35
Appendix C
Technical Aspects
the regional complexity of the skin’s microvasculature, its global variability
over the human body, and the fact that the complex nature of light scattering
in tissue makes LDF suitable for characterizing only relative changes in blood
perfusion. So, although the Blood FlowMeter can be absolutely calibrated
using an
in vitro
model, the complex and variable geometry of the vascular
tree precludes absolute calibration for
in vivo
applications. Th
is means that
even though the Blood Perfusion Unit (BPU) can be traced to a physical
standard, the measurements expressed in BPU must be considered as strictly
relative.
Zero BPU
Th
e zero reading of the Blood FlowMeter has been obtained by calibrating
the system against a special static scattering material where no movements
occur. In such cases the back-scattered light processed by the Blood
FlowMeter contains no Doppler shift ed frequencies and a true zero is
obtained. A zero reading therefore indicates zero motion in the measuring
volume under examination and also zero artifactual motion arising from
relative movements between the probe and the measuring volume. During
i
n vivo
measurements an absolute zero is rarely obtained. Even during total
occlusion of the tissue blood perfusion, there is oft en some small, residual
motion of blood cells trapped in the vessels, as well as some small muscle
and tissue movement in the measuring volume. Even aft er surgical removal
of tissue, localized cell movement and Brownian motion may still occur in
the severed blood vessels.
Th
e Blood FlowMeter’s internal soft ware allows the zeroing of the laser
Doppler signal when there is insuffi
cient light returning from the tissue to
the probe. In the default condition (power on), the cut-off threshold is set to
1%. Th
is means that if the backscatter signal falls below 1%, the laser Doppler
signal is automatically zeroed.
Motion Artifact Noise
Laser Doppler studies sometimes reveal changes in the blood perfusion signal
which are oft en unrelated to actual physiological changes in blood perfusion.
Th
ese artifacts in the blood perfusion signal can oft en be attributed to the
movement of the optical fi bers in the beam delivery/collection system and
are noticeable in situations where the subject moves or twitches. Th
is type
of artifact may be worse in situations where the probe moves with respect to
the tissue. Th
is eff ect can therefore be minimized by using a probe which is
connected to, rather than clamped over the tissue, for example by using the
standard right angle or saturable probes.
