R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Medical imaging for rapid prototyping 9
existing physical objects with computer-aided design (CAD) models.
Consequently, this process is often referred to as ‘reverse engineering’.
There are many types of surface scanner or digitizer available to the engineer
or designer. They can be separated into two main categories; ‘contact’
or ‘touch probe digitizers’ and non-contact scanners. Touch probe digitizers
use a pressure sensitive probe tip and calibrated motion to map out the
surface of an object point by point. Depending on the quality of its manufacture,
they can be extremely accurate. However, it is also a very slow and
laborious process, sometimes taking hours to capture the surface of an
object. Whilst this is acceptable when scanning inanimate objects, it is
clearly not appropriate to capturing the surface of human anatomy.
Therefore, non-contact scanners are typically used when capturing surface
data from people. Non-contact scanners utilise light and digital camera
technologies to capture many thousands of data points on the surface of an
object in a matter of seconds. The fast capture of data and the harmless
light used make these types of scanner ideal for capturing human anatomy.
These scanners are typically like very large cameras and may be tripod
mounted or in some instances even hand held. Despite the variety of
surface scanners available on the market, the general principles of their
operation and application are the same, and these principles are described
later in this chapter.
It is not intended to provide a defi nitive description of the technology
and practice of each scanning modality here but to establish some criteria
and guidelines that may be employed to optimise their use in the production
of virtual or physical medical models. Many texts are available that
describe each modality fully and some are listed in the recommended
reading list at the end of this chapter.
2.2 Computed Tomography (CT)
2.2.1 Background
Computed Tomography works by passing focused X-rays through the body
and measuring the amount of the X-ray energy absorbed. The amount of
X-ray energy absorbed by a known slice thickness is proportional to the
density of the body tissue. By taking many such measurements from many
angles the tissue densities can be composed as a cross-sectional image using
a computer. The computer generates a grey scale image where the tissue
density is indicated by shades of grey, ranging from black indicating the
density of air to white representing the density of the hardest bone.
As bones are much denser than surrounding soft tissues they show up
very clearly in CT images, as can be seen in Fig. 2.1. This makes CT an
important imaging modality when investigating skeletal anatomy. Similarly,