Radio Frequency Noise From the Modulator of a Linac

A novel approach to image-guided radiotherapy being undertaken by a few groups involves the integration of a linac with a magnetic resonance imager (MRI). In order to successfully combine a linac with a MRI, it is important to understand the characteristics and major sources of radio frequency (RF) noise from the pulse power modulator of a linac since these may interfere with the operation of the MRI. The RF noise power spectral density from the modulator of a linac, loaded separately with a magnetron and a resistive load, is measured. The RF fields emitted by the pulse-forming network (PFN) were determined by simulation and compared with measurements. Saturable reactors were introduced in the trigger circuits of the thyratron to reduce the injected voltage spikes into the trigger circuit to assess the impact of these spikes on the RF noise measurements. The results illustrate that the major source of RF noise from the modulator of a linac is the operation of a magnetron. It also eliminates the PFN coil and the grid voltage spikes of thyratron as possible major sources of RF noise. For linac-MRI systems the modulator of a linac should be housed in a separate RF cage from the MRI.