R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

16 Medical modelling
Movement
A good quality CT scan depends on the patient remaining perfectly still
throughout the acquisition. Movement during the acquisition will lead to
distortions in the data (analogous to a blurred photograph). This has
become less of a problem as acquisition times have decreased with the
advent of helical multi-slice CT. However, it can still present a problem in
some cases. For example, involuntary movement of the chest, neck, head
or mouth can occur through breathing or swallowing. Movement can be
particularly diffi cult to control when scanning babies, small children and
claustrophobic patients, in which case a sedative or even general anaesthetic
may be required.
X-ray image scatter by metal implants
Dense objects such as amalgam or gold fi llings, braces, bridges, screws,
plates and implants scatter X-rays resulting in a streaked appearance in the
scan image. The scatter results in signifi cant image errors where false data
appear with corresponding false missing data or shadows. Due to the nature
of X-rays, little can be done to eliminate these effects. Fig. 2.6 shows an
axial CT image with signifi cant artefact from scatter. Fig. 2.7 (left) shows
how the scatter will be demonstrated on a three-dimensional reconstruction
of the data, apparent as spikes radiating from the source of the scatter.
These effects can be manually edited in software to produce a normal
looking model (right). However, this does depend to some degree on the
expertise of the operator and consequently the accuracy of the model in
the affected areas cannot be guaranteed. In most cases, this does not affect
the usefulness of the whole model. As cases showing artefacts usually
occur in and around the teeth, a dental cast is typically used in conjunction
with the medical model and indeed may be combined with a physical
model.
Noise
Noise is a fundamental component of a CT image and is especially prevalent
in dense tissues. Although these images may be visually acceptable,
they are impractical for modelling. Good modelling from CT data depends
on identifying a smooth boundary between bone and soft tissue. Noise
reduces the boundary, which results in poor three-dimensional reconstructions
and consequently poor models. This commonly affects areas through
the shoulders and hence vertebrae in the lower neck and upper back (C6
to T4: see explanatory note 8.2.5). A typical example is shown in Fig. 2.8.
The effect becomes much more apparent when zooming into image data,