R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

14 Medical modelling
(assuming a 512 × 512 array), whilst a fi eld of view of 13 cm may be used
to capture data relating only to the mouth, which would result in a pixel
size of 0.25 mm. Whilst small pixel size is a desirable factor, it is more
important that the images cover all of the anatomy of interest plus some
margin.
Although exposure to X-rays should be minimised wherever possible, it
is more important that the scan covers all of the required anatomy, plus
some additional margin. It is better to perform one extensive CT scan than
a minimal one that is later found to be inadequate and necessitates subsequent
scans. Basic mistakes in capturing the required coverage can
be avoided through clear communication between the clinician and
radiographer.
2.2.4 Slice thickness
This is the distance between the axial scans taken to form the threedimensional
scan series. In the case of helical CT scanning, the parameter
applies to the distance between the images calculated during the scan (distance
between cuts). To maximise the data acquired, this distance should
be minimised. Some scanners can go as low as 0.5 mm, which gives excellent
results, but this must be balanced against increased X-ray dose. Typically,
distances of 1–1.5 mm produce acceptable results. A scan distance of 2 mm
may be adequate for larger structures such as the long bones or pelvis. A
scan distance greater than 2 mm will give poorer results as the scan distance
increases.
Collimation is the term used to describe the thickness of the X-ray
beam used to take the cross-sectional image. In combination with scan
distance, consideration may be given to collimation and overlap. In most
circumstances, the scan distance and collimation should be the same.
However, using a slice distance that is smaller than the collimation gives an
overlap. When scanning for very thin sections of bone that lie in the axial
plane, such as the orbital fl oor or palate, an overlap may give improved
results.
Even with a very small scan distance, some detail may be lost where thin
sections of bone exist between the scan planes (although this is not true
for volumetric acquisition). Typically, these are areas in the skull such as
the palate and orbital fl oors. In addition, very thin sections may prove too
delicate to survive the rapid prototyping build process.
2.2.5 Gantry tilt
Typically for the purposes of virtual or physical modelling gantry tilt should
be avoided as it does not signifi cantly improve the quality of the acquired