REVIEW

DICOM demystified: A review of digital file formatsand 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

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KEYWORDSInternet; Digital ima-ging and communi-cations in medicine(DICOM); PACS

09-9260/$ - see front matter Q 200i:10.1016/j.crad.2005.07.003

* Guarantor and correspondent: A.FRadiology, John Radcliffe Hospital, Hford OX3 9DU, UK. Tel.: C4465220801.E-mail address: andyscarsbrook1@a

Digital imaging and communications in medicine (DICOM) is the standard image fileformat used by radiological hardware devices. This article will provide an overview ofDICOM and attempt to demystify the bewildering number of image formats that arecommonly encountered. The characteristics and usefulness of different image filetypes will be explored and a variety of freely available web-based resources to aidviewing and manipulation of digital images will be reviewed. How best to harnessDICOM technology before the introduction of picture archiving and communicationsystems (PACS) will also be described.Q 2005 The Royal College of Radiologists. Published by Elsevier Ltd. All rightsreserved.

Introduction

Digital images are generated by a wide variety ofradiological hardware. Each device collects data,which are then encoded and stored electronically inDICOM format. This is a universal file type,developed to facilitate data exchange betweenhardware, irrespective of manufacturer. DICOMfiles store a large amount of data and usually needto be viewed on dedicated workstations but may betransferred electronically to other computerswhere they can be displayed provided appropriateDICOM viewing software is installed. DICOM files caneasily be converted to a variety of image formatsand edited before use in teaching and publications.There are a number of useful DICOM-relatedresources that are freely available on the internet.

5 The Royal College of Radiolo

. Scarsbrook, Departmenteadley Way, Headington,1865220815; fax: C44

ol.com (A.F. Scarsbrook).

What is the DICOM file format?

In response to the increased use of digital images inradiology the American College of Radiology (ACR)and the National Electrical Manufacturers Associ-ation (NEMA) formed a joint committee in 1983 tocreate a standard format for storing and transmit-ting medical images.1 The committee published theoriginal ACR-NEMA standard in 1985.1 This hassubsequently been revised and in 1993 the standardwas renamed DICOM.1 More recent improvements inDICOM (Version 3.0) have permitted transfer ofmedical images in a multi-vendor environment, andimportantly, have facilitated the development ofPACS and digital interfacing with medical infor-mation systems.1 DICOM is administered by theNEMA Diagnostic Imaging and Therapy Systemsdivision and each year the standard is updated.Details of recent improvements can be found on theNEMA website (http://medical.nema.org/).2

Each DICOM file has a header containing amongstother items, patient demographic information,acquisition parameters, referrer, practitioner andoperator identifiers and image dimensions. Theremaining portion of the DICOM file contains theimage data. Because they often contain multiplehigh-resolution images, DICOM files tend to be large

Clinical Radiology (2005) 60, 1133–1140

gists. Published by Elsevier Ltd. All rights reserved.

Figure 1 Lossless compression of a dataset: Repeatedvalues are coded to reduce the overall file size.

R.N.J. Graham et al.1134

[e.g., 35 megabytes for a pre and post-contrastcomputed tomography (CT) images of the brain]and are frequently compressed before storage andtransfer.

Digital image compression

Image compression is a method of reducing file sizeto increase the amount of data that can be archivedonto storage media and to speed up data trans-mission. DICOM images can be compressed byconverting the data into smaller image file typesusing specialized software, which will be describedin more detail later in the article.

There are two main types of data compression:Lossless and lossy. Lossless compression allows thefile size to be reduced without any loss ofinformation. This allows all the original data to berecovered if necessary. In lossless data compressionrepeated identical values within the dataset arereplaced with one value (Fig. 1) in a way that allowsunambiguous decoding without loss of information.This process uses a substantial amount of proces-sing power and makes compressed files slower toopen and save.

By contrast, lossy image compression perma-nently eliminates some of the file data and canresult in a remarkable reduction in file size. The aimis to eliminate redundant information from thedataset without adversely affecting image quality,but excessive compression inevitably results inimage degradation (Fig. 2).

