Effect of multiple scattering on heating induced by radio frequency energy

Heating induced by radio frequency (RF) energy may be generated in tissues near active or passive devices implanted in a patient undergoing magnetic resonance imaging (MRI) examination. Recent normative documents for the assessment of RF energy-induced heating state that the net temperature increase (ΔT) depends on the physical characteristics of the surrounding tissues and on the squared incident tangential electric field (Etan(l)). The documents do not address the possible effect of multiple scattering from surrounding reflective interfaces, such as, for example, a plastic enclosure of the container (“phantom”) used for implant testing. In our study, we investigated the effect of multiple scattering on ΔT/Etan(l)2 in relation to the implant location when tested in a phantom filled with a gel. The test implant was a titanium alloy rod of 100 mm in length, used as a reference implant in standard documents. The applied RF was in the 128 MHz band, corresponding to the RF generated in a 3 T MRI system. Both the numerical model and measurement provided substantial evidence that ΔT/Etan(l)2 depends on the distance between the rod and phantom box enclosure. In the numerical model, up to 38% variation of this quantity was observed, suggesting that location of the implant with respect to the phantom enclosure would need to be included as a parameter when evaluating RF-induced heating of implants