RADIATION-DOSE AND IMAGE QUALITY IN SPIRAL COMPUTED-TOMOGRAPHY - MULTICENTER EVALUATION AT 6 INSTITUTIONS

The purpose of this study was to evaluate the correlation of radiation dose with image quality in spiral CT. Seven clinical protocols were m easured in six different radiological departments provided with four d ifferent types of high specification spiral CT scanners. Central and s urface absorbed doses were measured in acrylic. The practical CT dose index (PCTDI) was calculated for seven clinical examination protocols and one standardized protocol using identical parameters on four diffe rent spiral CT scanners with a dedicated ionization chamber inserted i nto PMMA phantoms. For low contrast measurements, a cylindrical three- dimensional (3D) phantom (different sized spheres of defined contrast) was used. Image noise was measured with a cylindrical water phantom a nd high contrast resolution with a Perspex hole phantom. Image quality phantoms were scanned using the parameters of the clinical protocols. Images were randomized, blinded and read by six radiologists (one fro m each institution). PCTDI values for four different scanners varied u p to a factor between 1.5 (centre) and 2.2 (surface) for the standardi zed protocol. A greater degree of variation was observed for seven cli nical examination protocols of the six radiological departments. For e xample, PCTDI varied up to a Factor between 1.7 (cerebrum protocol) an d 8.3 (abdomen paediatric protocol). Low contrast resolution correlate s closely with dose. An improvement in detection from 8 mm to 4 min si zed spheres needs approximately a ten-fold increase in dose. Noise sho ws a moderate correlation with PCTDI. High contrast resolution of clin ical protocols is independent of PCTDI within a certain range. Differe nces in modem CT scanner technology seem to be of less importance for radiation exposure than selection of protocol parameters in different radiological institutes. Future discussion on guidelines regarding opt imal (patient adapted) tube current for clinical protocols is desirabl e.