Digital Imaging and Communications in Medicine (DICOM)

5.5 DICOM Objects 57
change the principle. A single rule determines the element order: all data elements
inside DICOM objects must be ordered by their tag “(group, element)”
number. This ordering serves at least two practical purposes:
1. Helps verify data integrity (similar to what even data length does). If, when
reading a DICOM object element after element, you run into an element
with a smaller tag number than the one you read before, the object is
corrupted.
2. Helps put data in the order in which it should really be interpreted. For example,
image width and height have lower tag numbers than the actual image
pixel buffer. This means that image width and height are read first, and only
after will you read the pixels, already knowing how to group them into the
width-by-height pixel matrix.
5.5.5
SQ: Encoding DICOM Object Sequences
As we have already seen, one VR type, SQ (sequence), plays a very special role
in DICOM data encoding. It allows us to store entire sequences of DICOM
objects in a single SQ VR (see Sect. 5.3.10). With the introduction of the SQ
type, DICOM object layout changes dramatically. The SQ element has no data
of its own. Instead, it contains a sequence of DICOM objects. These objects become
nested in the parent DICOM object (the object containing the SQ element).
Moreover, because the DICOM objects in the SQ sequence obey the same
DICOM object format, they can also contain SQ elements. As a result, we could
have multiple nesting levels as shown on Fig. 12. The nesting stops at the level
where nested DICOM objects have no more SQ elements. Thus, the entire concept
of object sequencing comes quite naturally. If it looks thorny to you, think
about the structure of this book. The book has chapters (data elements), but
some of these first-level chapters have second-level chapters (subsections) inside
them, and some second-level chapters have their own subchapters as well
(third level), and so on. So, if we consider this book as a large DICOM object,
chapters with subchapters would be our SQ elements. In fact, many current
data-representation languages (XML, for example) make full use of data nesting
just like DICOM does.
How do we encode SQ elements though? Without SQ everything is easy.
A DICOM object is a list of its data elements, so we encode it as a list as well.
For SQ elements, DICOM provides a special SQ encoding scheme that encapsulates
the DICOM object sequence within a single VR. This is similar to what
was learned in Sect. 5.5.1 and depends on the VR encoding method (explicit
or implicit) and our choice of data SQ item delimiters (known or unknown
length). The following three examples in Table 11 cover all possible SQ encoding
methods (nested DICOM objects are in gray cells). If you compare the
three examples, you might discover the SQ encoding rules on your own. The
rules include: