How to reverse anonymization?

We have a list of patients that we anonymized in the past. However, we need to know from the Anonymized ID what is the old Patient ID.

How can we do that?

If you did anonymized the study by Orthanc you will find a "Anonymized From" information in Studies API in Orthanc.

If the anonymization was not done in your current Orthanc session, it would be a good anonymization quality control to not be able to return to the original study, so the only way to do is to search for a breach : combination of non anonymized tag that give you the correct original study....

Compare pixel values (or hashes thereof)? Or DICOM fields that depend on the patient (like the precise x-ray techniques). If you did your anonymization properly that should be the only way.

Also - some anonymization tools (like CTP) use hashes to compute anonymized PatientID/AccessionNumber - you could run that same function over the original study values to create a map. Not sure if this is how Orthanc does it though.

Hi,

When you anonymize with Orthanc, by default, the IDs are generated randomly, so proposal of Sean Doyle will not work unless to have forced the IDs during the initial anonymization with some hash key (which you have not done probably seeing your question).

The solution proposed by Salim Kanoun is the easiest one, providing you kept the studies in Orthanc, as Orthanc will keep the link between the original and the anonymized study in the metadata, and you can access this metadata with the “Anonymized From” information.
But this solution won’t work if the anonymized study is no longer available on the Orthanc that hosts the original study as the information is stored in the metadata of Orthanc and not in the DICOM data of the study.

In case the solution of Salim Kanoun is not suitable to your setup, then I guess only the hardcore solution proposed by bobIQ morane can do the trick.

Cheers,

Michel