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Lossless and Diagnostically Acceptable Irreversible Compression

OnePacs's image compression settings are highly customizable, and may be modified to suit each individual radiology practice's preferences and standards, including using only lossless compression, if desired.

By default, the OnePacs Gateway software is configured to use Diagnostically acceptable irreversible compression (DAIC, or "lossy") compression for locally storing studies, as well as transmitting studies to OnePacs, for all study types other than conventional radiography and mammography studies. (CR and MG studies use lossless compression, at a compression ratio of approximately 3:1). For lossy compression, the JPEG2000 algorithm with a compression ratio of 8:1 is used. The JPEG2000 algorithm was selected due to an established body of research demonstrating JPEG2000 to have optimal characteristics as a compression algorithm for medical imaging, including the ability to maintain excellent image quality even at high compression ratios (see “JPEG2000 Compression of Medical Imagery”, Proc. of SPIE Vol. 3980, Feb. 2000).

DAIC in the manner employed by OnePacs is designed to not yield any human-perceptible difference in image characteristics. The 8:1 compression ratio chosen for the default OnePacs installation is a quite conservative/cautious figure; there is abundant evidence that images may be compressed significantly smaller than this without perceptible change in image characteristics. The DAIC used by OnePacs follows guidelines issued by the American College of Radiology and other societies regarding DAIC, including algorithm selection and compression ratio. The supervising radiologist(s) of a radiology practice, as always, bear the responsibility of reviewing and confirming image quality on an ongoing basis.

Society/Association Standards and Statements

Diagnostic imaging data sets are essentially the largest data sets actively used, transmitted, and stored in the course of clinical medical practice. The cost constraints, required technical resource capacities, and transmission times involved in storing such data sets and making them available at the point of needed use are considerable, and these factors are constrained by health care economics and technical feasibility. One way to address this issue. improving patient care (through more rapid transmission times) while simultaneously reducing costs is through the use of Diagnostically Acceptable Irreversible Compression (DAIC).

DAIC is defined by the ACR, AAPM, and SIIM as: "mathematically irreversible compression that does not affect a particular diagnostic task. DAIC may be used under the direction of a qualified physician with no reduction in clinical diagnostic performance by either the primary image interpreter or decision makers reviewing the images."

There is a broad consensus across radiology standard-setting associations worldwide that DAIC can be appropriately utilized in radiologic practice, provided that appropriate algorithms and compression ratios are used, and image quality is routinely reviewed by supervising radiologists. This includes for the purpose of long-term archiving.

While standards endorsed by ACR, AAPM, and SIIM, and other organizations/societies, may allow for trivial artifacts that do not interfere with interpretation, at OnePacs our DAIC endorses the higher and stricter standard of there being no human-perceptible difference between images subject to DAIC and images not subject to such compression, and is designed to achieve that purpose. Images should be routinely reviewed by supervising radiologists to ensure diagnostic adequacy.

Statements have been issued by the American College of Radiology, the American Association of Physicists in Medicine, Society of Imaging Informatics, the European Society of Radiology (ESR), Royal College of Radiologists in the UK, the German Roentgen Society, and the Canadian Association of Radiologists, all agreeing that the use of diagnostically acceptable irreversible (lossy) compression is possible without losing relevant clinical information, and can appropriately be used in clinical practice by radiologists. These guidelines all similarly emphasize that radiologists must supervise the utilization of DAIC to ensure image quality.

The American College of Radiology, American Association of Physicists in Medicine, and Society of Imaging Informatics jointly issues a "Technical Standard for Electronic Practice of Medical Imaging", last revised December 2017.  In part the standard reads:

Compression may be defined as mathematically reversible (lossless) or irreversible (lossy). Reversible compression may always be used, since by definition there is no impact on the image. Irreversible compression may be used to reduce transmission time or storage space only if the quality of the result is sufficient to reliably perform the clinical task. The type of body part, the modality, and the objective of the study will determine the amount of compression that can be tolerated.

