Digital Imaging and Communications in Medicine (DICOM)

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Chapter 16 DICOM Software Development
the same approach for IEs and IODs. Your module class should be nothing
more than a sorted array of VRs. Then, each more complex type (IE, then
IOD) will become an array of its simpler predecessor (remember our building
blocks). Do not forget XML conversion.
7. Based on the implicit/explicit VR encoding already implemented in guideline
1, add code to write your modules and IODs into binary buffers and to read
them back.
8. Implement the most common DICOM pixel representations (Chap. 6); this
includes uncompressed bitmaps at the very least. Ideally, you will have to
add image compression as well. It is inevitable, so consider doing it sooner
rather than later.
9. Never attempt to write image compression from scratch, even if you know
how it works. Understand that this is not about knowing the principles, this
is about having a solid implementation of a particular ISO compression
standard. The only exception will be DICOM RLE compression (described
in the DICOM standard, RLE is relatively easy to implement) and several
simple image pixel photometric conversions (from RGB into YBR, which
is clearly explained in PS3.3, and used mostly in ultrasound images). Then
you will need to add JPEG compression. Use the free and open-source Independent
JPEG Group library (IJG: http://www.ijg.org/), it’s a good place
to start. Unfortunately, the project has not released any updates since version
6b in 1998. It also has some bugs, their code style is very far from
contemporary object-oriented C + + , and performance optimization in this
library is virtually unknown. 55 Nevertheless, they have done a great job of
implementing a complete and transparent JPEG code, which is not as simple
as it might sound. 56
10. When you are done with generic IOD implementation, implement a few specific
IODs (CT, CR, and MR images, for example). Only now you can try to
apply your implementation to open (relatively easy) and write (more complex)
DICOM files, such as CT and MR image files. This part of your development
will be the most difficult and challenging, as you are now crossing
into a very practical area. Try to collect sample DICOM files from different
sources (different modalities, applications, online databases); this will help
you teach your code to deal with the natural differences and idiosyncrasies
in various DICOM implementations. When you write your DICOM files,
make sure the other DICOM software can open them.
11. Ultrasound images would be your next step. They are color images and come
with five different photometric representations (ways to encode color pixels
55 Many commercial JPEG implementations will try to make performance optimization
their strongest point, explaining to you why you need to pay them big bucks for something
you can get free with IJG.
56 For more compression options, search the Web; I suggest you start from www.datacompression.info.