Basic CD viewer requirements; extending PDI; software for sending images on CD media

Summary: IHE is defining requirements for basic CD viewers; PDI is being extended to add DVD, USB, compression and encryption; IHE PDI and DICOM CD media require viewers and importers to understand what is on the media; as compression, encryption and new types of images are used, receiving software struggles to keep up; this can be alleviated by executable software on the media that can decompress, decrypt and convert new image types to whatever has been negotiated with the recipient and then transmit them via the local DICOM network.

Long Version:

Since the cardiology community first began standardizing, promoting and adopting DICOM CDs as a means of interchange of images in the early 1990's, and radiology has rapidly caught up, CDs have proven to be wildly successful despite legitimate complaints about interoperability and ease of use. The PDI promotion effort by IHE initially focused on reducing confusion by insisting on only uncompressed images on CD, to reduce the burden on any device or software that the recipient may have installed. Dependence on on-board viewers was somewhat discouraged by IHE, both because of the potential security risk to executing externally supplied code and the variation in features that such viewers support.

As I have discussed previously, referring physicians who are the victims of a multitude of different viewers are "encouraging" us to improve the situation, both by endorsing the use of PDI as opposed to proprietary media, as well as joining with IHE to develop standards for what viewers are required to be able to do, in a manner that makes them intuitive to use. This latter effort is the Basic Image Review Profile. Last week we had our first Radiology Technical Committee meeting to discuss the requirements for this profile. The involvement of the users who are interested in this was extremely encouraging ... no fewer than three neurosurgeons attended the meeting to contribute! We discussed what features any basic viewer should have with respect to loading studies, navigating through them using thumbnails, comparing series side-by-side with synchronized scrolling, panning, zooming and windowing, making simple distance and angle measurements, displaying any if report present, and printing. We also discussed hardware and software requirements for such a viewer agreeing that it had to run on Windows (blech, but that's reality), and more controversially, to what extent elements of the user interface could be standardized in appearance to make unfamiliar viewers intuitively easy to use. Tooltips are one obvious means to assist with ease of use, but we also agreed to at least attempt to define what tools should be visible in the main interface and what they should look like (e.g., hand for pan, magnifying glass for zoom, etc.). We know there is a balance between consistency across vendors and the added value of proprietary look and feel, but hope that some consensus can be achieved on general principles. One item that everyone seems agreed on is the concept that the "basic" interface should be uncluttered, and "advanced" features should not be visible until they are called for, so the profile may well end up specifying what shall not be there in addition to what shall.

In the same meeting we also discussed extensions to PDI. For some time many applications have been limited by the size of datasets relative to the capacity and speed of uncompressed CD media. Accordingly, after our informal interoperability tests of DVD readability earlier this year at the Connectathon, the idea of extending PDI to support DVD as well as CD has been accepted, and at the same time it makes sense to add support for compression (as DICOM requires for DVD support) as well as for faster media like USB memory sticks and the like. The fuss about encryption of portable media makes this an opportune time to deal with that issue as well, to make sure that there is not a proliferation of proprietary alternatives to the DICOM secure media standard.

Yet extending PDI raises the bar for recipients that want to use their own pre-installed software or devices to display or to import media that may be compressed or encrypted in a manner that older software does not support. At the same time, we are well aware that any media may contain a multitude of different types of images, presentation states, key object selection and structured report documents, and IHE does not constrain this. What this means in practice is that though a viewer or importer (such as a PACS) may well support most of the image types, there may be content that is not successfully displayed or imported, the consequences of which may be unfortunate. The Basic Image Review Profile will address this for on-board viewers by adopting the fundamental principle that a compliant viewer on the media shall be able to view all the DICOM content on the media. That is a "no-brainer", but it doesn't help the pre-installed viewer or importer.

A solution that I have proposed for this that may help is to introduce the concept of "sending software" on the media. That is, even if one does not want to view the content on the media using an on-board viewer, which may or may not be present, easy to use, or even possible to execute on your hardware, it may be possible to execute software that helps to import the content into your own locally installed software. The requirements that I have drafted so far for the PDI extensions supplement include the ability to:

• allow the user to enter the recipients network location (IP, port, AET)
• read all the content of the media via the DICOMDIR
• select what to send
• coerce patient & study identifiers using local values supplied by the user
• decrypt content if encrypted using the password supplied by the user
  
• decompress content if the receiving devices doesn't support compression
• convert instances whose SOP classes the receiver does not support to one that it does
  
• transfer everything

The idea started out as a means to get around the fact that most of the installed base does not support encryption, or some of the more advanced compression schemes, but in fleshing this out it seemed reasonable to also address the fact that more recent SOP Classes like Digital X-Ray and Enhanced CT or MR are still not widely supported, and the same "fall back" strategy of converting SOP Classes that modalities use to deal with primitive PACS in this regard could be used on media too. A typical DX modality may, for instance, attempt to send a DX object that the PACS doesn't support, and can fall back to sending CR or even Secondary Capture instead if that is all that can be negotiated on the network; modern MR devices that support Enhanced MR do something similar. The key to this is the DICOM Association Negotiation mechanism that allows this to be figured out on the fly, rather than being pre-configured.

Ideally, the "sending software" present on the media would be multi-platform, and it is certainly possible to do that (say with Java and on-board JRE's for the popular platforms in case they are not already installed). But at the bare minimum, given the prevalence of Windows, the requirements are that it executes:

• from the media without installation
• on desktop Windows operating systems (XP or later)
• without requiring the presence of or installation of supporting frameworks (e.g., .NET or JRE), other than to be able to execute them from the media if required
• without requiring administrative privileges

Obviously, the requirements for "sending software" could be satisfied by an on-board viewer that had such functionality embedded within it, and in many cases that may be simpler than adding separate standalone software. That said, very few viewers supplied by commercial producers of CDs at least, include any kind of networking capability, at least not yet.

A potential problem is the need for the user to supply network parameters for the recipient (in the absence of self-discovery support, something not very widespread, unfortunately), and at the other end for the receiving PACS or workstation to be willing to accept inbound objects from a strange source (some are "promiscuous" in this respect, others are not). In the case where the media sending software is executed on the same machine as the "workstation" (or pre-installed viewer) into which the images are going to be imported, this should be less of a problem. Indeed defaulting to sending to port 104 or 11112 on the localhost with a pre-defined AET might well work for this and we could consider defining that in the IHE PDI profile option.

Anyway, though obviously the "sending software" option is not something ordinary users such as referring physicians will want to have to deal with, since their pre-installed or on-board Basic Image Review Profile viewer should cope most of the time, it provides a means of "last resort", if you will, for support personal to extract content from media that for some reason is unreadable locally through normal means. It also provides a means of helping the enterprise-to-enterprise interchange use-case, when the receiving PACS does not supported the more modern DICOM objects that advanced modalities produce, more modern compression techniques such as JPEG 2000, or the encryption that is being mandated by some jurisdictions specifically for this use-case.

David