Clinical Radiology (2005) 60, 1133–1140
REVIEW
DICOM demystified: A review of digital file formats and their use in radiological practice R.N.J. Graham, R.W. Perriss, A.F. Scarsbrook* Department of Radiology, John Radcliffe Hospital, Headington, Oxford, UK Received 28 June 2005; received in revised form 18 July 2005; accepted 28 July 2005
KEYWORDS Internet; Digital imaging and communications in medicine (DICOM); PACS
Digital imaging and communications in medicine (DICOM) is the standard image file format used by radiological hardware devices. This article will provide an overview of DICOM and attempt to demystify the bewildering number of image formats that are commonly encountered. The characteristics and usefulness of different image file types will be explored and a variety of freely available web-based resources to aid viewing and manipulation of digital images will be reviewed. How best to harness DICOM technology before the introduction of picture archiving and communication systems (PACS) will also be described. Q 2005 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction
What is the DICOM file format?
Digital images are generated by a wide variety of radiological hardware. Each device collects data, which are then encoded and stored electronically in DICOM format. This is a universal file type, developed to facilitate data exchange between hardware, irrespective of manufacturer. DICOM files store a large amount of data and usually need to be viewed on dedicated workstations but may be transferred electronically to other computers where they can be displayed provided appropriate DICOM viewing software is installed. DICOM files can easily be converted to a variety of image formats and edited before use in teaching and publications. There are a number of useful DICOM-related resources that are freely available on the internet.
In response to the increased use of digital images in radiology the American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA) formed a joint committee in 1983 to create a standard format for storing and transmitting medical images.1 The committee published the original ACR-NEMA standard in 1985.1 This has subsequently been revised and in 1993 the standard was renamed DICOM.1 More recent improvements in DICOM (Version 3.0) have permitted transfer of medical images in a multi-vendor environment, and importantly, have facilitated the development of PACS and digital interfacing with medical information systems.1 DICOM is administered by the NEMA Diagnostic Imaging and Therapy Systems division and each year the standard is updated. Details of recent improvements can be found on the NEMA website (http://medical.nema.org/).2 Each DICOM file has a header containing amongst other items, patient demographic information, acquisition parameters, referrer, practitioner and operator identifiers and image dimensions. The remaining portion of the DICOM file contains the image data. Because they often contain multiple high-resolution images, DICOM files tend to be large
* Guarantor and correspondent: A.F. Scarsbrook, Department of Radiology, John Radcliffe Hospital, Headley Way, Headington, Oxford OX3 9DU, UK. Tel.: C44 1865220815; fax: C44 1865220801. E-mail address:
[email protected] (A.F. Scarsbrook).
0009-9260/$ - see front matter Q 2005 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2005.07.003
1134
R.N.J. Graham et al.
Lossy compression file types
Figure 1 Lossless compression of a dataset: Repeated values are coded to reduce the overall file size.
[e.g., 35 megabytes for a pre and post-contrast computed tomography (CT) images of the brain] and are frequently compressed before storage and transfer.
Joint photographic experts group format (JPEG) This image type allows the user to specify how much compression is applied and hence how much of the original data are lost. JPEG files exploit the fact that the human eye perceives small colour changes less accurately than changes in brightness. The disadvantage with JPEG is that data are irreversibly lost and this may lead to an unacceptable level of image degradation (Fig. 2). The advantage, however, is a very small file size.
Digital image compression
Lossless compression file types
Image compression is a method of reducing file size to increase the amount of data that can be archived onto storage media and to speed up data transmission. DICOM images can be compressed by converting the data into smaller image file types using specialized software, which will be described in more detail later in the article. There are two main types of data compression: Lossless and lossy. Lossless compression allows the file size to be reduced without any loss of information. This allows all the original data to be recovered if necessary. In lossless data compression repeated identical values within the dataset are replaced with one value (Fig. 1) in a way that allows unambiguous decoding without loss of information. This process uses a substantial amount of processing power and makes compressed files slower to open and save. By contrast, lossy image compression permanently eliminates some of the file data and can result in a remarkable reduction in file size. The aim is to eliminate redundant information from the dataset without adversely affecting image quality, but excessive compression inevitably results in image degradation (Fig. 2).
