R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

4.4 Two-dimensional formats
Working with medical scan data 47
Typically in radiography the output of medical scanning modalities is in the
form of two-dimensional images. These images are usually prepared from
the scan data by the radiographer according to instructions from doctors
and surgeons, and they may be from the slices taken through the body or
three-dimensional reconstructions. Often these images are printed on fi lm
and treated in much the same way as X-ray fi lms. As medical scan data
images are made up of pixels, the images can be exported in familiar computer
graphics formats such as bit maps or JPEGs.
4.5 Pseudo three-dimensional formats
Data can be exported in formats that allow three-dimensional operations
to be undertaken without being true three-dimensional forms. The objects
are defi ned by a series of two-dimensional contours arranged in increments
in the third dimension. These types of fi le are often referred to as 2 1 / 2D
data or ‘slice’ formats. More typically, however, these formats are used
as an intermediate step in creating true three-dimensional CAD
representations.
The formats typically are in the form of lines delineating the inner and
outer boundaries of structures isolated by thresholding and region growing
techniques. The lines are usually smooth curves, or polylines that are
derived from the pixel data. This technique results in smooth contours that
more closely approximate the original anatomical shape than the pixelated
data. For example, if we consider the original CT data shown in Fig. 4.13
we can see that there is a high-density bone structure surrounded by lower
density soft tissue. The effect of specifying upper and lower threshold and
region growing is shown in Fig. 4.14. The inner and outer boundaries of the
selected region are smooth polylines.
The pseudo-three-dimensional effect arises when the two-dimensional
polylines are stacked in correct orientation and spacing to provide a layered
model, similar in effect to a contour map. Figure 4.15 shows a threedimensional
rendering derived from CT data of the proximal tibia alongside
the same data exported in a 2 1 / 2D polyline or ‘slice’ format. When such
formats are used in CAD, it is common to create surfaces between the slices
to generate true three-dimensional surfaces. However, when these formats
are used to interface directly with rapid prototyping machines, it is common
to interpolate intermediate layers between the original slices so that data
exists at layer intervals that correspond with the build layer thickness of
the machine being used. The formats that follow are essentially the same
and appear similar to that shown in Fig. 4.15. The differences between them
are concerned with the order and amount of information stored in them.