The other day I was asked by a prospect to explain the situation with digital slides and DICOM, and I thought it might be useful to all of you…
Terminology
DICOM is an acronym for Digital Imaging and COmmunications in Medicine. It is a standard for file formats and inter-system communication used in clinical markets (hospitals and labs) for storing radiology images (X-rays, MRIs, CTs, etc). The computer systems which support DICOM are called PACS, Picture Archive and Communication Systems. PACS are typically big and expensive, and are provided by large medical imaging vendors like GE, Philips, Siemens, and McKesson. Increasingly other medical disciplines also use DICOM to store images in PACS, such as cardiology; in this context such disciplines are called modalities. Pathology is the newest modality which is beginning to store images using DICOM in PACS.
A little background…
Digital slides are scans made of an entire microscope slide at high resolution. (These are sometimes called “whole-slide images”, or WSI.) These slides are stored as standard TIFF files, and are large – a typical 20mm x 15mm tissue sample scanned at 20X results in an image of 40,000 x 30,000 pixels, or 3.6GB of data, which is compressed to a file size of about 200MB; a more extreme sample of 25mm x 25mm scanned at 40X results in an image of 100,000 x 100,000 pixels, or 30GB of data, which is compressed to a file size of about 1.5GB.
The DICOM medical imaging standard presently does not accommodate such large images; both the pixel dimensions and the object size exceed the maximum values supported. NEMA Working Group 26, of which I am a member, is presently crafting a supplement to the DICOM standard enabling whole-slide images for pathology to be stored within the standard. The active proposal was made by Aperio and we have committed to supporting the enhanced standard when it is published. However it will take some time yet before this happens, and it will take longer before PACS vendors and digital pathology companies adopt the new standard and ship updates to their software.
In addition to the large object sizes, pathology imaging presents another incompatibility with DICOM in that the viewing pattern used by pathologists differs from their radiologist colleagues. As you know, pathologists pan through slides, zooming in and out often. A typical diagnosis involves a great deal of inspection at 4X, with occasional zooming to 10X, 20X, or 40X to inspect small regions at higher magnification. In contrast radiologists view an entire X-ray or MRI all at once, at full resolution (and they may have other needs such as rotating the point of view). For this reason present DICOM viewers are not efficient for pathologists to use in diagnosing digital slides. Currently specialized viewing software optimized for the needs of pathologists is used, although over time standard viewers will be enhanced for pathology as digital pathology becomes widely adopted.
So how does this come up…
When a hospital or lab is considering implementing digital pathology for some applications, and they already have a PACS, naturally they wonder if it would be possible to store the digital pathology images into the PACS. The considerations are financial (we already paid for the PACS, why not use it for this too) and logistical (it is helpful to have all the information for a case including all images in one system).
Implementing digital pathology and PACS systems…
Aperio has worked with many hospitals and labs who are using a PACS. The two issues noted above both arise; digital slides are too large to be stored with DICOM in a PACS, and the standard PACS viewing software does not enable pathologists to view digital slides efficiently. Therefore we’ve worked out a compromise. The full resolution digital slides are stored in Aperio’s SpectrumTM system, and can be viewed easily and efficiently using Aperio’s viewers (WebScope and ImageScope). A lower resolution version of each digital slide is stored in the PACS, imported as a standard TIFF file. This enables all images captured for a case to be kept together in the PACS, for all modalities. When a digital slide is viewed, it can be viewed at lower resolution in the PACS using standard DICOM viewing tools. A simple link is provided from the lower resolution image in the PACS to the full resolution image in Spectrum. Clicking the link within the PACS causes Aperio’s ImageScope viewer to automatically display the full resolution image, enabling the panning and zooming required for pathologists’ diagnosis.
This combination of PACS systems with Spectrum seems to work well and provide the maximum compatibility with existing workflow, while also providing support for the large image sizes of digital slides, and the way that pathologists work in viewing images.
Regarding the cost of storage
As noted above, hospitals and labs frequently think that storing digital slides in their existing PACS will save them money, since they’ve already paid for the PACS. However the main expense of storing digital slides is disk storage, and frequently the cost of adding additional disk storage to a PACS is several times more expensive than the cost of the same amount of storage in a Spectrum system. This happens because the PACS vendors mark up disk storage far beyond the actual cost of the hardware. For example adding 4TB of storage to a Spectrum DSR costs about $5,000 (4TB holds about 20,000 digital slides), while adding 4TB of storage to a PACS might cost over $100,000.
This cost discrepancy can be helpful to us; while initially prospects are anxious to store digital slides in their PACS and disappointed to find that they cannot do so, once they understand that using Spectrum to store the digital slides is less expensive, they feel better. When the incompatibility of digital slides with DICOM and PACS is the issue, we are the bad guys (even though we’re not responsible for the limitations of DICOM, and are actively working to overcome them), but when the cost of add-on storage for a PACS is the issue, we are the good guys.
Hope you find this helpful. As always please feel free to ask questions or leave comments...
Interesting article. We are glad to see that the better ue of concentrating images into the same repository is a well receive idea.
Our main company strategy is working in this concept. Based on the concept "Beyond PACS" we provide some technology called MIO that allows the movemtn of images between repositories.
Probably you could only store into PACS and use Aperio software for diagnosis and review (prefetching the image from the PACS). We have done some experiences integrating EKG's into PACS and retrive the images (and data) to put them on the clinical viewer (at any point of the hospital). Clinicians are satisfied.
Posted by: Oscar Lopez | August 10, 2008 at 04:23 AM