Multiple Area B1 Shimming: An efficient, low SAR approach for T2-weighted fMRI acquired in the Visual and Motor Cortices of the Human Brain at Ultra-High Field

2; matrix=128x128, 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). For comparison, a similar dataset but without the T2 preparation module was obtained. Identical readout was played in prepared and nonprepared acquisitions except for a smaller flip angle in the non prepared case to account for the reduced SNR due to the T2-weighing in the prepared case. Two B1 shim targets were defined based on two axial slices positioned in the center of the two slabs chosen for the subsequent fMRI series (one in the visual cortex, one in the motor cortex). Within each of these two axial reference slices an ROI was drawn defining the B1 shim target location. It was sufficient to utilize the center slice as transmit B1 varied only slowly along the Z direction, i.e. around those target slices. A 3D B1 Map of the whole brain was obtained with the AFI technique with a nominal flip angle of 70 degrees [8]. For each B1 shim target a series of 18 GE images were 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 [3]. 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 each target using the optimization toolbox in matlab. 3D B1 maps were measured again with the two B1 shim settings to validate the predicted B1 alterations. Note that only B1 phase modulation was utilized in the non linear optimization algorithm. RF power rescaling was performed in a second, optional step.