DCMTK Understand the "DIMSE No valid Presentation Context ID" error - c++

Question

I'm currently developing a simple application for querying/retrieving data on a PACS. I use DCMTK for this purpose, and a DCM4CHEE PACS as test server.
My goal is to implement simple C-FIND queries, and a C-MOVE retrieving system (coupled with a custom SCP to actually download the data).
To do so, I've created a CustomSCU class, that inherits the DCMTK DcmSCU class.
I first implemented a C-ECHO message, that worked great.
Then, I tried to implement C-FIND requesting, but I got the error "DIMSE No valid Presentation Context ID" (more on that in the next paragraph) from my application, but no other log from DCM4CHEE. I've then used the command tool findscu (from dcmtk) to see if there was some configuration issue but the tool just worked fine. So in order to implement my C-FIND request, I've read the source of findscu (here) and adapted it in my code (meaning that i'm not using DcmSCU::sendCFindRequest but the class DcmFindSU).
But now, i'm facing the same problem with C-MOVE request. My code is pretty straight-forward :
//transfer syntaxes
OFList<OFString> ts;
ts.push_back(UID_LittleEndianExplicitTransferSyntax);
ts.push_back(UID_BigEndianExplicitTransferSyntax);
ts.push_back(UID_LittleEndianImplicitTransferSyntax);
//sop class
OFString pc = UID_MOVEPatientRootQueryRetrieveInformationModel;
addPresentationContext(pc, ts);
DcmDataset query;
query.putAndInsertOFStringArray(DCM_QueryRetrieveLevel, "PATIENT");
query.putAndInsertOFStringArray(DCM_PatientID, <ThePatientId>);
OFCondition condition = sendMOVERequest(findPresentationContextID(pc, ""), getAETitle(), &query, nullptr);
return condition.good();
I've also tried using UID_MOVEStudyRootQueryRetrieveInformationModel instead of UID_MOVEPatientRootQueryRetrieveInformationModel, with the same result : my application shows the error
DIMSE No valid Presentation Context ID
As I understand, a presentation context is concatenation of one or more transfer syntax and one SOP class. I read that the problem could come from the PACS that won't accept my presentation contexts. To be sure, I used the movescu tool (from DCMTK). It worked, and I saw this in the logs from de server DCM4CHEE :
received AAssociatedRQ
pc-1 : as=<numbers>/Patient Root Q/R InfoModel = FIND
ts=<numbers>/Explicit VR Little Endian
ts=<numbers>/Explicit VR Big Endian
ts=<numbers>/Implicit VR Little Endian
That means that the movescu tool does a find before attempting an actual move ?
Therefore, I changed my application context creation with :
OFList<OFString> ts;
ts.push_back(UID_LittleEndianExplicitTransferSyntax);
ts.push_back(UID_BigEndianExplicitTransferSyntax);
ts.push_back(UID_LittleEndianImplicitTransferSyntax);
OFString pc1 = UID_FINDPatientRootQueryRetrieveInformationModel;
OFString pc = UID_MOVEPatientRootQueryRetrieveInformationModel;
addPresentationContext(pc1, ts);
addPresentationContext(pc, ts);
(also tried study root)
But this didn't do the trick.
The problem seems to lie on the client side, as findPresentationContextID(pc, ""); alwasy return 0, no matter what.
I don't feel like it's possible to adapt the code of the movescu tool, as it appears to be very complex and not adequat for simple retrieve operations.
I don't know what to try. I hope someone can help me understand what's going on. That's the last part of my application, as the storage SCP already works.
Regards

Answer 1

It looks like you are not negotiating the association with the PACS.
After adding the presentation contexts and before sending any command, the SCU must connect to the PACS and negotiate the PresentationContexts with DcmSCU::initNetwork and then DcmSCU::negotiateAssociation.