The phantom study of the effect of tube voltage on radiation dose and image quality of CT with automatic tube current modulation mode

Abstract

Objective To investigate the effect of the change of tube voltage on radiation dose and image quality in head-neck and chest scanning under automatic tube current modulation (ATCM) . Methods CT scanning was performed on the head-neck and chest phantom with ATCM and automatic tube voltage (CARE kV) . The tube voltage was manually selected at 70, 80, 100, 120 and 140 kV separately, and a routine CT scanning of the head-neck and chest with ATCM was performed. The scout was scanned for 3 times and a spiral scanning was performed once at each of tube voltage. The regions of interest (ROIs) were selected in the slices of orbital center and C5 upper edge level for the head-neck phantom, in the slices of apical and tracheal bifurcation level for the chest phantom. The contrast to noise ratios (CNRs) were measured and recorded. The organ dose of eye lens and mammary are measured with thermoluminescent dosimeters (TLDs) for every scanning (the average of 3 measurements) . The cumulative dose value of the scout and spiral scanning was calculated. The volume CT dose index (CTDIvol) of each scan was recorded, and the cumulative value of CTDIvol was calculated. Finally, the optimized tube voltage was obtained by calculating the FOM (figure of merit) . Results With ATCM and CARE kV, 120 kV and 108 mAs were chose automatically by system for head-neck phantom, 80 kV and 167 mAs for chest phantom. With ATCM, the radiation dose of eye lens and CTDIvol were minimal with manually selected 70 kV (0.779 and 4.070 mGy respectively) , and maximaum with manually selected 140 kV (2.571 and 25.670 mGy) . The radiation dose of the mammary gland and CTDIvol were minimal with manually selected 70 kV (0.698 and 0.900 mGy) , and maximal with manually selected 140 kV (3.452 and 7.400 mGy) . The CNR values of orbital center and C5 upper edge level were 51.30-118.36 and 80.78-173.12 respectively. The CNR values of the apical and tracheal bifurcation level were 50.15-129.58 and 49.63-115.40, respectively. The optimal FOM was appeared at orbital center slice with 80 kV, at C5 upper edge level slice with 120 kV and at both the apical and tracheal bifurcation level with 70 kV. Optimum tube voltage for head-neck phantom: manual 100 kV at orbital level, CARE kV mode (120 kV) at neck level. Optimal tube voltage for chest phantom: manual 100 kV. Conclusions The selection of tube voltage is responsible for the radiation dose and image quality of CT scanning. For conventional CT scan, manual 100 kV is suitable for orbital scanning, automatic 120 kV is suitable for neck scanning, manual 100 kV is suitable for chest scanning. Key words: Tomography, X-ray computed; Radiation dose; Tube voltage