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Image compression requirements and standards in PACS

Cost effective telemedicine and storage create a need for medical image compression. Compression saves communication bandwidth and reduces the size of the stored images. After clinicians become acquainted with the quality of the images using some of the newer algorithms, they accept the idea of lossy compression. The older algorithms, JPEG and MPEG in particular, are generally not adequate for high quality compression of medical images. The requirements for compression for medical images center on diagnostic quality images after the restoration of the images. The compression artifacts should not interfere with the viewing of the images for diagnosis. New requirements for compression arise from the fact that the images will likely be viewed on a computer workstation, where the images may be manipulated in ways that would bring out the artifacts. A medical imaging compression standard must be applicable across a large variety of image types from CT and MR to CR and ultrasound. To have one or a very few compression algorithms that are effective across a broad range of image types is desirable. Related series of images as for CT, MR, or cardiology require inter-image processing as well as intra-image processing for effective compression. Two preferred decompositions of the medical images are lapped orthogonal transforms and wavelet transforms. These transforms decompose the images in frequency in two different ways. The lapped orthogonal transforms groups the data according to the area where the data originated, while the wavelet transforms group the data by the frequency band of the image. The compression realized depends on the similarity of close transform coefficients. Huffman coding or the coding of the RICE algorithm are a beginning for the encoding. To be really effective the coding must have an extension for the areas where there is little information, the low entropy extension. In these areas there are less than one bit per pixel and multiple pixels must be coded together for the most effective compression. When the compression standard is available, it may be used in the interchange of medical images. The Digital Image Communications In Medicine (DICOM) standard is the interchange standard within which the compression is meant to be used. ACR-NEMA Working Group IV is the group that is considering compression standards for medical images. The concepts presented here are a suggestion for consideration by Working Group IV. Loral Medical Systems has one instantiation of a compression technique that satisfies the requirements outlined for a standard.