Maximising the mutual interoperability of an MRI scanner and a cancer therapy particle accelerator

Abstract

The work described in this PhD thesis was undertaken as part of a much larger research project: The Australian MRI-Linac program. The goal of this program is to merge two existing medical technologies – an MRI scanner and a Linear Accelerator (Linac) – thereby creating an advanced form of cancer treatment incorporating cutting edge anatomical and physiological imaging techniques. An overview of the background information necessary to understand the work presented in this thesis is provided in chapters 1 (overview of radiotherapy) and 2 (overview of electromagnetism and accelerator physics). The work in the remainder of this thesis can be split into two distinct sections, corresponding to the two quite different (but ultimately related) projects I worked on throughout this thesis: modelling the impact of external magnetic fields on electron beam transport within the linear accelerator, and the implementation of patient rotation in radiotherapy. The former project is the focus of Chapters 3-6. In Chapter 3 a finite element model of a clinical gridded electron gun is developed based on 3D laser scanning and electrical measurements, and the sensitivity of this gun in magnetic fields characterised. The results complement the existing literature in showing that conventional linear accelerator components are very sensitive to external magnetic fields – in fact this gun is over twice as sensitive to axial magnetic fields than the less realistic models existing in the literature. A first order approach to overcoming this sensitivity is to use magnetic shielding – however magnetic shielding of the linear accelerator can negatively impact on the performance of the MRI scanner. This magnetic shielding problem is explored in Chapter 4, where the fundamental principles of passive magnetic shielding are explored, and magnetic shields are implemented for the two possible MRI-linac configurations (in-line and perpendicular) for the 1.0 Tesla MRI magnet used in the Australian MRI Linac program. The efficacy of the shielding and the impact on the MRI is quantified, with the conclusion that passive shielding could be successfully implemented to allow acceptable operation of the linac without overly degrading the magnet performance of the MRI scanner. An alternative approach to magnetic shielding which would not have any impact on the magnet is to redesign the linear accelerator such that it functions robustly in an MRI environment without the need for shielding. This approach is explored in chapter 5, where a novel electron accelerator concept based on an RF-electron gun configuration is detailed. It is shown via particle in cell simulations that such a design would be able to operate in a wide range of axial magnetic fields with minimal current loss. In chapter 6, an experimental beam line based on this concept was constructed at Stanford Linear Accelerator Center (SLAC). This project is ongoing but progress so far is described in Chapter 6. In the second part of this thesis, a completely different project is explored, patient rotation. Patient rotation would be very beneficial for MRI-Linac systems as it would eliminate the complicated engineering that is used in conventional systems to rotate the beam around the patient, and the MRI could be used to adapt in real time for the resultant anatomic deformation. Patient rotation would also minimise some of the sources of electromagnetic interference explored in chapters 3-7. The two major obstacles to patient rotation are (1) Page 11 patient tolerance to rotation, and (2) anatomical deformation due to rotation. To quantify patient rotation, a clinical study of 15 patients was carried out and is detailed in chapter 7. The results of this study suggest that patient tolerance to rotation may not be a major issue, although this result needs to be verified in larger patient cohorts. In chapter 8, the design and construction of an MRI-compatible patient rotation device is detailed. This device is the first of its kind, and will allow data on anatomic deformation under rotation to be collected, enabling strategies to adapt for this motion to be developed. Thus far, MRI compatibility has been assessed and a volunteer imaging study undertaken, in which pelvic images were acquired under rotation angles of 360⁰ every 45⁰. In summary: In chapters 3-5, the impact of magnetic fields on conventional accelerator components was quantified; and two independent approaches to compensating for these effects (magnetic shielding and bespoke accelerator design) were explored. In chapter 6, an experimental beam is constructed to verify and support the findings of chapter 6. In chapter 7, a clinical study was undertaken quantifying patient tolerance of slow, single arc rotation. Finally, in chapter 8 a unique medical device was designed, constructed and tested, and through this device MRI images of anatomical distortion under lying rotation were collected and quantified.