Effective Methods To Reduce The RF-Induced Heating For Medical Implants In MRI
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Radiofrequency (RF) –induced heating is one of the primary safety concerns for patients with medical implants undergoing magnetic resonance imaging (MRI). Due to this potential hazard, patients with medical devices are usually contraindicated from the MRI process. With the steady growth in the number of patients with medical implants that require MRI, it is important to address the issue of RF heating in MRI. This dissertation focuses on techniques and methods to reduce the RF-induced heating so that patients with medical implants can undergo MRI safely. For the implantable leads, it was proposed to modify the electrode structure to reduce the RF-induced heating. We show that enlarging the electrode contact reduces the rate of RF energy deposition at the electrode, which results in less RF-induced heating at the lead tip. For the external fixation devices, the strategy of using capacitive structures has been investigated to reduce the RF-induced heating. Both simulations and experiments demonstrated that introducing proper capacitive structures to the external fixation device could reduce the temperature rise by more than 95% at both 1.5 T and 3 T. An equivalent circuit was developed to illustrate the heating reduction effect of the capacitive structures in the external fixation device. It is concluded from the circuit model that increasing the equivalent impedance of the external fixation device reduces the RF-induced heating. For the passive implants, we investigated the effects of two different incident RF fields generated by a transverse electromagnetic (TEM) coil and a birdcage (BC) coil on the RF-induced heating. Electromagnetic simulation results show that the BC coil generates a stronger incident electric field along the coil bore than the TEM coil does. Due to this reason, orthopedic plates and spinal fixations caused a higher RF heating in the BC coil than they did in the TEM coil. For passive implants with large dimensions along the coil bore, the TEM coil provides a greater safety margin for patients while the BC coil provides a more conservative heating evaluation.