Iterative multi-channel radio frequency pulse calibration with improving B(1) field uniformity in high field MRI

BACKGROUND: In high field MRI capable of multi-channel radio frequency (RF) transmission, B(1) shimming is a time-consuming job because conventional B(1) shimming techniques require B(1) mapping for each channel. After acquiring the complex-numbered B(1) field maps, the optimal amplitude and phase of the driving RF pulse are determined for each channel to maximize the B(1) field uniformity in conventional B(1) shimming. However, time-consuming B(1) shimming procedures at the pre-scan may not be tolerated in the clinical imaging in which patient throughput is one of the important factors. METHODS: To avoid the time-consuming B(1) mapping, the first spin echo and the stimulated echo were repeatedly acquired in the slice-selective stimulated echo sequence without imaging gradients. A cost function of the amplitudes and phases of the driving RF pulse for every channel was defined in a way that the ratio between the spin echo and stimulated echo amplitudes rapidly converged to √ 2. The amplitude and phase of the driving RF pulse were iteratively modified over the repeating RF pulse sequence so that the cost function was minimized. RESULTS: From the finite-difference-time-domain (FDTD) electromagnetic field simulations with a human body model placed in a birdcage coil operating at 3 T, it was observed that the RF pulse calibration with iterative cost function minimization can give improvement of B(1) field uniformity as well as flip-angle calibration. The experiments at 3 T also showed improvement of RF field uniformity in the phantom imaging studies. CONCLUSIONS: Since the proposed RF pulse calibration is not based on B(1) mapping, the RF pulse calibration time could be much shorter than the B(1)-mapping based methods. The proposed method is expected to be a practical substitute for the B(1)-mapping-based B(1) shimming methods when long pre-scan time is not tolerable.