Steven C. Horii, MD
Figure 4. Diagram illustrates a DICOM attribute (data element). The attribute includes a tag that identifies the element and consists of two 16-bit integers. The first integer is derived from the old ACR-NEMA group number and, for DICOM standard attributes, is always an even number. The second integer is also based on ACR-NEMA practice. The two numbers together form the attribute tag and are usually shown in parentheses -- for example, (0010, 0020), which happens to be a patient identification number. The second field (VR) is present only if the negotiated syntax includes DICOM explicit VR. It describes how the value field to follow is represented. The length field is a 16- or 32-bit field (16 bits if VR is explicit because the VR field takes up 16 bits itself) that gives the length of the value field that follows. In some DICOM-defined instances, the length may not be known; in such cases, the length field is filled with a representation that tells the system that the length is not known by the application that generated the attribute. The advantage of this attribute structure is that it allows for variable-length values.