Portable network graphics format (PNG) The PNG image format has several good features: Variable degree of transparency; image brightness control (gamma correction) and two-dimensional interlacing (initially every other line of the image data is displayed) for rapid image viewing.6 The lossless compression algorithm was written by Lempel and Ziv in 1977.7 A useful feature of the PNG format is the ability to embed text within the image file as so called “metadata”.3 This is rather like the header associated with a DICOM image, and is convenient when setting up a teaching file database as certain web-based search engines can find images based on this embedded information.
Image file formats
Graphic interchange format (GIF) This image format was first produced in 1987 and uses the LZW lossless compression algorithm.9 The compression of GIF images is less efficient than PNG files (by about 5–25%) and this file type lacks the wide-ranging features of the newer PNG.9 As a result GIF has been largely superseded by PNG.
Aside from DICOM there are a plethora of different digital image file formats, which can easily confuse the uninitiated. An extensive review of these is beyond the scope of this article, however, the interested reader may find additional information in an excellent article on image file formats by Wiggins et al.3 In simple terms, the different formats can be divided into those involving lossy compression such as Joint Photographic Experts Group (JPEG)4 and those using lossless compression, e.g., tagged image file format (TIFF).5
Tagged image file format (TIFF) With TIFF files either lossless or lossy data compression can be specified. Lossless compression is generally performed using the Lempel-Ziv-Welch (LZW) algorithm written by Abraham Lempel and Jacob Ziv in 1977 and 1978 and improved by Terry Welch in 1984, hence the name LZW.7,8 The disadvantage with TIFF is its relatively large file size making it less than ideal for Internet or PowerPoint (Microsoft, Redmond, WA, USA)-based applications.
Joint photographic experts group 2000 format (JPEG 2000) This is another example of an image file using lossless compression. JPEG 2000 images allow certain parts of the image to be defined as a region
DICOM demystified
1135
Figure 2 Image degradation resulting from excessive lossy data compression: (a) High-resolution JPEG image (144 kb) of an axial T2-weighted MRI image of the brain showing a right-sided acoustic neuroma (white arrow); (b) moderate compression of the same JPEG image (64 kb) showing no significant image degradation; (c) highly compressed JPEG (32 kb) demonstrating marked image degradation.
1136
R.N.J. Graham et al.
of interest (ROI), which can then be displayed before other parts of the image, or be losslessly compressed, whilst other less critical parts of the image undergo lossy compression. Like PNG, JPEG 2000 allows metadata to be embedded in the image file.4 JPEG 2000 is a new file format, and is not yet in wide use by radiologists. With its advanced features it is predicted that it is likely to be increasingly used in the future. So which of these file types should one use? Generally speaking, lossy files are perfectly suitable for image display in computer and web-based presentations where the small file size allows rapid image upload and facilitates easy image transfer between computers. In contrast, lossless formats are more suitable for archiving, teaching and for submission for publication. For example, Elsevier, the publisher of Clinical Radiology, provide guidance for submitting images electronically (http://authors.elsevier.com/ ArtworkInstructions.html?dcZAI1) the TIFF format is a preferred option. The most commonly used file formats in radiology are compared in Table 1.
Proprietary DICOM viewers Proprietary DICOM viewers tend to be written by the manufacturers of and supplied with medical imaging hardware, e.g., Advantage Workstation produced by General Electric Healthcare (Chalfont St Giles, UK). These dedicated workstations allow dynamic scrolling through stacks of images and many advanced functions, such as multiplanar reconstruction and three-dimensional volume rendering (Fig. 3). There is usually the ability to export images to portable storage media (e.g., CD-R) or to transfer images to other networked workstations. Exported files are typically converted by the proprietary software to smaller files (e.g., JPEG or PNG) that can then viewed on a PC without any special software. Once the DICOM images have been converted to other file formats the ability to view consecutive images from a series, as an interactive stack is lost. The downside is that workstations are often in constant use in radiology departments, particularly in those that do not yet have PACS installed and little time is available to use these for image manipulation purposes or for use in teaching. The use of third-party DICOM-viewing software can ameliorate this problem.
