Accelerated 3D multi‐channel B1+ mapping at 7 T for the brain and heart

Abstract

AbstractPurposeTo acquire accurate volumetric multi‐channel maps in under 14 s whole‐brain or 23 heartbeats whole‐heart for parallel transmit (pTx) applications at 7 T.Theory and MethodsWe evaluate the combination of three recently proposed techniques. The acquisition of multi‐channel transmit array maps is accelerated using transmit low rank (TxLR) with absolute mapping (Sandwich) acquired in a time‐interleaved acquisition of modes (B1TIAMO) fashion. Simulations using synthetic body images derived from Sim4Life were used to test the achievable acceleration for small scan matrices of 24 × 24. Next, we evaluated the method by retrospectively undersampling a fully sampled library of nine subjects in the brain. Finally, Cartesian undersampled phantom and in vivo images were acquired in both the brain of three subjects (8Tx/32 receive [Rx]) and the heart of another three subjects (8Tx/8Rx) at 7 T.ResultsSimulation and in vivo results show that volumetric multi‐channel maps can be acquired using acceleration factors of 4 in the body, reducing the acquisition time to within 23 heartbeats, which was previously not possible. In silico heart simulations demonstrated a RMS error to the fully sampled native resolution ground truth of 4.2° when combined in first‐order circularly polarized mode (mean flip angle 66°) at an acceleration factor of 4. The 14 s 3D maps acquired in the brain have a RMS error of 1.9° to the fully sampled (mean flip angle 86°).ConclusionThe proposed method is demonstrated as a fast pTx calibration technique in the brain and a promising method for pTx calibration in the body.