Iterative multi-channel radio frequency pulse calibration with improving B₁ field uniformity in high field MRI

Background: In high field MRI capable of multi-channel radio frequency (RF) transmission, B₁ shimming is a time-consuming job because conventional B₁ shimming techniques require B₁ mapping for each channel. After acquiring the complex-numbered B₁ field maps, the optimal amplitude and phase of the driving RF pulse are determined for each channel to maximize the B₁ field uniformity in conventional B₁ shimming. However, time-consuming B₁ 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 B₁ 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 B₁ 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 B₁ mapping, the RF pulse calibration time could be much shorter than the B₁-mapping based methods. The proposed method is expected to be a practical substitute for the B₁-mapping-based B₁ shimming methods when long pre-scan time is not tolerable.