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CT Dosimetry: Comparison of Measurement Techniques and Devices¹

¹ From the Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905. Presented as an education exhibit at the 2006 RSNA Annual Meeting. Received February 16, 2007; revision requested April 11 and received June 7; accepted June 11. J.A.B. receives research support from RTI Electronics; A.N.P. receives research support from Siemens; C.H.M. receives research support from Siemens and RTI Electronics; all other authors have no financial relationships to disclose. Address correspondence to C.H.M. (e-mail: mccollough.cynthia{at}mayo.edu).

In x-ray computed tomography (CT), the most common parameter used to estimate and minimize patient dose is the CT dose index (CTDI). The CTDI is a volume-averaged measure that is used in situations where the table is incremented in conjunction with the tube rotation. Variants of the CTDI correct for averaging across the field of view and for adjacent beam overlaps or gaps. CTDI is usually measured with a pencil-shaped ionization chamber, although methods have been developed that use alternative detectors, including an optically stimulated luminescence probe and a solid-state real-time dosimeter. Because the CTDI represents an averaged dose to a homogeneous cylindrical phantom, the measurements are only an approximation of the patient dose. Furthermore, dose from interventional or perfusion CT, in which the table remains stationary between multiple scans, is best evaluated with point dose measurements made with small detectors. CTDI and point dose values are nearly the same for measurement of surface dose from spiral CT. However, for measurement of surface dose from perfusion CT, the dose is overestimated by a factor of two or more with CTDI values in comparison with point dose values. Both CTDI and point dose measurement are valuable for evaluating CT scanner output and estimating patient dose.

© RSNA, 2008

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