Impact of a parallel magnetic field on radiation dose beneath thin copper and aluminum foils

Abstract

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.

How thin can you go? Performance of thin copper and aluminum RF coil conductors

“…All coils consisted of a square 15 × 15 cm 2 loop of 6 mm wide traces with 12.5 cm long leads that were connected to a matching network (Figure 1). The long leads were used to create a coil that could be struck by a radiation treatment beam, 14 whereas matching network components remained away from direct radiation. Total length of the trace was approximately λ/18 at 20.56 MHz, which satisfies the maximum length criterion 18 without requiring additional capacitive breaks.…”

“…8 Liquid metal 9,10 and braided conductors 11 also allow for stretchable coils that closely fit moving and flexing body parts. 9,11 Further alternate conductors include aluminum (conductivity 3.54 × 10 8 Sm −1 ), which has applications in multimodality systems such as X-ray MRI, 12 PET-MRI, 13 and linac-MR systems, 14,15 where interactions with ionizing radiation must be minimized. In X-ray MRI, 12 aluminum coils significantly reduce artifacts in the X-ray image compared to the copper coils.…”

“…In PET-MRI, 13 attenuation of the photons through aluminum is reduced compared to copper, but the difference is inconsequential because the required correction is equally effective for copper or aluminum conductors. In linac-MR systems, 14,15 the use of aluminum conductor of the same thickness as copper leads to interactions with the beam that are up to 3 times smaller.…”

“…Thinner conductors can also allow for reduced interactions with radiation, 14 as well as allowing form‐fitting coils made of flexible PCB (which use thinner copper than the up to 70 μm typically used in rigid PCBs). Quantifying the increased resistance expected due to using a thinner conductor is, however, not simple analytically.…”