Bringing the brain and body to light.
Near-infrared spectroscopy (NIRS) technology, such as that used in pulse oximetry, has been used and trusted in the world of medicine for decades. Several characteristics contribute to its widespread use, including its noninvasive nature, reliability and safety. The Somanetics' INVOS® System harnesses this power to safely and reliably "see inside" the brain and body. The result is the opportunity to improve outcomes.
The IN Vivo Optical Spectroscopy (INVOS®) System "reflects the color of Life®" by identifying hemoglobin and red-colored oxygenated hemoglobin molecules within red blood cells, and measuring the relative amounts of each to determine whether or not there is adequate oxygenation. Since brain cells and organ tissues die within minutes without proper oxygenation, this measurement provides potentially life-saving or life-changing information. When its regional oxygen saturation (rSO2) value shows a change in blood oxygenation toward or beyond threshold levels, the care team can intervene to potentially lessen or prevent complications.
3-D view of cerebral vasculature illustrates monitoring of the vulnerable watershed region at the extremes of the circulation in the anterior and middle cerebral arteries.
Graphic Footnotes: Moody DM, et al. AJNR 1990;11:431-439. (Cortex)
Courtesy of E. Bondio, MD (Watershed)
O2 Measurement Using NIRS
The theory behind Somanetics' noninvasive INVOS® System is conceptually simple. Near infrared (NIR) light photons penetrate the forehead and/or body tissue of interest to the clinician. After being scattered inside the scalp, brain or tissue, some fraction of the injected photons survive, returning to and exiting the skin (a property called "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.
Trans-illumination of body parts as an aid to medical diagnosis has been known and used for centuries. Near infrared light - as well as the red portion of incandescent light - easily passes through the skin and skull.
Human tissue is translucent to NIR photons having wavelengths between about 650 and 1100 nm. To see this first-hand, you can conduct your own experiment by illuminating your own tissues in a darkened room with a common red laser "pointer" used for slide presentations. These laser pointers produce light in the near-infrared band (typically 670 nm). You'll see that this light is easily transmitted through thin body parts (e.g., cheek, ear, fingers, etc.) and a "back-scattered" halo of light can be observed from 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 continue penetrating the tissue to considerable depths before meandering back to the surface at the point where a detector is located. Although this light is readily transmitted through human tissue, the scattering prevents it from being useful for imaging.
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.
Coloration of hemoglobin molecules is directly related to the oxygen they carry. 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 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/Somatic Oximeter is designed specifically to measure oxygen in brain or tissues directly beneath the sensor using two wavelengths, 730 and 810 nm, to measure changes in regional oxygen saturation (rSO2 index). Surface data from the skin and skull is subtracted out, to produce an rSO2 value for deeper tissues.
Detector Spacing and Photon Paths
Most photons reaching the detector will have taken some optimum course through the tissues of interest. Using sensitive photodiode detectors, light can be measured at considerable distances from the point of injection. 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 reaching 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. Therefore, mean photon path is neither deep nor shallow, but a moderate curve like a banana or canoe.
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The mean photon path is shaped like a banana or canoe with ends located at the light-emitting source and the corresponding detector. |
In a well-crafted set of experiments, Cui et al1, 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 al2, 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.
INVOS® System values have been validated to emphasize
brain signals and suppress scalp and surface matter.
Dr. Hongo's validation study was performed by injecting infrared absorbing green dye in the internal and external carotid arteries. The deep and shallow signals changed equally when dye was injected in the external carotid which feeds the external structures. The net difference is close to zero. However, when dye was injected into the internal carotid, providing blood to the cerebral cortex, the deep and shallow signals changed differently, corresponding to the amount of light that reaches the brain. The net difference is the signal of interest, originating in the brain. This study provides evidence that the INVOS® System measures predominantly cerebral oxygenation while suppressing external skin, skull and dura and that the spacing of its detectors from the emitter is appropriate to achieve this.
