High-Resolution Mapping of Neuronal Activation with Balanced SSFP at 9.4 T
In 2001, a novel method to detect BOLD response was proposed, which is based on the frequency sensitivity of the stop band of bSSFP. An excellent summary of further developments, modification and applications of BOLD bSSFP can be found in Karla Miller´s review paper (2). In this work, we demonstrate the feasibility of brain activation mapping based on rapid and high-resolution pass band bSSFP at 9.4T. Susceptibility-related frequency changes across the brain scale linearly with the main field strength, which produces more banding artifacts at increasingly higher fields. As a result, the repetition time of bSSFP has to be as short as possible, although several studies indicate an increased BOLD-related signal change with increasing repetition time. In addition to using a very short repetition time the temporal resolution of bSSFP was increased by parallel imaging methods and partial Fourier acquisition.
Activation patterns and signal changes were very stable and reproducible across subjects within the visual cortex, and comparable to reported values of SE-EPI at 7T and 9.4T. The acquired functional maps are without any spatial distortions allowing for a precise registration to anatomical images. Measured signal changes are slightly lower than SE-EPI at 9.4T, however, the reduced SAR budget easily allows for whole brain bSSFP coverage. The dominant spin echo contrast of bSSFP using very short repetition times is most sensitive to small vessel and thus potentially yields a higher spatial selectivity compared to GE-EPI. In addition, the absence of blurring along phase encoding direction provides a true isotropic point-spread function.