Abstract
Technical developments are ongoing in CT, and there has been a continually increasing trend in patient prescription, resulting in increased exposure. Currently, doses delivered during CT are generally evaluated using computed tomography dose index (CTDI), which is measured with a 10 cm pencil ionization chamber placed in a 14 cm PMMA phantom. However, shortfalls in CTDI have been identified by the American Association of Physicists in Medicine (AAPM) who have proposed a new method, dose equilibrium (DEq). In this paper, the dose equilibrium was used to estimate the dose in two protocols (thoracic and abdominopelvic) and compared to CTDI values. In addition, a retrospective correction was applied to 20 patient CTDI’s by characterizing the specific DEq profile of the system scans. The results indicated the dose equilibrium estimations of two protocols, thoracic and abdominopelvic, were 29% and 30% respectively, higher than those informed by the CT scanner. In addition, a retrospective dose correction estimation of a random sample of twenty patients demonstrated an annual underestimation in absorbed dose by between 26% and 28%. Continued use of the CTDI method in quality assurance of modern CT could result in greater patient risk. AAPM Task Group 111 presents a more accurate, safer method to estimate dose and its adoption is paramount.
Highlights
Computed Tomography (CT) comprises approximately 5% of global medical X-ray procedures [1]
Doses delivered during CT are generally evaluated using computed tomography dose index (CTDI), which is measured with a 10 cm pencil ionization chamber placed in a 14 cm PMMA phantom
Shortfalls in CTDI have been identified by the American Association of Physicists in Medicine (AAPM) who have proposed a new method, dose equilibrium (DEq)
Summary
Computed Tomography (CT) comprises approximately 5% of global medical X-ray procedures [1]. Computed Tomography Dose Index (CTDI) is used to determine CT quality assurance (QA) measurements and dose measurements, being the absorbed dose along the longitudinal axis (z-axis) during a single X-ray source rotation [3]. This measurement is usually conducted in a cylindrical phantom using a 100 mm ionization chamber. The chamber is responsible for a significant error in the dose profiles as it does not take into account some of the radiation scattered beyond the relatively short (100 mm) range of integration along the z-axis [4] [5] This is mostly due to over-beaming in multi-slice CT, where the z-collimation of the source radiation is broadened to achieve umbra-region incidence uniformly across detectors
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