Assessing the Difference between Equilibrium Dose and CTDI in Effective Dose Estimation

Abstract

Purpose: The dose of Computed Tomography (CT) scan exams consists of a large proportion of all medical imaging modalities' dose burdens. There are different methods to measure and describe radiation in CT. A standardized way is to measure the Computed Tomography Dose Index (CTDI). However, due to the increase in the detector system size along the z-axis in new CT scanner generations, new measurement methods are described in the American Association of Physicists in Medicine-Task Group No.111 (AAPM-TG111). This study aims to estimate the equilibrium dose and compare it with the amount displayed in the volume Computed Tomography Dose Index (CTDIvol) at the end of each exam. Eventually, the effective dose was calculated for both methods. Materials and Methods: Using a pencil ionization chamber and standard polymethylmethacrylate (PMMA phantom), the following values were calculated: CTDI100, CTDIvol, cumulative dose, equilibrium dose, and effective dose. Results: Six protocols performed in two centers, and the results indicated that the measurements with a standard CT dosimetry phantom, was varied between average equilibrium dose and CTDIvol, and the discrepancies ranged between 27 to 33. Conclusion: The CTDIvol is not suitable for evaluating the radiation dose at the end of each scan, and the use of an equilibrium dose for dosimetry of new systems is recommended. Copyright © 2026 Tehran University of Medical Sciences. This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International license (https://creativecommons.org/licenses/by-nc/4.0/). Noncommercial uses of the work are permitted, provided the original work is properly cited. https://creativecommons.org/licenses/by-nc/4.0/