R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Case studies 255
mummies. Mimics software (Materialise NV, Technologielaan 15, 3001
Leuven, Belgium) was instrumental in importing, segmenting, cleaning and
outputting the required fi les in order to produce the three-dimensional
model fi les for visualisation and manufacture. The physical models of the
skulls were produced by stereolithography (8). Stereolithography is increasingly
used to produce models of patients with many medical conditions and
has subsequently been applied to other archaeological and palaeontological
remains (9–11). Stereolithography and Laminated Object Manufacture
(LOM TM – see Section 5.8) were used to reproduce physical models of some
objects that were wrapped up with the mummy.
6.16.4 Case study
Mimics software is typically used in medicine to segment CT data to isolate
the tissue of interest. Frequently the tissue of interest is bone. This is accomplished
by using the ‘threshold’ functions to select the appropriate limits of
density and the numerous editing and segmentation tools to make adjustments
to ensure the desired bone structures are isolated.
In contrast, when attempting the fi rst case, ‘Jeni’, a number of challenges
made accurate reconstruction of the bony anatomy extremely challenging.
In living patients, the difference in density between bone and the adjacent
soft tissues is quite marked. This allows the segmentation of bone to be
performed relatively easily. However, in these ancient Egyptian mummy
cases, all of the soft tissues were completely desiccated by the mummifi cation
process. This resulted in the soft tissue remains having an artifi cially
high density compared to the remaining bone (see Figs 6.99 and 6.100).
This effect is confounded by the demineralisation of some bone structures,
also resulting from the mummifi cation process. Therefore, performing
the segmentation by density-threshold only results in a poor
three-dimensional reconstruction. It loses some data from the skull whilst
including unwanted elements of desiccated soft tissue. This effect can be
seen in the reconstruction on the left of Fig. 6.101.
In previous image reconstructions, higher thresholds had been used to
try to eliminate some of the desiccated soft tissue remains. Although this
improves matters, it results in the loss of low-density bone structures whilst
some soft-tissue structures remain. In addition, the high-density artifi cial
objects also remained present in the eyes, mouth and neck. To produce a
model of the skull from this data would have resulted in gaps in the surface
of the skull and the absence of some of the more delicate bone structures.
For example, gaps could be seen in the temporal bone and zygoma (cheekbones),
and the soft tissue of the ears is still present. As facial reconstruction
depends on imposing known depths of soft tissue on to the facial bones,
these defects would make the whole process more diffi cult and less
reliable.