The goal of this study is to investigate the accuracy of using CTDIw to estimate the averaged CTDI over x-y plane.
We used a Siemens Sensation 16 slice scanner, a nested 3 piece CTDI phantom with diameters of 10, 16, and 32 cm for each piece, a CT pencil ion chamber, and aluminum oxide OSL dosimeters. In axial mode, we measured the exposure with ion chamber at the 3, 6, 9, and 12 o'clock positions at distances of 4, 7, and 15 cm from the center of a 32 cm CTDI phantom, as well as at the center of the phantom. Measurements were performed at 80, 100, 120, and 140 kVp on the adult abdomen, adult head, and pediatric body phantom, using only the distances of 0, 4, and 7 cm from the phantom center for the smaller 16 cm diameter phantom. We did similar measurements using nanoDot dosimeters and the 32 cm phantom at 120 kVp in helical mode using a pitch of 0.8, 1.0, and 1.2.
The data obtained at four different outer clock positions was averaged and three models (linear, quadratic, and exponential) were used to fit exposure as a function of distance to the phantom center. We calculated the average CTDI over the x- y plane mathematically using the above models and compared the results with traditional CTDIw. In axial mode, the difference is within 6% for 32 cm phantom, with a slight increase in variance at low kVp, while the difference is within 1% for the 16 cm phantom. In helical mode with OSLs, the difference is within 2.5% for pitch 0.8-1.2.
The current CTDIw provides an accurate estimate of the averaged CTDI over the x-y plane for both axial and helical modes.
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