DICOM-viewing software Third-party software All of the image file types described above can easily be opened and viewed on a standard personal computer (PC) with a contemporary operating system such as Windows XP (Microsoft, Redmond, WA, USA) without the need for any special software. By contrast DICOM images require additional software to be installed before they can be opened and viewed. DICOM-viewing software falls into two main categories: Proprietary viewers, which are supplied with imaging systems such as CT or magnetic resonance imaging (MRI) machines; and third-party DICOM-viewing software, either in the form of PACS or as a stand-alone viewer for individual PCs. Table 1 formats.
There are a number of stand-alone DICOM-viewing packages that are commercially available. Probably the best known of these is eFilm (Merge eFilm, Milwaukee, WI, USA, available at: http://www. merge-efilm.com/products/efilmworkstation.asp) which has many advanced features and was, until
Comparison of lossy and lossless image file
File type
Compression method
Most suitable uses
JPEG TIFF
PowerPoint and website images Publications
PNG
Lossy Lossy or lossless Lossless
JPEG 2000 GIF
Lossless Lossless
Publications, teaching files and website images Publications and teaching files Website images
Figure 3 Screenshot from an example proprietary DICOM-viewing software package: GE Healthcare Advantage Windows workstation.
DICOM demystified
1137
relatively recently available free of charge (Fig. 4). This is no longer the case, and the current version is relatively expensive (Version 2.0.1 single licence costs $2500). Fortunately there are a number of free DICOM viewers that can be downloaded from the internet. These are of variable quality and other authors have reviewed a large number of the available resources.10,11 Good examples of free DICOM viewing software for use on individual PCs include Osiris (University Hospital of Geneva, Switzerland, available at: http://www.sim.hcuge. ch/osiris/01_Osiris_Presentation_EN.htm) and Dicom Works (Developed by Phillipe Puech & Loic Boussel, available at: http://dicom.online.fr/; Fig. 5). Free DICOM software is also available for use on Apple Mac computers and Osirix (Department of Radiology, David Geffen School of Medicine, University of California, Los Angeles, CA, USA, available at: http://homepage.mac.com/ rossetantoine/osirix/) is probably the best example (Fig. 6). All these programs have full DICOM functionality and allow individual or stacked images to be viewed and manipulated. A comparison of the main features of these example software packages is provided in Table 2. DICOM images can also be exported into Microsoft PowerPoint using a special plug in which can be downloaded, free of charge, from the internet (University of Toronto Medical Faculty, Toronto, Canada, available at: http://www.radfiler.com/ dicomppt.htm). A future article in this series will explore advanced uses of PowerPoint (Microsoft), including how to incorporate DICOM images into presentations. Figure 5 Screenshots from example freely available DICOM viewers for PCs: (a) Osiris and (b) Dicom Works.
Figure 4 Screenshot from an example commercially available DICOM viewer: Merge E-Film.
Figure 6 Screenshot from an example freely available DICOM viewer for Mac computers: Osirix.
1138
R.N.J. Graham et al.
