A geometrically decoupled, twisted solenoid single-axis gradient coil set for TRASE

Abstract

Purpose: TRASE uses phase gradients in the RF transmit field to encode MRI data. A highly efficient twisted solenoid coil was proposed recently for TRASE imaging for transverse $B_0$ geometries. This novel coil can be rotated to generate a phase gradient in any transverse direction, therefore, combining two such coils would double k-space coverage for single-axis encoding, resulting in higher spatial resolution. However, the strong inductive coupling between a pair of coaxial twisted solenoids must be overcome.

Methods: Here, we demonstrate that two concentric twisted solenoids, designed using previously described Biot-Savart calculations, can be geometrically decoupled by attaching to each a regular solenoid in series. The regular solenoid geometry resulting in minimization of mutual inductance was determined from simulations using the FastHenry2 tool. The effects on TRASE encoding performance due to the regular solenoids were assessed from simulations and experiments.

Results: The maximum resulting $B_1$ magnitude and phase distortions were 3.7% and $4.6^{\circ }$ , while a good isolation $S_{12}=-17.5$ dB between the coil pair was obtained. TRASE experiments confirmed the double k-space coverage, and achieved a rapid spin echo train with 128 k-space points collected within 80 ms, allowing short $T_2$ samples to be accurately imaged.

Conclusions: This study demonstrates that a pair of twisted solenoid phase gradient RF coils can be geometrically decoupled. Advantages over active PIN diode decoupling include faster switching, lower hardware complexity, and scalability.

Keywords: RF coil; TRASE MRI; geometric decoupling; twisted solenoid.