Just how difficult can those of us who write standards for a living actually make an implementer's life ? Pretty difficult, is the answer, though largely this occurs as we strive to avoid breaking the installed base of existing applications that might never be upgraded.
Today I was responding to a question from a software engineer at a vendor of veterinary radiology equipment who had come to realize the the "normal" attribute for encoding Exposure Time was insufficiently precise, given that it was restricted to being an Integer String, and small things, like cats, may have exposure times shorter than a whole second. I say "normal attribute", because the original CR IOD, and most other IODs since, have used this and other attributes with similarly constrained encoding to describe X-Ray technique, and in some cases made these attributes mandatory or conditional. The attributes I am talking about are:
- Exposure (0018,1152), which is IS VR
- Exposure Time (0018,1150), which is IS VR
- X-Ray Tube Current (0018,1151), which is IS VR
A naive approach would be to just change the VR for the existing data element, say from Integer String (IS) to Decimal String (DS), which would then allow fractional values. The problem with this solution would be that recipients that expected a string formatted in a particular manner might fail, for example if the parser, or display text field or database column did not expect decimal values. I.e., existing implementations might be broken, which is something we always try to avoid when "correcting" the standard.
You might well ask why the standard makes the distinction between integer strings and decimal strings in the first place, or indeed allows for both binary and string encoding of integers and floating point values. For example, a number might be encoded as an integer string (IS), decimal string (DS), unsigned 16 bit short (US) or 32 bit long (UL) or signed 16 bit (SS) or signed 32 bit (SL) binary integer, or as a 32 bit (FL) or 64 bit (FD) IEEE floating point binary value. The original ACR-NEMA standard had fewer and less specific encoding choices; it specified only four choices for value representation, 16 bit binary (BI), 32 bit binary (BD), ASCII numeric (AN) and ASCII text (AT). Note that there was no distinction between signed and unsigned binary values, and no distinction between integer and decimal string numeric values, and no way to encode floating point values in a binary form (indeed the standard for encoding binary floating point values, IEEE 754, was released in the same year as the first ACR-NEMA standard, 1985, and certainly was not universally adopted for many years). Anyway, if you review the list of data elements, the authors of the ACR-NEMA standard seem to have taken the approach of encoding:
- structural elements related to the encoding of the message (like lengths and offsets) and pixel value related (rows, columns, bits allocated) stuff as binary (16 or 32 bit as appropriate),
- "real world" things as ASCII numeric, even things things that could have been binary integers like counts of numbers of images, etc.
Unfortunately, even though the DICOM standard introduced the concept of sending not only the value of a data element but also its type in the message, using the so-called "explicit value representation" transfer syntaxes, the new standard continued to support, and indeed require as the default, the "implicit value representation" that was equivalent to the way some vendors had implemented the ACR-NEMA standard over the network. Requiring only explicit VR would have allowed recipients to use the VR transmitted to decide what to do with the value, and opened the door to "fixing" incorrect VRs in the data dictionary. One could have required that recipients check and use the explicit VR. Unfortunately, by permitting implicit VR transfer syntaxes, the VR has to remain fixed forever, otherwise receivers have no way of knowing what to do with a value that is of an unexpected form. I am told that there was significant discussion of this issue with respect to the 1992 RSNA demonstration, and that implicit VR was allowed for the demonstration to maximize participation, with the intent that it not be included in the standard published in 1993, but there was not sufficient support to follow through with this improvement after all. In hindsight it is easy to criticize this short-sighted decision. On interchange media, added in 1995, only explicit VR transfer syntaxes are permitted, but by then it was too late.
So what does all this mean for our exposure-related attributes ? Given that one cannot reasonably change the VR of an existing data element, the only option was to add a new one. So this is what CP 77 did:
- it described the problem with all three data elements
- it described the historic lack of constrains in ACR-NEMA
- it only fixed the problem for one of the data elements (Exposure (0018,1152)), without further explanation as to why only that one was addressed
- it add a new data element, Exposure in μAs (0018,1153), to the data dictionary and added it as an optional attribute in the CR Image Module
- it defined the new attribute to have a scaling factor 1,000 different than the original attribute, which was defined to be in mAs (as is normally displayed to the user)
- it gave the new attribute a VR of IS
- why CP 77 didn't just make the new data element a DS, keep the same units that were used previously and that are the normal units in which a user expects to see the value displayed ?