LED and Laser Light
As was stated earlier in O2 Measurement Using NIRS, the human body is easily penetrated by near-infrared light as it easily passes through the skin and skull. The amount of light absorbed by hemoglobin is extremely small, which is referred to as a low molar absorptivity. Tiny LEDs were chosen for use in the INVOS® System's sensors because LEDs give out appropriate amounts of light to take advantage of the low molar absorptivity.
In the graph titled Hemoglobin NIR Spectra, we see an example of low molar absorptivity. Note the lipid curve; it shows very low light absorption by typical tissue hydrocarbon molecules, hence the specificity for hemoglobin. The water curve shows that water does not absorb light at the wavelengths used by the INVOS® System. Therefore, it does not interfere with the measurement.
Hemoglobin NIR Spectra
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Hemoglobin has a very low absorptivity and hence the INVOS® System's use of LEDs, which give out very low intensity light, avoids overwhelming the measurement signal. |
In the graph titled Extinction Coefficients, we see that different hemoglobin species have different absorption characteristics. Note that the hemoglobin absorbs near-infrared light in a range where few other molecules absorb. Myoglobin, found in muscle, some oxidative enzymes and bilirubin absorb NIR light, but much less than hemoglobin, making tissue transparent to NIRS, enabling hemoglobin to be measured.
Extinction Coefficients
Trend and Absolute Values
Some noninvasive cerebral monitors market themselves as providing "absolute" values whereas INVOS® System values are trends. What's the difference? The INVOS® System uses two wavelength technology while some other NIRS devices use three or more wavelengths. While this might imply superiority of these devices, an examination of accuracy data, root mean square error (RMSE) and regression slopes of data used for FDA clearance shows the INVOS® System with spatial resolution provides accurate trending of brain oxygen saturation levels. If more wavelengths improved accuracy, one would expect to see a markedly superior difference demonstrated by other devices which is not the case3,4.
While "absolute" may be today's marketing buzz word, data remains to be seen that demonstrates the clinical value of "absolute." In fact, the majority of clinical references listed on one competitive device's bibliography are studies done with the INVOS® System, not their own device. Conversely, the INVOS® System is backed by more than 600 clinical references reporting threshold levels, intervention protocols and clinical benefits specifically related to the INVOS® System. It may be inappropriate to assume that any or all NIRS devices perform similarly. Therefore, it may be misleading to extrapolate these INVOS® System findings to other cerebral oximeters.
Surface Signal and Tissues Sampled
Between the Somanetics sensor and the brain or somatic tissue being monitored, there are several tissue layers with differing compositions and differing concentrations of blood. We refer to this as "surface tissue."
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The depth resolution of the INVOS® System refers to detectors, placed at different distances, subtracting the shallow measurement from the deep, providing rSO2. |
By placing detectors at different distances from the light source, two depths of penetration are measured. Subtracting the shallow measurement from the deep minimizes superficial signal contamination, and emphasizes changes in tissue oxygen saturation beneath the sensor. This is called "depth resolution."
To reduce the interference of surface tissue on the oxygenation measurement, the INVOS® System 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 photons reach deeper. Subtracting the near signal (surface data from the skin and skull) from the far signal results in an oxygenation value specific to the deeper tissues under the sensor.
Previous work indicates that, at a source-detector separation of 4 cm, some of the light injected into the head by the INVOS® System sensor and received by its detectors has penetrated to the cerebral cortex. Similarly, when placed on somatic tissue, readings are reflective of oxygenation approximately 1-2 cm beneath the sensor.
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By providing objective, real-time feedback on oxygen balance under the sensor, rSO2 transforms some intangible variables into concrete, actionable data. |
Footnotes:
1. Cui W, Kumar C, Chance B. Proceedings of time-resolved spectroscopy and imaging of tissues. SPIE 1991;1431:180-191.
2. Hongo K, Kobayashi S, Okudera H, Hokama M, Nakagawa F. Noninvasive cerebral optical spectroscopy: Depth-resolved measurements of cerebral haemodynamics using indocyanine green. Neurol Res. 1995 Apr;17(2):89-93.
3. Kim MB, et al. J Clin Monit Comput. 2000;16(3):191-9.
4. Nagdyman N Pediatr Anesth 2008 18 160.
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