DICOM primer unlocks standard for PACS novices

DICOM rookies, take heart. Researchers in the U.K. have developed a primer that demystifies the imaging standard and helps radiologists make the most of their soft-copy images.

The advent of the DICOM file format enables digital images generated on various modalities to be stored and transferred with ease irrespective of equipment manufacturer. The complex array of image formats, however, can be bewildering to the uninitiated. The primer attempts to illuminate DICOM for users new to the standard who may be contemplating PACS for the first time (Clin Radiol 2005;60(11):1133-1140).

"Digital images can be manipulated in many ways and converted to different formats for teaching and publication purposes," said Dr. Richard N. Graham of the radiology department at John Radcliffe Hospital in Headington, U.K. "We outline a variety of ways in which radiologists utilize digital images and how to make the most of the capabilities of DICOM prior to the introduction of PACS."

The Radcliffe paper maps and explains the contents of the DICOM header, then discusses why it is often necessary to compress images before storage and transfer. The primer also examines the advantages and disadvantages of lossless and lossy compression techniques as well as a number of image file formats:

• JPEG (joint photographic experts group) allows users to specify how much compression is applied. Compressed data are irreversibly lost, possibly leading to unacceptable image degradation.

• PNG (portable network graphics) offers image brightness control and 2D interlacing for rapid viewing. Users can embed text (called metadata) within the image file.

• TIFF (tagged image file format) can be used to specify either lossy or lossless compression. TIFF files are large, however, making them less than ideal for Internet or PowerPoint applications, according to Graham.

• GIF (graphic interchange format) also uses a lossless compression algorithm. GIF has largely been superseded by PNG because compression of GIF images is less efficient than PNG by about 5% to 25%.

• JPEG 2000 is a new format not yet in wide use by radiologists. It allows certain parts of the image to be defined as regions of interest, which can then be displayed before other parts of the image, or losslessly compressed while other parts of the image undergo lossy compression. JPEG 2000 also supports metadata embedding.

The DICOM standard emerged in response to the increased use of digital images in radiology. In 1983, the American College of Radiology and the National Electrical Manufacturers Association formed a committee to create a standard format for storing and transmitting digital images. The result was the original 1985 ACR-NEMA standard.

The standard was subsequently revised in 1993 and renamed Digital Imaging and Communication in Medicine, or DICOM.

Recent improvements in Version 3.0 of DICOM permit transfer of digital imaging studies in a multivendor environment and, more important, facilitated the development of PACS.