The purposes of this work were to determine the optimal peak voltage for chest computed radiography (CR) using visual grading scores and to compare visual grading characteristics (VGC) and ordinal regression in visual grading analysis. An Afga CR system was used to acquire images of an anthropomorphic chest phantom. Both entrance surface dose and detector surface dose were measured using the Piranha 657 dosimeter. The images were acquired under various voltages from 80 to 120 kVp and exposures from 0.5 to 12.5 mAs. The image qualities were evaluated by 5 experienced radiologists/radiographers based on modified European imaging criteria using 1-5 visual grading scale. The VGC, ordinal regression as well as the conventional visual grading analysis (VGA) were employed for the image quality analysis. Both VGC and ordinal regression yielded the same results with both 100 kVp and 120 kVp producing the best image quality. The image quality of the 120 kVp was slightly higher than that of the 100 kVp but its dose was also higher than that of the 100kVp. On balancing image quality with dose, the 100 kVp should be the optimal kVp for the chest imaging using the Afga CR system. The ordinal regression is a powerful tool in the analysis of image quality using visual grading scores and the VGC can be handled by the ordinal regression.
The purposes of this work were to find an optimal x-ray voltage for CT imaging and to determine the diagnostic
effectiveness of image reconstruction techniques by using the visual grading analysis (VGA). Images of the PH-5 CT
abdomen phantom (Kagaku Co, Kyoto) were acquired by the Toshiba Aquillion One 320 slices CT system with various
exposures (from 10 to 580 mAs) under different tube peak voltages (80, 100 and 120 kVp). The images were
reconstructed by employing the FBP and the AIDR 3D iterative reconstructions with Mild, Standard and Strong FBP
blending. Image quality was assessed by measuring noise, contrast to noise ratio and human observer’s VGA scores. The
CT dose index CTDIv was obtained from the values displayed on the images. The best fit for the curves of the image
quality VGA vs dose CTDIv is a logistic function from the SPSS estimation. A threshold dose Dt is defined as the
CTDIv at the just acceptable for diagnostic image quality and a figure of merit (FOM) is defined as the slope of the
standardised logistic function. The Dt and FOM were found to be 5.4, 8.1 and 9.1 mGy and 0.47, 0.51 and 0.38 under the
tube voltages of 80, 100 and 120 kVp, respectively, from images reconstructed by the FBP technique. The Dt and FOM
values were lower from the images reconstructed by the AIDR 3D in comparison with the FBP technique. The optimal xray
peak voltage for the imaging of the PH-5 abdomen phantom by the Aquillion One CT system was found to be at 100
kVp. The images reconstructed by the FBP are more diagnostically effective than that by the AIDR 3D but with a higher
dose Dt to the patients.
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