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Blood FlowMeter
Owner’s Guide
Laser Doppler Flowmetry
Laser Doppler fl owmetry (LDF) off ers a continuous measurement of blood
cell perfusion in the microcirculatory beds of skin tissue and other tissues
without infl uencing the blood perfusion. LDF is established as an eff ective
and reliable clinical medicine and microvascular research technique. Th
is
has been achieved largely because LDF satisfi es the need for continuous,
non-invasive and real-time measurement of blood perfusion in the
microvasculature.
Laser Doppler fl owmeters produce an output signal that is proportional to
the blood cell perfusion (or fl ux). Th
is represents the transport of blood cells
through microvasculature and is defi ned as:
Microvascular Perfusion = number of blood cells × mean velocity
Microvascular perfusion, therefore, is the product of the mean cell velocity
and mean blood cell concentration present in the small volume of tissue
under illumination from the laser beam.
LDF Theory
LDF makes use of the fact that when tissue is illuminated by a coherent,
low powered laser, light is scattered by both moving and static structures
within the microcirculatory beds. Photons, scattered by moving blood
cells are spectrally broadened according to the Doppler eff ect. Maximum
Doppler shift s occur when blood cells are moving in a direction parallel to
the incident light beam and the detected light (scattered light) from the cells
is detected in a direction opposite to its origin. For example, a Doppler shift
of about 4 kHz is obtained when laser light (of a wavelength of 830 nm) is
backscattered from a particle in water moving at 1 mm/s parallel to the light
beam. In general a continuous range of Doppler frequency shift s is expected.
Photons scattered by static structures alone do not undergo Doppler shift ing.
In order to detect these small frequency shift s it is necessary to use a
technique called ‘optical beating’. Th
is basically means that the original
output light is mixed with the backscattered light to eff ectively add the
signals. (In technical terms this is referred to as heterodyning). When these
signals are presented to the detector the result is an output which contains a
fl uctuating component related to the diff erence between the two beams.
Th
is is performed using a photo diode detector. Th
e frequency and magnitude
of the alternating component of the photocurrent from this device (i.e. the
power spectral density) is related to the mean velocity and concentration of
blood cells present in the measuring volume.