Editing exported images After choosing which images to export it is often necessary to anonymize them. If images are for use in teaching or for publication this may be essential to comply with the Data Protection Act. The Royal College of Radiologists have produced a document, “The Data Protection Act 1998—Practical Implications”12 that provides further guidance in this area. Removing the overlay containing patient demographic information on the image before export can anonymize images. Most DICOM viewers are able to do this, including the ones reviewed above. Exported image files can be imported into photo-editing software such as Adobe Photoshop (Adobe, San Jose, CA, USA, available at: http:// www.adobe.co.uk/products/photoshop/main.html) or Irfanview (Developed by Irfan Skiljan, available at: www.irfanview.com) for further manipulation (Fig. 7). Adobe Photoshop is the industry standard desktop digital imaging package with extensive image editing features but is relatively expensive (w£540). Adobe also produce Photoshop Elements (available at: http://www.adobe.co.uk/products/ photoshopelwin/main.html) which has much of the features of the more expensive product apart from some of the high-end professional editing functions and is available at a fraction of the cost (£49.99). Irfanview, whilst less comprehensive, has most of the basic image editing functions that a radiologist will need and can be downloaded from the internet free of charge. Once an image has been imported into editing software, cropping so that only the region of interest is displayed can reduce the size of the file. Cropping can also be used to remove any patient identifiers from the image. Altering the colour depth of the image can further reduce the file size. Saving a grey-scale image as an 8 bit grey scale rather than as a 24-bit colour image will decrease the file size by a factor of three without degrading the image. Often it is advantageous to place arrows on radiological images for presentations or publications. This can be done with ease in presentation packages such as Microsoft PowerPoint. The image Table 2
Figure 7 Screenshot from an example freely available image editing software package: Irfanview.
is imported into the presentation and arrows can easily be added as an overlay using the arrow tool from the draw toolbar. Arrows added in this manner do not alter the underlying image, so if the image is exported from PowerPoint it will be saved without any of the modifications. Photo-editing packages allow arrows or text to be added in so-called “layers”. Each layer is like an overlay on the image and individual arrows or items of text can be altered or deleted independently of each other. When the modified image is saved, the layers can be merged or kept separate. The advantage of keeping layers separate is that further manipulation can be performed at a later date; the file size is increased as a result. There are a vast number of other features within Adobe Photoshop, for example altering image contrast and brightness, rotating images, masking unwanted annotations and conversion from one image format to another. Many other image editing functions of use to radiologists are detailed in a number of articles.13–18
Image manipulation for clinical meetings or teaching purposes Before the widespread introduction of PACS into radiological departments, remote DICOM image display in clinico-radiological meetings and for teaching purposes was a technical challenge. Many proprietary
Comparative features of stand-alone DICOM viewing packages.
Feature
Osiris
DicomWorks
Osirix
eFilm
Export images as TIFF or JPEG View two series linked together Multiplanar reformating Windowing of images Measurement tools
Yes No Yes Yes Yes
Yes Yes No Yes Yes
Yes Yes Yes Yes Yes
Yes Yes Yes Yes Yes
DICOM demystified
workstations allow export of DICOM images to a CD-R along with a rudimentary DICOM viewer. The exported images can then be displayed on most PCs and use a Java-enabled web browser such as Internet Explorer (Microsoft) to display the images. However, these web viewers tend not to have many image manipulation or calibration features. Alternative methods of digital image display either require time-consuming manual transfer of images from workstations using portable storage media, or if available, direct electronic transmission via the local network to a dedicated PC for viewing. Any PC with sufficient memory and processing power can receive DICOM images if appropriate software is installed. Whilst DICOM receiving software is incorporated into commercial viewing packages, there are freely available DICOM receiving programs on the internet, which can be downloaded and installed such as SimpleDICOM (University of Pittsburgh Medical Centre, PA, USA, available at: http://www.radiology.upmc.edu/ Public/public_resources/software/index.html) (Fig. 8). This package is both a DICOM viewer and receiver, which can be installed together or individually. It is possible to use the SimpleDICOM receiver with a different DICOM viewing program such as one of those mentioned above. Setting up a DICOM receiver on a PC, while relatively simple, may require technical input from the local network manager to help configure the system correctly. Once the method of image transfer has been decided the next step is to choose how to display digital images at meetings or teaching sessions. In the absence of PACS there are several options. The first option is to transfer images in DICOM format to a laptop or PC with a DICOM viewer installed on it. Images can then be displayed via a digital projector and the DICOM stacks viewed rather like on a workstation. The advantage with this method is
1139
that all the images are available for review and can be windowed and reformatted (if supported by the DICOM viewer) during the meeting. Another option is to select specific images that demonstrate the salient information and export them as JPEG or TIFF files. This is a time-consuming process but is useful when preparing presentations for teaching. The easiest way to present these images is by importing them into a presentation package such as Microsoft PowerPoint. Clinical cases can easily be made into slides with relevant case history and images inserted. This method can be used by trainees to prepare cases for departmental meetings. The presentations could subsequently be used as teaching resources as part of a digital image library. The development of a digital teaching file has been discussed in a previous article in this series.19
Conversion of image file formats Image file types may be converted from one type to another without even viewing them. Irfanview has a very easy to use file conversion function. It allows conversion of most types of image file and will convert whole DICOM stacks to another image file format.
