Relative Contributions of Dielectric Relaxation Mechanisms to Effective Conductivity Relevant to MRI RF Heating

Abstract

Thermal injuries are important complications of magnetic resonance imaging (MRI) and are mediated by electric fields within the body. Although several mechanisms at the molecular and cellular level have been implicated in the specific interaction of electric fields with tissues, the relative contributions of these mechanisms have not been elucidated at the frequencies most often used in MRI, and there is controversy in the literature regarding which mechanisms contribute. These mechanisms include ionic conduction, water dipole rotation, ion channel and cell surface polarization, and the Maxwell–Wagner effect. Using a commonly employed tissue dielectric model together with available experimental data, the relative contributions of different terms of the model to the real effective conductivity are estimated for seventeen different tissue types. The conductivity is one of the factors controlling the specific absorption rate (SAR) and radiofrequency (RF) heating during MRI under fixed electric field and thermal assumptions. This model-based approach suggests that the contribution of effective conductivity to heating in MRI at the molecular and cellular level under these assumptions is more complex than previously indicated in the MRI literature. The differences in polarization and, hence, heating mechanisms between tissue types are interpreted in terms of tissue structure and function.