R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Medical imaging for rapid prototyping 29
With most scanning software, if there is suffi cient overlap between adjacent
scans they can be aligned by the computer. This means the patient
does not have to remain perfectly still between successive scans. However,
the same body posture or facial expression should be maintained throughout.
For the best results, the patient may have to be braced in a comfortable
position during each scan. Depending on the scanners being used, it may
be simpler to keep the subject still and move the scanner around them.
Alternatively, a series of multiple scanners can be positioned around the
subject to capture different views simultaneously or in rapid succession.
However, despite these steps some movement is likely to be encountered,
such as breathing, swallowing and blinking so care should be taken when
scanning the face, neck and chest. If the patient is able, a breath-hold may
help if the scan time is only a matter of seconds.
2.4.5 Noise
Non-contact scanners capture many thousands of points at a time. The vast
majority of these points will fall accurately on the surface of the object
being scanned. However, due to tolerances and optical effects some of these
points will deviate from the object surface. If enough points deviate from
the surface by a suffi cient amount, it will affect the quality of the data.
These errant points are usually referred to as ‘noise’ in the data. The
amount of noise present in captured data will depend on the type of scanner
and the optical properties of the surface being scanned. Smooth, matt surfaces
usually produce less noise than refl ective or textured surfaces.
Although it usually necessary to take multiple overlapping scans to cover
the whole surface of an object, large overlapping areas are likely to result
in increased levels of noise.
Noise can be reduced by fi ltering. Filtering selectively removes data
points that deviate greatly from the vast majority of neighbouring points.
This is illustrated schematically in steps 1–5 in Fig. 2.16: (1) the object
surface to be scanned; (2) the scan data points; (3) using all of the data
points creates a poor surface; (4) points are selectively fi ltered according
to their deviation from the majority of neighbouring points; (5) deleting
fi ltered points leaves a closer fi tting surface to the actual object surface. The
magnitude of the deviation can be defi ned by the user to vary the effect of
the fi ltering. Filtering functions are typically included in the software that
is used to operate the scanner.
The effect of noise can be seen in Figure 2.17 (left) showing a threedimensional
polygon model created from optical scan data of a dental cast
(in this example the STL fi le format is used). Noise in the original scan data
has resulted in a polygon surface that appears rough and pitted. Postprocessing
software can be used to improve the quality of the surface