Modified optical computed tomography scanner for low temperature scanning

Abstract

Large radiochromic hydrogels, such as ferrous xylenol formulations require several hours to warm from the storage temperature of 278 K to 293 K for optical CT scanning and irradiation. This warm-up time can be avoided if gels are scanned at 278 K, resulting in a more practical 3D dose measurement. In this study a Vista 16 optical CT scanner was modified to allow scanning at 278 K, below the temperature where condensation forms on the aquarium windows. The refractive index matching liquid was first cooled to 276K in order to cool the aquarium as it was filled. The modification involved pumping cool, dry air into the aquarium scanner compartment to provide a positive pressure, preventing condensation on the aquarium windows. A warming rate of 2 K per hour was achieved with passive cooling which relied on the liquid-filled aquarium’s thermal mass. The radiochromic reaction rate decreased with temperature, requiring an increased post-irradiation wait time from 1 hour at 293 K to 1.5 hours at 278 K. In addition, initial sample transmission was greater by avoiding the auto-darkening that occurred as the samples were warmed from 278 K to 293 K over several hours. This increase in transmission resulted in a larger dynamic range for the dosimeter system.