SU-FF-J-84: Shielding of Radio Frequency Noise From Clinical Linear Accelerators for the Linac-MR Project

Purpose: To determine the efficacy of shielding RFnoise generated by a linac during irradiation. Method and Materials: The electric (E) and magnetic (H) field strengths generated by a medicallinac 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 RFnoise as a function of frequency. Results: The data measured without the RF shield shows several peaks in the power spectrum approaching levels of 1mW/m2 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 RFnoise 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/m2 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 cm2. 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 RFnoise 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.