R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Working with medical scan data 45
Scans are usually performed in slices or a spiral form in a plane perpendicular
to the long axis of the patient. The interval between the slices may
be in the order of a millimetre or two, whereas the models are built in layers
between 0.1 and 0.2 mm in thickness. Therefore, additional software is used
to interpolate intermediate slices between the scan data slices. Another
interpolation is carried out within the plane of the scan to improve resolution.
Together, these operations result in the natural and accurate appearance
of the model. This interpolated data can then be exported in a number
of formats that may be transferred to computer-aided design packages or
rapid prototyping preparation software. Some of the different output data
formats are described later in this chapter. The same example as shown
here is used to enable comparison.
4.3 Point cloud data operations
Non-contact scanning typically produces a large number of points that correspond
to three-dimensional co-ordinate points on the surface of the target
object. The collection of points taken of an object is usually referred to as
a ‘point cloud’. The nature of point cloud data is completely different to
the pixel image data we obtain from CT and MR scanning and therefore
requires quite different software.
Point clouds in themselves are of little use. They are, therefore, usually
converted into three-dimensional surfaces. The simplest method is polygonisation.
This involves taking points and using them as vertices for polygon
facets. The collection of facets is usually known as a polygon mesh. The
simplest form of polygon is the triangle so it is frequently used. The steps
from point cloud data to polygon surface are shown in Fig. 4.11. Triangular
Point data Polygonised Rendered
4.11 A triangular polygon mesh created from a point cloud.