Alternative winding patterns for twisted solenoid coils with improved characteristics for TRASE MRI

Abstract

Magnetic resonance imaging (MRI) is a powerful noninvasive imaging modality; however, conventional high-field MRI systems rely on strong static magnetic fields and gradient coils, resulting in high system complexity, power consumption, acoustic noise, and limited accessibility. The development of low-field MRI systems has the potential to alleviate several of these challenges and motivate alternative spatial encoding techniques. One such approach is transmit array spatial encoding (TRASE). This gradient-free MRI method encodes spatial information using phase gradients in the radiofrequency (RF) magnetic field rather than static magnetic field gradients. The twisted solenoid is the most commonly used RF coil geometry for TRASE MRI studies employing vertical B0 magnets. While return wires must by necessity be included in experimental implementations of such coils, their effects are often neglected in theoretical modeling and simulation studies. This discrepancy motivates a systematic investigation of coil winding configurations and their influence on RF field characteristics relevant to TRASE imaging. In this thesis, four winding patterns for twisted solenoid–based RF coils are investigated, including conventional configurations with and without an explicitly modeled return wire, as well as two alternative designs proposed in this work. The proposed designs are derived using stream function and surface current density concepts and include a discretized loop-based model and a double-wound wire configuration composed of two simple twisted solenoids. These winding patterns are compared in terms of magnetic field uniformity, RF phase behavior, and phase gradient linearity in the context of low-field MRI, including the influence of the concomitant B1 field component. The results demonstrate that our proposed winding patterns significantly improve B1 field uniformity and phase gradient linearity compared to conventional twisted solenoid designs. These configurations eliminate the symmetry-breaking effects introduced by return wires and provide a practical and effective path forward for the development of RF phase-gradient coils for low-field TRASE MRI. Furthermore, truncated versions of all winding patterns are examined to assess their robustness to coil-length reduction. Numerical simulations based on the Biot–Savart law are used to quantify the impact of coil geometry on TRASE MRI performance. The results provide insight into the role of winding configuration in determining RF field quality and support the identification of optimized coil designs for low-field TRASE MRI.