TROMBONE: T₁-relaxation-oblivious mapping of transmit radio-frequency field (B₁) for MRI at high magnetic fields

Fast, 3D radio-frequency transmit field (B₁) mapping is important for parallel transmission, spatially selective pulse design and quantitative MRI applications. It has been shown that actual flip angle imaging - two interleaved spoiled gradient recalled echo images acquired in steady state with two very short time delays (TR₁, TR₂) - is an attractive method of B₁ mapping. Herein, we describe the TROMBONE method that efficiently integrates actual flip angle imaging with EPI imaging, alleviates very short TR requirement of actual flip angle imaging and through their synergy yields up to 16 times higher precision in B₁ estimation in the same experimental time. High precision of TROMBONE can be traded for faster scans. The map of B₁ reconstructed from the ratio of intensities of two images is insensitive to longitudinal relaxation time (T ₁) in the physiologically relevant range. A table of the optimal acquisition protocol parameters for various target experimental conditions is provided.