What identifiers to specify for this simple scenario?

[Tamás Sipos]

Consider this simple workflow:

John comes to the doctor with a broken rib.

The doctor opens my application to request a Chest CT for John:

He selects John from a list, selects Chest CT as the exam he wants to perform, and clicks on a button to add an order (or study? or requested procedure? or scheduled procedure step?) to dcm4chee's Modality Worklist, from where the CT machine in the room will query it and know which exam to perform on John.

I have to implement that function of the application that sends the HL7 message to dcm4chee, and for this I need help. I know HL7 and the technical details of sending such a message, but there is a confusion in my head as to which attributes the message should contain.

Should I set the Accession Number? What is it? Does it identify the Order (which is performing a Chest CT) ..or is it a Study? Which has a UID? Should I set the Study UID then? Or is that generated by the Modality?

What is actually the difference between Order and Study? Maybe both of them are completely irrelevant here, because the MWL has Procedures on it and not studies or orders? Is it a Requested Procedure that has to be added to the MWL with HL7 for John's Chest CT? Or are they Scheduled Procedures on the list? Scheduled Procedure Steps maybe?

So my problem is as you can see, that there are so many terms in DICOM, all of them with different numbers, IDs and UIDs as identifiers that I don't know which of them to define so that the CT modality will receive the order(?).

...and then the next step will be to retrieve the images created by the machine, but that's another story...

Could someone please enlighten me what concepts and identifiers are relevant for my situation? I've been investigating for days already but still don't fully understand the relationships between Order, Accession Number, Study, Study Instance UID, and Requested/Scheduled Procedure (Steps), and that which ones I need from these.

[fleetwoodfc]

It can be a confusing subject! The attached document is the best explanation that I have found - from the IHE Radiology Technical Framework
They also have this helpful code mapping white paper.

[Tamás Sipos]

This is enormous help, thank you!

Meanwhile I also found a document that clarifies some terms.

[fleetwoodfc]

The RIS section of the ClearCanvas Workstation User Guide is also a very good reference document for how this all works in practice.