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

TL;DR

This work evaluates alternative winding patterns for twisted solenoid RF phase-gradient coils in TRASE MRI to mitigate issues introduced by the return wire. Using Biot-Savart simulations, the authors compare the simple twisted solenoid (STS), return-wire twisted solenoid (RWTS), double-wound twisted solenoid (DWTS), and discrete-loop twisted solenoid (DLTS), focusing on $|B_1|$, phase, and phase gradient. The main finding is that DWTS and DLTS outperform RWTS, with DLTS offering similar central field and phase-gradient strength as the idealized STS while requiring substantially less wire than DWTS; truncated variants show DLTS remains advantageous. The results support switching to DLTS (or DWTS when appropriate) for practical TRASE MRI coil construction, including compatibility with 3D-printed formers and truncated designs, and provide a simpler strategy for relocating the RW return path when RWTS is retained. Key methodological elements include parametric winding models based on a stream-function formulation and Biot-Savart field calculations to quantify $|B_1|$, $\ ext{arg}(B_1)$, and $\nabla\phi_B$ across the imaging volume.

Abstract

Transmit Array Spatial Encoding (TRASE) is an MRI technique in which spatial encoding is achieved using phase gradients of the B1 field. This approach offers potential advantages such as hardware simplicity and reduced acoustic noise. In this study, we present an assessment of various winding patterns for twisted solenoid phase-gradient coils, including the simple twisted solenoid (with and without return wire), a double-wound twisted solenoid, and a discrete-loop twisted solenoid. We analyze the magnetic field uniformity and phase linearity of these configurations using Biot-Savart simulations. Our results show that both the double-wound and discrete loop designs offer similar improvements over the simple twisted solenoid with return wire. The discrete loop pattern requires less wire than the double-wound version, making it the preferred option for practical coil construction and operation in a TRASE MRI system.

Figures (12)

• Figure 1: Conceptual examples of wire-wound approximations of the uniform (top row) and twisted (bottom row) solenoid. Left-side: single-wire pitch models via Eq. \ref{['parametric_curve1']}. Right-side: discrete-loop models via Eq. \ref{['parametric_curve2b']}. For all drawings, the red lines are the wires, while the dashed lines indicate stream function contours (of Eq. \ref{['stream_function']}) containing equal integrated surface current. These contours are the same (left and right) for each of the two coil types (top with $A_n=0$ and bottom with $n=2$).
• Figure 2: Examples of the four archetypal twisted solenoid winding patterns consider in this work: (a) the simple twisted solenoid (STS), (b) the return-wire twisted solenoid (RWTS), (c) the double-wound twisted solenoid (DWTS), and (d) the discrete-loop twisted solenoid (DLTS). The red lines in the RWTS, DWTS and DLTS indicate return wires/windings. The following parameters were used for all coils here: $a = 78$ mm, $A = 40$ mm, $\varphi= 90^\circ$, $h = 50$ mm, and $N = 3$.
• Figure 3: Results for the simple twisted solenoid (STS). The left plot shows the STS winding pattern. The top row of contour plots on the right displays $B_1$ magnitude maps at three axial planes (left: $z = -5$ cm, middle: $z = 0$ cm and right: $z = 5$ cm.), while the bottom row shows the corresponding $B_1$ phase maps. The dashed line indicates the coil winding surface.
• Figure 4: Same as Figure \ref{['STS']}, but for the RWTS coil.
• Figure 5: Same as Figure \ref{['STS']}, but for the DWTS coil.