Image file formats

Aside from DICOM there are a plethora of differentdigital image file formats, which can easily confusethe uninitiated. An extensive review of these isbeyond the scope of this article, however, theinterested reader may find additional informationin an excellent article on image file formats byWiggins et al.3 In simple terms, the differentformats can be divided into those involving lossycompression such as Joint Photographic ExpertsGroup (JPEG)4 and those using lossless com-pression, e.g., tagged image file format (TIFF).5

Lossy compression file types

Joint photographic experts group format (JPEG)This image type allows the user to specify howmuchcompression is applied and hence how much of theoriginal data are lost. JPEG files exploit the factthat the human eye perceives small colour changesless accurately than changes in brightness. Thedisadvantage with JPEG is that data are irreversiblylost and this may lead to an unacceptable level ofimage degradation (Fig. 2). The advantage, how-ever, is a very small file size.

Lossless compression file types

Portable network graphics format (PNG)The PNG image format has several good features:Variable degree of transparency; image brightnesscontrol (gamma correction) and two-dimensionalinterlacing (initially every other line of the imagedata is displayed) for rapid image viewing.6 Thelossless compression algorithm was written byLempel and Ziv in 1977.7 A useful feature of thePNG format is the ability to embed text within theimage file as so called “metadata”.3 This is ratherlike the header associated with a DICOM image, andis convenient when setting up a teaching filedatabase as certain web-based search engines canfind images based on this embedded information.

Tagged image file format (TIFF)With TIFF files either lossless or lossy datacompression can be specified. Lossless compressionis generally performed using the Lempel-Ziv-Welch(LZW) algorithm written by Abraham Lempel andJacob Ziv in 1977 and 1978 and improved by TerryWelch in 1984, hence the name LZW.7,8 Thedisadvantage with TIFF is its relatively large filesize making it less than ideal for Internet orPowerPoint (Microsoft, Redmond, WA, USA)-basedapplications.

Graphic interchange format (GIF)This image format was first produced in 1987 anduses the LZW lossless compression algorithm.9 Thecompression of GIF images is less efficient than PNGfiles (by about 5–25%) and this file type lacks thewide-ranging features of the newer PNG.9 As aresult GIF has been largely superseded by PNG.

Joint photographic experts group 2000 format(JPEG 2000)This is another example of an image file usinglossless compression. JPEG 2000 images allowcertain parts of the image to be defined as a region

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) moderatecompression of the same JPEG image (64 kb) showing no significant image degradation; (c) highly compressed JPEG(32 kb) demonstrating marked image degradation.

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R.N.J. Graham et al.1136

of interest (ROI), which can then be displayedbefore other parts of the image, or be losslesslycompressed, whilst other less critical parts of theimage undergo lossy compression. Like PNG, JPEG2000 allows metadata to be embedded in the imagefile.4 JPEG 2000 is a new file format, and is not yetin wide use by radiologists. With its advancedfeatures it is predicted that it is likely to beincreasingly used in the future.

So which of these file types should one use?Generally speaking, lossy files are perfectly suitablefor image display in computer and web-basedpresentations where the small file size allowsrapid image upload and facilitates easy imagetransfer between computers. In contrast, losslessformats are more suitable for archiving, teachingand for submission for publication. For example,Elsevier, the publisher of Clinical Radiology,provide guidance for submitting imageselectronically (http://authors.elsevier.com/ArtworkInstructions.html?dcZAI1) the TIFF formatis a preferred option. The most commonly used fileformats in radiology are compared in Table 1.

DICOM-viewing software

All of the image file types described above caneasily be opened and viewed on a standard personalcomputer (PC) with a contemporary operatingsystem such as Windows XP (Microsoft, Redmond,WA, USA) without the need for any special soft-ware. By contrast DICOM images require additionalsoftware to be installed before they can be openedand viewed. DICOM-viewing software falls into twomain categories: Proprietary viewers, which aresupplied with imaging systems such as CT ormagnetic resonance imaging (MRI) machines; andthird-party DICOM-viewing software, either in theform of PACS or as a stand-alone viewer forindividual PCs.

Table 1 Comparison of lossy and lossless image fileformats.

