Thursday, December 15, 2011

Introduction to DICOM - Chapter 5 – Solving a DICOM Communication Problem

Today we are going to diagnose a communication problem between two DICOM applications and hopefully find the reason for the problem and solve it. I know, we didn’t even start talking about the DICOM network protocol, but hey, we’re not going to read all this 3,000 pages standard together before getting our hands dirty, right?
In this post we'll discuss:
  1. Application Entities (AE’s) – the nodes in the DICOM network and their name – AE Title
  2. Association – a network peer-to-peer session between two DICOM applications
  3. Association Negotiation – The first part of the association in which the two AE’s agree on what can and can’t be done during the Association
  4. The Verification Service using the C-ECHO command – a DICOM Service Class that is used to verify a connection, sort of application level ‘ping’.
  5. The Storage Service using the C-STORE command – a DICOM Service that allows one AE to send a DICOM object to another AE
The C in C-ECHO and C-STORE commands stands for Composite. If you remember, in chapter 4 when discussing the DICOM Data Model, we said that DICOM applications exchange composite objects (the DICOM images that we already know) that are composites of modules from different IE's where IE's are the information entities of the Normalized DICOM data model.

Here's the story:
Complaint 20123

Burt Simpson from Springfield Memorial Hospital reports that he can’t send the screen capture to the PACS. He kept clicking the green “Send” button but he always gets the same error: “Operation Failed!”. The log file Burt copied from the system is attached.
You may ask yourself, what’s the point in analyzing a log of an application that we are never going to use? Well, the truth is that all DICOM logs look alike. Actually, most DICOM applications are quite similar because DICOM software implementations have common ancient ancestors. If it’s a C library it may be the DICOM test node, CTN. If it’s Java than it might be dcm4che. Even if it's PHP or other newer languages, the libraries were transcribed and ported from the old C implementations so all DICOM logs are similar.

Thursday, December 1, 2011

Introduction to DICOM - Chapter 4 - DICOM Objects

Chapter 4 – DICOM Objects
In chapter 3 we’ve learned about DICOM elements. Every element is one piece of typed data with a pre defined, well specified meaning. There are thousands of DICOM elements (See chapter 6 of the standard) from the very basic attributes of patient name and birth date to the most esoteric uses of 3D surface vortices. In this chapter we’re going to collect elements into image object that is called Secondary Capture Image. 

The guys at DICOM did a lot of very good work and created well defined classes for a very detailed Data Model. This is why I always advise to dig in the DICOM standard before designing your imaging device software because there’s a very good chance that the DICOM technical committees already did the work for you and you can save a lot of expansive design time this way. 

In a way DICOM objects definitions are similar to object oriented programming. I prefer though the analog to interfaces specifications. The motivation to adhere to a standard is to enable interoperability. By detailing information object definitions (IOD’s) DICOM enables us to exchange virtual objects between applications without knowing in advance anything about the application we are going to interface with. 

In this chapter I'm going to complete chapter’s 3 examples by adding elements to the object until it’s a valid Secondary Capture Image according to the DICOM standard. Secondary Capture Image is the simplest DICOM image object. Secondary Captures is not related to any specific device. It has the very basic set of elements that a DICOM application needs in order to display and archive a DICOM image properly.