Investigation of CT dosimetry techniques for use in optimisation of Automatic Tube Current Modulation (ATCM) performance

QC Physics

Sookpeng, Supawitoo ; Martin, Colin

University:University of Glasgow

Department:School of Medicine, Dentistry & Nursing

关键词: Computed Tomography (CT), Automatic Tube Current Modulation (ATCM), Automatic Exposure Control (AEC), CT Dosimetry;   

Others  :  http://theses.gla.ac.uk/5252/7/2014sookpengphd.pdf

来源: University of Glasgow

【 摘 要 】

The PhD project aimed to develop methodologies for optimisation of automatic tube current modulation (ATCM) performance for the four most important computed tomography (CT) scanner manufacturers (Toshiba, GE, Philips and Siemens scanners). Since the human body to which ATCM response is elliptical in shape, the project started with an investigation of differences between dosimetry in cylindrical and elliptical phantoms. The ATCM systems modulate the tube current in both the x-y plane and z-axis, therefore phantoms made from multiple sections of different size ellipses were designed for quality control of the ATCM performances and evaluated by comparing with a conical phantom developed by imaging performance and assessment of CT (ImPACT) evaluation centre, UK. In order to link the project into patient dose optimisation, CT scanners in which patient doses were high were identified and the link with patient size evaluated. Since the large variations in patient dose may be influenced by scan parameters, the phantom developed was used to carry out measurements on CT scanners and investigate some factors and attribute reasons for the high doses. Finally changes in CT scanner protocols were recommended. The results from the elliptical phantom showed that the doses in the centre and anterior were larger than in the cylinder, while doses in the lateral periphery were similar. Differences in ratios of doses between the two phantoms for different CT scanners are linked to the beam profiles produced by the individual bow tie filters. Phantoms made from multi elliptical sections demonstrated similar trends for the Philips and Siemens ATCM systems. However, the abrupt changes in attenuation provoked the ATCMs to increase tube current aggressively with the GE and Toshiba systems. A phantom like a wedding cake with broader sections and smaller differences in attenuation circumvented these effects. The volume weighted CT dose index (CTDIvol) increased significantly with patient size for Toshiba and GE scanners whilst the changes for Siemens and Philips scanners were less marked. However, the use of Philips the D-DOM ATCM option led to a significant increase in patient dose. The reconstruction filter and image thickness are major factors influencing patient dose for the Toshiba CT scanner.