Ultra-high-field CAIPIRINHA modulated parallel transmit excitation for homogenous image reconstruction without RF shimming

Ultra-high-field (UHF) provides higher SNR than conventional, clinically available field strengths. However, UHF suffers from heterogeneous transmit B₁⁺ fields. At 7 T, the shortened transmit radio-frequency (RF) wavelengths have a similar value to the dimensions of the human head/thorax which may result in signal cancellation and local signal dropouts. In this paper, we propose a novel imaging scheme based on simultaneous excitation with all transmit channels. Controlled aliasing is used to encode each transmit channel independently which we term Tx-CAIPIRINHA.

Tx-CAIPIRINHA has been demonstrated in-vivo. The concept uses the linear superposition of B₁⁺ fields via the excitation flip angle which only holds true in the low flip angle regime. When normalizing to transmit efficiency, Tx-CAIPIRINHA achieved a marginally higher SNR than B₁⁺ shimming, demonstrating the constructive combination of transmit sensitives throughout the image.