Reduction of RF power for magnetization transfer with optimized application of RF pulses in k-space.

More efficient use of RF power for RF-intensive applications such as magnetization transfer (MT) is necessary at high field strength (3.0 T or greater) to keep the specific absorption rate (SAR) within regulatory limits. It has been demonstrated that RF power deposition can be reduced with minimal impact on image quality by preferential application of MT pulses to the central phase-encoding views. This work extends that approach to both phase-encoding directions in a 3D acquisition (i.e., phase and slice) and further improves it by modulating the flip angle of the MT pulse according to the phase-encoded view's distance to the center of k-space. This technique is implemented for 3D time-of-flight (TOF) MR angiography (MRA) and the parameters for MT pulse are optimized based on phantom studies at 3.0 T. MT applied with this method at 3.0 T is shown to improve the blood vessel detectability in high-resolution intracranial 3D TOF MRA exams of 11 patients.