The term “diagnostically acceptable irreversible compression” (DAIC) refers to mathematically irreversible compression that does not affect a particular diagnostic task. DAIC may be used under the direction of a qualified physician with no reduction in clinical diagnostic performance by either the primary image interpreter or decision makers reviewing the images. From a practical perspective, this means that any artifacts generated by the compression scheme should not be perceptible by the human viewer or are at such a low level that they do not interfere with interpretation.

The ACR and this technical standard make no general statement on the type or amount of compression that is appropriate to any particular modality, disease, or clinical application to achieve the diagnostically acceptable goal. The scientific literature and other national guidelines may assist the responsible physician in choosing appropriate types and amounts of compression, weighing the risk of degraded performance against the benefits of reduced storage space or transmission time. The type and amount of compression applied to different imaging studies transmitted and stored by the system should be initially selected and periodically reviewed by the responsible physician to ensure appropriate clinical image quality, always considering that it may be difficult to evaluate the impact on observer performance objectively and reliably.



Usability of irreversible image compression in radiological imaging. A position paper by the European Society of Radiology (ESR)

Royal College of Radiologists (RCR, UK) “The adoption of lossy data compression for the purpose of clinical interpretation” (April 2008)

German Röntgen Society (DRG, Germany) “Compression of digital images in radiology—results of a consensus conference” (Rofo 2009)

Canadian Association of Radiologists (CAR, Canada) “Pan-Canadian evaluation of irreversible compression ratios (“lossy” compression) for the development of national guidelines” (J Digit Imaging, 2009)

Modification of Default Compression Settings

Some users may wish to change the default compression settings, either to utilize reversible compression, or to increase or decrease the compression ratio.

The compression used can be configured in the jmx-console, reached in a web browser at the following URL:

(On the gateway machine itself) http://localhost:8080/jmx-console

After login to the jmx console, select the “Service=StoreSCP” option. Scroll down the screen to find the “CompressionRules” item. Into the text box containing the compression rules, you may paste in one of the following options:

JPLLCompress all studies with JPEG lossless
J2KICompress all studies with JPEG2000 lossy

You may also edit the “LossyCompressionQuality” item to change the compression ratio. The default value is 0.12, signifying an approximately 8 to 1 compression ratio (that is, the final image will be 0.12 times the size of the original uncompressed image). JPLL generally leads to a ratio of about 3.5, with the compressed image approximately 0.29 the size of the original image. Thus, as configured by default in the OnePacs system, J2KI will generally result in approximately 2.4 x greater achieved compression than J2LL.

It is also possible to customize compression characteristics per modality type and per SOP class. OnePacs support can provide further assistance on this.

Implications of DAIC for Archiving

As noted above, there is a broad consensus across radiological societies and standard-setting organizations world-wide that Diagnostically Acceptable Irreversible Compression can be appropriately used for image storage, including permanent records/long-term archives. OnePacs's default settings, as noted, are to apply such compression to all studies, other than Mammography (which is explicitly required to use lossless compression in the US due to regulations), and plain films/CR/DX (which are not required to use lossless by rule/regulation, though it is OnePacs's preference to use lossless compression for these studies).

Whatever compression method you choose at the Gateway will determine how studies are stored in the OnePacs system. If you are archiving studies with OnePacs, you may choose to either archive studies in a lossless format, or with lossy compression. Using lossy compression has the advantage of reducing file size, which reduces storage costs and transmission times. However, some facilities/groups may wish to archive certain types of studies, or all studies, only using lossless compression.

If you wish to have all studies for one or more facilities managed in the OnePacs system using only lossless compression, simply configure the Gateway(s) for such facility(ies) to utilize lossless compression. If studies are compressed using lossless compression at the Gateway, they will remain compressed with lossless compression for the remainder of their life cycle with OnePacs, including permanent archiving, if applicable.

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