Radial TRASE: 2D RF encoding through mechanical rotation and active digital decoupling

Abstract

Purpose: Two-dimensional (2D) transmit array spatial encoding (TRASE) previously required four radiofrequency fields; however, interactions between transmit (Tx) array elements caused significant challenges for 2D imaging. Here, we present a low-cost, 2D radial encoding scheme (Radial TRASE) using a simplified two-coil array.

Theory and methods: The system consists of two B₁ phase gradient coils capable of encoding any one transverse direction. By incremental mechanical rotation over a 90° range, the encoding axis can be changed, allowing a complete radial k-space acquisition. As a first demonstration, a wrist-sized coil pair was experimentally verified on a 2-MHz Halbach magnet, incorporating a static B₀ slice-selection gradient. Although a high level of isolation is achievable geometrically, for a more robust implementation, we demonstrate the capability of active digital decoupling in eliminating residual coupling through a parallel-transmit system.

Results: Radial TRASE-encoded images of water phantoms were acquired, achieving a resolution better than 1.67 mm. Rotation of the Tx array was performed during the recovery period, which caused no imaging delays. All acquired images show minimal distortions, indicating the advantage of the simplified Tx array. The active digital decoupling technique is demonstrated to eliminate residual coupled currents, effectively increasing the isolation of the two-coil array to -50 dB. Sequential axial slice images were demonstrated using a uniform B₀ coil to shift the slice position.

Conclusion: Two-coil Radial TRASE can encode a 2D slice without rapidly switched B₀ gradients. Compared with previous three-coil or four-coil Cartesian TRASE, the design and isolation of the Tx array are significantly simplified.

Keywords: RF coil rotation; TRASE MRI; active digital decoupling; radial encoding; radiofrequency (RF) transmit array; twisted solenoid.