Bruce Kuo Ting Ho

THE EFFECT OF IRREVERSIBLE IMAGE COMPRESSION ON DIAGNOSTIC ACCURACY IN THORACIC IMAGING

RATIONALE AND OBJECTIVES.Digital image compression reduces the storage requirements and network traffic on picture archiving and communications systems. Full-frame bitallocation (FFBA) is an irreversible image-compression method based on the discrete cosine transform that provides for high compression ratios with a high degree of image fidelity. METHODS.One hundred twenty-two posteroanterior chest radiographs were obtained on patients in an ambulatory patient setting, including 30 cases of interstitial lung disease, 45 images containing combinations of lung nodules (N=37) or mediastinal masses (N=39), and 47 normal images containing none of the pathology for which we were testing. The images were digitized (nominal 2 K X 2 K X 12-bit resolution), printed on a 35 x 35-cm hard copy format, and compressed at an approximate compression ratio of 20:1. Observer performance tests were conducted with five radiologists using receiver operating characteristic analysis on digitized uncompressed and compressed hard copy images. RESULTS.There were no significant differences between the two display conditions for the detectability of any of the thoracic abnormalities. CONCLUSIONS.Our preliminary results suggest that irreversible image compression at ratios of 20:1 may be acceptable for use in digital thoracic imaging.

Implementation of a large-scale picture archiving and communication system

This paper describes the implementation of a large-scale picture archiving and communication system (PACS) in a clinical environment. The system consists of a PACS infrastructure, composed of a PACS controller, a database management system, communication networks, and optical disk archive. It connects to three MR units, four CT scanners, three computed radiography systems, and two laser film digitizers. Seven display stations are on line 24 h/day, 7 days/wk in genitourinary radiology (2K), pediatric radiology in-patient (1K and 2K) and outpatient (2K), neuroradiology (2K), pediatric ICU (1K), coronary care unit (1K), and one laser film printing station. The PACS is integrated with the hospital information system and the radiology information system. The system has been in operation since February 1992. We have integrated this PACS as a clinical component in daily radiology practice. It archives an average of 2.0-gigabyte image data per workday. A 3-mo system performance of various components are tabulated. The deployment of this large-scale PACS signifies a milestone in our PACS research and development effort. Radiologists, fellows, residents, and clinicians use it for case review, conferences, and occasionally for primary diagnosis. With this large-scale PACS in place, it will allow us to investigate the two critical issues raised when PACS research first started 10 yrs ago: system performance and cost effectiveness between a digital-based and a film-based system.

Applying wavelet transforms with arithmetic coding to radiological image compression

Radiological archives need the images to be compressed at a moderate compression ratio between 10:1 to 20:1 while retaining good diagnostic quality. We have developed a compression algorithm based on discrete wavelet transforms (DWTs) and arithmetic coding (AC) that satisfies those requirements. This new method is superior to the previously developed full frame discrete cosine transform (FFDCT) method, as well as the industrial standard developed by the joint photographic expert group (JPEG). Since DWT is localized in both spatial and scale domains, the error due to quantization of coefficients does not propagate throughout the reconstructed picture as in FFDCT. Because it is a global transformation, it does not suffer the limitation of block transform methods such as JPEG. The severity of the error as measured by the normalized mean square error (NMSE) and maximum difference technique increases very slowly with compression ratio compared to the FFDCT. Normalized nearest neighbor difference (NNND), which is a measure of blockiness, stays approximately constant, while JPEG NNND increases rapidly with compression ratio. Furthermore, DWT has an efficient finite response filter FlR implementation that can be put in parallel hardware. DWT also offers total flexibility in the image format; the size of the image does not have to be a power of two as in the case of FFDCT. >

Radiological image compression using error-free irreversible two-dimensional direct-cosine-transform coding techniques

Some error-free and irreversible two-dimensional direct-cosine-transform (2D-DCT) coding, image-compression techniques applied to radiological images are discussed in this paper. Run-length coding and Huffman coding are described, and examples are given for error-free image compression. In the case of irreversible 2D-DCT coding, the block-quantization technique and the full-frame bit-allocation (FFBA) technique are described. Error-free image compression can achieve a compression ratio from 2:1 to 3:1, whereas the irreversible 2D-DCT coding compression technique can, in general, achieve a much higher acceptable compression ratio. The currently available block-quantization hardware may lead to visible block artifacts at certain compression ratios, but FFBA may be employed with the same or higher compression ratios without generating such artifacts. An even higher compression ratio can be achieved if the image is compressed by using first FFBA and then Huffman coding. The disadvantages of FFBA are that it is sensitive to sharp edges and no hardware is available. This paper also describes the design of the FFBA technique.

