0° vs. 180° CT localiser: The effect of vertical off‐centring, phantom positioning and tube voltage on dose optimisation in multidetector computed tomography

Abstract

Introduction: Patient positioning is an essential consideration for the optimisation of radiation dose during CT examinations. The study objectives seek to explore the effects of vertical off-centring, localiser direction (0°and 180°), and phantom positioning (supine and prone) on radiation dose, using three different tube voltages in multidetector computed tomography (MDCT) imaging. Methods: The trunk of a PBU-60 anthropomorphic phantom was imaged using a Discovery CT750 HD -128 slice (GE Healthcare). Images employing 0°and 180°localisers were acquired in supine and prone orientation for each combination of vertical off-centring (AE100, AE60 and AE30 mm) and different tube voltages (80, 120 and 140 kVp), using the system's automatic tube current modulation (ATCM) function. The displayed volume CT dose index (CTDI vol ) and dose length product (DLP) were recorded. Results: With incremental table off-centring of AE100 mm, the dose at 120 kVp in the supine position ranged from 63% to 196% (0°localiser) and from 66% to 191% (180°l ocaliser) as compared to iso-centre. While in the prone position, the dose ranged from 62% to 195% (0°localiser); and 62% to 193% (180°localiser), with a notable dose increase at higher tube voltages. Dose variation and vertical off-centring showed a significant relationship for both 0°and 180°localisers (r = 0.94 and 0.96, respectively, P < 0.001). The CTDI vol variation between supine and prone phantom positions at AE100 mm off-centring was 0.22 mGy (2.9%), and 0.19 mGy (2.3%) when the 0°and 180 °localisers were utilised, respectively. Conclusions: Phantom off-centring and localiser direction evidenced large dose variation. It is recommended that the 0°localiser is employed during CT examinations, in order to minimise the potential additional radiation dose which may result from off-centring and the use of lower tube voltages where clinically appropriate.

Influence of tube and patient positioning in thoracoabdominal CT examinations on radiation exposure–towards a better patient positioning

“…We barely saw patients being positioned above the iso-centre (29/684, 4%). This is in line with previously published articles [12]. Interestingly, in a study of Ryu et al, off-centreing has no clinical impact on radiation dose in the routine clinical practice [16].…”

“…The effect is well described in literature. Al-Hayek et al and Paolicchi et al even noticed that the magnification effect was more pronounced for 180 • -localisers, compared to 0 • -localisers [12,13]. Kaasalainen et al evaluated the effect of phantom positioning in chest CTs.…”

Impacts of Phantom Off-Center Positioning on CT Numbers and Dose Index CTDIv: An Evaluation of Two CT Scanners from GE

“…That explains why the cross-over of the CTDIv is located at a point above the system's iso-center as shown in Figure 6a. It also explains why using a PA localizer has a higher CTDIv than that of using an AP localizer when patients are positioned at the iso-center [12][13][14][15][16], because the iso-center I is at a magnifying position in the PA projection according to O. This mismatch could be avoided if manufacturers use both AP and PA projections to define the projection center for the ATCM.…”

“…ATCM takes the reduced photon flux through the body of a patient as a thicker body, and therefore applies a higher tube current. As a result, PA localizer has a higher CTDIv than that of an AP localizer in ATCM [12][13][14][15][16]. The American Association of Physicists in Medicine (AAPM) recommends AP and LAT projection images be used for the calculations of body sizes [10].…”

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

“…The study reported here represents a further analysis of data collected as a part of a larger study, which details the experiment and dose measurements [ 20 ]. The trunk of a PBU-60 anthropomorphic phantom (Kyoto Kagaku Co., Ltd., Kyoto, Japan) representing an average-sized adult was scanned using a 128-slice MDCT system (Discovery CT750 HD, GE Healthcare).…”