File type Compressionmethod

Most suitable uses

JPEG Lossy PowerPoint and website imagesTIFF Lossy or

losslessPublications

PNG Lossless Publications, teaching files andwebsite images

JPEG 2000 Lossless Publications and teaching filesGIF Lossless Website images

Proprietary DICOM viewers

Proprietary DICOM viewers tend to be written bythe manufacturers of and supplied with medicalimaging hardware, e.g., Advantage Workstationproduced by General Electric Healthcare (ChalfontSt Giles, UK). These dedicated workstations allowdynamic scrolling through stacks of images andmany advanced functions, such as multiplanarreconstruction and three-dimensional volume ren-dering (Fig. 3). There is usually the ability to exportimages to portable storage media (e.g., CD-R) or totransfer images to other networked workstations.Exported files are typically converted by theproprietary software to smaller files (e.g., JPEG orPNG) that can then viewed on a PC without anyspecial software. Once the DICOM images have beenconverted to other file formats the ability to viewconsecutive images from a series, as an interactivestack is lost. The downside is that workstations areoften in constant use in radiology departments,particularly in those that do not yet have PACSinstalled and little time is available to use these forimage manipulation purposes or for use in teaching.The use of third-party DICOM-viewing software canameliorate this problem.

Third-party software

There are a number of stand-alone DICOM-viewingpackages that are commercially available. Probablythe 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

Figure 3 Screenshot from an example proprietaryDICOM-viewing software package: GE Healthcare Advan-tage Windows workstation.

DICOM demystified 1137

relatively recently available free of charge (Fig. 4).This is no longer the case, and the current version isrelatively expensive (Version 2.0.1 single licencecosts $2500). Fortunately there are a number offree DICOM viewers that can be downloaded fromthe internet. These are of variable quality and otherauthors have reviewed a large number of theavailable resources.10,11 Good examples of freeDICOM viewing software for use on individual PCsinclude Osiris (University Hospital of Geneva,Switzerland, available at: http://www.sim.hcuge.ch/osiris/01_Osiris_Presentation_EN.htm) andDicom Works (Developed by Phillipe Puech & LoicBoussel, available at: http://dicom.online.fr/;Fig. 5). Free DICOM software is also available foruse on Apple Mac computers and Osirix (Depart-ment of Radiology, David Geffen School of Medi-cine, 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 DICOMfunctionality and allow individual or stacked imagesto be viewed and manipulated. A comparison of themain features of these example software packagesis provided in Table 2.

DICOM images can also be exported into Micro-soft PowerPoint using a special plug in which can bedownloaded, 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 willexplore advanced uses of PowerPoint (Microsoft),including how to incorporate DICOM images intopresentations.

Figure 4 Screenshot from an example commerciallyavailable DICOM viewer: Merge E-Film.

Figure 5 Screenshots from example freely availableDICOM viewers for PCs: (a) Osiris and (b) Dicom Works.

Figure 6 Screenshot from an example freely availableDICOM viewer for Mac computers: Osirix.

Figure 7 Screenshot from an example freely availableimage editing software package: Irfanview.

R.N.J. Graham et al.1138

Editing exported images

After choosing which images to export it is oftennecessary to anonymize them. If images are for usein teaching or for publication this may be essentialto comply with the Data Protection Act. The RoyalCollege of Radiologists have produced a document,“The Data Protection Act 1998—Practical Impli-cations”12 that provides further guidance in thisarea. Removing the overlay containing patientdemographic information on the image beforeexport can anonymize images. Most DICOM viewersare able to do this, including the ones reviewedabove. Exported image files can be imported intophoto-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, availableat: www.irfanview.com) for further manipulation(Fig. 7). Adobe Photoshop is the industry standarddesktop digital imaging package with extensiveimage 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 thefeatures of the more expensive product apart fromsome of the high-end professional editing functionsand is available at a fraction of the cost (£49.99).Irfanview, whilst less comprehensive, has most ofthe basic image editing functions that a radiologistwill need and can be downloaded from the internetfree of charge.

Once an image has been imported into editingsoftware, cropping so that only the region ofinterest is displayed can reduce the size of thefile. Cropping can also be used to remove anypatient identifiers from the image. Altering thecolour depth of the image can further reduce thefile size. Saving a grey-scale image as an 8 bit greyscale rather than as a 24-bit colour image willdecrease the file size by a factor of three withoutdegrading the image.

