Sampling and evaluation of specific absorption rates during patient examinations performed on 1.5-Tesla MR systems

It was the purpose of present study, to evaluate a large number of exposure-time courses measured during patient examinations in clinical routine in relation to the current IEC standard and the draft version of the revised standard and, moreover, to investigate whether there is a correlation between the subjective heat perception of the patients during the MR examination and the intensity of RF power deposition. To this end, radiofrequency exposure to 591 patients undergoing MR examinations performed on 1.5-Tesla MR systems was monitored in five clinics and evaluated in accordance with both IEC standards. For each of the 7902 sequences applied, whole body and partial body SARs were estimated on the basis of a simple patient model. Following the examinations, 149 patients were willing to provide information in a questionnaire regarding their body weight and their subjective heat perception during the examination. Although patient masses entered into the MR system were in some cases too high, reliable masses could be estimated by the SAR monitor. In relation to our data, the revision of the IEC standard results in a tightening of the restrictions, but still more than 96% of the examinations did not exceed the SAR limits recommended for the normal operating mode. For the exposure conditions examined, no statistically significant correlation was found between the subjective heat perception of the patients and the intensity of power deposition. Taking advantage of the possibility to compute running SAR averages, MR sequences can be employed in clinical practice for which SAR levels exceed the defined IEC limits, if the acquisition time is short in relation to the averaging period and energy deposition has been low previous to the applied high-power sequence.