Fat Suppression with Low SAR for SE EPI fMRI at 7T

Introduction: Functional imaging studies at 7 Tesla have given encouraging results, with increased CNR and stronger BOLD contrast [1]. However, a remaining problem at such field strengths is the increased specific absorption rate (SAR). A possible solution would be the omission or the modification of the RF pulses used in the imaging sequence. The SAR for gradient-echo EPI at 7 Tesla can be reduced significantly by omitting the fat suppression pulse, since at the optimal echo time for BOLD weighting (~25 ms) the signal from subcutaneous fat has already decayed [2]. T2* of the tissue region outside the scull at 7 Tesla is about 10 ms [2]. However, the T2 of fat is closer to 65 ms [3] so that for a spin-echo (SE) BOLD EPI acquisition, where the fat signal is refocussed, the fat saturation pulse cannot easily be omitted. Here we describe a simple method to remove the fat signal, by increasing the duration of the refocussing pulse to approximately the T2* value of fat, which has the additional virtue of reducing the SAR by a factor of 2.