title: Influence of CT automatic tube current modulation on uncertainty in effective dose.
abstract: Computed tomography (CT) scanners are equipped with automatic tube current modulation (ATCM) systems that adjust the current to compensate for variations in patient attenuation. CT dosimetry variables are not defined for ATCM situations and, thus, only the averaged values are displayed and analysed. The patient effective dose (E), which is derived from a weighted sum of organ equivalent doses, will be modified by the ATCM. Values for E for chest-abdomen-pelvis CT scans have been calculated using the ImPACT spreadsheet for patients on five CT scanners. Values for E resulting from the z-axis modulation under ATCM have been compared with results assessed using the same effective mAs values with constant tube currents. Mean values for E under ATCM were within ±10 % of those for fixed tube currents for all scanners. Cumulative dose distributions under ATCM have been simulated for two patient scans using single-slice dose profiles measured in elliptical and cylindrical phantoms on one scanner. Contributions to the effective dose from organs in the upper thorax under ATCM are 30-35 % lower for superficial tissues (e.g. breast) and 15-20 % lower for deeper organs (e.g. lungs). The effect on doses to organs in the abdomen depends on body shape, and they can be 10-22 % higher for larger patients. Results indicate that scan dosimetry parameters, dose-length product and effective mAs averaged over the whole scan can provide an assessment in terms of E that is sufficiently accurate to quantify relative risk for routine patient exposures under ATCM.
authors: Department of Radiological Technology, Faculty of Allied Health Sciences, Naresuan University, Phitsanulok 65000, Thailand supawitoos@nu.ac.th.
authors: S
authors: S
authors: Sookpeng
authors: Department of Clinical Physics, University of Glasgow, Glasgow G12 8QQ, UK.
authors: C J
authors: CJ
authors: Martin
authors: Health Physics, Gartnavel Royal Hospital, 1055 Great Western Road, Glasgow G12 0XH, UK.
authors: D J
authors: DJ
authors: Gentle
mesh_terms: D000465:Algorithms; D001331:Automation; D003198:Computer Simulation; D006801:Humans; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011835:Radiation Protection / Q000379:methods; D011874:Radiometry / Q000295:instrumentation / Q000379:methods; D012189:Retrospective Studies; D012306:Risk; D013909:Thorax / Q000528:radiation effects; D014018:Tissue Distribution; D015898:Tomography Scanners, X-Ray Computed; D014057:Tomography, X-Ray Computed; D035501:Uncertainty