Digital Imaging and Communications in Medicine (DICOM)

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Chapter 5 Parlez-Vous DICOM?
DICOM UIDs are strings such as “1.2.840.10008.1.2” that are built from
numerical components separated by periods; that is, they conform to the UI
VR type (see UI type in Table 2 in 5.3). UID strings are supposed to be globally
unique to guarantee distinction across multiple countries, sites, vendors, and
equipment. In our globalized universe, this is the only way to ensure that your
image or transaction does not run the risk of being confused with any other, no
matter where it comes from. Therefore, DICOM uses the following UID encoding
convention:
UID = .
Here, the “” portion of the UID uniquely identifies an organization
(i.e., manufacturer, research organization, NEMA, and so on). Ideally, each organization
is supposed to apply and receive its own root ID (which does not
always happen) to guarantee that this root is not in use (see Annex C, DICOM
PS3.5, for the root registration rules). Also, the “1.2.840.10008” string is reserved
as the universal “” for all DICOM transaction UIDs, and cannot
be used elsewhere.
Tip: recognizing DICOM files
Open an unknown file in WordPad and search it for 1.2.840.10008. If this
string is found, you are likely dealing with a DICOM file.
The four DICM letters at the very top of the file (characters 129–132, counting
from the file start) are another way of identifying DICOM files. However,
some old or incorrect DICOM implementations might not write them,
while multiple UID prefixes like 1.2.840.10008 should be easier to spot.
The “” portion of the UID is also composed of several numeric components,
and “shall be unique within the scope of the ” (see section 9
in DICOM PS3.5). While roots are relatively short, these are the suffixes that
are used to capture the uniqueness of the instance. For example, if you build a
suffix like:
...
you could be pretty sure that no one in your enterprise will have the same UID.
This is why all 64 UID-type characters come in so handy; the more that are
used, the lower the chance of having two equal UIDs.
For the same reason, UIDs are often used as DICOM file names. This is not
really DICOM-compliant (see 10.1.4): true DICOM file names should come
from eight-character components containing only capital letters, digits, and
underscore characters (for example, DIR1\SKW12AB5). However, if the eightcharacter
format is mandatory for file-exporting applications (writing DICOM
files to external, removable media such as CDs/DVDs or flash drives, see
Chap. 10), UID-based names are widely used internally by nearly all DICOM
applications to store their images on local hard drives. The UID-based DICOM