Since this Jira was written, we've clarified the presence of an accession number. Ultimately, we're dependent upon the FHIR data model here  – See ImagingStudy-open's resource profile –  Relative to FHIR R4, the ImagingStudy resource’s change relevant to FHIRcast is the guidance FHIR R5 provides in specifying the accession number. In FHIR R4, the guidance is to provide the accession number in the  identifier  array. In FHIR R5, the accession number is provided in the ImagingStudy’s  basedOn  array as a reference using a ServiceRequest reference type. Since this version of FHIRcast promotes the use of FHIR R4 resources, the guidance to provide the accession number in the  identifier  array SHOULD be used and this approach is shown in all ImagingStudy examples in the FHIRcast specification. However, if Subscribers agree to use FHIR R5 resources, the FHIR R5 recommendation MAY be used. For a more detailed explanation of these business identifiers, see the  FHIR R4 implementation notes of the FHIR ImagingStudy resource  (and the  FHIR R5 implementation notes of the FHIR ImagingStudy resource ). Ideally both the accession number and Study Instance UID are available and present in an ImagingStudy resource used in FHIRcast. The presence of both business identifiers ensures that all Subscribers will be able to be able to identify the appropriate imaging study.