In integrated linac-MRI systems, a measurable radiation induced current (RIC) is caused in RF coils by pulsed irradiation. This work (1) tests a buildup method of RIC removal in planar conductors; (2) validates a Monte Carlo method of RIC calculation in metal conductors; and (3) uses the Monte Carlo method to examine the effects of magnetic fields on both planar conductor and practical cylindrical coil geometries.Methods:
The RIC was measured in copper and aluminum plates, taken as the RF coilconductor surrogates, as a function of increasing thickness of buildup materials (teflon and copper). Based on the Penelope Monte Carlo code, a method of RIC calculation was implemented and validated against measurements. This method was then used to calculate the RIC in cylindrical coil geometries with various air gaps between the coilconductor and the enclosed water phantom. Magnetic fields, both parallel and perpendicular to the radiation beam direction, were then included in the simulation program. The effect of magnetic fields on the effectiveness of RIC removal with the application of buildup material was examined in both the planar and the cylindrical geometries.Results:
Buildup reduced RIC in metal plate conductors. For copper detector/copper buildup case, the RIC amplitude was reduced to zero value with 0.15 cm copper buildup. However, when the copper is replaced with teflon as buildup atop the copperconductor, the RIC was only reduced to 80% of its value at zero buildup since the true electronic equilibrium cannot be obtained in this case. For the aluminum detector/teflon buildup case, the initial amplitude of the RIC was reduced by 90% and 92% in planar aluminumconductor and a surface coil, respectively. In case of cylindrical coils made of aluminum, teflon buildup around the coil's outer surface was generally effective but failed to remove RIC when there was an air gap between the coil and the phantom. Stronger magnetic fields (>0.5 T) perpendicular to the beam direction showed a modest decrease in the RIC for planar conductors with buildup. In the cylindrical geometries, the effect of magnetic fields was very small compared to the effect of introducing air gaps. Loss in signal-to-noise ratio(SNR) due to RIC was reduced from 11% to 5% when a simple buildup was applied to the solenoid in a preliminary experiment.Conclusions:
The RIC in RF coils results from the lack of electronic equilibrium in the coilconductor as the RIC in planar conductor was completely removed by identical buildup of adequate thickness to create electronic equilibrium. The buildup method of RIC removal is effective in cylindrical coil geometry when the coilconductor is in direct contact with the patient. The presence of air makes this method of RIC removal less effective although placing buildup still reduces the RIC by up to 60%. The RIC Monte Carlo simulation is a useful tool for practical coil design where radiation effects must be considered. The SNR is improved in the images obtained concurrently withradiation if buildup is applied to the coil.
II.A. Effect of RIC on signal-to-noise ratio in MRI
II.B. Radiation effect or radiofrequency interference
III. METHODS AND MATERIALS
III.A. Confirmation of buildup method for RIC removal
III.A.1. Detector-buildup combinations
III.A.2. Aluminum surface coil with teflon buildup
III.B. Monte Carlo RIC simulation algorithm: Description and validation
III.B.1. Description of the algorithm
III.B.2. Validation of RIC Monte Carlo algorithm
III.C. Cylindrical coil geometry simulations
III.C.1. Effect of air gaps between patient and RF coil on RIC removal
III.D. Monte Carlo simulation of magnetic field effects RIC
III.E. Proof of concept: Imaging with buildup applied
IV.A. Confirmation of buildup method for RIC removal
IV.A.1. Detector-buildup combinations
IV.A.2. Aluminum surface coil with teflon buildup
IV.B. Monte Carlo simulation algorithm validation
IV.C. Cylindrical coil geometry simulations: Effect of air gaps
IV.D. Monte Carlo simulation of magnetic field effects on RIC
IV.D.1. Planar geometry
IV.D.2. Cylindrical coil geometry
IV.E. Proof of concept: Imaging with buildup applied
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