- why not just call the data element something like Exposure (Decimal), or indeed use the same name and rename the old one to Exposure (Retired) or similar ?
- why was the old attribute in the CR Image Module not simply retired or deprecated in some other way ?
- Exposure Time in μS (0018,8150), which is DS VR
- Exposure in μAs (0018,1153), which is IS VR
- X-Ray Tube Current in μA (0018,8151), which is DS VR
There are several other problems than the VR and the scaling factor with this approach of fixing inappropriate VRs by adding optional attributes that mean the same thing as what they are intended to "replace", without actually retiring and removing the old attribute. Specifically:
- How is a poor receiver to know which to use if it receives both (the sensible answer is to use the more precise one instead of the less precise one, but the standard does not require that) ?
- What about an old receiver that has never heard of the new attribute (it will display the old less precise one) ?
- Should a sender send both a less precise and a precise value, just to be able to allow such old receivers to display something rather than nothing (almost certainly yes) ?
- Exposure Time in ms (0018,9328), which is FD VR
- X-Ray Tube Current in mA (0018,9330), which is FD VR
- Exposure in mAs (0018,9332), which is FD VR
The problem with these new data elements is that now that they are in the data dictionary, some creative implementers of non-enhanced images have started to stuff them into the "old" IODs in order to send values with greater precision, instead of sending the intended CP 77 and CP 187 data elements. Strictly speaking this is legal as a so-called "Standard Extended SOP Class", but it creates an even greater problem for the receivers. When I first encountered someone doing this, I added a specific check to my dciodvfy validator to display an error if these attributes are present when they should not be in the DX IOD, and I have subsequently the check to other "old" IODs as well, including CR, XA/XRF and CT; I also implemented some limited consistency checking when multiple attributes for the same concept are present, since I encountered examples where completely different values were present that made no sense at all. As more and more modalities implement the Enhanced family of objects, however, and include the ability to "fall back" to sending the "old" objects if the SCP does not support the new ones, and do it by copying the "new" attributes from the functional group sequences into the top level datasets of old IOD objects rather than converting them to the "old" attributes, we may see more proliferation of a multitude of different data elements in which the exposure parameters might be encoded.
So back to the problem of what a poor receiver (of non-enhanced IOD) images is to do ? The bottom line in my opinion is that a modern receiver should check for the presence of any of the alternative attributes that encode the exposure parameters, and use whatever they find in order of greater precision. I implemented this rather crudely recently in the com.pixelmed.display.DemographicAndTechniqueAnnotations class in my PixelMed toolkit, if you are interested in taking a look at one approach to this; look for the use of the getOneOfThreeNumericAttributesOrNull() method.
If the foregoing sounds a little critical and sarcastic, it is intended to be. I continue to amaze myself with my own poor expedient decisions, lack of consistency and frequent carelessness when working on corrections and additions to the DICOM standard, and so this missive is intended to be as self-deprecating as it is critical of my contemporaries and predecessors. Much as we would like to change DICOM to make it "perfect", the need to correct problems and add functionality yet avoid breaking things that already work and avoid raising the implementation hurdle too high to be realistic are overriding; the result of compromise is significant "impurity".
If we ever had the chance to start DICOM all over again and "do it right", I am sure that despite our best intentions we would still manage to screw it up in equally egregious ways. We sometimes joke about doing a new standard called just "4", so-called because it would be the successor to DICOM 3.0, would not necessarily be just about images, and which would be an opportunity to skip the past the morass that is HL7 version 3. I doubt that we would really do much better and would no doubt encounter Fred Brooks' "second system syndrome". Indeed, DICOM 3.0 being the successor to ACR-NEMA already suffers in that respect, perhaps being accurately described as an "elephantine, feature-laden monstrosity". From what little I know about HL7 v3, it is not exempt either.