Biomedical Engineering Reference
In-Depth Information
Fig. 26.2
Side view of a
human liver as captured by
the CCD-camera of the
aspiration probe; the
extracted contour is indicated
as white line with the apex
defined as point with the
maximum deflection along
the contour, denoted '
P
'on
the picture
be controlled according to a prescribed time history. The experiment is performed
by establishing thorough contact between the probe head and the tissue, and cre-
ating a time-variable vacuum in the aspiration tube such that the tissue is sucked
in through the aspiration hole. A sufficiently smooth surface region of the organ of
about 30 mm diameter is required to enable proper contact. During a typical ex-
periment on human liver, a maximum suction pressure of 200 mbar is applied such
that the displacement of the tissue apex is in the order of 3 mm (point P, Figs.
26.2
and
26.3
). Nominal strains up to 30 % are induced into the involved tissue portion.
Tissue from the surface down to about 10-15 mm is considered to contribute to the
observed response, i.e. represents the characterized tissue volume. A monochrome
CCD-camera at the top of the body captures the full side view profile of the de-
forming tissue mirrored in a prism installed in the probe head. A cold light source is
used to illuminate the tissue surface by dint of an optical fiber. A pressure sensor is
installed in the handle of the device. The silicone pressure conduit, the optical fibers
and electronic cables for the pressure sensor are maintained and sealed in a standard
medical cable tube suitable for sterilization. Pressure data and images are recorded
at 25 Hz. The assembled probe weighs approximately 0.5 kg. The probe is designed
such that it can be completely disassembled for cleaning and ethylene oxide (EO)
gas sterilization. The pneumatic system is composed of a diaphragm pump, an air
reservoir, two pressure sensors, and two three-way isolation valves. The pressure
sensors, the valves, the pump, and the CCD-camera are linked to a computer for
controlling and data acquisition by means of a custom-written LabView program.
The pictures are processed off-line with Matlab to extract the contour of the side
view of the aspirated tissue. Time histories of measured pressure and deformation
profiles are the input data for further analysis. Further technical details and descrip-
tions of all components are reported in Hollenstein (
2011
).
26.2.1.1 Inverse Analysis
The time histories of applied negative pressure and resulting tissue deformation pro-
files are used to determine parameters describing the mechanical behavior of the
tissue. One possible evaluation method is based on the iterative comparison be-
tween predicted and measured tissue response, i.e. on the solution of the so called
