Digital Imaging and Communications in Medicine (DICOM)

10.1 DICOM File Format 223
10.1.2
Group 0002: DICOM File Meta Information
Right after the DICM prefix, beginning at byte 133, we find the DICOM File
Meta Information. Unlike the preamble and the DICM prefix, meta information
is encoded as a set of DICOM attributes with explicit VRs. All attributes in File
Meta Information belong to DICOM group 0002, and are shown in Table 49.
The last four elements in our table are optional, and can therefore be omitted.
The required elements, as you can see, are either derived from the DICOM data
object properties (SOP, Transfer Syntax), or record the properties of the application
that created the file. The most important element in the entire 0002 group
is probably the “Transfer Syntax UID” (0002,0010), which defines how the subsequent
DICOM data object is encoded. If you remember (see 9.4), in DICOM
networking the Transfer Syntaxes are communicated at DICOM association establishment
time; now they are written into the DICOM file headers.
You will find another example of group 0002 use a little bit later (in 10.2.1)
when the structure of the DICOMDIR file is discussed. It will also give you an
example of group 0004 application, which we did not discuss here for brevity,
but which is used quite similarly to group 0002.
Table 49 File Meta Information
Attribute name Tag VR Attribute description
Group Length (0002,0000) UL Number of bytes following this File Meta
Element (end of the value field) up to and
including the last File Meta Element of the
Group 0002
File Meta
Information
Version
Media Storage
SOP Class UID
Media Storage
SOP Instance
UID
(0002,0001) OB This is a 2-byte field where each bit identifies
a version of this File Meta Information header.
In current DICOM version, the first byte
value is 00H and the second byte value is 01H
(0002,0002) UI Uniquely identifies the SOP Class associated
with the DICOM data object. The value of
this element is based on the image modality
type; in other words, it should contain the C-
Store SOP for image modality (see 7.3)
Example: 1.2.840.10008.5.1.4.1.1.1
(CT image storage)
(0002,0003) UI Uniquely identifies the SOP Instance associated
with the DICOM data object placed in
the file and following the File Meta Information.
This value comes from the Image SOP
Instance UID attribute (0008,0018)
Example: 1.2.840.10008.2008.03.25.12.33.55.7