R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

122 Medical modelling
These effects can be avoided by eliminating the creation of a threedimensional
surface mesh and creating the RP build data directly from
the CT image data. This essentially creates the 2 1 / 2D layer data for the RP
machine from the CT images. Interpolation is used to create accurate intermediate
layers between the CT images. This route not only eliminates
surface modelling effects but also results in much smaller computer fi les
and faster preparation.
Metal artefact
Metal artefact was present within CT scans of the maxilla and mandible
due to the presence of metal within fi llings of the teeth or the presence of
dental implants. This was manifest as high signal intensities (in the form of
scattered rays) around the upper and lower mandible. Fig. 6.10 shows
an FDM TM model with signifi cant metal artefact around the teeth. These
ray appearances extended from a couple of millimetres to over one centimetre
in length. In some circumstances, the artefact may be reduced by
software during CT image reconstruction (32). This artefact was plainly
visible and added many superfl uous structures to the medical models.
Although no signifi cant geometric distortion was observed on models, large
spikes were visible emanating from around the teeth which distorted the
bone in the local area. The artefact may be removed by detailed slice-by-
6.10 Metal artefact due to scattering of X-rays from metal within the
teeth.