DICOM Query/Retrieve Part I

It all started when I was sitting in a cubicle with a customer, looking at the code of their workstation performing a Query/Retrieve cycle and though everything did look familiar and pretty much straight forward something bothered me.

Query/Retrieve, or Q/R for short, is the DICOM service for searching images on the PACS and getting a copy of them to the workstation where they can be displayed.

Q/R is a fundamental service and every workstation implements it. This sounds like a trivial task, just like downloading a zip file from a web site but there are a lot of details to take care of and while writing this post I realized that I will have to split it to a little sub-series. Today's post will be about the Query part and in the next post I'll get to the Retrieve.

To search the PACS we use the DICOM command C-FIND. This command takes as an argument a DICOM object that represent a query. The PACS transforms the object that we send to a query, probably to SQL, runs it and then transform every result record back into a DICOM object and send it back to us in a C-FIND response. The PACS sends one C-FIND response for every result record. While still running, the status field of the C-FIND response command is pending (0xFF00). The last response has a status success. It may of course fail and then RZDCX will throw an exception with the failure reason and status. It may also succeed but with no matches (empty results set).

Let's do some examples. This code constructs a query for searching patients:

            // Fill the query object

            DCXOBJ obj = new DCXOBJ();

            DCXELM  el = new DCXELM();

            el.Init((int)DICOM_TAGS_ENUM.QueryRetrieveLevel);

            el.Value = "PATIENT";

            obj.insertElement(el);

            el.Init(0x00100010);

            el.Value = "R*";

            obj.insertElement(el);

            el.Init(0x00100020);

            obj.insertElement(el);

            el.Init((int)DICOM_TAGS_ENUM.PatientsSex);

            obj.insertElement(el);

            el.Init((int)DICOM_TAGS_ENUM.PatientsBirthDate);

            obj.insertElement(el);

Introduction to DICOM - Chapter 6 - Transfer Syntax

Transfer syntax defines how DICOM objects are serialized. When holding an object in memory, the only thing that matters is that your application can use it. The internal representation of the object is your own business. However, when sharing objects with other applications, everyone should be able to use the same object. The common solution for such problems is serialization.

Serialization is the process of writing a data structure or object state to wire i.e in a format that can be stored in a file or memory buffer, or transmitted across a network so it can be read on the other side of the wire or later by the same or by another process.

There's no shared memory in DICOM but it can be easily made using the same mechanism that is utilized for networking and files alike i.e. serializing the object into memory according to the rules dictated by the standard i.e. using transfer syntax.

In this post I'll cover the following issues:

• Present the term Transfer Syntax, 
• Why Transfer Syntax is required 
• What is Transfer Syntax used for 
• How Transfer Syntax is set when using 
• DICOM files 
• DICOM network

So, as I said, the serialization in DICOM is governed by a term called Transfer Syntax.

Transfer Syntax is defined at the object level and is the syntax for serializing a DICOM object. We have seen transfer syntaxes already in chapter 5 when dealing with association negotiation but did not discuss them. In order for an application to read a DICOM object from a network wire, it has to know the rules that were used to write the object into the wire. In the association request the calling AE sends a list of abstract syntaxes with SOP Class UID's. For every SOP Class, the calling AE sends a list of transfer syntax UID's. In the association response the called AE selects one of the transfer syntax UID's for every SOP class it accepts.