PACS workstation design

This paper covers some of the recent concepts in designing a digital imaging workstation in a multimodality Picture Archiving and Communications Systems (PACS) network. A workstation in a multimodality PACS network must access, display, and analyze digital images from different imaging modalities with very different formats. The user interface should allow clinicians with minimal or no computer manipulation skills to use complex analysis tools. General guidelines of a graphics oriented user interface, based on windows and icons, are proposed. Instantaneous (real-time) response in the primary display and processing functions is vital for user acceptance. The hardware architectural concepts to achieve such a performance speed are described. Finally, a workstation environment conducive to comfortable viewing by the radiologists is discussed.

Three-dimensional transform compression of images from dynamic studies

Transform based compression methods achieve their effect by taking advantage of the correlations between adjacent pLtels in an image. The increasing use of three-dimensional imaging studies in radiology requires new techniques for image compression. For time-sequenced studies such as digital subtraction angiography, pixels are correlated between images, as well as within an image. By using three-dimensional cosine transforms, correlations in time as well as space can be exploited for image compression. Sequences of up to eight 512 x 512 x 8-bit images were compressed using a single full volume three-dimensional cosine transform, followed by quantization and bit-allocation. The quantization process is a uniform thresholding type and an adaptive three-dimensional bit-allocation table is used. The resultant image fidelity vs. compression ratio was shown to be superior to that achieved by compressing each image individually.

Volumetric image compression by 3D discrete wavelet transform (DWT)

The newly developed discrete wavelet transform (DWT) compression method is far superior to previous full frame discrete cosine transform (FFDCT) as well as industrial standard JPEG. Due to its localization properties both in spatial and transform domain, the quantization error introduced in DWT will not propagate globally as in FFDCT. Also DWT transform is a global technique that avoids the JPEG type block artifacts. As in all techniques, correlation among pixels makes compression possible. In volumetric image sets, such as CT and MR, inter-slice correlation can be exploited in addition to in-slice correlation. In this 3D DWT study, inter- slice correlation has also been investigated for CT and MR image set. Different numbers of slices are grouped together to perform wavelet transform in the transaxiale direction as a mean of testing relationship between correlation and compression efficiency. The 3D DWT is developed on UNIX platform. Significant higher compression ratio is achieved by compressing CT data as a volume versus one slice at a time. DWT is an excellent technique for exploiting inter-slice correlation to gain additional compression efficiency.

Mathematical model to quantify JPEG block artifacts

A method for quantifying the magnitude of block artifacts caused by JPEG type of image compression has been developed. The behavior of block artifacts as a function of compression ratio and edge enhancement operations can now be mathematically tracked. This mathematical technique is found to be useful for development of image compression techniques with regard to eliminating block artifacts.

Effect of data compression on diagnostic accuracy in digital hand and chest radiography

Image compression is essential to handle a large volume of digital images including CT, MR, CR, and digitized films in a digital radiology operation. The full-frame bit allocation using the cosine transform technique developed during the last few years has been proven to be an excellent irreversible image compression method. This paper describes the effect of using the hardware compression module on diagnostic accuracy in hand radiographs with subperiosteal resorption and chest radiographs with interstitial disease. Receiver operating characteristic analysis using 71 hand radiographs and 52 chest radiographs with five observers each demonstrates that there is no statistical significant difference in diagnostic accuracy between the original films and the compressed images with a compression ratio as high as 20:1.

Full-frame cosine transform image compression for medical and industrial applications

Full-frame image compression capable of a 10∶1 ratio or higher was developed for radiological applications where block artifacts are not acceptable. Applications in machine vision with similar stringencies can utilize the same principles and hardware design. Subsecond compression speed for image sizes up to 1K × 1K × 8 can be achieved by DMA and DSP designs using off-the-shelf components and customized bus architecture. Successions of completed hardware module design and proposed enhancements are reported with technical details. The possibility of 512 × 512 × 8 image compression in real time is also explored with design examples.