Biomedical Engineering Reference
In-Depth Information
affect the intrinsic signal in different ways and to various extents.
Because these optical properties of the tissue vary in time, multi-
ple illumination wavelengths (e.g., 600-630 nm for oximetry) are
necessary to calculate the desired parameters. The intrinsic signals
are based on small reflectance signal changes (0.1-1%) which peak
shortly (2-3 s) after stimulus onset.
VSD uses small exogenous probes (weight
<
500 Da; length
<
200 nm) that are designed to preferentially bind to the
periphery of the cell membrane, and therefore, act as molecular
transducers
(71)
. Changes in the membrane potential or ion con-
centration can then be transformed into an optical signal. There
are two key factors to a successful dye used in VSD imaging. The
optical yield (or contrast-to-noise ratio) when the molecule binds
to the membrane is obviously critical. Another factor, which partly
limits its application more widely, is successful delivery of the dye
to the site of interest. Since the blood-brain barrier limits most
foreign molecules from entering the extracellular milieu, direct
injection of the dye into the site of interest is preferred. Because
different events at the nerve terminal are linked to intracellular
Ca
2
+
ion, dyes sensitive to Ca
2
+
voltage-gated ion channels have
wide use in imaging activities, both neurons and astrocytes, with
good temporal resolution (
10
2
ms). The VSD signal changes
can be quite large (2-4%) which peak quite rapidly (
<
<
1 s) after
stimulus onset.
NIRS is a spectroscopic method which utilizes the near infra-
red region (650-950 nm) of the electromagnetic spectrum to
sense changes in the hemoglobin oxygenation state
(72)
. Because
NIRS is non-invasive and has better tissue penetration (cm instead
of mm) than visible light, the method has found wide use in both
clinical monitoring of tissue oxygenation and functional imaging
in humans and small animals
(73)
. Since near-infrared absorption
spectra of oxyhemoglobin and deoxyhemoglobin allow separation
of these two species, NIRS measurements commonly utilize three
separate wavelengths to measure these components for blood oxy-
genation, and later sum them to get blood volume. The spatial
resolution (in mm range) is lower than other optical methods
because of the diffuse nature of photon migration through tissue
which can affect sensitivity and quantitation of oxyhemoglobin
and deoxyhemoglobin
(74)
. The NIRS signal changes are larger
than intrinsic signal changes (
>
2%) and peak slowly (within 5 s)
after stimulus onset.
LDF is an optical technique for measuring movement of
red blood cells in the microvascular bed
(75)
. The principle is
based on the “Doppler” frequency shift that arises in light that
is scattered by moving red blood cells. By illuminating the tis-
sue with single-frequency (low power) laser light and detect-
ing the frequency distribution of the back-scattered light, red
blood cell velocity and/or flux can be estimated. Since the LDF
