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

Soheila Sharifian  Jazi; Saman  Dalvand; Hamed  Zamani; Fahimeh Hossein  Beigi; Mohammad  Ghaderian; Reihaneh  Faraji; Daryoush  Shahbazi-Gahrouei

doi:10.18502/fbt.v13i2.21942

Soheila Sharifian Jazi Department of Medical Physics, School of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran
Saman Dalvand Department of Medical Physics, School of Medicine, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran
Hamed Zamani Department of Medical Physics, School of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran
Fahimeh Hossein Beigi Department of Imaging, 9th Dey of Manzariyeh Hospital, Isfahan, Iran
Mohammad Ghaderian Department of Medical Physics, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran
Reihaneh Faraji Department of Medical Physics, Medicine Faculty, Mashhad University of Medical Sciences, Mashhad, Iran
Daryoush Shahbazi-Gahrouei Department of Medical Physics, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran

DOI: https://doi.org/10.18502/fbt.v13i2.21942

Keywords: Multidetector Computed Tomography; Equilibrium Dose; Computed Tomography Volume Dose Index, American Association of Physicists in Medicine-Task Group No.111; Radiation Dosimetry.

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 (CTDI_vol) 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, CTDI_vol, 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 CTDI_vol, and the discrepancies ranged between 27% to 33%.

Conclusion: The CTDI_Vol 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.