Radiation induced current in the RF coils of integrated linac-MR systems: The effect of buildup and magnetic field
Sample MR Images acquired on linac-MR system. Images acquired with (a) no radiation beam and (b) with linac producing radiation at rate of 250 MU/min. There is an 11% reduction in SNR in the image obtained with radiation.
K-space data for images shown in Fig. 1. The top image data were acquired with no radiation and the bottom image data were acquired with the linac producing radiation at 250 MU/min. The window and level in the image has been adjusted to make the RIC induced streaks clearly visible. The missing streaks in the regular pattern are due to the dropping of pulses by the linac.
Schematic representation of a metal plate detector inside the Faraday cage (dotted line). The linac's pulsed radiation beam is focused on the detector. The RIC is then amplified and detected by a digital oscilloscope being triggered by the linac's magnetron pulses.
Schematics of the (a) copper detector/copper buildup measurement setup, (b) aluminum detector/teflon buildup measurement setup, and (c) aluminum surface coil setup for RIC measurements. The styrofoam represents an air gap between the surface coil and the solid water serving as a patient surrogate. The irradiated portion of the detectors in (a) and (b) is 7.5 × 7.5 cm2. The detectors are electrically insulated from both the backscatter and buildup materials. The copper buildup and backscatter materials in (a) are grounded. The teflon buildup sheets in (b) and (c) have grounded aluminum tape applied to the top and bottom of the stack.
Generic algorithm for PENELOPE RIC script: Mat is a variable used by Penelope to track the material in which the simulated particle currently resides. Det_Mat represents the material in the input file, defined as the detector; it is not used for any other geometric region. Old_mat is a variable introduced to store the previous material along the particle transport track. It is used for the comparisons shown in the figure. Q in is a counter of electrons originating outside the detector volume and entering in the detector volume. Q out is a counter of electrons originating inside the detector volume and then being ejected from it. The last step in the algorithm is to subtract Q in from Q out to determine the net loss of charge in the detector.
Phantom geometries used for coil simulations: (a) No air gap between water phantom and coil, (b) a uniform air gap between the water phantom and the RF coil (1 cm gap shown), and (c) an air gap which begins at 1 cm at the top of the phantom and decreases to zero at the bottom of the phantom to simulate patient sag. The aluminum coil windings are visible in (b) and the solid outer surface is the teflon buildup.
Measurement results for the reduction of RIC with buildup in the various detector/buildup combinations.
Reduction of RIC in an aluminum surface coil through the application of teflon buildup. The initial RIC amplitude is reduced by 92% with 0.9 cm of teflon.
Validation of Monte Carlo RIC algorithm through comparison of measurements and simulations. The measured data were acquired with the buildup and backscatter biased at ±10 V. Both the measured and simulated data are normalized at zero buildup thickness.
Monte Carlo simulation of RIC reduction through the use of teflon buildup in a cylindrical RF coil. Four different scenarios were examined: (1) no air gap between the coil and the phantom, (2) air gap decreases from top to bottom of the phantom, (3) a uniform 1 cm air gap surrounding the phantom, and (4) a uniform 2 cm air gap surrounding the phantom.
The RIC simulation results for the planar geometry in the presence of parallel and perpendicular orientation of uniform magnetic fields. In each orientation of magnetic field, three values of uniform magnetic field are simulated to examine the effect of magnetic field strength on the reduction of RIC in an aluminum detector by teflon buildup.
The results of RIC simulation in the presence of parallel and perpendicular magnetic fields as a function of thickness of teflon buildup. This figure shows the results of all three coil setups from Fig. 6; the (a), (b), and (c) labels match those from Fig. 6.
K-space data for images acquired with the linac producing radiation at 250 MU/min and incident upon the coil with no buildup (top image) and with teflon buildup in place (bottom image). The same window and level was used to display each image.
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