Multi-Line Balanced SSFP for Rapid Functional Imaging at Ultra High-Field
Balanced steady-state free precession (bSSFP) is a BOLD-sensitive acquisition method that is highly sensitive to small vessels in the range of 5 to 20 µm. Thus, it shows a high potential to represent oxygenation changes within the microvasculature in contrast to gradient echoes with their huge and unspecific sensitivity to larger draining veins. A major drawback of bSSFP is its significantly reduced imaging speed compared to EPI. In this work we present a multi-line bSSFP for functional BOLD imaging that approaches the speed of EPI, and that shows an increased BOLD sensitivity compared to single-line bSSFP. Depending on the length of the echo train, we were able to acquire 16 slices with a 0.9 mm isotropic resolution in a volume TR of 1.21 to 1.66 s, compared to 3.14 s for a single-line acquisition. We analyze the acquisition efficiency, temporal and thermal signal to noise, as well as the observed BOLD signal changes upon a visual task on a 9.4 T system. With increasing TR and echo train length, the BOLD-related signal change as well as thermal and temporal noise was improved. Activation patterns and signal changes were stable and reproducible across subjects.