[Radiofrequency magnetic fields and SAR in magnetic resonance for clinical use]

The exposure to radiofrequency (RF) electromagnetic fields is one of the main aspects to be controlled in order to ensure the safety of patients in an NMR environment. Starting from the description of the main physical characteristics of RF fields active in imaging instrumentation, a simple model is obtained which accounts for the experimental interdependence between the parameters of the stimulus and SAR values deposited on the patients. This model can yield a preliminary/approximate prediction of exposure levels for a given test. Some NMR equipment admit the estimation of the actual SAR values during the test on the basis of electrical measurements. The paper illustrates the basic principles of these measurements. The biological effects associated with the absorption of RF energy are discussed with particular attention to the physical and biological factors which may result in local heating on the body, a danger which may not be prevented by simply observing the prescribed thresholds of mean exposure.