In a transmit apparatus, a multi-channel radio frequency transmitter (30, 46) includes a plurality of transmit elements (32) defining at least two independently operable transmit channels. A transmit configuration selector (54) determines a selected transmit configuration (60) specifying amplitude and phase applied to each transmit channel to generate a B.sub.1 field in a corresponding selected region (90) of a subject (38) coupled with the radio frequency transmitter. The transmit configuration selector determines the selected transmit configuration based on B.sub.1 mapping (58) of the subject and a B.sub.1 field quality assessment employing at least two different B.sub.1 field quality measures.