title: Utilizing a simple CT dosimetry phantom for the comprehension of the operational characteristics of CT AEC systems.
abstract: OBJECTIVE
  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 phantom's 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.
authors: Medical Physics Department, Anticancer-Oncology Hospital of Athens "Agios Savvas," 171 Alexandras Avenue, 115 22 Athens, Greece.
authors: Ioannis A
authors: IA
authors: Tsalafoutas
authors: Athanasios
authors: A
authors: Varsamidis
authors: Stella
authors: S
authors: Thalassinou
authors: Efstathios P
authors: EP
authors: Efstathopoulos
mesh_terms: D001330:Electronic Data Processing; D004867:Equipment Design; D006257:Head / Q000000981:diagnostic imaging; D006801:Humans; D007091:Image Processing, Computer-Assisted; D019047:Phantoms, Imaging; D019904:Polymethyl Methacrylate / Q000737:chemistry; D011829:Radiation Dosage; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods*; D011874:Radiometry / Q000379:methods*; D015203:Reproducibility of Results; D015898:Tomography Scanners, X-Ray Computed; D014057:Tomography, X-Ray Computed
chemical_list: D019904:Polymethyl Methacrylate