Digital Imaging and Communications in Medicine (DICOM)

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10.1
DICOM File Format
Chapter 10 DICOM Media: Files, Folders, and DICOMDIRs
When talking about DICOM media, we are talking about DICOM files. DICOM
files store DICOM data objects (also referred to as Data Sets), most frequently,
DICOM images. The data objects are written into the DICOM files with the exact
same encoding rules as we used in DICOM networking: implicit or explicit
VR encoding (see 5.5). The only difference is in the DICOM file header, which
precedes the data object, as shown on Fig. 72. The DICOM header plays the
role of the missing DICOM association establishment: it explains to any filereading
application that the file stores DICOM data of certain a SOP type, and
in certain Transfer Syntax format. The DICOM header includes a preamble, a
DICM prefix, and a pinch of DICOM file attributes (file meta elements).
10.1.1
Preamble and DICM Prefix
The preamble is a string of 128 bytes, which opens any DICOM file. The use of a
preamble is common in many imaging and data formats (consider TIFF images,
for example), and DICOM adapted the same style. However, the DICOM standard
does not define any particular preamble structure or content. It is up to each
DICOM application to use the 128 preamble bytes to its advantage. Obviously,
this makes preamble content application-dependent; different applications can
use it differently. For that reason, the preamble in DICOM is generally ignored
and filled with 0 bytes; in DICOM this simply means “unused preamble”.
The DICM prefix (indicating the DICOM file format) follows the 128-byte
preamble verbatim. It simply consists of the four uppercase letters (D I C M)
written into bytes 129–132. The use of a format prefix (often called the magic
number) is also very common in many file formats (imaging included) and
DICOM follows the same convention.
Neither the preamble nor the DICM prefix use DICOM VR encoding rules.
They are simply stored in the first 128 + 4 = 132 bytes. If you are writing a program
to identify DICOM files, make it skip the first 128 bytes, and then verify
the DICM prefix.
Fig. 72 DICOM file structure