[BitBucket user: Sébastien Jodogne]
[BitBucket date: 2016-06-09.07:06:51]

In Orthanc Explorer, the patients whose PatientID tag is empty are all grouped together as a single patient (cf. https://orthanc.chu.ulg.ac.be/book/faq/orthanc-ids.html). If you open this "pseudo-patient", the individual studies corresponding to all the original patients will show up. This is because Orthanc Explorer is designed as a low-level, administrative user interface. In particular, Orthanc Explorer only supports the patient-study-series-instance access scheme, not the study-series-instance scheme that you wish. If this behavior is insufficient for you, we kindly invite you to create a GUI on the top of the REST API of Orthanc (cf. https://orthanc.chu.ulg.ac.be/book/faq/improving-interface.html).