Conclusion The advent of the DICOM file format has been a major step forward in clinical radiology by allowing digital images to be easily stored and transferred electronically. Digital images can be manipulated in many ways and converted to different formats for teaching and publication purposes. We have outlined a variety of ways in which radiologists may utilize digital images and how to make the most of the capabilities of DICOM before the introduction of PACS.
References
Figure 8 Screenshot from an example freely available DICOM receiver: SimpleDICOM.
1. Digital imaging and communication in medicine strategic document version 4.0. Available at: http://medical.nema. org/dicom/geninfo/Strategy.htm; accessed 27 June, 2005. 2. National Electrical Manufacturers Association DICOM website. Available at: http://medical.nema.org/; accessed 27 June, 2005. 3. Wiggins RH, Davidson HC, Harnsberger HR, Lauman JR, Goede PA. Image file formats: Past, present, and future. RadioGraphics 2001;21:789—98. 4. Joint Photographic Experts Group website. Available at: http://www.jpeg.org/index.html; accessed 27 June, 2005.
1140
5. The unofficial TIFF homepage website. Available at: http:// home.earthlink.net/writter/tiff/; accessed 27 June, 2005. 6. Portable Network Graphics website, maintained by Roelofs G. Available at: http://www.libpng.org/pub/png/; accessed 27 June, 2005. 7. Ziv J, Lempel A. A universal algorithm for sequential data compression. IEEE Trans Inf Theory 1977;23:337—43. 8. Welch TA. A technique for high performance data compression. IEEE Comput 1984;17:8—19. 9. Graphics interchange format—general information. Available at: http://256.com/gray/docs/gifspecs/general.html; accessed 27 June, 2005. 10. Escott EJ, Rubinstein D. Free DICOM image viewing and processing software for your desktop computer: What’s available and what it can do for you. RadioGraphics 2003;23: 1341—57. 11. Escott EJ, Rubinstein D. Free DICOM image viewing and processing software for the Machintosh computer: What’s available and what it can do for you. RadioGraphics 2004;24: 1763—77. 12. The Data Protection Act 1998—practical implications, Royal
R.N.J. Graham et al.
13. 14. 15. 16.
17. 18.
19.
College of Radiologists, UK. Available at: http://www.rcr. ac.uk/index.asp?PageIDZ310&PublicationIDZ177; accessed 27 June, 2005. Caruso RD, Postel GC. Image editing with Adobe Photoshop 6.0. RadioGraphics 2002;22:993—1002. Taylor GA. Photoshop for radiologists, initial steps in image preparation. AJR Am J Roentgenol 2002;179:1411—3. Taylor GA. Photoshop for radiologists, improving image contrast. AJR Am J Roentgenol 2003;180:329—31. Taylor GA. Photoshop for radiologists, removal of dust and scratches from electronic image files. AJR Am J Roentgenol 2003;180:1533—6. Taylor GA. Photoshop for radiologists, sharpening the image. AJR Am J Roentgenol 2003;181:43—5. Taylor GA. Photoshop for radiologists, color correction and automating repetitive tasks. AJR Am J Roentgenol 2003; 181:383—6. Scarsbrook AF, Foley PT, Perriss RW, Graham RNJ. Radiological digital teaching file development: An overview. Clin Radiol 2005;60:831—7.
