Towards Whole Brain T2-Weighted fMRI at Ultra-High Fields using an Integrated Approach

Introduction/Synopsis Ultra-High Magnetic Fields offer large advantages, including higher image SNR, higher functional contrast and increased spatial specificity (i.e. accuracy) for T2-weighted fMRI [1]. Short transverse relaxation times, increased magnetic susceptibility effects, specific absorption rate (SAR) and B1 inhomogeneities [2,3], however, can all undermine these advantages. Here we present an integrated approach consisting of a T2 weighted sequence that reduces SAR significantly (SPIF-T2) [4], a large volume B1 shim to improve T2 contrast and either a 16 channel or a 30 channel transceiver array coil that enable and improve RF (B1) shimming for large volumes of the human brain. Robust activation is demonstrated in both the visual and motor areas of the human brain. Methods A slab wise magnetization Preparation for Functional Imaging with a T2 weight (SPIF-T2) [4] is used to provide the T2 weighting for the more accurate Spin Echo (SE) fMRI [1,4,5], while reducing SAR significantly (~3 fold for 10 slices when compared to a standard multi slice Spin Echo (SE) sequence). Ten slices were positioned to go through either the visualor the motor-cortex. This technique is used in conjunction with Parallel Imaging with X4 acceleration (1D) and a half-Fourier technique (5/8) to allow for whole brain coverage while maintaining short acquisition times necessary to keep Gradient Echo (GE) contributions small. Two normal subject of similar physiology, one for each coil, participated in this study. A 16 channel [6] and a 30 channel transceiver array coil (Fig. 1) were used. The 30 channel coil consisted of two concentric rings (14 and 16 elements respectively) of short (8 cm) line elements to cover the superior and inferior part of the human head, allowing for efficient B1 manipulation along the z-direction. Two elements have been omitted from the lower ring to allow for task presentation. 16 channels, distributed among the two rings in an approximately interleaved fashion were used for transmit and receive. Experiments were performed on a 7T system (Siemens). The motor (finger tapping) and visual (flashing red checker board) paradigms consisted of 10 blocks of 30s stimulus and 30s rest with a total duration of about 10 minutes. Each 30s period consisted of 5 acquisitions. Each acquisition consisted of the same T2 prepared 120 mm slab going through the visualand the motor-cortex. The slab selective T2 magnetization preparation (Fig. 2), consisted of a (90o |180o|-90o) RF sequence to flip back the magnetization along the z axis, followed by 10, interleaved GE EPI slices of 2 mm thickness each. (FOV = 19.2 x 19.2 cm; matrix = 128 x 128, single shot; α=90°); TE for the preparation slab was 55 ms; TE for the EPI readout with half-Fourier was 5.9 ms. TR in the multi slice EPI train was ~ 25 ms per slice leading to 250 ms for the 10 slice acquisition following each T2 preparation module; this includes a 12.2 ms fat suppression module for each slice). Two B1 shim targets within one large volume were defined based on three axial slices positioned in each of the two slabs chosen for the subsequent fMRI series (one in the visual cortex, one in the motor cortex). Within each of these six (2*3) axial reference slices an ROI was drawn defining the B1 shim target location. A 3D B1 Map of the whole brain was obtained with the AFI technique with a nominal flip angle of 70 degrees [7]. A series of 18 GE images was obtained with a small flip angle (16 images one channel transmitting at a time, one image all coils transmitting, one image without pulsing RF) to produce relative B1 maps [8]. Those relative maps merged with the 3D B1 map yielded 16 magnitude and phase B1 maps for each channel [9]). A B1 shim solution was calculated for all targets using the optimization toolbox in matlab. 3D B1 maps were measured again with the two B1 shim settings to validate the predicted B1 alterations and efficiencies.