Radio frequency noise from clinical linear accelerators

Abstract

There is a great deal of interest in image-guided radiotherapy (IGRT), and to advance the state of IGRT, an integrated linear accelerator-magnetic resonance (linac-MR) system has been proposed. Knowledge of the radiofrequency (RF) emissions near a linac is important for the design of appropriate RF shielding to facilitate the successful integration of these two devices. The frequency spectra of both electric and magnetic fields of RF emission are measured using commercially available measurement probes near the treatment couch in three clinical linac vaults with distinct physical layouts. The magnitude spectrum of the RF power emitted from these three linacs is then estimated. The electric field spectrum was also measured at several distances from the linac modulator in order to assess the effects of variations in spatial location in the treatment vault. A large fraction of RF power is emitted at frequencies below 5 MHz. However, the measured RF power at the Larmor frequency (8.5 MHz) of the proposed 0.2 T MR in the linac-MR (0.4-14.6 μW m −2 ) is still large enough to cause artifacts in MR images. Magnetron-based linacs generally emit much larger RF power than klystron-based linacs. In the frequency range of 1-50 MHz, only slight variation in the measured electric field is observed as a function of measurement position. This study suggests that the RF emissions are strong enough to cause image artifacts in MRI systems.

Impact of the MLC on the MRI field distortion of a prototype MRI‐linac

“…15 In the latter case, various studies exist which looked at the ability to take MRI images of phantoms while during linac irradiation 7,16 and investigated the effect of the radiation and RF noise from the linac on the gradient RF coils. 17,18 Recently, a proof-ofconcept study for tracking of a 1D pencil-beam navigator 19 and a study on tracking of phantom motion on 2D MRI images 20 have demonstrated that image acquisition is possible with MRI-linac prototypes incorporating a MLC.…”

Radiation induced current in the RF coils of integrated linac‐MR systems: The effect of buildup and magnetic field

“…The whole setup was placed inside a Faraday RF Cage (Model FC-10, LBA Technologies, Greenville, NC) to remove any RF noise emanating from the linac's magnetron. 12,13 The detector was connected to a high speed low current amplifier (59-179 Edmund Optics) and all the measurements were taken with a nominal amplifier gain setting of 10 5 VA −1 . The radiation beam was focused on the detector through the RF cage, and the amplifier was not irradiated.…”

“…9 in Ref. 8), which would not be the case if this signal was RF noise, as Burke et al 12 showed that the RF noise is essentially constant everywhere within the linac vault (see Fig. 5 in Ref.…”

“…5 in Ref. 12). For the preliminary k-space data presented here, a lead block was placed in the radiation beam path to completely attenuate the radiation from reaching the RF coil.…”

“…2 by Lamey et al 13 shows there is very little RF noise present inside the RF cage. Also, our previous work 12 showed that the majority of the RF noise in linear accelerators emanates from the pulsed power modulator system. In the linac-MR proto-type, the modulator is located externally to the RF cage in an adjacent room.…”