Purpose
To improve pseudo‐continuous arterial spin labeling (pcASL) at 7T by exploiting a hybrid homogeneity‐ and efficiency‐optimized ‐shim with adapted gradient strength as well as background suppression.
Methods
The following three experiments were performed at 7T, each employing five volunteers: (1) A hybrid (ie, homogeneity‐efficiency optimized) ‐shim was introduced and evaluated for variable‐rate selective excitation pcASL labeling. Therefore, ‐maps in the V3 segment and time‐of‐flight images were acquired to identify the feeding arteries. For validation, a gradient‐echo sequence was applied in circular polarized (CP) mode and with the hybrid ‐shim. Additionally, the gray matter (temporal) signal‐to‐noise ratio (tSNR) in pcASL perfusion images was evaluated. (2) Bloch simulations for the pcASL labeling were conducted and validated experimentally, with a focus on the slice‐selective gradients. (3) Background suppression was added to the ‐shimmed, gradient‐adapted 7T sequence and this was then compared to a matched sequence at 3T.
Results
The ‐shim improved the signal within the labeling plane (23.6%) and the SNR/tSNR increased (+11%/+11%) compared to its value in CP mode; however, the increase was not significant. In accordance with the simulations, the adapted gradients increased the tSNR (35%) and SNR (45%) significantly. Background suppression further improved the perfusion images at 7T, and this protocol performed as well as a resolution‐matched protocol at 3T.
Conclusion
The combination of the proposed hybrid ‐phase‐shim with the adapted slice‐selective gradients and background suppression shows great potential for improved pcASL labeling under suboptimal conditions at 7T.