To maximize diagnostic accuracy and minimize costs, magnetic resonance imaging (MRI) scanners expose patients to electromagnetic exposure levels well above the established maximum, but in a well-controlled environment. In this paper, we discuss a novel safety assessment model that offers maximum flexibility while ensuring no local tissue damage due to radiofrequency induced heating occurs. This model is based on the cumulative equivalent minutes at 43 °C (CEM43) thermal dose concept, which naturally considers exposure duration, tissue sensitivity and the transient nature of heating, and permits rapid assessment of exposure safety of a given MRI scan using information about the transient specific absorption rate (SAR). It builds upon theoretical considerations (e.g., relating peak temperatures in the presence and absence of local thermoregulation) as well as data extracted from simulations involving anatomical models (e.g., to determine the characteristic time of temperature changes). The model is capable of predicting CEM43 for patients with either uncompromised thermoregulation or absent thermoregulation. The model predictions approximate detailed simulations well and results illustrate the importance of adequately considering changes in perfusion. The model presented herein offers an MRI safety assessment approach that overcomes problems associated with traditional SAR-based limits. Its limitations and the associated uncertainties are discussed together with remaining open questions.
Keywords: CEM43; MR safety; fast estimation; local heating; modeling.
© 2015 Wiley Periodicals, Inc.