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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
Figure 12. P
Figure 13. Imprint on pig skin
Figure 14. Imprint on human skin
Using this analysis we are able to determine the coefficients of
human and pig skin and precisely model the two. The
characteristics of the imprint or indentations were the same in
all testing. The mechanics of this can be best described by the
following.
For the cycle of indentations from pressure: External pressure
on the skin causes tissue compression and distortion. If the
external pressure is greater than the capillary pressure keeping
the capillaries open, then capillary occlusion occurs.
Susceptible areas are over boney prominences.
Occlusion of capillaries leads to the secretion of endogenous
chemicals that have multiple effects. One is to signal all the
capillaries in the area to "open up". Another set of chemicals is
secreted to attract the immune system to send cells to "clean
up" any damaged tissue. Yet another set of chemicals cause the
capillaries to become "leaky" to allow the cells attracted to get
to the tissue affected.
Initially, the area of capillary occusion will have no blood flow
and appear white in color. If the circulation is interrupted by
death, then the area of pressure will never receive any further
blood flow and remain permanently white. As tissue re-
expansion requires blood flow, the tissue will remain
permanently indented.
If blood flow is restored, blood resumes flowing to an area that
has capillaries that are "wide open" and "leaky". The net result
is a red discoloration of the area, and the area begins to swell.
The swelling follows a pattern similar to the shape of the
pressure applied; the areas with less pressure will swell first,
followed by the areas of greater pressure. This will give
palpable ridges in the area in the shape of the object applying
the pressure.
Eventually the tissue recovers, and normal capillary flow is
restored, again following the pattern of least pressure
recovering first and most pressure recovering last.
3.2 Ante and post mortem imprint marks
This study covered areas of anti and post mortem marks related
to forensic applications. We all know that when pressure is
applied to skin, an imprint mark will be made. We also know
that over a period of time this mark will dissipate and the skin
will return to normal. There are distinct differences between
ante and post mortem imprint marks. To model these
differences we had to model the characteristics at time of
death. We obviously used pigs for this phase. Figure 15.
illustrates the procedure. We placed objects on the skin to
produce indentation or imprints on the pig. À lethal injection is
made in the heart. Using our targeting and imaging as
described we monitor the reaction to the imprints in addition to
imprints made prior to death. The test provided very
interesting results.
Figure 15. Lethal Injection to the Heart
The test provided precise method of monitoring the major
differences between ante and post mortem marks. As stated
earlier, when pressure is applied to skin, an imprint mark will
be made. We also know that over a period of time this mark
will dissipate and the skin will return to normal. It was
discovered that at the instant of death the dispersion rate of the
indention stops. It is also noted that the amount of pressure and
length of time the object that causes the indention does not
affect the overall depth of the imprint in most cases. This
opens a new area for forensic study. By modelling the elasticity
of skin we can calculate the dispersion or lapse rate of the skin
returning to normal. This can provide the ability to determine
the time of death after the wound or imprint is made on the
skin. This observation and characteristics were duplicated in
subsequent testing. The following Figures 16. and 17. illustrate
the indentations on the skin.
