The Relationship between Subjective and Objective Parameters in CT Phantom Image Evaluation

OBJECTIVE: To evaluate whether there is a relationship between subjective parameters determined by a reviewer (spatial resolution, low contrast resolution, and artifacts) and objective parameters (the CT number of water, noise, and image uniformity) in CT phantom image evaluations. MATERIALS AND METHODS: We reviewed the CT results of phantom image evaluations conducted by Korean Institute for Accreditation of Medical Image (KIAMI) from May 2007 to June 2007. We compared the objective parameters against the pass or fail groups for the subjective parameters. We also evaluated whether there is a relationship between the artifact types and the other subjective parameters. RESULTS: The mean noise value was significantly higher in the fail groups for the subjective parameters compared to the pass groups (p = 0.006). Specifically, noise and low contrast resolution were found to have a statistically significant positive correlation (r = 0.183, p < 0.001). In the fail group for low contrast resolution, the failure due to artifacts was significantly higher than the pass group (p < 0.001). In contrast, no statistically significant differences were found for the mean CT number of water, noise, or image uniformity based on the types of artifacts. CONCLUSION: Subjective CT image parameters evaluated by a reviewer correlate with objectively measured parameters, especially noise. Therefore, a stricter noise standard might be able to improve the subjective parameters results, such as low contrast resolution.

1024 Matrix Image Reconstruction: Usefulness in High Resolution chest CT

PURPOSE: We tried to evaluate whether high resolution chest CT with a 1,024 matrix has a significant advantage in image quality compared to a 512 matrix. MATERIALS AND METHODS: Each set of 512 and 1024 matrix high resolution chest CT scans with both 0.625 mm and 1.25 mm slice thickness were obtained from 26 patients. Seventy locations that contained twenty-four low density lesions without sharp boundary such as emphysema, and forty-six sharp linear densities such as linear fibrosis were selected; these were randomly displayed on a five mega pixel LCD monitor. All the images were masked for information concerning the matrix size and slice thickness. Two chest radiologists scored the image quality of each arrowed lesion as follows: (1) undistinguishable, (2) poorly distinguishable, (3) fairly distinguishable, (4) well visible and (5) excellently visible. The scores were compared from the the aspects of matrix size, slice thickness and the different observers by using ANOVA tests. RESULTS: The average and standard deviation of image quality were 3.09 (+/-.92) for the 0.625 mmx512 matrix, 3.16 (+/-.84) for the 0.625 mmx1024 matrix, 2.49 (+/-1.02) for the 1.25 mmx512 matrix, and 2.35 (+/-1.02) for the 1.25 mmx1024 matrix, respectively. The image quality on both matrices of the high resolution chest CT scans with a 0.625 mm slice thickness was significantly better than that on the 1.25 mm slice thickness (p < 0.001). However, the image quality on the 1024 matrix high resolution chest CT scans was not significantly different from that on the 512 matrix high resolution chest CT scans (p = 0.678). The interobserver variation between the two observers was not significant (p = 0.691). CONCLUSION: We think that 1024 matrix image reconstruction for high resolution chest CT may not be clinically useful.