Digital Imaging and Communications in Medicine (DICOM)

6.1 DICOM BMPs 85
Chapter 6
Medical Images in DICOM
Let me assume, my dear reader, that the time we used to wander in the land of
VRs and DICOM objects has been well spent, and that you have a much better
idea of how DICOM deals with medical data. This has prepared us for the
next important step in our voyage: looking at how DICOM works with medical
images. Certainly, images possess some well-known properties (width, height,
bits per pixel), which can be found in the DICOM Data Dictionary, and which
DICOM encodes with explicit or implicit VRs, as we have already learned. But
the most interesting image attribute is the image itself, the sequence of image
pixel values, that DICOM stores in the standard (7FE0, 0010) “Pixel Data”
attribute, using either OB (for 1-byte pixel samples) or OW (for 2-byte pixel
samples) encoding.
DICOM supports a wide range of image formats for storing these (7FE0,
0010) pixels. The formats can be loosely broken into two main groups:
1. DICOM-specific: the formats that are used by DICOM only. They are typically
the oldest ones, introduced at the dawn of the computer era before
better image formats had been developed. They resemble raw BMPs with
varying ways of packing the pixel bytes.
2. Independent standard formats accepted by DICOM. These include such
well-known formats as JPEG, RLE (run-length encoding), ZIP, and the lessknown
(but becoming more and more popular) JPEG2000, and JPEG-LS.
All of these standards are also associated with various image-compression
techniques, both reversible and irreversible, which makes them particularly
useful in medical imaging (reducing image data size is important). The modular
standard approach, when the main standard (DICOM) includes the
other standards (such as JPEG) to be used for particular tasks (such as image
encoding), is very convenient, consistent, and makes good practical sense.
We will start with the first type, because it is the easiest and the oldest, and
because it is still used most of the time as the DICOM default. Then we will
review the most important points of the independent standards. You can read
more about them elsewhere, but some of their properties have tremendous effects
on medical imaging storage and analysis, so we will try not to miss them.
6.1
DICOM BMPs
A digital image, as you might already know, is a rectangular matrix of pixels
(picture elements), tiny dots of different colors that form the actual picture.
For example, a typical CT image is 512 pixels wide and 512 pixels high; that
is, it contains 512 × 512 = 262,144 pixels. If you write these pixels line-by-line