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.

a) Overlay of MP2RAGE and bSSFP data. Top: Gray matter outlines obtained from MP2RAGE segmentation on top of bSSFP images. Bottom: bSSFP activation maps on top of MP2RAGE images. As can be seen, the distortion-free bSSFP data allow for accurate registration. b) shows a bSSFP image, a T2*-weighted image, as well as an overlay of a bSSFP activation map (0.6 mm isotropic resolution, same z-score scaling as in a)) on top of the T2*-weighted image. The strong intravascular signal from a draining vein (marked by a white arrow) is clearly visible.

Ehses P, Scheffler K.:
Multiline balanced SSFP for rapid functional imaging at ultrahigh field.
Magn Reson Med. 2018 Feb;79(2):994-1000. Epub 2017 May 25. PMID: 28547846
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