Digital Imaging and Communications in Medicine (DICOM)

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Chapter 5 Parlez-Vous DICOM?
Fig. 9 Example of a group length element: element (0010,0000) at the very beginning
of group 0010 contains data value equal to L, where L is the total number of bytes in
the encoded group 0010 data elements (following right after element (0010,0000)). The
number of bytes in (0010,0000) is not included in L
The reason for group length encoding is exactly the same as it is for element
length encoding. If your application does not need to read the group gggg data,
it might read only the group’s length L from the (gggg,0000) tag, and then fastforward
L bytes, thus proceeding to the next available group. Because the number
of elements in gggg can be large, this forwarding can substantially speed up
DICOM object reading (a technique known as partial parsing). This becomes
particularly handy when dealing with proprietary, odd-numbered groups. Such
groups can be correctly interpreted only by their manufacturers and have to be
ignored by everyone else. If group length is provided in element (gggg,0000),
skipping such groups becomes a very easy exercise.
Group length can also be viewed as a basic security feature. Group length
(just like data checksums) makes it harder to modify something in part of a
DICOM object without destroying the entire object structure. There is no free
lunch, however, and this method has its own principal drawback. To be able to
write the (gggg,0000) tag, your software application must know the length L of
all data elements from the gggg group that are present in the DICOM object.
That is, it needs to know the total length of all gggg elements, which will be
written after the very first (gggg,0000) group length element. This implies that
all gggg elements must already be available to you with their final values and
encoded ahead of encoding the (gggg,0000) element. To DICOM software developers,
this means two things:
1. Writing DICOM objects on-the-fly becomes practically impossible because
all data must be collected first.