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Effect of Digital Imaging Systems on Image Quality and Diagnostic Accuracy

1. Development of a High-Quality Film Duplication System Using a Laser Digitizer

We developed a high quality film-duplication system in order to improve the image quality of duplicated radiographs and to recover improperly exposed films. The system consisted of a laser film digitizer, a laser film printer, a workstation, and a magneto-optical disk. Radiographs were digitized by the laser digitizer, processed by the computer for image enhancement, and then printed on a film by the laser printer. A nonlinear density-correction technique was employed in recovering improperly exposed radiographs using the H & D curve of the screen-film system. Using the new duplication system in our department, the average recovery rate was over 80% for chest and abdominal films rejected due to over- or under-exposure. The basic imaging properties of the duplication system were compared with those of a Computed Radiography (CR) system and a conventional screen-film system. For low spatial frequencies, the MTF of the CR system was superior to that of the digital duplication system; however, for high spatial frequencies, the MTF of the duplication system was superior. The noise in the duplication system was about half of that in the CR system. (381, 383, 401, 414)

2. Comparison of Imaging Properties of Computed Radiography and Screen-Film Systems

To compare the diagnostic quality of images obtained with a CR system based on storage phosphor technology with that obtained with conventional screen-film systems, we devised a dual-image recording technique. With this technique, a CR imaging plate is placed behind a screen-film system in a conventional cassette. This made it possible to obtain two images simultaneously, one from each system, in a clinical examination with the same patient positioning, the same degree of patient motion, the same geometric unsharpness, and no additional exposure. The MTFs of the CR system with and without the dual-image recording technique were greater at low frequencies, but lower at high frequencies, than the MTFs of the screen-film systems used. The noise Wiener spectra of the CR images at the plane of the imaging plate were greater than those of the screen-film systems, but were comparable to those of the screen-film systems at the plane of the printed film due to the reduction in image size. Clinical chest images obtained with the dual-image recording technique appeared comparable, probably because of the image size reduction and the use of mild unsharp mask processing. (328)


3. Effect of Data Compression on Diagnostic Accuracy in Digital Chest Radiography

High-resolution digital images comprise very large data sets which are relatively slow to transmit and expensive to store. Data compression techniques are being developed to address this problem, but significant image deterioration can occur at high compression ratios. In this study, we evaluated a new compression technique called adaptive block cosine transform coding, which allows considerable compression of digital radiographs with minimal degradation of image quality. To determine the effect of data compression on diagnostic accuracy, we performed observer tests using 60 digitized chest radiographs (2048 x 2048 matrix, and 1024 shades of gray) containing subtle examples of pneumothorax, interstitial infiltrate, nodules and bone lesions. These cases were presented in randomized order to 12 radiologists with no compression, with 25:1 compression, and with 50:1 compression ratios. Detection accuracy for these findings was evaluated by ROC analysis using a five-point confidence rating scale. The results indicated that, with this compression scheme, compression ratios as high as 25:1 might be acceptable for primary diagnosis in chest radiology. (318)

4. Effect of Display Format on Diagnostic Accuracy: Hardcopy, Video, and Reversed Gray Scale

To compare the effects of using hardcopy (film) versus video display on diagnostic accuracy, and also to determine the diagnostic merits of using conventional negative ("white bone") versus positive ("black bone") video displays, we conducted observer performance tests. The subjective preferences of each observer for each display modality were elicited, and diagnostic accuracy was determined by ROC analysis. Digitized chest radiographs were used, including normal and abnormal cases with a variety of subtle abnormalities. The hardcopy was printed with a 1024 x 1024 matrix by a high quality drum scanner in conventional "white bone" format only. The video images were displayed on a 1024 line monitor (30 Hz, interlaced) in both "white bone" and "black bone" formats with fixed window and brightness settings. A majority of our observers preferred hardcopy to video, though preferences were sharply divided between "white bone" and "blackbone" video. Diagnostic accuracy was significantly greater with hardcopy than with video display. In the case of video, accuracy was significantly superior with the conventional "white bone" format than with the "black bone" display. The improved accuracy obtained with film can be explained by its superior image quality in terms of MTF and contrast. The reason for the observers' inferior performance with reversed gray scale video compared to conventional video display is uncertain. (246)

5. Digital Mammography: Effects of Pixel Size on Detection of Subtle Microcalcifications

We investigated the spatial resolution requirement and the effect of unsharp-mask filtering on the detectability of subtle microcalcifications in digital mammography. Digital images were obtained by digitizing conventional screen-film mammograms with a 0.1 x 0.1 mm pixel size, processed with unsharp masking, and then reconstituted on film with a Fuji image processing/simulation system. Twenty normal cases and 12 cases with subtle microcalcifications were included. Observer performance experiments were conducted to assess the detectability of subtle microcalcifications in the conventional, the unprocessed digital, and the unsharp-masked mammograms. The observer response data were evaluated using ROC and LROC (ROC with localization) analyses. Our results indicate that digital mammograms obtained with 0.1 x 0.1 mm pixels provide lower detectability than the conventional screen-film mammograms. The detectability of microcalcifications in the digital mammograms was improved by unsharp-mask filtering; the processed mammograms still provided lower accuracy than the conventional mammograms, however, chiefly because of increased false-positive detection rates for the processed images at each subjective confidence level. Viewing unprocessed digital and unsharp-masked images in pairs resulted in approximately the same detectability as that obtained with the unsharp-masked images alone. However, this result may be influenced by the fact that the same limited viewing time was necessarily divided between the two images. (213)

6. Density Correction of Peripheral Breast Tissue on Digital Mammograms

When digital mammograms are viewed on video displays, evaluation of the skin and subcutaneous tissue is often difficult and may require special window settings. An algorithm was developed for selective enhancement (i.e., density correction) of the dark peripheral portions of the breast on mammograms. After an automated segmentation of the digital mammogram and identification of the skin line, a fitted enhancement curve was generated to selectively enhance all pixels within a certain distance from the skin to match the density of the center part of the breast. After enhancement, skin and breast parenchyma can be evaluated simultaneously without the need for different window settings. When tested on a set of 400 digitized mammograms, the density correction algorithm significantly (P<0.001) increased the maximum area of breast tissue visualized simultaneously at window width settings of 0.5-2.0 optical densities. Artifacts interfering with interpretation were observed in less than 1%. The algorithm for correcting the density of peripheral breast tissue substantially facilitated and improved the display of digital mammograms and thus will be a valuable component of an integrated workstation for computer-aided diagnosis in mammography. (442, 481)



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