Room: Karl Dean Ballroom B1
Purpose: Passive shielding is used in MRI-Linacs to isolate the Linac from the MRI scanner. Analytic shielding theory suggests that reducing the radial dimension of a passive shield will both improve shield effectiveness and reduce shield mass, decreasing interference with the MRI-scanner. To exploit these effects, the linac radial dimension must be reduced. Previous work focused on standard linac designs without considering whether they could be altered to improve shielding efficiency. Here, we describe two methods to reduce the radial dimension of a linear accelerator, and test how they improve shielding efficiency in both the in-line and perpendicular MRI-Linac configurations.
Methods: Three standing wave 6MeV Linac designs were considered: S-band side-coupled (standard), S-band on-axis-coupled and X-band on-axis-coupled. The latter two designs substantially reduce the radial dimension of the linac: the respective inner radial dimensions of each shield were 165mm, 101mm, and 44mm. Shield length was held constant at 400 mm. Each shield design was simulated using an FEM model (Comsol) of a 1T MRI magnet for an SID of 1.5m and shield thicknesses between 2 and 20mm. The heterogeneity induced in the MRI imaging volume, average shielding factor (ratio of field without/with shield in sensitive volume), and shielding efficiency (ratio of shielding factor to heterogeneity) was quantified.
Results: In the perpendicular configuration, average improvements in shielding efficiency by factors of 6.0 (S-band on-axis) and 53.2 (X-band on-axis) were seen. For the in-line configuration average improvements were 1.7 (S-band on-axis) and 1.8 (X-band on-axis) and the maximum improvement in shielding efficiency was 2.8 (X-band on-axis).
Conclusion: Two independent mechanisms to reduce a linac's radial dimension have been demonstrated. These strategies simultaneously reduce the distortion induced by a shield and increase shielding efficiency, with particularly stark improvement for the perpendicular configuration. This will help enable smaller footprint 2nd generation MRI-Linac systems.
Funding Support, Disclosures, and Conflict of Interest: Paul Keall is a stakeholder in SeeTreat, a start-up company to commercialize intellectual property generated through NHMRC program grant APP1036075