The NaI (Tl) scintillater detector of a head CT scanner (Hitachi CT-250) in our hospital has recently been replaced by BGO sintillater. The reason for the replacement was to exclude the size dependence of a CT scanner. An Alderson Catphan has been used to evaluate the performance of the two different detectors. Half value layers were measured under usual scanning condition of 120Kv, 30mA tube voltage with additional filter of 0.1mm Cu+0.5mm Al. The results were as follows : 7.5mm H.V.L.Al for NaI (Tl) scintillater, and 7.1mm H.V.L.Al for BGO sintillater. Half value layer for NaI (Tl) detector was, therefore, significantly thicker than that for BGO detector. In addition, CT numbers of water were measured for both detectors. The results showed that the CT numbers of water were decreased as the tube voltage increased in NaI (Tl) scintillater. The results were opposite in BGO scintillater. In conclusion, the size independence and linearity has been improved in BGO scintillater detector.
