The effects of coupled B₁ fields in B₁ encoded TRASE MRI - A simulation study

Transmit Array Spatial Encoding (TRASE) is a novel MRI technique that encodes spatial information by introducing phase gradients in the transmit RF (B₁) magnetic field. Since TRASE relies on the use of multiple RF fields (B₁ fields with different phase gradients) for k-space traversal, a TRASE pulse sequence requires RF pulses that are produced by switching between the transmit coils (B₁ fields). However, interactions among the transmit RF coils can cause un-driven coils to produce unwanted B₁ fields that impair the spatial encoding. Therefore, TRASE is sensitive to B₁ field perturbations arising from inductive coupling among the RF transmit coils and any B₁ field isolation (coil decoupling) technique requires an understanding of the effects of the B₁ field interactions. The purpose of this study was to investigate the effects of B₁ field coupling using Bloch equation based simulations and to determine the acceptable level of B₁ field interactions for 2D TRASE imaging. The simulations show that 2D TRASE MRI (using a 3-coil setup) displays ideal performance for pairwise coupling constant lower than k = 0.01 while having acceptable performance up to k = 0.1. This translates into S₁₂ measurements of range ~(- 50 dB to -30 dB) required for successful 2D TRASE MRI in this study. This result is of crucial importance for designers of practical TRASE transmit array systems.