A radiation exposure index for CT

The purpose of this study is to define an Exposure Index for CT (EICT) and to estimate the magnitude of the EICT 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 (EICT) is defined as the average Air Kerma that would be incident on an extended 360 deg. detector array completely surrounding the patient. For an axial scan of a uniform cylindrical phantom, EICT can be approximated as T x [(CTDIair)/4] x [βdeg./360 deg.] where T is the fractional transmission through the cylinder, CTDIair 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 CTDIair can be estimated from the weighted CTDI (CTDIw) 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 CTDIair/CTDIw 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 = 61 cm and r =8 cm) to 85.38 (R = 55 cm and r=20 cm). For an adult head CT examination, EICT was estimated to be∼70 μGy at a CTDIvol of 75 mGy (16 cm), and for an adult abdominal CT examination, EICT was estimated to be∼11 μGy at a CTDIvol of 25 mGy (32 cm). For an abdomen CT examination in a 5-y-old child, EICT was estimated to be ∼21 μGy at a CTDIvol of 20 mGy (16 cm). The EICT is introduced that provides a quantitative measure of the amount of the radiation used to generate images in any CT examination and is analogous to the average image receptor Exposure Index recently proposed for use in projection imaging. The EICT 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. (authors)