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 Data Element Tag is a pair of 16-bit unsigned integers representing the group number and the element number.

When migrating from ACR-NEMA to DICOM, this initial notion of the two components (Group, Element) was kept intact, even though it has no meaning anymore. All of the standardized tags are listed in the data dictionary. Some examples of data element tags are:
(0008,0020) Study Date
(0008,0030) Study Time
(0008,0060) Modality
(0010,0010) Patient’s Name
(0010,0020) Patient ID
(0028,0010) Number of pixel rows in the image
(0028,0011) Number of pixel columns in the image
(0038,001A) Scheduled admission date
(0038,001B) Scheduled admission time

The tags are identified by hexa-decimal number, and they can range from 0000 to FFFF. They are always sorted in ascending order in a DICOM header to make it easily searchable and possible to parse.

A typical DICOM message contains a long series of these data element groups, and is sometimes called a data set.




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