Twisted solenoid B1 field and isolation requirements for two-coil radial TRASE

Transmit Array Spatial Encoding (TRASE) is an MRI technique that utilizes multiple radiofrequency (RF) phase-gradient fields (B₁) for k-space encoding. Here, we demonstrate that the recently developed Radial TRASE variant mitigates the primary limitation of earlier versions, which required very high isolation between RF coil elements. Whereas earlier 2D Cartesian TRASE used three or four RF coils, 2D Radial TRASE requires only two coils, together with mechanical rotation. The reduced coil count and 1D nature of Radial TRASE encoding contribute to a simpler implementation, easier reconstruction, and reduced artifacts. Motivated by encouraging early experimental results, this study systematically evaluated Radial TRASE under imperfect B₁ field conditions to establish practical design criteria for the RF coils and their isolation. Bloch-equation-based simulations were performed using ideal twisted solenoid (TS) phase-gradient fields. The TS geometry was varied to quantify the trade-off between increasing the phase-gradient strength (and thus resolution) and decreasing B₁ uniformity. Coil coupling was modelled by varying the current amplitude within the idle coil relative to the active coil. Reconstructed images were compared to a reference using the structural similarity index (SSIM). Simulations indicate a modulation-to-radius ratio (A/a) of approximately 0.7 is optimal for TS coil design with a refocusing flip-angle error within ±15°. Under these conditions, isolation levels between -20 and -16.5 dB are shown to produce minimal Radial TRASE distortions, whereas prior studies indicate -30 dB is required for three-coil Cartesian TRASE. This represents a far more achievable isolation in practice, reinforcing the practicality of Radial TRASE imaging.