Digital Imaging and Communications in Medicine (DICOM)

5.6 DICOM Information Hierarchy 71
patient identification will only grow. The use of smart cards, radiofrequency
identification tags, and biological patient profiles (which can be done without
violating patient’s privacy) to encode and differentiate between patients is already
gaining support in medical information technology.
Real case: twins
Once, I was involved in a case with twin sisters. Both came to the same
doctor at the same time with the same right ankle problem. Both girls had
the same last name (as you might expect), and to make things even more
interesting, their name was about 20 letters long. When it was first entered
in the standard “Patient Name” box on the technologist’s workstation, there
was no space left for their first names. Fortunately, the sisters were assigned
different IDs, but the IDs were not descriptive; even the doctor could not
tell whose images he was looking at.
Do not consider this situation to be exceptional.
5.6.2
Problems with Study, Series, and Image UIDs
One good thing about Study, Series, and Image UIDs is that, unlike Patient IDs,
they are almost never entered or assigned manually. Instead, they are autogenerated
on modalities and automatically inserted into the DICOM tags of their
images.
Another good thing is that Study, Series, and Image data are collected during
a single patient scan. With Patient IDs, we have to worry about keeping them
the same between the studies or at different locations. Study, Series, and Image
UIDs are collected in a single, uninterrupted imaging procedure, which they
identify. This also simplifies the method of generating Study, Series, and Image
UIDs. For example, they can be created as current date/time strings written to
the fraction of a second accuracy, as discussed in 5.5.8. This definitely makes the
entire process more reliable, but not without a few very common problems.
The most annoying problem arises when, for whatever technical reason,
some of these IDs are generated incorrectly. For example, I have seen many
times the situation with different Series Instance UIDs, assigned to images
from the same series. In any DICOM software, images are grouped in series
solely based on their Series UID, so the series with inconsistent Series UID
values would simply fall apart into sets of unrelated images. That is, instead
of a CT series with 2000 images nicely ordered by time and 3D location, you
might see 2000 separate images with no means to view them as a whole. If the
image viewing software has image-sorting functionality based on image time,
that feature can at least put the images in order temporarily, but this would be
a mere band aid on the problem. Unlike Patient ID, DICOM software rarely of-