides a unique method for acquiring quantitative estimates of flow as well as morphological imaging. The quantitative aspects of phase contrast magnetic resonance angiography (PC‐MRA) provide unique relationships between measurement parameters and resulting signal to noise ratio of the velocity measurements. This article introduces a new method to exploit these relationships providing increased efficiency, and therefore, higher vessel conspicuity. Signal to noise ratio gains in high‐moment PC‐MRA are limited by the ability to unalias phase measurements that fall outside the −π to π interval. Unaliasing phase on a per pixel basis is limited by errors in the measurements due to noise and intravoxel flow distributions. Current dual‐VENC methods have been shown to be robust to these errors and provide high velocity to noise ratio gains, however, the collection of a required high‐VENC set can be inefficient. The presented method provides more time efficient gains in velocity to noise ratio compared to a dual‐VENC approach by eliminating the high‐VENC acquisitions and using shared information between nonorthogonal measurements. Simulations, phantom, and in vivo angiography are used to characterize the noise performance of each method. The velocity to noise ratio efficiency of the proposed method is shown to be ∼1.7 times greater than the dual‐VENC method at the same gradient moment. Magn Reson Med, 2013.

Analytical three-point Dixon method: With applications for spiral water-fat imaging

The goal of this work is to present a new three‐point analytical approach with flexible even or uneven echo increments for water–fat separation and to evaluate its feasibility with spiral imaging.

Fast, variable system delay correction for spiral MRI

Time‐varying system delays and eddy currents can substantially reduce the image quality of spiral images. A new method is proposed to estimate variable system delays for spiral‐based trajectories.