The effects of coupled B fields in B 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 (B) magnetic field. Since TRASE relies on the use of multiple RF fields (B 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 (B fields). However, interactions among the transmit RF coils can cause un-driven coils to produce unwanted B fields that impair the spatial encoding. Therefore, TRASE is sensitive to B field perturbations arising from inductive coupling among the RF transmit coils and any B field isolation (coil decoupling) technique requires an understanding of the effects of the B field interactions. The purpose of this study was to investigate the effects of B field coupling using Bloch equation based simulations and to determine the acceptable level of B 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 S 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.