As expected, placing coil materials in direct contact with the phantom surface increases the surface dose considerably. The surface dose is reduced by creating a gap between the coil materials and phantom surface. This dose reduction happens more rapidly in the presence of a transverse magnetic field. However, the surface dose stays relatively large irrespective of the gap in the presence of a parallel magnetic field. Thus, the standard, off-the-shelf RF coils should be used with caution in integrated linac-MR systems, especially those using a parallel magnetic field orientation in which case the RF coils will probably need to be reconfigured to create open ports for the radiation beam.
Purpose: Integrating a linac with a magnetic resonance imager (MRI) will revolutionize the accuracy of external beam radiation treatments. Irradiating in the presence of a strong magnetic field, however, will modify the dose distribution. These dose modifications have been investigated previously, mainly using Monte Carlo simulations. The purpose of this work is to experimentally verify the use of the EGSnrc Monte Carlo (MC) package for calculating percent depth doses (PDDs) in a homogeneous phantom, in the presence of a realistic parallel magnetic field. Methods: Two cylindrical electromagnets were used to produce a 0.207 T magnetic field parallel to the central axis of a 6 MV photon beam from a clinical linac. The magnetic field was measured at discrete points along orthogonal axes, and these measurements were used to validate a full 3D magnetic field map generated using COMSOL Multiphysics. Using a small parallel plate ion chamber, the depth dose was measured in a polystyrene phantom placed inside the electromagnet bore at two separate locations: phantom top surface coinciding with top of bore, and phantom top surface coinciding with center of bore. BEAMnrc MC was used to model the linac head which was benchmarked against the linac's commissioning measurements. The depth dose in polystyrene was simulated using DOSXYZnrc MC. For the magnetic field case, the DOSXYZnrc code was slightly modified to implement the previously calculated 3D magnetic field map to be used in the standard electromagnetic macros. Results: The calculated magnetic field matched the measurements within 2% of the maximum central field (0.207 T) with most points within the experimental uncertainty (1.5%). For the MC linac head model, over 93% of all simulated points passed the 2%, 2 mm c acceptance criterion, when comparing measured and simulated lateral beam and depth dose profiles. The parallel magnetic field caused a surface dose increase, compared to the no magnetic field case, due to the Lorentz force confining contaminant electrons within the beam. The surface dose increase was measured to be approximately 10% (relative to no field D max ) when the phantom surface coincided with the top of the electromagnet's bore. This effect was enhanced by moving the phantom surface to the center of the magnet's bore in relatively high magnetic field (> 0.13 T). The surface dose for this setup increased by 30% and the entire buildup region was affected. When the dimensions and composition of the ion chamber air cavity and entrance window were included, EGSnrc was able to accurately simulate these dose increases, both at the surface and in the buildup region. All the simulated points were within 1% of the measurements for both setups. The ferromagnetic linac head was determined to have a negligible effect on the final PDD comparison. Conclusions: Irradiating in the presence of a parallel magnetic field causes measurable surface and buildup depth dose increases. We have experimentally verified that the EGSnrc Monte Carlo package is able to accurately ...
The RIC in RF coils results from the lack of electronic equilibrium in the coil conductor 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 coil conductor 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.
Purpose: The RF coils for magnetic resonance image guided radiotherapy (MRIgRT) may be constructed using thin and/or low-density conductors, along with thinner enclosure materials. This work measures the surface dose increases for lightweight conductors and enclosure materials in a magnetic field parallel to a 6 MV photon beam. Methods: Aluminum and copper foils (9-127 μm thick), as well as samples of polyimide (17 μm) and polyester (127 μm) films are positioned atop a polystyrene phantom. A parallel plate ion chamber embedded into the top of the phantom measures the surface dose in 6 MV photon beam. Measurements (% of dose at the depth of maximum dose) are performed with and without a parallel magnetic field (0.22T at magnet center). Results: In the presence of a magnetic field, the unobstructed surface dose is higher (31.9%D max versus 22.2%D max ). The surface dose is found to increase linearly with thickness for thin (<25 μm) copper (0.339%D max μm −1 ) and aluminum (0.116%D max μm −1 ) foils. In the presence of a magnetic field the slope is lower (copper: 0.16%D max μm −1 , aluminum: 0.06%D max μm −1 ). The effect of in-beam foils is reduced due to partial shielding of the surface from contaminant electrons. Copper causes a surface dose increase ≈3 times higher than aluminum of the same thickness, consistent with their relative electron density. Polyester film (127μm) increases the surface dose (to 35% D max with field) about as much as a gown (36% D max with field), while the increase with polyimide film (17μm) is less than 1% above the open field dose. Conclusions: Thin copper and aluminum conductors increase surface dose by an amount comparable to a hospital gown. Similarly, enclosure materials made of thin polyester or polyimide film increase surface dose by only a few %D max in excess of an unobstructed beam. Based on measurements in this study, inbeam, surface RF coils are feasible for MRIgRT systems.
Purpose: The integration of a medical linac with magnetic resonance imaging (MRI) has the potential to provide exquisite soft tissue contrast and real time imaging during the radiation treatment. However, during a real time treatment‐imaging session, the direct irradiation of MRIˈs radiofrequency (RF) coil by the pulsed x‐ray beam induces a current in the conductor of the RF coil due to the release of Compton electrons. This radiation induced current (RIC) can potentially degrade the SNR in acquired MRI images. The present work investigates methods of minimizing this RIC both experimentally and through Monte Carlo simulations.Methods: Copper and aluminum metal plates emulating the conductors used in RF coils were connected to an amplifier and placed in an RF cage. The plates (i.e. “detector”) were irradiated by the linacˈs pulsed 6 MV beam through the RF cage. The induced signal was measured by an oscilloscope and recorded using a PC. Various materials were used as buildup in an attempt to establish electronic equilibrium in the “detectors” ‐ thus removing the undesired RIC. A Monte Carlo script was written which counts the amount of charge entering and leaving a specified “detector volume”, and determines the net change in charge per primary history as a measure of RIC. The simulation geometry mimics the experimental setup.Results: It has been clearly demonstrated by both measurements and simulations that buildup of the same material as the conductor will reestablish electronic equilibrium and remove the RIC. Also, using a polymer with a density close to that of the conductor (i.e. Teflon with aluminum) for buildup will reduce the RIC to negligible amplitude Conclusions: With the proper combination of coil conductor and buildup, the RIC can be reduced to negligible amplitudes. Future work will assess the importance of RIC for the SNR in MRI images.
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