Evaluation of the image quality of temporal subtraction images produced automatically in a PACS environment

Abstract

The aim of the study is to evaluate the reliable production of temporal subtraction images in a picture archiving and communication system environment and to establish objective criteria for the evaluation of image quality. A total of 117 temporal subtraction chest images (55 in the upright position, 62 in the supine position) were obtained in five consecutive days. In all of these, we confirmed that there were no interval changes on the original images, and cases with diffuse lung disease were excluded. The temporal subtraction images were classified by three chest radiologists into five levels: 5, excellent; 4, good; 3, acceptable; 2, poor; and 1, very poor. The following were examined: (1) the yield of adequate quality of the temporal subtraction images; (2) whether the temporal subtraction images were obtained in the warping or nonwarping mode; and (3) the correlation of the overall subjective image quality with the relative shift angles, relative shift distances, and the standard deviation of gray levels in the temporal subtraction images. The percentages of acceptable temporal subtraction images were 100% and 66% in the upright and supine positions, respectively. Sixteen (26%) of the 62 supine-position images were made in nonwarping mode, whereas all upright images were made in warping mode. Significant correlations were obtained in the relative shift angle (P < 0.05), relative horizontal shift distance (P < 0.05), and standard deviation of gray levels (P < 0.0001). Temporal subtraction images with acceptable image quality were obtained in the upright position. The objective criteria may be useful for the evaluation of image quality.

Fig 1

Examples of temporal subtraction images with each of the five different scores. (a) Excellent (5 points); (b) good (4 points); (c) acceptable (3 points; (d) poor (2 points), and (e) very poor (1 point). Misregistration artifacts come into prominence in images with the lesser scores.

Fig 2

(a) Previous chest radiograph, (b) current radiograph, and (c) temporal subtraction image produced by warping mode and scored at 5 points. The temporal subtraction image shows only slight misregistration artifacts on the margin of the heart. The standard deviation of the grayscale of the temporal subtraction image was 41. The relative shift angle, vertical shift, and horizontal shift between the previous and current chest radiographs were 0.3°, 16 pixels, and 15 pixels, respectively.

Fig 3

(a) Previous chest radiograph, (b) current radiograph, and (c) temporal subtraction image produced by warping mode and scored at 2 points. The temporal subtraction image shows remarkable misregistration artifact image of the thoracic cage. The standard deviation of the grayscale of the temporal subtraction image was 96. The relative shift angle, vertical shift, and horizontal shift between the previous and current chest radiographs were 1.8°, 0 pixels, and 12 pixels, respectively.

Fig 4

Correlation between overall subjective image quality and the following parameters: (a) relative shift angle, (b) relative horizontal shift distance, and (c) relative vertical shift distance between previous and current chest radiographs. (d) Correlation between subjective image quality and standard deviation in temporal subtraction images.