Master's Thesis - Studierstube Augmented Reality Project - Graz ...

2.1 Technical Visualization
cial needs of medical image data and physiological flow data. The upcoming sections
identify dedicated visualization techniques for both.
2.1.1.1 Representation Techniques for Volumetric Morphological Data
To be able to perform medical volumetric data visualization one first has to consider the
underlying two dimensional images which build-up an image stack for further volumetric
approaches together. The following considerations refer predominantly to morphological
data which means images presenting an accurate representation for the shape of
different tissues.
Displaying even two dimensional medical image data is in general not as trivial as decoding
and presenting an image in a common format. Unlike a picture taken with an ordinary
picturing device providing red, green and blue components R[0...255], G[0...255],
B[0...255], structural projection or scanning techniques deliver only gray values (R =
G = B) but with a greater range. In a commonly known medical imaging format called
Digital Imaging and Communications in Medicine (DICOM), [DICOM2007], the intensity
values are encoded with 12 bits, which allows to encode more information in
one pixel. The problem hereby is the limitation of all display devices to 256 values and
the constraints on the human visual system. [Barlow1956; Lu1999; Barten1992] have
identified these constraints by estimating the required signal stimulus energy required
for an observer to maintain a perception and found that the perception is dependent
on a non-linear function. Due to these limitations a windowing mechanism has been
developed among others by [NEMA2003] to provide a mapping from the measured 12-
bit data to a 256 gray value gradient. Such a mapping is called windowing and briefly
outlined in figure 2.5.
This constraint of pixel value mapping has to be kept in mind when working on
medical data, especially when using them with some special volumetric rendering techniques
for image stacks as they are appearing next. Later used techniques, which are
described in more detail, use transfer functions which implicate the windowing in their
parameters. The remaining rendering techniques are itemized in the end of this section
along with the definition of volume rendering transfer functions.
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