Often it is advantageous to place arrows onradiological images for presentations or publi-cations. This can be done with ease in presentationpackages such as Microsoft PowerPoint. The image

Table 2 Comparative features of stand-alone DICOM viewing pac

Feature Osiris Di

Export images as TIFF or JPEG Yes YeView two series linked together No YeMultiplanar reformating Yes NoWindowing of images Yes YeMeasurement tools Yes Ye

is imported into the presentation and arrows caneasily be added as an overlay using the arrow toolfrom the draw toolbar. Arrows added in this mannerdo not alter the underlying image, so if the image isexported from PowerPoint it will be saved withoutany of the modifications. Photo-editing packagesallow arrows or text to be added in so-called“layers”. Each layer is like an overlay on the imageand individual arrows or items of text can be alteredor deleted independently of each other. When themodified image is saved, the layers can be mergedor kept separate. The advantage of keeping layersseparate is that further manipulation can beperformed at a later date; the file size is increasedas a result. There are a vast number of otherfeatures within Adobe Photoshop, for examplealtering image contrast and brightness, rotatingimages, masking unwanted annotations and con-version from one image format to another. Manyother image editing functions of use to radiologistsare detailed in a number of articles.13–18

Image manipulation for clinical meetingsor teaching purposes

Before the widespread introduction of PACS intoradiological departments, remote DICOM image dis-play in clinico-radiological meetings and for teachingpurposeswas a technical challenge.Manyproprietary

kages.

comWorks Osirix eFilm

s Yes Yess Yes Yes

Yes Yess Yes Yess Yes Yes

DICOM demystified 1139

workstations allow export of DICOM images to a CD-Ralong with a rudimentary DICOM viewer. Theexported images can then be displayed on most PCsand use a Java-enabled web browser such as InternetExplorer (Microsoft) to display the images. However,these web viewers tend not to have many imagemanipulation or calibration features. Alternativemethods of digital image display either requiretime-consuming manual transfer of images fromworkstations using portable storage media, or ifavailable, direct electronic transmission via the localnetwork to a dedicated PC for viewing.

Any PC with sufficient memory and processingpower can receive DICOM images if appropriatesoftware is installed. Whilst DICOM receiving soft-ware is incorporated into commercial viewingpackages, there are freely available DICOM receiv-ing programs on the internet, which can be down-loaded 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 andreceiver, which can be installed together orindividually. It is possible to use the SimpleDICOMreceiver with a different DICOM viewing programsuch as one of those mentioned above. Setting up aDICOM receiver on a PC, while relatively simple,may require technical input from the local networkmanager to help configure the system correctly.

Once the method of image transfer has beendecided the next step is to choose how to displaydigital images at meetings or teaching sessions. Inthe absence of PACS there are several options. Thefirst option is to transfer images in DICOM format toa laptop or PC with a DICOM viewer installed on it.Images can then be displayed via a digital projectorand the DICOM stacks viewed rather like on aworkstation. The advantage with this method is

Figure 8 Screenshot from an example freely availableDICOM receiver: SimpleDICOM.

that all the images are available for review and canbe windowed and reformatted (if supported by theDICOM viewer) during the meeting.

Another option is to select specific images thatdemonstrate the salient information and exportthem as JPEG or TIFF files. This is a time-consumingprocess but is useful when preparing presentationsfor teaching. The easiest way to present theseimages is by importing them into a presentationpackage such as Microsoft PowerPoint. Clinicalcases can easily be made into slides with relevantcase history and images inserted. This method canbe used by trainees to prepare cases for depart-mental meetings. The presentations could sub-sequently be used as teaching resources as part ofa digital image library. The development of a digitalteaching file has been discussed in a previous articlein this series.19

Conversion of image file formats

Image file types may be converted from one type toanother without even viewing them. Irfanview has avery easy to use file conversion function. It allowsconversion of most types of image file and willconvert whole DICOM stacks to another image fileformat.

Conclusion

The advent of the DICOM file format has been amajor step forward in clinical radiology by allowingdigital images to be easily stored and transferredelectronically. Digital images can be manipulated inmany ways and converted to different formats forteaching and publication purposes. We have out-lined a variety of ways in which radiologists mayutilize digital images and how to make the most ofthe capabilities of DICOM before the introduction ofPACS.

References

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2. National Electrical Manufacturers Association DICOM web-site. Available at: http://medical.nema.org/; accessed 27June, 2005.

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4. Joint Photographic Experts Group website. Available at:http://www.jpeg.org/index.html; accessed 27 June, 2005.

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