Comparison of dosimetric properties among four commercial multi-detector computed tomography scanners

This study compared dosimetric properties among four commercial multi-detector CT (MDCT) scanners. The X-ray beam characteristics were obtained from photon intensity attenuation curves of aluminum and off-center ratio (OCR) profiles in air, which were measured with four commercial MDCT scanners. The absorbed dose for MDCT scanners was evaluated with Farmer ionization chamber measurements at the center and four peripheral points in the body- and head-type cylindrical water phantoms. Measured collected charge was converted to absorbed dose using a 60Co absorbed dose-to-water calibration factor and Monte Carlo (MC)-calculated correction factors. Four MDCT scanners were modeled to correspond with measured X-ray beam characteristics using GMctdospp (IMPS, Germany) software. Al half-value layers (Al-HVLs) with a body-bowtie filter determined from measured Al-attenuation curves ranged 7.2‒9.1 mm at 120 kVp and 6.1‒8.0 mm at 100 kVp. MC-calculated Al-HVLs and OCRs in air were in acceptable agreement within 0.5 mm and 5% of measured values, respectively. The percentage difference between nominal and actual beam width was greater with decreasing collimation width. The absorbed doses for MDCT scanners at 120 kVp ranged 5.1‒7.1 mGy and 10.8‒17.5 mGy per 100 mAs at the center in the body- and head-type water phantoms, respectively. Measured doses at four peripheral points were within 5% agreement of MC-calculated values. The absorbed dose at the center in both water phantoms increased with decreasing Al-HVL for the same charge on the focus. In this study the X-ray beam characteristics and the absorbed dose were measured and compared with calculated values for four MDCT scanners.