High-resolution digital images make up very large data sets that 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, the authors evaluated a form of adaptive block cosine transform coding, a new compression technique that allows considerable compression of digital radiographs with minimal degradation of image quality. To determine the effect of data compression on diagnostic accuracy, observer tests were performed with 60 digitized chest radiographs (2,048 x 2,048 matrix, 1,024 shades of gray) containing subtle examples of pneumothorax, interstitial infiltrate, nodules, and bone lesions. Radiographs with no compression, with 25:1 compression, and with 50:1 compression ratios were presented in randomized order to 12 radiologists. The results suggest that, with this compression scheme, compression ratios as high as 25:1 may be acceptable for primary diagnosis in chest radiology.

The digital mammography system evaluated here is a digital full-field phase contrast mammography (PCM) system that employs a practical molybdenum x-ray tube with a 0.1mm focal spot size. By using custom apparatus to position 14"x17" photostimulable phosphor plate 49cm from the object, a distance of 65cm was obtained between the object and the focal spot of the x-ray tube. A plate optimized for the PCM system acquired images magnified 1.75 times with a 14"x17" and was scanned at a sampling pitch of 43.75µm by using a CR system. The diagnostic images were reduced in printing to actual object size with a pixel size of 25µm on 8"x10" dry-processed film with a maximum density of 4. This study compares the performance of the system with that of a usual contact CR mammography system.

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A new scatter rejection technique using the modified air gap method is presented. Two x-ray cassettes are used per exposure. One cassette (front cassette) was placed immediately behind the object. The other (back cassette) was positioned 60 cm behind the object. Thefront radiograph has good spatial resolution but includes a relatively large amount of scatter. The back radiograph has poor high frequency resolution but little scatter due to the air gap. The spectral composition technique extracts highfrequency informationfrom thefront radiograph and combines it with the lowfrequency information of the back radiograph to produce an improved composite synthesized image. No artifacts due to the spectral composition technique were observed. In the lung and heart regions, the modified air gap method resulted in comparable or superior scatter rejection performance as compared to the conventional grid technique.

We have studied the problem of adaptive cosine transform coding, and developed an original quantization algorithm which can minimize block artifacts. There are three features in our compression technique.The first feauture is variable quantization interval. The quantization interval is determined adaptively depending on the statistical characteristics of the block. In our technique, the number of AC components which are rounded off to zero by quantization is used for the quantization parameter. The second feature is the adaptive cutoff of the high frequency components of the transform coefficients.High frequency components may be ignored adaptively in our technique.The third feature is the double quantization algorithm. Non -adaptive quantization is carried out first. The second quantization is performed adaptively. ABSTRACTWe have studied the problem of adaptive cosine transform coding, and developed an original quantization algorithm which can minimize block artifacts. There are three features in our compression technique.The first feauture is variable quantization interval. The quantization interval is determined adaptively depending on the statistical characteristics of the block. In our technique, the number of AC components which are rounded off to zero by quantization is used for the quantization parameter. The second feature is the adaptive cutoff of the high frequency components of the transform coefficients. High frequency components may be ignored adaptively in our technique.The third feature is the double quantization algorithm. Non-adaptive quantization is carried out first. The second quantization is performed adaptively. 474 /SPIEVol. 1093 Medical Imaging III: PACS System Design and Evaluation (1989) Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx SPIE Vol. 1093 Medical Imaging III: PACS System Design and Evaluation (1989) / 475 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx