BOLD fMRI using a modified HASTE sequence.

For more than a decade, turbo spin echo (TSE) pulse sequences have been suggested as an alternative to echo planar imaging (EPI) sequences for fMRI studies. Recent development in parallel imaging has renewed the interest in developing more robust TSE sequences for fMRI. In this study, a modified half Fourier acquisition single-shot TSE (mHASTE) sequence has been developed with a three-fold GRAPPA to improve temporal resolution as well as a preparation time to enhance BOLD sensitivity. Using a classical flashing checkerboard block design, the BOLD signal characteristics of this novel method have been systematically analyzed as a function of several sequence parameters and compared to those of gradient-echo and spin-echo EPI sequences. Experimental studies on visual cortex of five volunteers have provided evidence suggesting that mHASTE can be more sensitive to extra-vascular BOLD effects around microvascular networks, which leads to more accurate function localization. The studies also show that the activation cluster size in mHASTE increases with the refocusing RF flip angle and TE while decreasing with the echo number (n(center)) used to sample the k-space center. Compared to spin-echo EPI, mHASTE incurs an approximately 50% reduction in activation cluster size and an approximately 20% decrease in BOLD contrast. However a higher signal-to-noise ratio and a spatially more uniform temporal stability have been observed in mHASTE as compared to the EPI sequences when the scan times are held constant. With further refinement and optimization, mHASTE can become a viable alternative for fMRI in situations where the conventional EPI sequences are limited or prohibitive.