High-resolution spiral imaging on a whole-body 7T scanner with minimized image blurring

Abstract

High-resolution (~0.22 mm) images are preferably acquired on whole-body 7T scanners to visualize minianatomic structures in human brain. They usually need long acquisition time (~12 min) in three-dimensional scans, even with both parallel imaging and partial Fourier samplings. The combined use of both fast imaging techniques, however, leads to occasionally visible undersampling artifacts. Spiral imaging has an advantage in acquisition efficiency over rectangular sampling, but its implementations are limited due to image blurring caused by a strong off-resonance effect at 7T. This study proposes a solution for minimizing image blurring while keeping spiral efficient. Image blurring at 7T was, first, quantitatively investigated using computer simulations and point-spread functions. A combined use of multishot spirals and ultrashort echo time acquisitions was then employed to minimize offresonance-induced image blurring. Experiments on phantoms and healthy subjects were performed on a whole-body 7T scanner to show the performance of the proposed method. The three-dimensional brain images of human subjects were obtained at echo time 5 1.18 ms, resolution 5 0.22mm (field of view 5 220mm, matrix size 5 1024), and in-plane spiral shots 5 128, using a home-developed ultrashort echo time sequence (acquisition-weighted stack of spirals). The total acquisition time for 60 partitions at pulse repetition time 5 100 ms was 12.8 min without use of parallel imaging and partial Fourier sampling. The blurring in these spiral images was minimized to a level comparable to that in gradient-echo images with rectangular acquisitions, while the spiral acquisition efficiency was maintained at eight. These images showed that spiral imaging at 7T was feasible. Magn Reson Med 63:543-552,

High‐resolution sodium imaging of human brain at 7 T

“…This difference resulted from the sampling volume in the k ‐space and windowing (i.e., k ‐space filtering) during image reconstruction. The in‐plane spiral data acquisition was performed in a disk region, instead of a rectangular region as usually used in Cartesian data acquisitions, leading to 41% wider of point spread function in the spiral sampling than in Cartesian sampling (27, 28). A two‐dimensional Hann window was applied to the spiral data for reducing noise at high‐spatial frequencies, contributing additional ∼30% to the point spread function widening (22).…”

Single‐shot spiral imaging at 7 T

“…However, single‐shot readouts are equally attractive at higher main field and resolution, particularly for BOLD fMRI and ASL, which benefit greatly from enhanced baseline sensitivity . At 7 T, spiral imaging has only been reported with segmented readouts up to 20 ms, targeting structural contrast . Toward single‐shot high‐resolution acquisition at 7 T, the main obstacle is that high field exacerbates the off‐resonance challenge.…”

Fast low-specific absorption rate B0 -mapping along projections at high field using two-dimensional radiofrequency pulses

“…B 0 variations can distort the ideally cylindrical excitation pattern because of the long RF pulse duration . At 7T and for a 5‐ms pulse, moderate blurring of the profile is expected for small B 0 perturbations (0–1 ppm).…”

“…Using an iterative shimming method makes the exact fidelity of the excitation profile less important, and fast B 0 mapping can be achieved using simple numerical tools and minimal RF energy deposition. B 0 variations can distort the ideally cylindrical excitation pattern because of the long RF pulse duration (14,15). At 7T and for a 5-ms pulse, moderate blurring of the profile is expected for small B 0 perturbations (0-1 ppm).…”