A DICOM object is identified with its Information Object Definition, or IOD. These IODs are specified using modules, which consist of the most basic DICOM unit, the data element or attribute.

The modules are described as a group of data elements. The DICOM standard uses modules to group information items that are similar for multiple objects so they only have to be defined once in the standard. For example, a patient module is related to the real-world patient information entity, and contains patient name, birth date, sex, etc.

The IOD definitions start with a module table, and list all of the modules included in that IOD. Some modules are mandatory (M), some optional (U) and others conditional (C). A module is composed of a set of attributes, tags, attribute type and attribute description. The module specifies the individual attributes and their type.
The type specifies whether or not an attribute is required, conditional or optional.

Attributes that are optional are identified as Type 3.




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