Numerical and Experimental Assessment of MRI RF Coil Induced Heating for External Fixation Devices and Technique to Reduce the Thermal Effects

Tissue heating under MRI environment is one of the primary concerns in MRI related safety, especially with the presence of medical devices. This dissertation focuses on the MRI RF coil induced heating effects of external fixation devices and the technique to reduce such thermal impact. Different categories of external fixation devices, including modular external fixation devices and circular external fixation devices are investigated both numerically and experimentally. Published literature has studied the RF induced heating effects on modular external fixation devices. The shortest insertion depth, largest pin spacing and conductive bar will result in worst-case heating. Using absorption material is proposed as an effective way to reduce the RF induced heating effects. With the proper choice of absorption material, the maximum 1g-averaged SAR can be significantly reduced by more than 70% through simulation study. Experimental results also show the effectiveness of the technique. When we assess the MRI RF coil induced heating for circular external fixation devices, the conventional testing approach does not apply. A novel leg phantom was designed. Good agreement can be seen from a comparison of the electric field or SAR distribution between the human model and leg phantom. Numerical study suggests that all screw configurations with fewer screws, large ring frame size and relatively large strut length and the maximum insertion angle pointing outwards may lead to the worst case MRI RF coil induced heating. The usage of absorption material has the same impact on circular external fixation devices. For the worst case scenario, using low permittivity absorption material with appropriate conductivity can reduce the 1g-averaged SAR from 346 W/kg to 26 W/kg. Based on the induced current behavior, we propose a circuit model of MRI RF coil induced heating on external fixation devices, which gives qualitative insights on the mechanism of the heating reduction using absorption material. Preliminary error quantification is discussed for simulations and experiments. By using a simple case for which analytical solutions are possible, the simulation uncertainty is bounded within 6.7%. By using an uncertainty budget, the combined uncertainty is shown to be 14.99% for standard testing method for MRI RF experiment system.