Utilizing a simple CT dosimetry phantom for the comprehension of the operational characteristics of CT AEC systems. Issue 11 (24th October 2013)

• Record Type: Journal Article
• Authors: Tsalafoutas, Ioannis A.; Varsamidis, Athanasios; Thalassinou, Stella; Efstathopoulos, Efstathios P.
• Is Part Of: Medical physics. Volume 40:Issue 11(2013)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2013-10-24
• Subjects: Dosimetry/exposure assessment -- Computed tomography -- Reconstruction; computerised tomography -- diagnostic radiography -- dosimetry -- image reconstruction -- medical image processing -- phantoms -- polymers; computed tomography -- tube current modulation -- automatic exposure control -- radiation dose -- image quality; Computerised tomographs -- Biological material, e.g. blood, urine; Haemocytometers -- Digital computing or data processing equipment or methods, specially adapted for specific applications -- Image data processing or generation, in general -- Scintigraphy; Computed tomography -- Medical imaging -- Medical image reconstruction -- Medical image noise -- Medical image quality -- Dosimetry -- Image scanners -- Image reconstruction -- Control systems -- Radiography; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4826160
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: : To investigate the utility of the nested polymethylacrylate (PMMA) phantom (which is available in many CT facilities for CTDI measurements), as a tool for the presentation and comparison of the ways that two different CT automatic exposure control (AEC) systems respond to a phantom when various scan parameters and AEC protocols are modified. Methods: : By offsetting the two phantomˈs components (the head phantom and the body ring) half‐way along their longitudinal axis, a phantom with three sections of different x‐ray attenuation was created. Scan projection radiographs (SPRs) and helical scans of the three‐section phantom were performed on a Toshiba Aquilion 64 and a Philips Brilliance 64 CT scanners, with different scan parameter selections [scan direction, pitch factor, slice thickness, and reconstruction interval (ST/RI), AEC protocol, and tube potential used for the SPRs]. The dose length product (DLP) values of each scan were recorded and the tube current (mA) values of the reconstructed CT images were plotted against the respective Z‐axis positions on the phantom. Furthermore, measurements of the noise levels at the center of each phantom section were performed to assess the impact of mA modulation on image quality. Results: : The mA modulation patterns of the two CT scanners were very dissimilar. The mA variations were more pronounced for Aquilion 64, where changes in any of the aforementioned scan parameters affected both the mA modulations curves and DLP values. However, the noise levels were affected only by changes in pitch, ST/RI, and AEC protocol selections. For Brilliance 64, changes in pitch affected the mA modulation curves but not the DLP values, whereas only AEC protocol and SPR tube potential selection variations affected both the mA modulation curves and DLP values. The noise levels increased for smaller ST/RI, larger weight category AEC protocol, and larger SPR tube potential selection. Conclusions: : The nested PMMA dosimetry phantom can be effectively utilized for the comprehension of CT AEC systems performance and the way that different scan conditions affect the mA modulation patterns, DLP values, and image noise. However, in depth analysis of the reasons why these two systems exhibited such different behaviors in response to the same phantom requires further investigation which is beyond the scope of this study.