R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

118 Medical modelling
6.6 Effect of gantry tilt on a three-dimensional surface rendered skull
(left) and the same after correction (right).
Model stair-step artefact
Two elements contribute to the stepped effect seen in medical models. One
contribution is from the discrete layer thickness at which the model is built.
This is a characteristic of the particular RP process and material being used.
Typically, these range from 0.1–0.3 mm. This effect can be minimised by
selecting processes and parameters that minimise the build layer thickness.
However, thinner layers result in longer build times and increased costs,
and an economic compromise is typically found for each RP process. As
the layer thickness is typically an order of magnitude smaller than the scan
distance of the CT images, it does not have an overriding effect on the
quality of the model.
The second effect arises from the slice thickness of the acquired CT or
MR images and any potential gap between them. The stair-step artefact is
a common feature on conventional and single slice helical CT scans where
the slice thickness is near to an order of magnitude greater than the in-plane
pixel size (30). The artefact is manifest as a series of concentric axial rings
around the model. The depth and size of these rings depends on the CT
imaging protocol, but may be very slight where there is a thin slice used
(e.g. 3 mm acquisition with 1 mm reconstruction interval). In thick-slice
acquisitions (e.g. >3 mm with similar reconstruction interval to the slice
thickness), the stair-step artefact will cause signifi cant distortion to the
model. Fig. 6.7 shows a stereolithography model of a full skull. The CT scan
was performed on a conventional CT scanner with 5 mm slice thickness and
no interpolation of the image data to create thin slices. Note that there was
signifi cant stair-step artefact around the top of the skull and on the lower
edge of the mandible. The stair-step artefact was most prominent on