No abstract

Purpose: To estimate absolute brain temperature using proton MR spectroscopy ( 1 H-MRS) and mean brain-body temperature difference of healthy human volunteers. Materials and Methods:Chemical shift difference between temperature-dependent water spectral line position and temperature-stable metabolite spectral reference was used for the estimations of absolute brain temperature. Temperature calibrations constants were obtained from the spectra of the N-acetyl aspartate (NAA line at $2.0 ppm), glycero-phosphocholine (GPC line at $3.2 ppm), and creatine (Cr line at $3.0 ppm) aqueous solutions with pH values within physiologically pertinent ranges. Single-voxel PRESS sequence (TR/TE 2000/ 80 ms) was used for this purpose. Brain temperature was determined by averaging the temperatures computed from water-Cho, water-Cr, and water-NAA chemical shift differences. Results:The mean brain temperature of 18 healthy volunteers was 38.1 6 0.4 C and mean brain-body (rectal) temperature difference was 1.3 6 0.4 C.Conclusion: Improved accuracy of the temperature constants and averaging the temperatures computed from water-Cho, water-Cr, and water-NAA chemical shift differences increased the reliability of the brain temperature estimations. CURRENT KNOWLEDGE ABOUT brain temperature in healthy humans is limited because direct and accurate measurements cannot be made without the need for surgery. The importance of brain temperature and its fluctuations due to biochemical and physical processes is, in the meantime, well acknowledged (1).The factors thought responsible for brain temperature regulation are the temperature of incoming arterial blood, metabolic heat production, heat removal by blood flow, heat conductance of the tissues, and heat exchange with the environment. In mammals, the most important factors are the temperature of the incoming arterial blood, brain metabolism, and external temperature (2). It has been theoretically estimated that the normal human brain produces ca 0.66 J of heat every minute per gram of brain tissue (3). The internal heat is removed mainly by blood flow. The cooling effect of external temperature seems to be less important because it is limited to superficial brain regions of several millimeters of thickness and depends on brain size and ambient temperature (4,5). Theoretical simulations (3,6), animal studies (7), and measurements of venous (internal jugular vein) versus arterial (aortic artery) temperatures in healthy volunteers (8) suggest that there is a positive difference 0.3-0.5 C between brain and body. These observations are in the qualitative agreement with direct measurements of injured head or stroke patients that reveal the brain-body temperature difference in the range 1-2 C (9).Because invasive measurement of the brain temperature cannot be made in healthy volunteers, research has focused on noninvasive MR techniques. Proton MR spectroscopy ( 1 H-MRS) and MR spectroscopic imaging (MRSI) are now validated methods that are capable of estimating absolute brain temperatures (10-17). Thes...

Both MR techniques revealed brain temperature reductions after 60 min of intranasal cooling with balloons circulated with saline at 20°C in awake, unsedated volunteers.