title: Modality for radiology orders in ORM^O01 message
body: <p>Our DICOM Modality Worklist server is currently receiving HL7 <code>ORM^O01</code> orders from our hospital RIS. To map each order to a modality we are currently using the <code>OBR-24</code> field in the message. The codes contained in <code>OBR-24</code> are mapped to AE titles and modality by a lookup table in the MWL server. The current <code>OBR-24</code> values are something along the lines of "LOC_STATION", where LOC is the location (on building level) for the scheduled station and STATION is the scheduled station for the current order.</p>
  
  <p>Now a need has arisen to direct the same incoming message feed to another system for prefetching prior studies from a slower long-time archive based on the orders. An important input parameter for the prefetch engine rules is the modality of the ordered study. Therefore we would like to add the DICOM modality code in the incoming order messages, since we do not want to duplicate the entire lookup tables system and manage it separately in two places.</p>
  
  <p>What would the best field for this kind information be inside the <code>ORM^O01</code> structure as defined in v2.3.1 of HL7 standard? I have skimmed the standard and went through our MWL server vendor reference materials, but the closest I have found, is the same <code>OBR-24</code> field, which is already in use in our solution. Or should we look at implementing some kind of custom Z-segment?</p>
  
  <p>The situation is additionally complicated by the fact, that we are an independent PACS service provider, therefore we do not control the development of the HIS/RIS softwares in the hospitals and mostly we have to integrate with existing systems with minimum modifications on their side. Therefore it is quite difficult to change or move any of the existing fields in our messaging standard, but it's easier to implement new unused fields for new purposes.</p>