Receive Radio-Frequency Coils for a Parallel B0 Linac-MR
Creators: Barta, Radim
Publisher: name: University of Alberta Library
Year: 2023

<p>The rotating B0 linac-MR is a new type of radiotherapy treatment machine that combines radiation delivery from a linear accelerator (linac) with a magnetic resonance imaging (MRI) system for real-time tumour tracking. This kind of combined technology aims to achieve tighter margins in cancer treatments with better tumour control and reduced damage to healthy tissue.<br>
A combination system like the linac-MR still requires receive radio-frequency (RF) coils to capture MRI images before and during the treatment. All of the standard requirements, like high efficiency coils for high SNR images, apply for RF-coils for the rotating B0 linac-MR. In addition, two additional design hurdles need to be considered: one, if RF-coils are in the path of the radiation beam, they will interact with the radiation to increase the radiation dose to the skin, and two, specific to the Alberta linac-MR design, the shared gantry of the main magnet and linac means the orientation of B0 will change as the treatment beam angle changes, which also changes the orientation of the MRI’s signal detection plane. RF-coils must be sensitive to this plane through 360° of linac rotation.<br>
In this work the impact on surface dose due to copper and aluminum conductors of various thicknesses is investigated. The surface dose increase due to aluminum is three times lower than with copper of the same thickness. The increase in surface dose is determined to be linear with thickness below 25 micrometers. Beyond this, the surface dose increases more slowly and non-linearly with increasing thickness of conductor.<br>
A further investigation looked at the resulting image quality penalty in terms of signal-to-noise ratio (SNR) when using thin aluminum and copper conductors. This work investigates the practical limits for using thin conductors in RF-coils for the linac-MR. This work includes an experimental verification of the relationship between bench measured coil efficiency and the achieved image SNR, which is relevant in a variety of novel coil applications.<br>
The second challenge, the rotating B0, is investigated through a proposed solution which combines three orthogonal RF-coils into a three channel array. A single turn coil is combined with two figure-eight or butterfly coils. The butterfly coil consists of two D-shaped loops connected in parallel across a capacitor. The three coils are stacked in a pancake arrangement, with the two butterfly coils rotated 90° with respect to one another. The stack sits in the transverse plane adjacent to the patient and the butterfly coils are thus sensitive to the transverse plane. The vertical butterfly coil is primarily sensitive to magnetization along the patient anterior-posterior (AP) axis, while the horizontal butterfly coil is sensitive to magnetization along the patient left-right (LR) axis. The single turn coil is primarily sensitive to magnetization along the patient superior-inferior (SI) axis.<br>
This array approximately maintains uniform SNR across all gantry angles (θ) of the rotating B0 linac-MR, where the magnetic field B0 rotates in the patient’s transverse plane. Consequently, the SI axis is perpendicular to the transverse plane at all gantry angles and the magnitude of the component of transverse magnetization along the SI axis is also the same at all gantry angles. The signal received by the single turn coil is therefore constant across gantry angles. Each individual butterfly coil, however, will perform best when B0 is perpendicular to its sensitive axis and worst when B0 is parallel. The performance varies as the cross-product of B0 and the sensitive axis: cos(θ) for the horizontal butterfly coil and sin(θ) for the vertical butterfly coil. Due to the identical structure, but 90° rotation of the two butterfly coils, the root sum of squares of the two butterfly coils is constant with θ. The sum of the two butterfly coils gives the component of magnetization in the transverse plane, thus combining it with the single turn coil, the array achieves quadrature-like reception with constant SNR at all gantry angles.<br>
This work thus describes several essential elements of surface coils and surface arrays for the linac-MR. It describes how interactions between the radiation and the coils can be reduced without sacrificing image quality and how a combination of coils with orthogonal sensitive axes leads to uniform SNR across gantry angles in a rotating<br>
B0 linac-MR.</p>