Numerical evaluation of heating during magnetic resonance imaging

Magnetic Resonance Imaging (MRI) has become an indispensable tool of modern medicine. Its popularity can be chiefly attributed to its ability to distinguish among many soft tissue types producing, in most cases, a more detailed and clinically useful image than most other methods. Moreover, unlike other medical imaging methods such as Xray or Computer Aided Tomography (CAT), MRI does not expose the patient to harmful ionizing radiation. It is consequently considered a safe and low risk procedure. As MRI continues to grow in popularity researchers are striving to develop new machines with increased capabilities. One particular approach is to utilize much larger magnetic fields than are currently in clinical use. These high fields will theoretically improve the signal-to-noise ratio, which will subsequently improve the resolution and quality of the final MRI images. In doing so, however, these high field MRI machines will require radio frequency (RF) fields that are much higher in frequency than currently used in clinical magnets. Since the MRI process requires the patient to be exposed to RF electromagnetic fields for an extended period of time, tissue temperatures may significantly increase with long-term exposure to these high frequency RF fields. Therefore, it is essential to accurately quantify not only the energy absorption but also the temperature elevation caused by high field MRI and to predict if any potential hazards may exist. In this research, detailed three dimensional models were developed to accurately calculate the energy absorption and the associated heating during MRI for frequencies from 63 MHz up to 500 MHz. Finite-difference time-domain (FDTD) and finite different methods were used to calculate the energy absorption and the associated temperature distributions, respectively. In addition, experimental validations of the models were performed at the MRI laboratory facility at the FDA's Center for Devices and Radiological Health (CDRH).