The Impacts of Vertical Off-Centring, Localiser Direction, Phantom Positioning and Tube Voltage on CT Number Accuracy: An Experimental Study

Background: This study investigates the effects of vertical off-centring, localiser direction, tube voltage, and phantom positioning (supine and prone) on computed tomography (CT) numbers and radiation dose. Methods: An anthropomorphic phantom was scanned using a Discovery CT750 HD-128 slice (GE Healthcare) scanner at different tube voltages (80, 120, and 140 kVp). Images employing 0 degrees and 180 degrees localisers were acquired in supine and prone positions for each vertical off-centring (+/- 100, +/- 60, and +/- 30 mm from the iso-centre). CT numbers and displayed volume CT dose index (CTDIvo1) were recorded. The relationship between dose variation and CT number was investigated. Results: The maximum changes in CT number between the two phantom positions as a function of vertical-off-centring were for the upper thorax 34 HU (0 degrees localiser, 120 kVp), mid thorax 43 HU (180 degrees localiser, 80 kVp), and for the abdominal section 31 HU (0 degrees localiser, 80 kVp) in the prone position. A strong positive correlation was reported between the variation in dose and CT number (r = 0.969, p < 0.001); 95% CI (0.93, 0.99). Conclusions: Patient positioning demands an approach with a high degree of accuracy, especially in cases where clinical decisions depend on CT number accuracy for tissue lesion characterisation.