Cerebral TOF angiography at 7T: Impact of B1 (+) shimming with a 16-channel transceiver array.

PURPOSE: Time-of-flight (TOF) MR imaging is clinically among the most common cerebral noncontrast enhanced MR angiography techniques allowing for high spatial resolution. As shown by several groups TOF contrast significantly improves at ultrahigh field of B0 = 7T, however, spatially varying transmit B1 (B1 (+)) fields at 7T reduce TOF contrast uniformity, typically resulting in suboptimal contrast and reduced vessel conspicuity in the brain periphery.
METHODS: Using a 16-channel B1 (+) shimming system, we compare different dynamically applied B1 (+) phase shimming approaches on the radiofrequency excitation to improve contrast homogeneity for a (0.5 mm)(3) resolution multislab TOF acquisition. In addition, B1 (+) shimming applied on the venous saturation pulse was investigated to improve venous suppression, subcutaneous fat signal reduction and enhanced background suppression originating from MT effect.
RESULTS: B1 (+) excitation homogeneity was improved by a factor 2.2-2.6 on average depending on the shimming approach, compared to a standard CP-like phase setting, leading to improved vessel conspicuity particularly in the periphery. Stronger saturation, higher fat suppression and improved background suppression were observed when dynamically applying B1 (+) shimming on the venous saturation pulse.
CONCLUSION: B1+ shimming can significantly improve high resolution TOF vascular investigations at ultrahigh field, holding strong promise for non contrast-enhanced clinical applications.