Impact of RF imperfections on MRF-based and reference B1+ mapping methods with a commercial 1Tx/32Rx head coil

Motivation: Magnetic Resonance Fingerprinting (MRF) and preparation-based reference B1+ mapping approaches yielded an unexpected B1+ offset. Goal(s): Identify the origin of the B1+ offset. Approach: Pick-up coil measurements to evaluate the actual RF pulse shape using a commercial 1Tx/32Rx head-coil at 7T. Results: For the preparation-based B1+ mapping method with rectangular pulses, longer RF rise times (~100 µs) and phase variations (~10°) were observed. Furthermore, during the 21s-long MRF acquisition using sinc-pulses, an increased RF amplitude (~2.5%) was observed. By incorporating the measured RF alterations, the B1+ offset could be substantially reduced. Impact: Unidentified effects of the RF transmission such as deviation in envelope and temporal instability affect measurements. Characterizing the RF transmission enables quantification and correction of these deviations.