Single-shot pseudo-centric EPI for magnetization-prepared imaging.

PURPOSE: To implement a single-shot centric-reordered EPI (1sh-CenEPI), which reduces TE significantly, thus enabling to improve SNR for magnetization-prepared imaging. METHODS: We proposed a 1sh-CenEPI in which grouped oscillating readout gradients, phase-encoding blips within each group, and big phase-encoding jumps between two consecutive groups are incorporated to encode whole k-space from the center to the edges in a single shot. The concept was tested on phantoms and human brains at 3 T. In addition, the proposed reordering scheme was applied to pseudo-continuous arterial spin labeling for evaluating the efficiency of the centric reordering in magnetization-prepared imaging. RESULTS: The proposed 1sh-CenEPI reduced TE from 50 ms to 1.4 ms for gradient-echo EPI, and from 100 ms to 7 ms for spin-echo EPI, while the elongation of readout duration was below 10% of the whole readout duration in most cases. The 1sh-CenEPI images exhibited no distinct geometric distortion both in phantom and human brain, comparable to the conventional two-shot center-out EPI. In pseudo-continuous arterial spin labeling results, 3-fold temporal SNR increase and 2-fold spatial SNR increase in the perfusion-weighted images were achieved with 1sh-CenEPI compared with the conventional linear ordering, whereas the cerebral blood flow values were consistent with previous studies. CONCLUSION: The proposed 1sh-CenEPI significantly reduced TE while maintaining similar readout window and providing images comparable to the conventional linear and multishot center-out EPI images. It can be a qualified candidate as a new readout for various magnetization-prepared imaging techniques.

Rapid, theoretically artifact-free calculation of static magnetic field induced by voxelated susceptibility distribution in an arbitrary volume of interest.

PURPOSE: To demonstrate a computationally efficient and theoretically artifact-free method to calculate static field (B0 ) inhomogeneity in a volume of interest induced by an arbitrary voxelated susceptibility distribution. METHODS: Our method computes B0 by circular convolution between a zero-filled susceptibility matrix and a shifted, voxel-integrated dipolar field kernel on a grid of size NS +NT - 1 in each dimension, where NS and NT are the sizes of the susceptibility source and B0 target grids, respectively. The computational resource requirement is independent of source-target separation. The method, called generalized susceptibility voxel convolution, is demonstrated on three susceptibility models: an ellipsoid, MR-compatible screws, and a dynamic human heartbeat model. RESULTS: B0 in an ellipsoid calculated by generalized susceptibility voxel convolution matched an analytical solution nearly exactly. The method also calculated screw-induced B0 in agreement with experimental data. Dynamic simulation demonstrated its computational efficiency for repeated B0 calculations on time-varying susceptibility. On the contrary, conventional and alias-subtracted k-space-discretized Fourier convolution methods showed nonnegligible aliasing and Gibbs ringing artifacts in the tested models. CONCLUSION: Generalized susceptibility voxel convolution can be a fast and reliable way to compute susceptibility-induced B0 when the susceptibility source is not colocated with the B0 target volume of interest, as in modeling B0 variations from motion and foreign objects.

Distortion correction using topup algorithm by single k-space (TASK) for echo planar imaging.

Distortion of echo planar imaging (EPI) can be corrected using B0 field maps, which can be estimated with the topup algorithm that requires two EPI images with opposite distortions. In this study, we propose a new algorithm, termed topup algorithm by single K-space (TASK), to generate two input images from a single k-space for the topup algorithm to correct EPI distortions. The centric EPI contains the opposite phase-encoding polarities in one k-space, which can be divided into two halves with opposite distortions. Therefore, two inputs could be extracted by dividing the k-space into halves and processing them using the proposed procedure including an iterative procedure of automatic brain masking and uniformity correction. The efficiency of TASK was evaluated using 3D EPI. Quantitative evaluations showed that TASK corrected EPI distortion at a similar level to the traditional methods. The estimated field maps from the conventional topup and TASK showed a high correlation ([Formula: see text]). An ablation study showed the validity of every suggested step. Furthermore, it was confirmed that TASK was effective for distortion correction of two-shot centric EPI as well, demonstrating its wider applicability. In conclusion, TASK can correct EPI distortions by its own single k-space information with no additional scan.