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Abstract:
Purpose/Introduction: Sub-millimeter single-shot echo-planar
imaging at ultra-high field is challenging due to stronger distortions evoked by increased B0-inhomogeneities as well as increased blurring and a reduction of the echo-time yielding the maximum BOLD contrast caused by faster T2 * relaxation. Therefore, a shorter echo-train length (ETL) is required, either by utilizing partial Fourier acquisition
or parallel imaging techniques. A further reduction of the ETL can be achieved with the reduced field of view (rFOV) method [1]. But even highly accelerated images can suffer from distortions which can be accurately measured and corrected by the point-spread-function (psf) mapping technique [2, 3]. Another issue of high resolution fMRI, is
the increased vulnerability to subject motion. Here, we combined rFOV imaging with psf distortion correction and prospective motion correction for high-resolution functional imaging at 9.4 Tesla.
Subjects and Methods: Measurements were performed on a whole
body scanner (Siemens Healthcare, Germany) using a 16-Tx/31-Rx array [4]. An outer volume suppression pulse was implemented in an EPI sequence [1]. The slice positions of the psf-mapping and functional measurements were updated using data from a camera (Metria Innovation Inc., USA) which tracked a moire´ phase marker fixed to the subject’s bite bar [6]. Parameters: TR = 2000 ms, TE = 25 ms, FA = 80, GRAPPA = 3 (FLEET ACS-scan, [7]), 6/8 partial Fourier,
FoV = 150 9 75 mm2, 17 slices, 0.65 mm isotropic, ETL = 36 ms. Paradigm: 20 periods, 10 s flickering circular
checkerboard (8 Hz, 7 visual field) + 26.5 s isolimuninant background. Images were reconstructed using in-house developed Matlab routines. From the fMRI data, residual motion was estimated before distortion correction.
Results: Although the short ETL used here resulted in already quite small distortions (Fig. 1: a,b), a clear improvement of anatomical matching can be seen when the distortion correction is applied (Fig. 1: c). According to the camera data, the subject presented here moved less than 0.24 mm/0.57. However, the retrospective estimate was
even smaller (max. 0.21 mm/0.09).
Discussion/Conclusion: We demonstrated anatomically matching
sub-millimeter fMRI with reduced interpolation blurring due to avoiding retrospective motion correction. In addition, the echo-train length was shortened in order to limit T2 * -blurring. However, the increased SAR caused by the additional outer volume suppression required a reduction of the number of slices. To reduce SAR, implementation of static B1 +-shimming or even 2D-selective excitation may
help [7]. Nevertheless, the presented approach can probably help to investigate the actual resolution of single-shot EPI at high field and can possibly be also used for measuring differences in functional layers.
