DICOM basics

The DICOM Standard

DICOM stands for “Digital Imaging and Communications in Medicine”. It is a standard that defines how medical imaging (primarily) data is stored and moved over the network. It’s been around since the '80s and eventual adoption of this standard by all manufacturers of medical imaging equipment has been a huge enabler for medical data interoperability and clinical research in general.

It is an open standard that is maintained by the National Electrical Manufacturers Association (NEMA) and is available at http://dicom.nema.org/. The standard is updated a few times per year, as we update our views on how medical imaging data needs to be stored, and all prior versions are also available. When referencing the DICOM standard in your documentation, it is important to be clear about whether you are referencing the current or past versions of the standard. In this course, I will be referencing version 2020a of the standard.

DICOM is a vast and quite complex standard, and you don’t need to know it all unless you are developing a medical imaging modality or storage software. However, since you are very likely to get data as DICOM, it is important to know what is there, and how to look up things.

In this lesson, we will focus on the storage portion of the standard - the part that defines how the data acquired by the scanners is stored as files on filesystems, and what metadata accompanies these files.

DICOM Entity-Relationship Model

DICOM standard defines Information Entities that represent various real-world entities and relationships between them. The cornerstone of the DICOM standard are the following objects and relationships:

Patient is, naturally, the patient undergoing the imaging study. A patient object contains one or more studies.

Study - a representation of a “medical study” performed on a patient. You can think of a study as a single visit to a hospital for the purpose of taking one or more images, usually within. A Study contains one or more series.

Series - a representation of a single “acquisition sweep”. I.e., a CT scanner took multiple slices to compose a 3D image would be one image series. A set of MRI T1 images at different axial levels would also be called one image series. Series, among other things, consists of one or more instances.

Instance - (or Image Information Entity instance) is an entity that represents a single scan, like a 2D image that is a result of filtered backprojection from CT or reconstruction at a given level for MR. Instances contain pixel data and metadata (Data Elements in DICOM lingo).

There are many more entities defined by the DICOM standard, but we will focus on these for the purposes of this course. You can look up the comprehensive list in Section A.1.2 of Part 3 of the standard.

New Vocabulary

• DICOM - “Digital Imaging and Communications in Medicine”. A standard that defines how medical imaging (primarily) data is stored and moved over the network.