Assessment of RF heating by MR-based measurements and models

Abstract

RF transmit signals are an inherent part of MRI that is crucial for spin excitation. Current developments in MRI are moving towards higher magnetic field strengths and more transmit coils leading to more inhomogeneous RF distributions. This increases the possibility of SAR hotspots, creating higher risks of local heating. In order to cope with these developments and still perform reliable safety assessments the MRI safety community is discussing to revise the guidelines on RF induced tissue heating. Instead of restricting the scanners on a derived measure, i.e. SAR, the scanners should be limited by the guidelines that are direct measures for tissue damage such as absolute temperature. In this debate the translation from SAR to temperature estimations is crucial. In order to make this translation thermal modeling is required. In this thesis MR-based methods were developed in order to measure the influence of RF exposure in human subjects, both in temperature and perfusion. Those results could subsequently be used to test whether current thermal modeling was able to predict those effects for a given subject. In vivo temperature measurements in the calf resulted in precise representations of the heating that could be used as a reliable gold standard. Furthermore, perfusion increases over the complete leg due to this local temperature increase could be observed. Although the simulations matched with the experiments in the EM regime, the accuracy of the estimated temperature distributions was not sufficient for a safety assessment. This showed that more work on thermal modeling is required before MRI scanners can be restricted on measures based on temperature. Therefore it would be unwise to immediately discard all SAR related restrictions. Local SAR still proved to be a very useful restrictive measure since it can be quickly determined by simulations and can more easily be verified. Correct thermal modeling proved to rely on inputs that are not easily measured such as subject specific thermoregulation. Although the first steps in this direction were made in this thesis, obtaining a proper estimate of thermal behavior over the whole human body is a highly challenging pursuit.