The study on the effect of tube voltage on the dose of superficial radiation sensitive organ in the same CT dose index

Abstract

Objective To investigate the effect of tube voltage on the organ dose of the superficial sensitive organs (lens, thyroid and mammary gland) and the image quality under the same volume CT dose index (CTDIvol). Methods Combining the appropriate scout image and scanning center, the head-neck and chest phantom were scanned by CT under CARE kV semi mode. Under the same CTDIvol, the tube voltage was set by 70,80,100,120,140 kV respectively, and the scout image was scanned three times under each tube voltage. Then each tube voltage was used for spiral scan. For head-neck phantom ROIs were selected in orbital center and C5 upper edge level, for chest phantom ROIs were selected in the apical and tracheal bifurcation level.The contrast noise ratio (CNR) was measured and recorded.The CTDIvol values of the head-neck and chest phantom were set by 15.30 mGy, 2.60 mGy respectively. The organ dose of lens, thyroid and mammary were measured by thermoluminescent dosimeters (TLD) in every scanning(take the average of 3 measurements),calculating the cumulative value of scout image and spiral scanning. Finally, the appropriate tube voltage was sought by calculating the FOM (figure of merit). Results When CTDIvol was 15.30 mGy, the lens radiation dose was the smallest at 70 kV(2.440 mGy), the lens radiation dose was the largest at 140 kV(2.823 mGy), the thyroid radiation dose was the smallest at 100 kV(4.052 mGy), and the largest at 80 kV(4.914 mGy). When CTDIvol was 2.60 mGy, the mammary radiation dose was the smallest at 70 kV(2.353 mGy), the largest at 120 kV(2.735 mGy). When the tube voltage was 120 kV, the CNR value was the highest at the orbital center and C5 upper edge level, and the range was 82.83 to 111.67 and 117.22 to 162.51 respectively. When the tube voltage was 140 kV, the CNR value was the highest at the apical and tracheal bifurcation level, and the range was 71.26 to 112.81 and 76.21 to 88.52 respectively. The optimal FOM factor was 100 kV at orbital center and 120 kV at C5 upper edge level (4 575.01, 6 198.94 respectively). The optimal FOM factor was 140 kV at the apical and 70 kV at tracheal bifurcation level (4 937.01, 3 208.01 respectively). Conclusion The change of the tube voltage has a certain effect on the dose of the superficial sensitive organs and the image quality under the premise of the same CTDIvol. Key words: Tomography, X-ray computed; Radiation dose; Tube voltage