There is no question that digital mammography is booming. One of the reasons is probably due to the results of the recent National Cancer Institutes multi-center Digital Mammographic Imaging Screening Trial (DMIST) which concluded that for certain age groups of women it might be better to have a digital mammogram performed, rather than conducting conventional film-screen mammography. In addition, the use of computer-assisted diagnosis (CAD) is becoming mainstream in many mammography practices (a recent survey showed about 30 percent regularly use CAD). Of course, it is possible to use CAD by digitizing film, but from a workflow perspective, it is much easier to utilize CAD when it is integrated with a digital system. Lastly, the U.S. market will soon see the introduction of CR-based digital mammography systems from different vendors once they clear the FDA (which is expected soon), offering competitive alternatives to the current DR-based systems.
The FDA approved the initial digital mammography systems as complete, single vendor acquisition and printing or viewing systems. Since than, several third-party workstation and PACS vendors have received FDA clearance for their viewing stations to be used for digital mammography (when used with an approved monitor), which means that users now have a choice. However, this introduced a whole set of new interconnectivity and interoperability issues. As a matter of fact, early adopters of this new technology that were brave enough to use different vendors for digital mammography and diagnostic workstations have truly felt what it meant to be “on the bleeding edge” of technology. It can hurt.
The good news is that there has been a flurry of activity to define a set of requirements in the form of a special profile by the Integrating the Healthcare Enterprise (IHE) organization to address the issues that came to the surface while exchanging images between the acquisition devices and CAD devices and displaying the results on image displays. (See www.ihe.net/mammo for details.) One should be aware that compliance from the vendors with these profiles might take a while, with some estimates forecasting at least a year. One might reasonably expect that facilities jumping into digital mammography today will have to deal with similar workflow issues and challenges, especially if they want to use PACS workstations and digital mammography systems from different vendors during this interim period.
What are some of the issues addressed by this IHE profile? First of all, the profile requires the use of the DICOM Digital Mammography object for all mammography images, whether a digital detector performs the acquisition, computed radiography is used or film is digitized (and not the CR or Secondary Capture objects). The DICOM image definition allows some identification, technique and dose attributes to be left empty by a manufacturer, and where these are considered sufficiently important by users, regulatory authorities and professional associations, the IHE profile raises the bar and requires them to be filled in by the acquisition device and displayed on the workstation as well as printed on film. This includes information required by MQSA for patient and unit identification. A means of identifying and encoding the background air outside the breast skin edge also is required to be present; this allows window width/level adjustments by the user without changing the background air intensity. Different vendors have used various different mechanisms available in DICOM for specifying how the image contrast is to be encoded and adjusted, and the profile requires that the workstation support all of these mechanisms so that images will have the expected appearance regardless of which combination of modality and workstation is used. Of course, all workstations are required to be calibrated to the standard DICOM grayscale function, though it will likely be necessary to use the same technology (LCD or CRT) throughout the organization as well as to choose the same minimum and maximum luminance levels to achieve consistency.
Since CAD also is expected to be an important factor in the widespread use of digital mammography for screening, the IHE profile makes the ability to display CAD marks encoded in standard DICOM mammography CAD SR objects a required capability for the workstation. Mammography images need to be processed into a suitable form (“For Presentation” images) before display to the user or printing. CAD systems, however, typically require the unprocessed, raw data (“For Processing” images). Therefore, there is a requirement in the IHE profile for acquisition devices to support sending both image types, to allow them to be sent to different destinations, and for the PACS to support both as well. The profile also resolves the issue of how to display the locations of CAD findings on the “For Presentation” images available to the workstation, when the CAD output typically references the “For Processing” images.
Efficient workflow in screening mammography reporting requires that the current and prior images be automatically displayed to the user in the correct sequence and orientation, so as to minimize manipulation required by the radiologist. The DICOM standard provides mandatory attributes that contain the necessary information for this but is silent on the behavior expected by the workstation; the IHE profile is not, and requires that specific mandatory coded attributes be used to control the default display or hanging protocols, without going so far as to dictate the use of any particular hanging protocol. The workstation is required to properly orient the images based on the image laterality, view code and patient orientation. An additional requirement is the capability to justify each displayed image to the chest-wall side of the view port instead of centering the images so they can be displayed in the back-to-back format in which mammography images are always viewed. The display of distracting tools and/or other “window dressing” such as scroll bars in between images, which might make sense for other modalities, is forbidden for mammography.
Displayed image size is another important issue, particularly when current and prior images have been obtained on different vendors’ equipment, such as when the prior examination was performed at another facility. Regardless of the physical size of the detector plate or pixels or gantry geometry, the radiologists require that the images be displayed with the same relative physical size. The DICOM standard already requires that the necessary attributes be present and completed; the IHE profile requires that the workstation make use of them. In addition, the IHE profile also requires workstations to be able to display images at “true size,” so that side-by-side comparison can be made with film-screen mammograms hung on a viewer. Last but not least, it is necessary for a facility to be capable of printing digitally acquired images for the patient or the surgeon; the IHE profile includes specific requirements of the workstation and printer for true size printing as well as the required annotation and additional features such as the presence of a distance.
If a user would try to interpret the IHE profile for mammography, he or she might easily become overwhelmed with the level of technical detail. That is why the IHE Integration Statements are critical. These allow vendors to concisely specify that a product adheres to the profile or not and gives a higher level of confidence to the user that the issues are addressed as described above. It should simply be necessary for the user to require from the vendor that all of the equipment used for digital mammography support the appropriate actors in the IHE Mammography profile. The acquisition device (FFDM or CR system or film digitizer) needs to support the profile as an Acquisition Modality actor. All the workstations used for primary diagnosis, whether the modality vendor or a third-party supplies them or they are a built-in component of the PACS, need to support the profile as Image Display actors. The PACS or any standalone archive also needs to support the profile as an Image Manager/Archive actor. Any device from which printing is to be performed needs to support the profile as a Print Composer actor, and any printer used for mammography needs to support the profile as a Print Server actor. Any CAD device needs to support the profile as an Evidence Creator actor. Note that if a PACS with its own built-in workstations are to be used, it is necessary to specify that the PACS support both the Image Manager/Archive and Image Display actors; the former alone is not sufficient. It is very likely that the user also will want to specify that all of the equipment also comply with other IHE profiles, such as Scheduled Workflow (SWF), to ensure accurate exchange of identification information, Portable Data for Imaging (PDI), in order to exchange mammography images on CD.
Now, the IHE mammography profile is brand new, and was developed according to an aggressive timeline. So, do not expect it to be a panacea for all woes. However, it is going to be a major step towards better interoperability. Needless to say that any user buying a mammography device, whether it is a acquisition, workstation or CAD system needs to specify future compliance with this profile.
You can view the archived winter ’06 e-conference by OTech (www.otechimg.com) on-line, which contains several interviews with a digital mammography user and the editor of the IHE profile.
David Clunie, CTO of Radpharm, is the editor of the IHE Mammography Profile. Herman Oosterwijk is president of OTech, Inc. He can be reached via OTech’s website at www.otechimg.com.