ABSTRACT
Rigorous phantom dose equations have been derived which illustrate the significant limitations and common misconceptions concerning the computed tomography dose index (CTDI) paradigm. The necessity for shift-invariance in CTDI can also be operationally understood from the nature of a pencil chamber acquisition; namely, all profiles in the scan series must be identical to the single profile integrated by the pencil chamber in order for the equations and predictive nature of the CTDI paradigm to apply. The International Electrotechnical Commission has felt it an imperative to report a CTDI value for any shift-variant scan technique such as tube modulation, and also including stationary phantom techniques. The basic Size-Specific Dose Estimates dose index concept presented in the Report of American Association of Physicists in Medicine Task Group 204 is an approach to develop a more reasonable estimate of patient dose using the scanner-reported CTDIvol and conversion factors that account for patient size.
