What exactly is a DICOM IOD (Information Object Definition)?

Question

I'm trying to understand DICOM better in preparation for a presentation about SOP.

What I still haven't really understood is what an IOD (Information Object Definition) is practically.

If I for example had a dataset with an encapsulated document inside, what exactly would be considered an IOD? Is it the whole dataset, the PDF inside, single tags or tag groups?

Answer 1

If I for example had a dataset with an encapsulated document inside, what exactly would be considered an IOD? Is it the whole dataset, the PDF inside, single tags or tag groups?

None of it will be considered an IOD because you are talking about instance of IOD here. IOD should be looked as class instead.

Please refer to the diagram below. A SOP Class is defined as a combination of DICOM Service Element (command), and an Object. The object is defined by an Information Object Definition (IOD). So, your dataset is an instance of IOD; it itself is not an IOD. You can look at IOD as a template for simplicity; you create actual instances (dataset) from this template. IODs are further split in Composite IOD and Normalized IOD.

If you see this through programming language point of view, you can think IOD as class in your programming language. Its a class defined; not yet instantiated. Then somewhere, you create an instance of this class and set the attributes/properties. This is your dataset. Single tag/element resembles with property. The PDF document in dataset is just a value of property. The concept of Modules resembles with Composition concept of Object Oriented Programming.

6.1 Information Object Definition
An Information Object Definition (IOD) is an object-oriented abstract data model used to specify information about Real-World Objects. An IOD provides communicating Application Entities with a common view of the information to be exchanged.

_{Major Structures of DICOM Information Model}

An IOD does not represent a specific instance of a Real-World Object, but rather a class of Real-World Objects that share the same properties. An IOD used to represent a single class of Real-World Objects is called a Normalized Information Object. An IOD that includes information about related Real-World Objects is called a Composite Information Object.

_{Source - Specifications (DICOM PS3.4 2020a)}

Further, Roni on DicomIsEasy explains this far better:

The classes of the DICOM static data model are called SOP Classes and are defined by IOD’s – Information Object Definition. IOD’s are specified in Appendix A of chapter 3 of the standard. An IOD is a collection of Modules and a Module is a collection of elements from one information entity that together represent something. The modules are also defined in chapter 3 of the DICOM standard in appendix C. Two object oriented concepts, composition and reuse, that are used by DICOM is the Modules that are parts shared between different IOD’s.

The other good resource that explains it in simple language.