| The basis of radiofrequency (RF) safety (ICNRP, ICES) is the exact understanding of the induced temperature rise in tissue. It must be realized that the only scientifically proven effect of RF exposure, in the frequency range used for telecommunication, is a local temperature rise and its related physiological reactions and local burns. This temperature rise is determined in a two step process. First the absorption of the radiofrequency (specific absorption rate, SAR) is calculated, secondly this SAR is used to calculate the resulting temperature rise. The accuracy of this method is hampered by the limited spatial resolution of the computational methods used and the uncertainties in tissue properties and perfusion. Another limitation is the bioheat transfer equation used for the temperature calculation. This equation does not take into account all aspects of blood flow related heat transfer. As a consequence of the uncertainties in thermal modelling, the present safety procedures use the computed absorbed power (SAR) inside the anatomy integrated over a certain volume as surrogate for the temperature rise.The clinical application of high field MRI (3-7 T), with its deep absorption of RF power and high intensity pulses, is an example where the present safety approach fails. Due to the coarse resolution of the simulation grids, anatomy related field singularities are missed in the computations, while they may result in severe internal burns. Both the SAR distributions and the resulting temperature prove to be highly anatomy dependent and must be evaluated with high quality modelling at a high spatial resolution. In vivo verification is essential. Uncertainties may result in strict RF safety guidelines. In clinical MRI applications too strict guidelines may reduce the MRI pulse sequences allowed and hamper the exploitation of high field MRI for improving functional and morphological imaging. This may lead to suboptimal diagnostic imaging and may result in more harm to the patient compared to the harm induced by the presence of hot spots. Exact knowledge on the RF and temperature distributions is needed to balance these clinical risks.In the application of MRI RF safety we have the unique situation that the RF excitation pulses of the MRI scanner cause the RF absorption and local temperature rises, but that the MRI also provides the possibility to measure these radiofrequency fields (B1+ mapping) and the resulting temperature distributions. MRI thus causes RF safety problems but also provides a tool to investigate these problems in general and to test the computational methods in high resolution and 3D. This makes MRI the ideal application to verify in vivo the quality of the present computational methods and also to investigate the present methodology in the safety guidelines. This process may result in better guidelines in general but also in a better understanding of the specific patient safety aspects in clinical MRI. |
