Combined caLculation of Ultra‐high field Biases ( CLUB ) With Sandwich: Fast, Simultaneous Estimation of 3D B 0 and Multi‐Channel B 1 + Maps at 7 T

ABSTRACT Purpose A method for simultaneous mapping of static (B 0 ) and transmit (B 1 + ) field inhomogeneities at ultra‐high field (UHF) was developed and validated. The utility of accelerating the proposed sequence using deep learning (DL) and joint low‐rank tensor completion (TxLR) reconstruction methods was evaluated to enable rapid online implementation. Methods A 3D sequence, Combined caLculation of UHF Biases (CLUB)‐Sandwich, was developed by incorporating a multi‐echo readout into the unsaturated segment of the Sandwich B 1 + mapping sequence, enabling simultaneous B 0 estimation. Data from 11 healthy volunteers were acquired at 7 T. Estimated ΔB 0 and B 1 + maps were compared with established, separate reference scans. Retrospectively and prospectively undersampled data were reconstructed using TxLR and a DL‐based algorithm. The resulting maps were compared with fully sampled data. Results CLUB‐Sandwich maps showed strong agreement with reference methods. A strong correlation ( r > 0.97) and low mean volumetric root mean squared errors were found for both ΔB 0 (9.5 ± 1.8 Hz) and absolute B 1 + (3.5° ± 0.3°). Both reconstruction methods enabled acquisitions in under 10 s of acquisition time. DL reconstruction was found to be substantially faster (5 s) than the TxLR algorithm (4 min) while producing comparable map quality. Prospective validation confirmed the feasibility of online mapping with acceptable accuracy. Conclusion The CLUB‐Sandwich method was developed for fast, accurate, and simultaneous ΔB 0 and B 1 + mapping. When combined with a DL‐based reconstruction, the proposed framework provides maps in under 10 s of acquisition time, presenting a feasible solution for rapid online inhomogeneity estimation in UHF applications.