Small Angle Light Scattering (SALS) Device
Background
The planar fibrous connective tissues of the body
are composed of a dense extra-cellular network of collagen
and elastin fibers embedded in a ground matrix, hence
can be thought of as biocomposites. Thus, the quantification
of fiber architecture an important step in developing
an understanding of the mechanics of planar tissues
in health and disease. We have used small angle light
scattering (SALS) to map the gross fiber orientation
of several soft membrane connective tissues.
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Anterior leaflet of a porcine
mitral valve in light to dark (top) to
show laser scattering pattern (bottom). |
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We have
developed an improved SALS device that allows for rapid
data acquisition, automated high spatial resolution
specimen positioning, and new analysis methods suitable
for large-scale mapping studies. Extensive validation
experiments revealed that the SALS device can accurately
measure fiber orientation for up to a tissue thickness
of at least 500 microns to an angular resolution of
~1° and a spatial resolution of ± 50 microns.
The Device
The SALS device consists of an unpolarized
4 mW HeNe laser (l =632.8 nm, Uniphase, Manteca, CA),
spatial filter-beam expander, sample positioner, projection
screen, and a CCD NTSC-compatible auto-iris video camera
(Sony Inc., Japan) equipped with a telecentric lens
to minimize image distortion (Edmund Scientific, Barrington,
NJ), all mounted on a rigid optical platform. Since
the laser has a Gaussian intensity profile, we used
the common definition of the beam diameter as that
diameter of the laser beam core that contains all but
1/e2 (i.e. 13.5%) of the total radiant beam power.
Thus, all beam diameters reported in this study contain
86.5% of the total beam intensity.
As the laser light
passes through the tissue it is scattered by the
fibrous structures, and the resultant scattering pattern
is
cast onto the projection screen. The video image
of the scattered light is digitized to a resolution
of
640x480 8-bit pixels using a data translation image
grabber board mounted in the PC. The image grabber board
has sufficient video memory to allow for image analysis
while the subsequent image is being acquired.
Accessory devices have been designed to work in
conjunction with the SALS device to explore additional
influences on tissue structures. One such apparatus
is a biaxial stretching device, as shown below. With this, tissues
can be stretched, and the fiber architecture can be
analyzed to provide a better understanding of the
structure-function relationship of soft tissues.
Movement
The sample positioner allows for two axes of movement in a plane
perpendicular to the optic axis. Both axes utilize a precision linear
travel system using 16 threads/inch lead screws with anti-backlash
nuts. Positioning of each axis is accomplished by rotating the lead
screws using two PC controlled stepper motors with 400 steps/rev
resolution, resulting in a net movement resolution of 4 mm/step
and a total travel range of 25 cm along each axis. All motion control,
video processing, and data analysis is performed by custom written
C++ programs.
This system allows for video acquisition, initial
data analysis, storage of the analyzed data to the
hard disk, and movement to the next test location, all in ~1 second.
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