Temperature and SAR calculations for a human head within volume and surface coils at 64 and 300 MHz

Authors

  • Christopher M. Collins PhD,

    Corresponding author
    1. Center for NMR Research, Department of Radiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania
    • Center for NMR Research, NMR/MRI Building, Department of Radiology H066, The Pennsylvania State University College of Medicine, 500 University Drive, Hershey, PA 17033
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  • Wanzhan Liu MS,

    1. Center for NMR Research, Department of Radiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania
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  • Jinghua Wang PhD,

    1. Center for NMR Research, Department of Radiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania
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  • Rolf Gruetter PhD,

    1. Center for MR Research, Department of Radiology, School of Medicine, University of Minnesota, Minneapolis, Minnesota
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  • J. Thomas Vaughan PhD,

    1. Center for MR Research, Department of Radiology, School of Medicine, University of Minnesota, Minneapolis, Minnesota
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  • Kamil Ugurbil PhD,

    1. Center for MR Research, Department of Radiology, School of Medicine, University of Minnesota, Minneapolis, Minnesota
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  • Michael B. Smith PhD

    1. Center for NMR Research, Department of Radiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania
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Abstract

Purpose

To examine relationships between specific energy absorption rate (SAR) and temperature distributions in the human head during radio frequency energy deposition in MRI.

Materials and Methods

A multi-tissue numerical model of the head was developed that considered thermal conductivity, heat capacity, perfusion, heat of metabolism, electrical properties, and density. Calculations of SAR and the resulting temperature increase were performed for different coils at different frequencies.

Results

Because of tissue-dependant perfusion rates and thermal conduction, there is not a good overall spatial correlation between SAR and temperature increase. When a volume coil is driven to induce a head average SAR level of either 3.0 or 3.2 W/kg, it is unlikely that a significant temperature increase in the brain will occur due to its high rate of perfusion, although limits on SAR in any 1 g of tissue in the head may be exceeded.

Conclusion

Attempts to ensure RF safety in MRI often rely on assumptions about local temperature from local SAR levels. The relationship between local SAR and local temperature is not, however, straightforward. In cases where high SAR levels are required due to pulse sequence demands, calculations of temperature may be preferable to calculations of SAR because of the more direct relationship between temperature and safety. J. Magn. Reson. Imaging 2004;19:650–656. © 2004 Wiley-Liss, Inc.

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