Effect of Tube Projection Angle of Scout Images on Radiation Dose and Image Quality in Head CT: Based on ATCM Combined with Spectral Shaping Technology

Objective: To investigate the effect of different tube projection angles in scout imaging using automatic tube-current modulation (ATCM) and spectral-shaping technology on radiation dose and image quality in head CT examinations. Methods: Head-neck phantoms are used for helical scans with ATCM. Combinations of tube projection angle include anteroposterior (AP), posteroanterior (PA), lateral (LAT), AP+LAT, and PA+LAT. The helical scan tube voltages are 120 kV, 80/140 kV, Sn 150 kV, 70/Sn 150 kV, 80/Sn 150 kV, 90/Sn 150 kV, and 100/Sn 150 kV. Each scan method is repeated thrice. The volume CT dose index (CTDI_vol) and dose-length product (DLP) are recorded, and the lens organ dose is measured using a thermoluminescent dosimeter. At the central orbital plane, regions of interest (ROIs) are delineated, with the centers of the left and right eyeballs as well as the brainstem designated as signal regions, and air as the background region. The contrast-to-noise ratio (CNR) is calculated based on the mean CT values and standard deviations of the ROIs. Based on the ocular and brainstem CNRs as well as the lens organ dose, the figures-of-merit (FOMs) for the eyes and brainstem are calculated. A generalized estimation equation is established for the FOM of different scout-image combinations, and the model is constructed using an equicorrelation structure matrix. Results: CTDI_vol and lens organ dose: Both single- and dual-tube conventional and spectral filtration groups yield the highest values for PA positioning and the lowest values for lateral positioning. CNRs of eye and brainstem: For 120 kV, Sn 150 kV, and dual-tube spectral filtration groups, the highest CNR is observed in PA positioning, whereas the lowest in lateral positioning. For 80/140 kV, the highest CNR is observed in (PA+lateral) positioning, and the lowest in lateral positioning. The projection angles of different scout-image tubes significantly affect the FOM. At 120 kV, the highest FOM is indicated by (PA+lateral) positioning, whereas at Sn 150 kV, the highest is indicated by PA positioning. For dual-tube conventional and spectral filtration groups, the lateral positioning indicates the highest FOM. Conclusion: The selection of scout images affects the effectiveness of ATCM under spectral shaping. Under Sn 150kV, the use of PA scout images is recommended, whereas the use of LAT scout images is recommended for dual-source spectral shaping.