Digital Imaging and Communications in Medicine (DICOM)

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Chapter 5 Parlez-Vous DICOM?
fers any merge function for series. So the only way of dealing with this problem
is to fix it where it happens: on the modality unit.
Another typical but less problematic dilemma arises from the uniqueness of
the Series, Study, or Image (SOP) UID. The letter U in UID stands for “Unique”
(Identifier), implying that identifier values must be globally unique. If anything
ever changes in an image (for example, we crop/rotate it, or we compress it with
lossy compression), then the image is not the same anymore, even if all clinically
important information remains the same. Consequently, the Image (SOP)
UID for this modified image needs to be changed (see 5.5.8). Do not be surprised
if you see two image series that look exactly the same, but are still stored
separately on your imaging archive; there could be a reason.
It is good to have Study, Series, and Image IDs generated automatically on
a modality, but what if this does not happen? The most typical example involves
secondary capture (SC) DICOM images, such as scanned (digitized)
films. When you digitize films or import non-DICOM images into DICOM
format, the relationship between several films or images are known only to you,
and definitely not to your digitizer. Do the images belong to the same patient,
study, or series? Your DICOM digitizer should provide sufficient functionality
for assigning Study, Series, and Image UIDs manually to record the original
image relationships in the digitized DICOM data. Unfortunately, not all SC devices
treat this requirement seriously. If you are planning on buying one, please
verify that it has ample support for entering DICOM data and for assigning the
images to the same Patient, Study, or Series groups.
5.6.3
Hierarchical and Relational Data
To wrap up this discussion on the DICOM Patient-Study-Series-Image information
model, let us just briefly mention that it represents a central part of a
more complex DICOM model of the real world. This model can be found in
the standard (section 7 in PS3.3) adding more items to the basic Patient-Study-
Series-Image hierarchy without changing its essence. Virtually all DICOM devices
function on the Patient-Study-Series-Image steps, performing hierarchical
data searches, retrievals, and transactions.
To identify a series, you will need first to find the patient, and the study
this series belongs to. This logic is clearly reflected in all DICOM interfaces.
They always start browsing their data from the top, from the highest Patient
(sometimes Study) level, then gradually descending to Series and Images.
Moreover, hierarchical DICOM queries and data retrievals will fail on any of
the four Patient-Study-Series-Image levels if the information about the higher
level IDs is not known. For example, a hierarchical search for a particular series
will fail if its Study and Patient IDs are not known beforehand.
The only DICOM alternative to hierarchical data processing is known as
relational data processing (Fig. 16). Unlike hierarchical data processing, rela-