R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

116 Medical modelling
bone. However, the process is still not conducted in the large volumes
associated with industrial rapid prototyping and, as such, practitioners
applying these techniques to medicine often confront problems that are
not encountered in industry. The small turnover associated with medical
modelling also means that many manufacturers and vendors cannot justify
investment in specifi c software, processes and materials for this sector.
These characteristics combine to make medical modelling a challenging
fi eld of work with many potential pitfalls.
The authors’ many years of practical experience in medical modelling
has resulted in a knowledge base that has identifi ed the problems that may
be encountered, many of which are simple or procedural in nature. This
study aims to highlight some of these common problems, the effect they
have on the resultant models and suggest methods that can be employed
to avoid or minimise their occurrence or impact on the usefulness of the
models produced.
6.2.5 Medical rapid prototyped model artefacts
Associated with all medical imaging modalities are unusual or unexpected
image appearances referred to as artefacts. Some imaging modalities are
prone to geometric distortion like MR (27), and this should be accounted
for in soft tissue models manufactured from this source. CT does not suffer
from the same distortion as MR and models produced from this source have
been proven to be dimensionally accurate (28). In some circumstances
artefacts are easily recognisable and taken into account by the viewer,
whilst in other circumstances they can be problematic and diffi cult to
explain. Artefacts present in the image data may subsequently be transferred
to a medical model. In addition, due to the image processing steps
and surface modelling required in the production of medical models there
is scope for the appearance of a wide range of artefacts. This section
describes and illustrates some of the problems and pitfalls encountered in
the production of medical models.
The procedures and potential problems associated with transferring and
translating medical scan data are described in Chapters 2 and 3. However,
the following observations serve to illustrate particular examples of some
of the problems encountered by the authors. An example of some of the
practical implications of transferring data from a hospital to an RP service
provider is given in Section 6.1 Implementation case study 1.
CT data import errors
CT data consists of a series of pixel images of slices through the human
body. When importing data the key characteristics that determine size and