How does Orthanc assign an old for patients with no PatientID tag?

Hi all –

I just noticed that I can upload an image to Orthanc without a PatientID explicitly assigned – it seems like having a PatientID used to be a hard requirement, because the PatientID is hashed to create the OID at every level. I’m curious how Orthanc infers a consistent PatientID substitute for OID hashing if it doesn’t find one explicitly? It would be useful for me to be able to predict OIDs for some unfortunately over-anonymized files with no PatientID without needing to impute my own dummy PatientIDs before sending them to Orthanc.



Hi Derek,

If the PatientID is null, it will be considered as an empty string and therefore, all patient whose PatientID is null or empty will be mapped to the same orthanc id ( -

These patient ids collisions are the reason why it’s usually safer to work at the study level.

Best regards,


Thanks for confirming, Alain. D–