Abstract
Magnetic Resonance Imaging (MRI) is one of the most widely used clinical diagnostic procedures. Evaluation of safety due to Radio Frequency (RF) energy deposition and tissue heating in patients during MRI, especially in the presence of implantable prosthetic devices is significant for MR safety. The work presented in this thesis aims to characterize the interactions between the pulsed RF fields during MRI and biological tissues of a patient with a Retinal Prosthesis (RP) implant, in terms of Specific Absorption Rate (SAR) and temperature elevation. A logarithmically expanding grid Finite-Difference Time-Domain (FDTD) is used for computational modeling of the MR environment at 64, 128 and 256 MHz. Unlike traditional methods, expanding grid FDTD facilitates in accurate modeling of the region of the implant where a finer grid with cell sizes of the order of micrometers is used. Also, this technique greatly helps to reduce the constraints on computational memory and time. It was found that, while the RF magnetic field, B1, homogeneity decreases with frequency; power deposition in the tissues increases slightly. However, thermal elevation resulting from the SAR distribution as well as the induced currents in the RP implant, evaluated using the bio-heat equation, is observed to be minimal at these frequencies. These results provide useful information for RF safety guidelines during MRI at high fields.
|
