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SU‐FF‐J‐84: Shielding of Radio Frequency Noise From Clinical Linear Accelerators for the Linac‐MR Project
Author(s) -
Lamey M,
Burke B,
Rathee S,
Murray B,
Fallone B
Publication year - 2009
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3181376
Subject(s) - electromagnetic shielding , linear particle accelerator , radio frequency , noise (video) , attenuation , electromagnetic coil , rf power amplifier , physics , shield , radiofrequency coil , optics , nuclear magnetic resonance , materials science , electrical engineering , beam (structure) , optoelectronics , computer science , engineering , petrology , amplifier , cmos , quantum mechanics , geology , artificial intelligence , image (mathematics)
Purpose: To determine the efficacy of shielding RF noise generated by a linac during irradiation. Method and Materials: The electric (E) and magnetic (H) field strengths generated by a medical linac during irradiation were measured using near field E and H probes connected with a 2 GHz digital oscilloscope and IEEE 488 data acquisition system. These measurements were performed both with and without our RF shield, which houses our 0.2 T MRI. These field measurements were then used to determine the power spectral density of RF noise as a function of frequency. Results: The data measured without the RF shield shows several peaks in the power spectrum approaching levels of 1mW/m 2 in the frequency range 1–20 MHz. Above 5 MHz the measured data with the RF shield shows significantly reduced power levels. For example at 35 MHz the attenuation is 38.5 dB. The RF noise generated by a linac during the MR imaging k‐space acquisition must be less than μWs. In a 50 kHz bin, our data indicates power densities of ∼250, 69, 7.2 and 2.6 nW/m 2 at 8.5, 21.25, 42.5 and 63.75 MHz, corresponding to 0.2, 0.5 1.0 and 1.5 T MR resonant frequencies, respectively. For a typical surface coil used for human imaging, the surface area might be 20×20 cm 2 . Thus the power available to a typical surface coil, according to our measurements, would be a maximum of 10, 2.76, 0.29 and 0.1 nW respectively. This does not include the coil efficiency which would further reduce the actual RF noise reaching the acquisition system. Conclusion: The noise power levels measured using our RF shielding is much lower than those detected in typical human MR imaging.