Radiation induced currents in MRI RF coils: application to linac/MRI integration

Abstract

The integration of medical linear accelerators (linac) with magnetic resonance imaging (MRI) systems is advancing the current state of image-guided radiotherapy. The MRI in these integrated units will provide real-time, accurate tumor locations for radiotherapy treatment, thus decreasing geometric margins around tumors and reducing normal tissue damage. In the real-time operation of these integrated systems, the radiofrequency (RF) coils of MRI will be irradiated with radiation pulses from the linac. The effect of pulsed radiation on MRI radio frequency (RF) coils is not known and must be studied. The instantaneous radiation induced current (RIC) in two different MRI RF coils were measured and presented. The frequency spectra of the induced currents were calculated. Some basic characterization of the RIC was also done: isolation of the RF coil component responsible for RIC, dependence of RIC on dose rate, and effect of wax buildup placed on coil on RIC. Both the time and frequency characteristics of the RIC were seen to vary with the MRI RF coil used. The copper windings of the RF coils were isolated as the main source of RIC. A linear dependence on dose rate was seen. The RIC was decreased with wax buildup, suggesting an electronic disequilibrium as the cause of RIC. This study shows a measurable RIC present in MRI RF coils. This unwanted current could be possibly detrimental to the signal to noise ratio in MRI and produce image artifacts.

Effect of radiation induced current on the quality of MR images in an integrated linac‐MR system

“…This has been shown to result in instantaneous currents being induced in the MR coils-called radiation induced current (RIC). 4 RIC has been widely reported on in various materials when exposed to various sources of radiation. [5][6][7][8][9] These extraneous currents have the potential to adversely affect MR imaging by distorting the RF signal being measured by the RF coils.…”

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

“…In the work presented in Ref. 8, we used a well designed, commercial RF cage (Model FC-10, LBA Technologies, Greenville, NC), with up to 70dB RF isolation, that housed the coil being irradiated. The observed signal in RF coil inside the RF cage was a well-formed pulse whose amplitude followed the well-known inverse square law as the coil was systematically moved away from the radiation source.…”

“…The pulsed radiation has been shown to create a measurable radiation induced current (RIC) in the RF coil. 8 The RIC has also been observed and found to be the main cause of the polarity effect in parallel plate ion chambers. 9,10 The RIC in RF coils was hypothesized to arise from the ejection of Compton electrons from the RF coil's metal windings, leading to an electronic disequilibrium in the windings.…”

“…9,10 The RIC in RF coils was hypothesized to arise from the ejection of Compton electrons from the RF coil's metal windings, leading to an electronic disequilibrium in the windings. 8 Based on this hypothesis, a method was devised that uses buildup material, placed on the metal conductor, in order to restore electronic equilibrium in the conductors thereby reducing the RIC. The following considerations are important: the buildup and the RF coil conductor materials should have a similar density, and a sufficient thickness of buildup must be placed on top of the conductor.…”

Quantitative analysis of image quality for acceptance and commissioning of an MRI simulator with a semiautomatic method

“…The procedure of MRI simulation is complicated by comparison of CT simulation, because except the scanner, the RF coils also should be applied to receive the MR signal 14. The parameters setting for each pulse sequence in MRI scanning are also complicated and flexible 15.…”