Pushing functional MRI spatial and temporal resolution further: High‐density receive arrays combined with shot‐selective 2D CAIPIRINHA for 3D echo‐planar imaging at 7 T

Abstract

To be able to examine dynamic and detailed brain functions, the spatial and temporal resolution of 7 T MRI needs to improve. In this study, it was investigated whether submillimeter multishot 3D EPI fMRI scans, acquired with high-density receive arrays, can benefit from a 2D CAIPIRINHA sampling pattern, in terms of noise amplification (g-factor), temporal SNR and fMRI sensitivity. High-density receive arrays were combined with a shot-selective 2D CAIPIRINHA implementation for multishot 3D EPI sequences at 7 T. In this implementation, in contrast to conventional inclusion of extra k z gradient blips, specific EPI shots are left out to create a CAIPIRINHA shift and reduction of scan time. First, the implementation of the CAIPIRINHA sequence was evaluated with a standard receive setup by acquiring submillimeter whole brain T 2 *-weighted anatomy images. Second, the CAIPIRINHA sequence was combined with high-density receive arrays to push the temporal resolution of submillimeter 3D EPI fMRI scans of the visual cortex. Results show that the shot-selective 2D CAIPIRI-NHA sequence enables a reduction in scan time for 0.5 mm isotropic 3D EPI T 2 *weighted anatomy scans by a factor of 4 compared with earlier reports. The use of the 2D CAIPIRINHA implementation in combination with high-density receive arrays, enhances the image quality of submillimeter 3D EPI scans of the visual cortex at high acceleration as compared to conventional SENSE. Both the g-factor and temporal SNR improved, resulting in a method that is more sensitive to the fMRI signal. Using this method, it is possible to acquire submillimeter single volume 3D EPI scans of the visual cortex in a subsecond timeframe. Overall, high-density receive arrays in combination with shot-selective 2D CAIPIRINHA for 3D EPI scans prove to be valuable for reducing the scan time of submillimeter MRI acquisitions. K E Y W O R D S3D EPI, 7T, CAIPI, high resolution, receive arrays, shot selective, visual cortex Abbreviations used: BOLD, blood oxygenation level-dependent; CAIPIRINHA, controlled aliasing in parallel imaging; EPI, echo-planar imaging; FOV, field of view; MB, multiband; RF, radio frequency; SENSE, sensitivity encoding; tSNR, temporal signal-to-noise ratio.

Segmented K-Space Blipped-Controlled Aliasing in Parallel Imaging (Skipped-CAIPI) for High Spatiotemporal Resolution Echo Planar Imaging

“…On the other hand, high-resolution MRI, which may benefit most from skipped-CAIPI sampling, also benefits most from volumetric (3D) imaging (33,52). For multi-echo fMRI (53–56) or ultra-high-field fMRI, where often partial Fourier sampling and large R y are employed to minimize EF , TE and geometric distortions, alternative skipped-CAIPI sampling is valuable, in particular with high-density receive arrays (13,18) or for laminar resolution imaging (57,58). Furthermore, eased matching of the phase encode bandwidth between scans with different matrix sizes or CAIPI patterns fosters rapid, distortion-matched high-contrast acquisitions (19,59,60) even at resolutions differing from the corresponding functional scan.…”

“…(28,29). Such a shot-selective CAIPI approach has recently been applied with SMS-EPI (5) and 3D-EPI (18) to achieve greater y -blips. Each CAIPI pattern corresponds to one unique shot-selective trajectory without z -blips.…”

“…It is generally appealing to combine the inherent signal-to-noise ratio (SNR) efficiency of EPI (23,24) with controlled aliasing in parallel imaging (CAIPIRINHA, short: CAIPI (25)) to minimize the geometrydependent parallel imaging noise penalty (g-factor (26)). However, previous sampling approaches limit each CAIPI pattern to only two possible EPI k-space trajectories and corresponding EPI factors: single-shot blipped-CAIPI (27) and multi-shot shot-selective CAIPI (18,28,29). Thus, higher-priority parameters affecting the EPI factor as well (e.g.…”

Segmented K‐space blipped‐controlled aliasing in parallel imaging for high spatiotemporal resolution EPI

“…On the other hand, highresolution MRI, which may benefit most from skipped-CAIPI sampling, also benefits most from volumetric (3D) imaging. 33,52 For multi-echo fMRI [53][54][55][56] or ultrahigh-field fMRI, where often partial Fourier sampling and large R y are employed to minimize EF, TE, and geometric distortions, alternative skipped-CAIPI sampling is valuable, in particular with high-density receive arrays 13,18 or for laminar resolution imaging. 57,58 Furthermore, eased matching of the phase encode bandwidth between scans with different matrix sizes or CAIPI patterns fosters rapid, distortion-matched highcontrast acquisitions, 19,59,60 even at resolutions differing from the corresponding functional scan.…”

“…An increasing number of structural imaging, [1][2][3][4][5][6] quantitative imaging, [7][8][9][10] or high spatiotemporal resolution functional imaging [11][12][13][14][15][16][17][18][19][20][21] applications have been approached by dedicated EPI 22 implementations. Many of those aim at higher spatial resolution than usual, for example, in fMRI.…”