Thermoregulatory responses to clinical magnetic resonance imaging of the head at 1.5 tesla. Lack of evidence for direct effects on the hypothalamus.

Warming temperature sensitive neurons in the hypothalamus will induce thermoregulatory heat dissipation. Application of radiofrequency (RF) radiation to the head during magnetic resonance imaging (MRI) could, conceivably, heat the brain, causing a generalized peripheral vasodilation and result in a paradoxical and unnecessary decrease in body temperature. To evaluate the thermoregulatory responses to the RF power deposition used during MRI, we measured body (sublingual pocket) and skin temperatures in 15 patients immediately before and after MRI scans of the head. Ear-skin blood flow was determined by laser-Doppler velocimetry to assess local vasomotor tone. A high-field (1.5 tesla/64 MHz) MRI device (General Electric Company) with a coil designed for head/brain imaging was used in this study. Ambient conditions were room temperature 20-24 degrees C and relative humidity 40-50%. The specific absorption rate averaged over the head ranged from 0.83 to 1.20 W/kg. There was a slight but statistically significant elevation in body temperature (36.5 +/- 0.5 to 36.7 +/- 0.4 degrees C). Skin temperatures of the ear (30.0 +/- 1.2 to 32.0 +/- 0.9 degrees C) and forehead (32.4 +/- 0.5 to 32.8 +/- 0.5 degrees C) increased significantly, while hand (29.9 +/- 1.4 to 29.8 +/- 2.1) skin temperature was unchanged. Ear-skin blood flow also increased a statistically significant amount (average change 36%). The data indicate that there was predominantly surface heating associated with MRI of the head which stimulated a local vasodilating response (i.e. significant increase in ear skin blood flow).(ABSTRACT TRUNCATED AT 250 WORDS)