Effects of different scout images on radiation dose in CT scanning using automatic tube current modulation

Objective: To investigate the influence of different scout image on the radiation dose of head and chest spiral scanning in CT using automatic tube current modulation (ATCM). Methods: CT scanning was performed on the head-neck and chest phantom with ATCM. Five different scout images through the five various positions that include anteroposterior (AP), posteroanterior (PA), lateral, AP, lateral, PA and lateral. The phantom was scanned three times for each position. Then each scout image was scanned by spiral technique once more. ROI in orbital center and C5 upper edge level were selected for Head and neck phantom, ROI in the apical and tracheal bifurcation level were selected for chest phantom. The contrast to noise ratio (CNR) was measured and recorded. The organ dose of eye lens and mammary glands were measured by thermoluminescent dosimeters (TLD) in every scanning (the average of 3 measurements), The cumulative value of scout image and spiral scanning were calculated.The volume CT dose index (CTDIvol) of each scan was recorded, and the cumulative value of CTDIvol was calculated. Results: In the five scout images of the head-neck phantom mode, the maximum value of accumulated radiation dose of eye lens and CTDIvol appear on PA scout image (18.354 and 26.43 mGy respectively), while the minimum value appear on the lateral scout image (11.847 and 18.20 mGy respectively). In the chest phantom mode, the maximum value of the accumulated radiation dose of the mammary gland and CTDIvol emerge from the AP scout image (6.873 and 9.42 mGy respectively), while the minimum value of the mammary accumulated radiation dose appear on lateral scout image (4.592 mGy), the minimum value of CTDIvol appear on AP plus lateral scout images(3.94 mGy). The accumulated radiation dose of eye lens and CTDIvol value in PA scout image mode were 54.9% (6.507/11.847) and 45.2% (8.23/18.20) higher than those of the lateral scout image mode. The accumulated radiation dose of mammary gland and CTDIvol value in AP scout image mode were 42.5% (2.051/4.822) and 136.7% (5.44/3.98) higher than that of PA plus lateral scout images mode. In the head-neck phantom mode, the CNR value of orbital center and C5 upper edge level were 102.55 to 115.89, 161.01 to 204.52 respectively. In the chest phantom mode, the CNR value of the apical and tracheal bifurcation level were 82.74 to 164.00, 83.12 to 121.49 respectively. Conclusion: The choice of the scout image had significant effect on the radiation dose and the sensitive organ dose in CT. (authors)