The phantom study on the influence of radiation dose in CT scanning with different scanning centers combined with the techniques of the ATCM and CARE kV

Objective To investigate the influence of different scanning centers on the radiation dose of head-neck and chest spiral scanning in CT with automatic tube current modulation(ATCM) and automatic tube voltage modulation(CARE kV). Methods Combined with ATCM and CARE kV techniques, spiral CT scanning was performed on head-neck and chest phantoms. The head-neck phantom was scanned using 5 different scanning centers, with the levels of 4 cm above the eye, the eye, the midpoint of the eye and the outer ear hole, the external ear hole, 5 cm below the outer ear hole, respectively, according to different heights of check bed. The chest phantom was scanned using 6 different scanning centers with the levels of 5 cm above breast, 4 cm above breast, anterior axillary line, midaxillary line, posterior axillary line, respectively. At each scanning center they were scanned three times of scout and one spiral. ROIs were selected at the slices of orbital center and C5 upper edge level for head-neck phantom, and at the slices of the apical and tracheal bifurcation level for chest phantom. The values of contrast-to-noise ratios (CNRs) were measured and recorded. The organ dose of eye lens and mammary gland were measured with thermoluminescent dosimeters (TLD) for all of scans. The volume CT dose index (CTDIvol) of each scan was recorded. Results With 5 different scanning centers for the head-neck phantom, the maximum eye lens dose appeared at the level of midpoint of the eye and the outer ear hole(8.851 mGy), while the maximum CTDIvol and minimum eye lens dose at the level of 5 cm below the outer ear hole(15.850 mGy and 7.096 mGy). With 6 different scanning centers for the chest phantom, the maximum mammary gland dose emerged from the level of breast(6.467 mGy), while the maximum CTDIvol from the level of the anterior axillary line(4.120 mGy), the minimum gland dose and CTDIvol from the level of the posterior axillary line(4.794 mGy and 3.540 mGy). In the head-neck phantom images, the CNR values at the level of orbital center and C5 upper edge were 87.22 to 108.88, 136.13 to 175.57 respectively. In the chest phantom images, the CNR values at the level of the apical and tracheal bifurcation were 75.19 to 116.92, 42.85 to 86.78 respectively. Conclusions The selection of CT scanning center has great influence on the radiation dose of CT scanning, especially for radiation sensitive tissues and organs. Key words: Tomography, X-ray computed; Radiation dose; Scanning center