OPTIMIZATION OF RADIATION DOSE AND IMAGE QUALITY IN ABDOMINAL RADIOGRAPHY USING DIGITAL MOBILE X-RAY SYSTEM

The abdominal radiography using digital mobile x-ray system is commonly requested for the immobilized patient. The selection of the exposure parameters needs the experience of the operator for radiation dose and image quality. The purpose of this study was to optimize the radiation dose and image quality for abdominal radiography using digital mobile x-ray system in phantom at King Chulalongkorn Memorial Hospital. Digital mobile x-ray system model OptimaXR220amx and the Kyoto Kagaku phantom model PBU-60 with 21 cm of abdominal thickness were used. The buildup layer was enclosed on the phantom to simulate a larger body type of 25 and 29 cm thickness of abdomen. The setting for experimental exposure parameters, kVp range was 70-90 (in 5 kVp increments) and mAs was 3.2, 6.3, 12.5, and 25. The setting for routine clinical exposure parameters of 21, 25, and 29 cm thickness of abdomen were 75 kVp 32 mAs, 80 kVp 32 mAs, and 85 kVp 32 mAs, respectively. The setting source to image receptor distance (SID) was 100 cm. The experimental and routine clinical exposure parameters were performed and compared in terms of radiation dose and image quality evaluation. The determination of image quality score based on IAEA protocol (5 from 7 points) and qualitative noise (1 to 2 points) were scored by three observers. The quantitative analysis was evaluated in terms of signal-to-noise ratio (SNR) by 3 regions at liver (1st ROI), transverse process in 4th lumbar spine (2nd ROI), and pelvis (3rd ROI) and contrast-to-noise ratio (CNR) was evaluated on 3 areas at liver area (1st area), left kidney and transverse process in 4th lumbar spine (2nd area), and right kidney and pelvis (3rd area) on the image. The exposure index was also recorded. The optimal parameter for 21 cm thickness of abdomen at 100 cm SID was 80 kVp, 6.3 mAs with the exposure index of 381. The image quality scoring and qualitative noise from 3 observers were 5.67 and 1, respectively. The average SNR in 1st, 2nd, and 3rd ROIs were 39.56, 61.55, and 18.24, respectively and the average of CNR in 1st, 2nd, and 3rd areas were 5.97, 7.27, and 1.50, respectively. For 25 cm thickness, the optimal parameter was 85 kVp, 6.3 mAs with the exposure index of 395. The image quality scoring and qualitative noise were 5.17 and 2, respectively. The average SNR 1st, 2nd, and 3rd ROIs were 43.67, 75.08, and 19.53, respectively and the average of CNR 1st, 2nd, and 3rd areas were 6.42, 7.54, and 1.75, respectively. For 29 cm thickness, the optimal parameter was 85 kVp, 12.5 mAs with exposure index 409. The image quality scoring and qualitative noise were 5.33 and 1, respectively. The average SNR in 1st, 2nd, and 3rd ROIs were 35.47, 67.70, and 19.43, respectively and the average of CNR in 1st, 2nd, and 3rd areas were 6.16, 5.59, and 1.69, respectively. Currently, there is no dose reference level of abdominal radiography for Thai population. The comparison between the routine clinical and optimal parameters, the entrance surface air kerma (ESAK) in the optimal parameter of 21, 25, and 29 cm thicknesses of abdomen were lower than 77%, 78%, and 61% to the routine clinical parameter. However, the optimal parameters in this study can maintain the image quality for acceptable diagnosis in various abdominal thicknesses. The optimal parameters from this study should be applied to the patient for the routine clinical parameter in the future.