Radio frequency noise studies for a linac-MRI system.

摘要

A prototype system which has integrated a linear accelerator (linac) with a magnetic resonance imager (MRI) has been constructed at the Cross Cancer Institute. The real time operation of a linac-MRI system will require proper radio frequency (RF) shielding such that the MRI images can be acquired without extraneous RF noise from the linac. This thesis reports on the steps taken to successfully RF-shield the linac from the MRI such that the two devices can operate independently of one another. The RF noise from functioning multileaf collimators (MLC) is measured using near field probes and MRI images are acquired with the MLC near the MRI. This included measuring the RF noise as a function of applied magnetic field strength. Several measurement and simulation scenarios are discussed to determine the major sources of RF noise generation from the modulator of a linac. Finally RF power density levels are reported internally and externally to the RF cage which houses the linac and the MRI. The shielding effectiveness of the RF cage has been measured in the frequency range 1 -- 50 MHz and is presented. MRI images of two phantoms are presented during linac operation.;The MLC studies illustrate that the small RF noise produced by functioning MLC motors can be effectively shielded to avoid signal-to-noise degradation in the MRI image. A functioning MLC can be incorporated into a linac-MRI unit. The RF noise source investigations of the modulator of a linac illustrate that the major source of RF noise involves the operation of a magnetron. These studies also eliminate the pulse forming network (PFN) coil and the grid voltage spikes on the thyratron as possible major sources of RF noise. The main result is that for linac-MRI systems the modulator of a linac should be housed in a separate RF cage from the MRI. Finally imaging work with the linac operating illustrates that the accelerating structure of a linac and an MRI can be housed within the same RF cage. The 6 MV linac can be operated to produce radiation with no experientially measurable degradation in image quality due to RF effects.