Cerebral TOF Angiography at 7T: Impact of B1+ Shimming with a 16-Channel Transceiver Array

PurposeTime-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 B-0=7T, however, spatially varying transmit B-1 (B-1(+)) fields at 7T reduce TOF contrast uniformity, typically resulting in suboptimal contrast and reduced vessel conspicuity in the brain periphery. MethodsUsing a 16-channel B-1(+) shimming system, we compare different dynamically applied B-1(+) phase shimming approaches on the radiofrequency excitation to improve contrast homogeneity for a (0.5 mm)(3) resolution multislab TOF acquisition. In addition, B-1(+) 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. ResultsB(1)(+) 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 B-1(+) shimming on the venous saturation pulse. ConclusionB(1)+ shimming can significantly improve high resolution TOF vascular investigations at ultrahigh field, holding strong promise for non contrast-enhanced clinical applications. Magn Reson Med 71:966-977, 2014. (c) 2013 Wiley Periodicals, Inc.