THE DETERMINATION OF OPTIMAL PROTOCOL FOR DIGITAL CHEST TOMOSYNTHESIS

Abstract

Recently, digital chest tomosynthesis (DTS) is introduced as alternative technique in digital chest radiography for evaluating pulmonary disease and enhancing the internal structures in different slices. However, the radiation dose is higher compared to general chest radiography. The present study was to determine the optimal protocol for DTS in order to reduce the radiation dose to patients while maintaining the image quality. The multipurpose chest phantom N1 "LUNGMAN’’ was scanned by digital radiographic systems model Definium 8000.Such phantom was inserted with simulated nodules with size diameter of 3, 5, 8, 10, 12 mm, and the data were acquired using chest VolumeRAD protocol with AEC technique. Parameters were varied in tube voltage (100, 110, 120 kVp) copper filter (0.0, 0.1, 0.2, 0.3 mm) and dose ratio (1:5, 1:8, 1:10) for evaluating the optimal protocol. All of protocols were performed three times. The entrance surface dose (ESD) was measured using glass dosimeter attached at the mid-chest level of the phantom. The effective dose (ED) was calculated using the recorded DAP value. The signal-to-noise ratio (SNR) was measured for qualitative image quality evaluation. The image criteria and nodule detection capability were scored by two experienced radiologists. The results indicated that the average±SD of ESD obtained from vendor’s default protocol at 120 kVp, dose ratio 1:10 and no copper filter was 1.68±0.15 mGy. The optimal parameter for DTS was obtained at 110 kVp, dose ratio 1:5, and copper filter at 0.3 mm with the ESD of 0.47±0.02 mGy. The effective doses for the default protocol and optimal protocol were 313.98±0.72 µSv and 100.55±0.28 µSv, respectively. There were slightly different of the image criteria and nodule detection between optimal and default protocols using visual assessment by two radiologists. In the clinical study, the average patient’s thickness of 22.51±1.70 cm (range 19.30-25.80 cm) was obtained. The average±SD effective dose of 98.87±0.08 µSv was obtained after applied the optimal protocol in 30 patients. The dose ratio and tube voltage were in slightly correlation with the radiation dose since the AEC technique was applied. A copper filter has a potential to reduce radiation dose to the patients. In conclusion, the optimal protocol can reduce radiation dose substantially while preserving the image quality compared to the vendor default protocol.