Bio-medical Ontologies Maintenance and Change Management

80 L. Stanescu, D. Dan Burdescu, and M. Brezovan
1985, after the joining of the two organizations and formed a standard committee.
In that period only the computed tomography and MRI devices could decode the
produced images and the radiologists wanted to use the images for dose planning
for radiation therapy. The initial goal in developing a standard for the transmission
of digital images was to enable users to retrieve images and associated information
from digital imaging equipment in a standard format that would be the same
across multiple manufacturers. In 1988 the second version was released with
improvements. The third version appeared in 1992, when new service classes were
defined, network support added and the Conformance Statement was introduced.
Officially, the latest version of the standard is still 3.0, however, it has been
constantly updated and extended since 1992. Instead of using the version number
the standard is often version-numbered using the release year, like “the 2007 version
of DICOM”.
2.2 The Organization of DICOM Files
A DICOM file has the following structure [19, 20, 21, 78, 55]:
• A preamble of 128 bytes
• Prefix (4 bytes) that stores the letters ‘D’, ‘I’, ‘C’, ‘M’ which represent the signature
of the DICOM file
• Data Set, which stores a set of information such as: patient name, type of image,
size of the image, etc.
• Pixels that compose the image(s) included into the DICOM file.
Figure 2.1 shows a small part of a DICOM file [22]. In figure 2.2, the structure
of the DICOM file is sketched.
The Data Set is composed of a number of Data Elements. It represents an
instance of a real world information object and the Data Elements contain the
encoded values of attributes of that object [55].
A Data Element is composed of several fields [55]:
Data Element Tag – identifies the information in a unique way. This tag
contains two parts: a Group Number (2 bytes) and an Element Number (2 bytes).
For example, in (0010, 0020) tag the Group Number is 0010 and the Element
Number is 0020. It is important the group with the number 0002 and the element
with the number 0010 from this group which represent the Transfer Syntax
Unique Identifier. The Transfer Syntax UID defines the byte order for raw data.
The integer values can be stored using the Big Endian or the Little Endian
ordering.
Little Endian byte ordering is defined as follows: in a binary number consisting
of multiple bytes (e.g. a 32-bit unsigned integer value, the Group Number, the
Element Number, etc.), the least significant byte is encoded first; with the
remaining bytes encoded in increasing order of significance. In a character string
consisting of multiple 8-bit single byte codes, the characters will be encoded in the
order of occurrence in the string (left to right). Big Endian byte ordering differs