Assessment of Neurovascular Coupling - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 346

changes in fl ow due to its high time resolution. Major drawbacks
of LDF devices are the one-point measurement and the fact that
LDF instruments do not measure perfusion changes in absolute
units of blood fl ow per unit volume of tissue. The main reason for
the latter is that the sensitivity to a particular fl ow rate in a vessel
varies with the distance of the vessel to the tip of the probe. Thus,
LDF cannot be calibrated in absolute units. In addition, the nature
of the method implies that the measurements are sensitive to
motion artifacts. Therefore, it should be avoided to place vibrating
instruments (e.g., perfusion pumps) close to the measuring site or
fi bers. Dislocation of the probe results in a change of baseline read-
ings and should be prevented by positioning of the probe with a
suitable micromanipulator. LDF shows a trend to overestimate
fl ow changes at very high rCBF values and large surface vessels
should be avoided when the probe is placed.
LDF has been extensively reviewed ( 37, 38 ). The standard
laser Doppler perfusion monitor uses fi ber-optic probes with one
fi ber delivering the laser light from the laser diode to the tissue
and another fi ber to detect and deliver the backscattered photons
to the photodetector. The interference of Doppler-shifted with
non-Doppler-shifted light produces a dynamic speckle pattern that
causes fl uctuations of the photodetector's current. The power
spectrum of the current fl uctuations is then analyzed to extract fl ux
and concentration of moving blood cells. rCBF is expressed in
arbitrary units. The LDF records integrated perfusion in the sam-
pling volume. The precise measurement depth and sampling vol-
ume depend on the fi ber separation and wavelength of the laser.
The probes used for rCBF measurements typically have a fi ber sep-
aration of 0.25 mm and a 780-nm wavelength. It is assumed that
rCBF is measured in a volume of ~1 mm 3 with such a confi gura-
tion; the measuring depth is ~1 mm. LDF is a reliable method to
record relative rCBF changes in the brain, usually expressed as
changes compared to baseline under control conditions ( 29,
39-41 ). To achieve a certain degree of depth resolution of cortical
perfusion changes, combined fi ber probes can be used with differ-
ent fi ber separations and different wavelengths. This enabled anal-
ysis of rCBF changes with a 200-300- m spatial depth resolution

and has been used for laminar analysis of rCBF changes in response
to functional activation and SD ( 42-44 ). Laser Doppler perfusion
measurements are easy to handle and commercially available from
a number of manufacturers, like Perimed AB, Järfalla, Sweden;
Moor Instruments, Axminster, Devon, UK; LEA Medizintechnik,
Giessen, Germany; and Oxford Optronix Ltd., Oxford, UK.

μ

4.2. Laser Speckle
Contrast Analysis

By means of laser speckle contrast analysis (LASCA), two-
dimensional maps of cortical rCBF can be obtained with good spa-
tial and temporal resolution ( 33, 45 ). Laser speckle is a random
interference pattern produced by the coherent addition of scattered