A DICOM message can be visualized as a stream of data elements, where each element is made up of four data fields: element tag, optional value representation, value length and the value itself.
The Value Field is the actual value being sent. It must always contain an even number of bytes. The value field is used to send all the attribute contents, including image pixel data.
The requirement for a value field to be an even number requires the addition (or padding) of the fields with a dummy character (space, null) should it happen to be odd.
For example, to value “M” for the sex field, identifying a male patient would require to be encoded as “M_”. This is a rule dating back to the early days of DICOM when most software was written in assembly code.
There are 2 types of choices for those types of fields; either they are identified in the standard as defined terms or enumerated values.
Enumerated values have only a fixed number of choices. A good example of this is the field containing the patient sex which can be M, F or O (Male, Female, or Other).
In contrast, defined terms can be added by later revisions or extensions of the DICOM standard, or even by manufacturers. They are not as tightly controlled. A good example of a defined terms field is the modality field, which can contain new values as new modalities are introduced.