Multi-Banded T 2-Weighted fMRI with a z-Encoding RF Coil Array for Whole Brain Coverage at 7 T

Introduction. Due to significant B1 inhomogeneities and SAR limitations at ultra-high fields it is challenging to achieve effective whole brain T2weighted fMRI. However, it has recently [1] been demonstrated at 7 T that B1-shim homogeneity, T2-weighted contrast and hence activation in different target volumes of the human brain can be improved significantly with a multi-region B1-shim [2-4]. Here we built upon this technique and present a new integrated approach, consisting of a novel multi banded sequence, a 3D B1 shim and a z-encoding T/R coil array. The combination of these components allow to effectively address SAR and B1 inhomogeneity challenges and enable whole head T2-weighted fMRI. Methods Three normal subjects participated in this study. A 16-channel transceiver array (Fig. 1) consisting of two concentric rings (8 plus 8 elements) of short (8 cm) transmission line elements was used [5]. The staggering of the T/R elements in the z direction is essential for efficient B1 manipulation along the z-direction and additionally allows to unalias ten pairs of simultaneously acquired axial slices [6]. Two elements have been omitted from the lower ring to allow for task presentation. Experiments were performed on a 7T system (Siemens) with 16 independently controlled Tx channels. A motor (finger tapping) and visual (flashing checker board) paradigm was used for fMRI. The slab-selective T2 magnetization preparation SPIF-T2 [7], consisted of a (90o |180o|-90o) RF sequence to prepare and flip back the magnetization along the z axis. It was implemented with adiabatic refocusing pulses. It is followed by 10, interleaved GE EPI slices (single band or double band) of 2 mm thickness each. (FOV=19.2x19.2cm; matrix=128x128, single shot; α=90°; TE for the preparation slab was 55 ms; TE for the EPI readout with GRAPPA=4 and half-Fourier (5/8) was 6.5 ms). The whole brain B1 shim was defined based on five axial slices within the entire head. Within each of these five axial reference slices an ROI encompassing the brain in that respective slice was drawn defining the B1 shim target location [6]