Multi‐echo balanced SSFP with a sequential phase‐encoding order for functional MR imaging at 7T

Abstract

To develop a 2D multi-echo passband balanced SSFP (bSSFP) sequence using an echo-train readout with a sequential phase-encoding order (sequential multi-echo bSSFP), and evaluate its performance in fast functional brain imaging at 7 T. Methods:As images of sequential multi-echo bSSFP exhibit multiple ghosts due to periodic k-space modulations, a GRAPPA-based reconstruction method was proposed to eliminate ghosting artifacts. MRI experiments were performed to compare the image quality of multi-echo bSSFP and conventional single-echo bSSFP. Submillimeter-resolution fMRI using a checkerboard visual stimulus was conducted to compare the activation characteristics of multi-echo bSSFP, conventional single-echo bSSFP and standard gradient-echo EPI (GE-EPI).Results: A higher mean structural similarity index was found between images of single-echo bSSFP and multi-echo bSSFP with a shorter echo train length (ETL).Multi-echo bSSFP (ETL = 3) showed higher temporal SNR (tSNR) values than GRAPPA-accelerated single-echo bSSFP (R = 2). In submillimeter-resolution fMRI experiments, multi-echo bSSFP (ETL = 3) approached the imaging speed of GRAPPA-accelerated single-echo bSSFP (R = 2), but without tSNR penalty and reduced activation due to acceleration. The median t-value and the number of significantly activated voxels were comparable between GE-EPI and multi-echo bSSFP (ETL = 3) that provides virtually distortion-free functional images and inherits the activation patterns of conventional bSSFP.

The laminar pattern of proprioceptive activation in human primary motor cortex

“…Nevertheless, considering the limitations of the applied correction strategy (see Figure S4B), the corrected profiles should be interpreted with caution. Liang et al, 2022;Miller, 2012;Scheffler et al, 2018). Nevertheless, as visible in activation maps, we did observe strong signals towards CSF, which likely reflects high sensitivity of bSSFP-BOLD to intravascular signals and inflow effects (Liang et al, 2022).…”

“…This is supported by both subjects’ laminar profiles (Figure 3B) showing significant sub-clusters of activation in the deep layers (p = 0.005 and p = 0.001 for Subject 1 and Subject 2, respectively). Due to bSSFP being relatively insensitive to T2* contributions, it is expected to have less bias towards large veins than gradient echo EPI (Báez-Yánez et al, 2017; Liang et al, 2022; Miller, 2012; Scheffler et al, 2018). Nevertheless, as visible in activation maps, we did observe strong signals towards CSF, which likely reflects high sensitivity of bSSFP-BOLD to intravascular signals and inflow effects (Liang et al, 2022).…”

“…Due to bSSFP being relatively insensitive to T2* contributions, it is expected to have less bias towards large veins than gradient echo EPI (Báez-Yánez et al, 2017; Liang et al, 2022; Miller, 2012; Scheffler et al, 2018). Nevertheless, as visible in activation maps, we did observe strong signals towards CSF, which likely reflects high sensitivity of bSSFP-BOLD to intravascular signals and inflow effects (Liang et al, 2022). To minimize the effect of large vessels, voxels with percent signal change exceeding 4% in the across-condition averaged bSSFP map were excluded from the map of each condition before generating laminar profiles (uncorrected profiles are shown in Figure S5).…”

“…The spin echo-like contrast mechanism of bSSFP results in BOLD contrast with higher tissue specificity than conventional gradient echo BOLD (Báez-Yánez et al, 2017; Miller, 2012; Scheffler et al, 2018). It additionally benefits from minimal spatial distortions and blurring-effects because of its very short read-out time, which further improves the ability to resolve laminar activation (Liang et al, 2022; Miller, 2012). One subject was scanned twice across different days to assess the reliability of 2D-bSSFP activation maps (Figure S1).…”

“…The bSSFP data was preprocessed using AFNI (Cox, 1996), including removal of outliers ( 3dToutcount function), rigid body motion correction, brain masking, and coregistration of the T1 weighted anatomical volume to the mean functional image with localized pearson correlation as the cost function. Functional bSSFP images has minimal distortions, analogous to MP2RAGE, and high-quality coregistration was thus possible based on rigid and affine transformations only (Liang et al, 2022).…”