Investigation of the characteristics of Automatic Exposure Control (AEC) of a Computed Tomography (CT) scanner by utilising cylindrical and anthropomorphic phantoms

Abstract

Abstract.One method to optimise the use of x-rays in CT and hence a reduction in patient dose is the application of automatic exposure control (AEC). This study measured the effective mAs, image noise and volume CT dose index (CTDIvol) as the result of changing the AEC index on a Siemens Somatom Definition 64 slices dual source CT scanner. The scans were performed on four phantoms of different geometries, namely the 16 and 32 cm cylindrical CTDI phantoms and two anthropomorphic phantoms, RANDO (20 cm effective diameter) and ATOM (19.8 cm effective diameter). Results showed that the effective mAs increased with increasing tube potential (kVp) and Quality Reference mAs (QRM), therefore increasing CTDIvol while reducing image noise. Meanwhile, no changes of radiation dose and image noise were observed when the pitch was increased. However, for the largest phantom (32 cm effective diameter), a constant effective mAs was found between 120 and 140 kVp. The same trend was also found with increasing QRM from 300 mAs to 400 mAs suggesting a certain limitation of the AEC has been reached. In conclusion, this study showed that AEC is affected by kVp and QRM but not by pitch selection. Further work is required to quantify the characteristics of the AEC system in relation to the mentioned parameters for better optimisation.

Establishing how patient size and degree of miscentring affect CTDIvol, using patient data from a dose tracking system

“…The literature suggests that the effective mAs in the Siemens AEC system remains constant or exhibits only a slight decrease for such changes in pitch. 13 For other manufacturer, AEC systems doubling the pitch has been reported to lead to a decrease in CTDI vol by as much as 66-77%. 13 There appears to be limited data available relating the changes in slice thickness with CT output when using AEC; however, for the TAP protocols in this work, a 15% reduction in output was observed when comparing the 5 mm to the 1 mm slice thickness protocol.…”

“…13 For other manufacturer, AEC systems doubling the pitch has been reported to lead to a decrease in CTDI vol by as much as 66-77%. 13 There appears to be limited data available relating the changes in slice thickness with CT output when using AEC; however, for the TAP protocols in this work, a 15% reduction in output was observed when comparing the 5 mm to the 1 mm slice thickness protocol. It should be noted that image quality metrics have not been evaluated in this work, but it has been assumed that the image quality metric demands of the AEC system have been met leading to the observed and modelled CTDI vol data for clinical data.…”