Temperature and SAR calculations for a human head within volume and surface coils at 64 and 300 MHz.
OBJECTIVE
To examine relationships between specific energy absorption rate (SAR) and temperature distributions in the human head during radio frequency energy deposition in MRI.


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-dependent 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.


CONCLUSIONS
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.
Collins
Christopher M
CM
Center for NMR Research, Department of Radiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA. cmcollins@psu.edu
Liu
Wanzhan
W
Wang
Jinghua
J
Gruetter
Rolf
R
Vaughan
J Thomas
JT
Ugurbil
Kamil
K
Smith
Michael B
MB
D004869:Equipment Safety; D020342:Finite Element Analysis; D006257:Head / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D008954:Models, Biological; D013696:Temperature; D013814:Thermal Conductivity