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INVOS® Cerebral Oximeter
Principles of Operation
For visitors who want
to learn more about the theory behind the noninvasive INVOS Cerebral
Oximeter, this section is for you.
Theory of Measurement
and Technology Description
Tissue Sampled
Suppression of Extracerebral Signal
Theory of Measurement and Technology
Description
The theory behind Somanetics' noninvasive INVOS Cerebral Oximeter
is conceptually simple. Near infrared ("NIR") light photons are
injected into the skin over the forehead. After being scattered
about inside the scalp, skull, and brain, some fraction of the injected
photons survive to return and exit the skin ("reflectance"). By
measuring the quantity of returning photons as a function of wavelength,
one can infer the spectral absorption of the underlying tissue and
make some conclusions about its average oxygenation.
Human tissue is translucent to NIR photons
having wavelengths between about 650 and 1100 nm. Transillumination
of body parts as an aid to medical diagnosis has been known and
used for centuries. Although this light is readily transmitted through
human tissue, scattering prevents successful imaging of internal
features that are not near the surface.
What is bad for imaging is good for spectroscopy.
The long, torturous paths taken by the scattered photons make them
exceedingly sensitive to the optical properties of tissue. Even
small amounts of colored materials ("chromophores") will cause wavelength-dependent
absorption of photons which produces characteristic signatures in
the spectrum of the emerging light. As early as 1977 Jöbsis reported
measuring the absorption spectrum of NIR light passing through the
head of a cat and was also able to get enough light through the
human brain from temple to contralateral temple to detect an increase
in light transmission during hyperventilation.
The chromophore with the highest absorption
in body tissue is in the 280 million, red-colored hemoglobin molecules
found within each of the 1013 red blood
cells circulating in the blood. It looks red in white light because
it absorbs shorter wavelengths (green and blue).
Hemoglobin is of vital importance to
us because it transports oxygen from the lungs to the cells of the
body which cannot live without it - the cells of the brain will
die within a few minutes. The exact shade of red of each hemoglobin
molecule depends on the amount of oxygen it is carrying, a property
that forms the basis of a number of blood oxygenation measurement
devices ("oximeters"). The Somanetics INVOS Cerebral Oximeter system
is designed specifically for measuring oxygen in the blood of the
brain in the area underlying the sensor and uses two wavelengths,
730 and 810 nm, to measure changes in regional hemoglobin oxygen
saturation (rSO2 index).
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Tissue Sampled
Previous work indicates that, at a source-detector separation of
4 cm, some of the light injected into the head by the SomaSensor
and received by its detectors has penetrated to the cerebral cortex.
The transmission of light through tissue can be verified by probing
one's own tissues in a darkened room with a red laser "pointer"
used for slide presentations. They produce light in the near-infrared
band (typically 670 nm). This light is easily transmitted through
thin body parts (cheek, ear, fingers, etc.) and a "back-scattered"
halo of light can be observed from all thick tissues.
In the latter case, light can be seen
emerging from the skin at distances of a couple of centimeters from
the point of injection (even farther for fatty tissue). In the absence
of light absorbing materials some photons will penetrate the tissue
to considerable depths before meandering back to the surface at
the point where a detector is located. Using sensitive photodiode
detectors, light can be measured at considerable distances from
the point of injection.
Intuitively, the greater the separation
of source and detector, the greater the average depth of penetration.
Photons that happen to meander close to the surface are very likely
to be lost out of the skin before getting to a distant detector.
Large source-detector spacings are therefore biased against "shallow"
photons except in the tissues directly under the source and detector.
On the other hand, geometry and absorption also make it unlikely
that very deeply penetrating photons will find their way back to
the detector. Most of the photons reaching the detector will have
taken some optimum middle course. This mean photon path is shaped
approximately like a "banana" or "canoe" with ends located at the
source and detector.
In a well-crafted set of experiments,
Cui et al. found that the most likely penetration depth is about
a third of the spacing between the light source and the detector.
They used a tank filled with a liquid scattering material (intralipid)
which approximated tissue and noted the changes in light received
by the detector, at various source-detector distances, as they inserted
and removed small absorbers (black cylinders 2.5 mm diameter by
1 mm thick) at different depths. They reported the same "banana-shaped"
sensitivity distributions found by others using both experiments
and computer simulations.
These results have been confirmed by
Hongo et al. in the human forehead by injecting a bolus of infrared
absorbing dye (indocyanine green) into the internal carotid artery
and observing the transient decrease in signals at various source-detector
spacings. The larger signals at increasing source-detector spacings
indicated deeper penetration into the head and the very short duration
of the signals (~5 seconds) verified cerebral circulation as the
source.
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Suppression of Extracerebral Signal
Between the Somanetics sensor where it contacts the skin and the
cerebral cortex there are several layers of tissue of differing
composition and with different concentrations of blood. We refer
to this extracerebral tissue as simply "scalp and skull" or occasionally
just "scalp."
To reduce the interference of extracerebral
oxygen on the oxygenation measurement, the Somanetics Cerebral Oximeter
uses two source-detector spacings: a "near" (shallow) spacing of
3 cm and a "far" (deep) spacing of 4 cm. Both sample about equally
the shallow layers in the tissue volumes directly under the light
sources and detectors, but the far spacing "sees" deeper than the
near spacing. Subtraction of some of the near signals from the far
theoretically should leave a signal originating predominantly in
the brain cortex.
It is not feasible to directly measure
the contribution of oxygen in overlying tissue to the Oximeter reading.
However, using mathematical modeling based on the diffusion theory
as it is applied to turbid biological tissue, we can estimate this
contribution by calculating oxygenation sampling density (OSD) distributions
which estimate the probability that a given photon will interact
with a hemoglobin molecule in various locations of the skin, scalp,
skull and brain on its way to the sensor's detector.
Using the model at a 4 cm source-detector
spacing and no signal subtraction,
the overlying tissue and skull contribute, on average, about 45
percent of the signal while 55% is cerebral in origin. Subtracting
the data from the 3 cm spacing (as the Oximeter does) reduces this
extracerebral contribution to less than
15 percent. While the potential exists to develop an instrument
that will reduce the extracerebral contribution to zero, subject-dependent
variations in anatomy and physiology will likely cause variations
of ±10%. While the extracerebral contribution is not zero, the noninvasive
Somanetics INVOS Cerebral Oximeter provides a "predominately cerebral"
measurement where over 85 percent of the signal, on average, is
exclusively from the brain.
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More information about cerebral oximetry
can be found here in the INVOS Cerebral Oximeter section at the
INVOS Cerebral Oximeter Photo Gallery
and our Product Information Publications
section. And, visit our Clinical Center
for the latest clinical reports.
We will be happy to send a scientific
package of comprehensive background information about the INVOS
Cerebral Oximeter. Complete a Request
Form and we'll get information to you promptly.
Customers can order our products online
by completing an Order Form or Request
for Quotation Form.
INVOS Cerebral Oximeter
INVOS Cerebral Oximeter Photo Gallery
Product Information Publications
Request Form
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Troy Michigan
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