Thermal Tissue Damage Model Analyzed for Different Whole-Body SAR and Scan Durations for Standard MR Body Coils

Manuel Murbach, Esra Neufeld, Myles Capstick, Wolfgang Kainz, David O. Brunner, Theodoros Samaras, Klaas P. Pruessmann, and Niels Kuster, Magnetic Resonance in Medicine, Volume 71, Issue 1, pp. 421-431, January 2014, online February 14, 2013

This article investigates the thermal safety of magnetic resonance imaging (MRI) by estimating the transient local tissue heating both in a 1.5T MRI system and in four anatomical human models in different Z-positions with and without local thermoregulation. To determine the effective thermal stress of the tissues, the thermal dose model "cumulative equivalent minutes at 43°C" (CEM43) was applied. This model allows predicting thermal tissue damage risk and identifying potentially hazardous MR scan-scenarios. The numerical results were validated by B1+ and skin temperature measurements. Under continuous exposure conditions at the guideline limit value, peak tissue temperatures of up to 42.8°C were computed for the thermoregulated model. When applying tissue-specific damage thresholds, adverse effects cannot be excluded after 25 min of scanning if the current scanning limits are fully exploited. As realistic exposures are considerably below the maximum levels allowed, the results are found to be consistent with the history of safe use in MR scanning, but not with current safety guidelines. For future safety concepts, we suggest to use thermal dose models instead of temperatures or SAR. Special safety concerns for patients with impaired thermoregulation (e.g., elderly or diabetic patients) should be addressed. 

The scientific and technical impact of this study can be summarized as:

  • Demonstration of the accurate translation of different MRI exposure scenarios to transient temperature increases in realistic human models and various scanning positions.
  • Modeling of realistic local thermoregulation.
  • Application of the thermal dose model (CEM43) and estimation of the maximum MR scan times tolerable before causing potential thermal tissue damage.
  • Numerical results are found to be consistent with the history of safe use in MR scanning.
  • Simulation models were validated with skin-temperature measurements in one healthy subject.
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