Improving TOF Angiography Contrast Homogeneity with B1+ Shimming at 7 Tesla: Benefits & Challenges

Sebastian Schmitter¹, Edward J. Auerbach¹, Gregor Adriany¹, Kamil Ugurbil¹, Pierre-Francois Van De Moortele¹

In Time-of-flight (TOF) angiography a contrast is generated between static tissues, partially saturated due to a high flip angle (&alpha=20-30ｺ) with short TR<<T1, and vessels, with strong signal from inflowing blood in the slab. Benefiting from high SNR and long T1, TOF studies at 7T provide good vessel-to-background contrast. However, a significant drawback observed at 7T is the spatially inhomogeneous magnitude of transmit B1 (B1+), resulting in non-uniform TOF contrast. The aim of this work was to investigate the potential and challenges of using B1+ phase shimming to improve the uniformity of TOF contrast in the brain at 7T.