Noninvasive thermometry with a clinical x-ray CT scanner.

A CT scanner yields CT numbers which are proportional to the fractional difference in effective local electron density of the subject material with respect to that of calibration material. A homogeneous water-equivalent material is used as a calibration phantom under isothermal conditions. Any temperature variation (spatial or temporal) in the subject material subsequently scanned, will generate a CT-number shift in the CT image because of density changes due to thermal expansion. The potential use of the thermally generated CT-number shift in noninvasive thermometry during cancer hyperthermia was studied in vitro in samples of water and muscle tissue. The reproducibility of the area-averaged CT-number measurement on our EMI-7070 scanner was found to depend strongly on the time interval between successive counterclockwise scans and on the size of the region of interest used for area averaging. A linear relationship was found between the CT number and water density in the water temperature range from 10 to 55 degrees C. In the hyperthermia temperature range (36-50 degrees C), the relationship between the CT number and temperature itself is linear, with a CT-number thermal shift of about 0.4 and 0.45 HU/degrees C for water and muscle tissue, respectively. The achievable temperature discrimination is a fraction of degree C at spatial resolutions of the order of a centimeter.