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A radiation exposure index for CT

Radiation Protection Dosimetry, ISSN: 0144-8420, Vol: 157, Issue: 2, Page: 172-180
2013
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Article Description

The purpose of this study is to define an Exposure Index for CT (EI) and to estimate the magnitude of the EI for common clinical CT examinations. For a single-axial rotation of a CT X-ray tube that includes only rays that pass through the patient, the CT Exposure Index (EI) is defined as the average Air Kerma that would be incident on an extended 360 detector array completely surrounding the patient. For an axial scan of a uniform cylindrical phantom, EI can be approximated as T 3 [(CTDI)/4] 3 [β/360] where T is the fractional transmission through the cylinder, CTDI is the CT Dosimetry Index-determined 'free in air' at isocentre, and β/2 is the fan beam angle that will completely irradiate a cylindrical phantom at isocentre. The value of CTDI can be estimated from the weighted CTDI (CTDI) for a given CT examination, and the angle β depends on the irradiation geometry that can be obtained from the cylinder diameter (r) and the focus to isocentre distance (R). At a voltage of 120 kV, transmission through an adult head was ~2.6%, through an adult abdomen~0.4% and through a 5-y-old paediatric abdomen ~3%. Average ratios of CTDI/CTDI were 1.42±0.12 in 16-cm dosimetry phantom and 2.82±0.37 in 32-cm phantom. Values of b ranged from 30.18 (R 5 61 cm and r58 cm) to 85.38 (R 5 55 cm and r520 cm). For an adult head CTexamination, EI was estimated to be~70 μGy at a CTDI of 75 mGy (16 cm), and for an adult abdominal CTexamination, EI was estimated to be~11 μGy at a CTDI of 25 mGy (32 cm). For an abdomen CTexamination in a 5-y-old child, EI was estimated to be ~21 μGy at a CTDI of 20 mGy (16 cm). The EI is introduced that provides a quantitative measure of the amount of the radiation used to generate images in any CTexamination and is analogous to the average image receptor Exposure Index recently proposed for use in projection imaging. The EI metric provides operators with an objective index of the amount of the radiation used to create CT images and can be used to control quantum mottle in CT. © The Author 2013. Published by Oxford University Press. All rights reserved.

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