Performance characteristics of Automatic Exposure Control (AEC) system for computed tomography (CT) using phantom study

Abstract

The number of CT examinations has increased as the growing number of the installed scanners. This resulted in the increased patient dose from the CT scan when compared to other radiographic examinations. In order to optimize the radiation dose from CT scan and maintain the image quality, several methods were proposed by the manufacturer and the users. One method to obtain the patient dose reduction from CT scan is the use of Automatic Exposure Control (AEC) system. The objective of this study is to evaluate the characteristics of AEC and non AEC systems on radiation dose (CTDIvol) and image noise using an agar cone phantom of various diameters represent the pediatric abdomen. A cone phantom diameters 10, 15 20 and 25 cm, was scanned by a multi-slice computed tomography with the parameters of kVp at 80, 100 and 120 for AEC and non AEC, mAs at 50, 70, 90 and 110. Both systems were applied for each phantom diameter which CTDIvol and image noise were studied. The image quality of 12 AEC and 48 non AEC images had been assessed by the qualified radiologists. The image quality scoring was related to the CTDIvol. The AEC characteristic was studied on the effect of phantom sizes and the effective mAs. The range of effective mAs was 45-116 at 80 kVp, 43-117 at 100 kVp and 43-117 at 120 kVp respectively. The CTDIvol was 1.17-3.02, 2.15-5.85 and 3.35-9.13 mGy and image noise was 9.36-28.42, 6.88-16.98 and 5.32-13.08 HU for each kVp and diameter. For non AEC, the CTDIvol were 1.3, 1.82, 2.34 and 2.86 mGy at 80 kVp, 2.5, 3.5, 4.5 and 5.5 mGy at 100 kVp and 3.9, 5.46, 7.02 and 8.58 mGy at 120 kVp. The ranges of noise were 5.68-41.68 HU at 80 kVp, 4.0-24.4 HU at 100 kVp and 3.46-18.76 HU at 120 kVp. The qualitative image quality scored by a radiologist was 1- not acceptable, 2- acceptable, 3 - good and 4- very good. The result on AEC was: score 1 -16.7%, 2- 58.3%, 3- 25% and 4- 0% and non AEC the score was 1- 14.5%, 2- 37.5%, 3- 33.3% and 4- 14.5%. As the number of AEC image was less than non AEC, and the AEC system modulated mAs to optimal values, the percentage of very good quality is none. The AEC system had influenced on the dose and noise of the phantom sizes. The limitation on the use of non AEC, the CTDIvol is constant at all phantom diameters and increases with increasing kVp. Therefore, the optimal protocols were set for both AEC and non AEC modes at each phantom diameter. For the diameters of 10, 15, 20, 25 cm the protocols are kVp 80, 80, 100 and 120 kVp for AEC mode and kVp 80/ mAs 50, kVp 80/ mAs 70, kVp 100/ mAs 90 and kVp 120/ mAs 70 for non AEC mode. The pediatric abdomen protocol should be studied for each CT system to optimize the patient dose and reasonable image quality.