R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

4.1 Pixel data operations
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Working with medical scan data
As described in earlier chapters, both CT and MR images are made up of
grey scale pixels. In CT, the grey scale is proportional to the X-ray density.
In MR, the grey scale will be proportional to the magnetic resonance of the
soft tissues. In many cases, it is advisable to work with the original data
rather than any three-dimensional reconstruction derived from it. Therefore,
CT and MR image data is often manipulated in the pixel format. Much of
this data manipulation is similar in concept to popular photo-editing software
such as Adobe ® Photoshop ® (Adobe Systems Inc., 345 Park Avenue,
San Jose, CA 95110-2704, USA). Many software packages are available
that utilise such pixel manipulation to allow specifi c individual anatomical
structures to be isolated from a CT or MR data set and exported in an
appropriate format. Many of these packages operate in a similar manner
to the software that radiographers routinely use to generate images in
radiology departments.
4.1.1 Thresholding
Thresholding is the term used for selecting anatomical structures depending
on their density, or grey scale value. By specifying upper and lower density
thresholds, tissues of a certain density range can be isolated from surrounding
tissues. Due to the partial pixel effect described in Chapter 2, small
variations in the thresholds may affect the quality of the anatomical structures
isolated. The effect may be to make them slightly larger or smaller as
illustrated in Figs. 4.1–4.3. However, thresholding will select all pixels within
the specifi ed density range regardless of their relationship to individual
anatomical structures. This may be overcome using region growing.
The effect can be clearly seen in the real example shown in Fig. 4.4. In
this example, bone is selected by setting a high upper threshold and an
appropriate lower threshold, with the resulting region shown in Fig. 4.5.
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