The Impact of Vertical Off-Centring, Tube Voltage and Phantom Size on CT Numbers: An Experimental Study

Abstract

Objectives: This pilot experimental study explores the effect of vertical off-centring on CT numbers in combination with various tube voltages and phantom sizes.Methods: A CIRS Model 062 Electron Density and Combined Head and Body phantom underwent imaging using a Siemens Emotion 16-slice CT and GEMINI GXL scanners. Uniformity was evaluated as a function of vertical off-centring (20, 40, 60, and 80 mm above the gantry iso-centre) using different phantom sizes and tube voltage for ROI positions across the X and Y axis of each phantom. CT number change was assessed by comparing the measured values between anterior (upper) versus posterior (lower) ROIs.Results: The results showed that vertical off-centring and phantom size could account for 92% of the recorded variance and resultant CT number change. The uniformity test recorded a maximum change of 27.2 and 14 HU for peripheral ROIs at 80 mm phantom shift above the gantry iso-centre using the GEMINI GXL and Siemens scanners, respectively. The absolute CT number differences between anterior and posterior ROIs were 13.7 for the 30 cm phantom and 4.8 for the 20 cm phantom at 80 mm vertical off-centring. The most remarkable differences were observed at lower tube voltages.Conclusions: It is essential to highlight the significance of optimal patient centring for CT examinations and the consequences of CT numbers variation on clinical decision making. Phantom off-centring and ROI location have been demonstrated to affect CT number uniformity in this pilot experimental study. This was more evident at peripheral phantom areas, lower tube voltages and larger phantom size.