Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2000-12-27
2002-07-16
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C356S039000
Reexamination Certificate
active
06420709
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to improvements in optical systems and their uses for the measurement of concentration and temperature in scattering media, and the related discrimination of subsurface features. More particularly, the invention provides methods and apparatus which minimize the ratio of diffusely scattered radiation to directly transmitted radiation reaching the detector(s) in optical concentration measurement and imaging apparatus. The methods and apparatus of the invention have special applicability to noninvasive testing, particularly for concentration measurements of materials such as glucose and hemoglobin in blood.
Recent literature is replete with articles describing attempts at performing non-invasive testing using optical measurements (e.g., infrared systems). Part of this expansion has been fueled by the spread of acquired immunodeficiency disease syndrome (AIDS), and the associated fear among public and health care personnel of AIDS. AIDS and other diseases such as hepatitis are born in the blood and can be spread by improper practice of invasive procedures. In addition, the diabetic population has also been anxiously awaiting non-invasive test instruments for many years. Many diabetics must test their blood glucose levels four or more time a day. The modern battery powered instruments for home use require a finger prick to obtain the sample. The extracted blood samples are placed on a chemically-treated carrier which is inserted into the instrument to obtain a glucose reading. This finger prick is painful and can be objectionable when required frequently. In addition, although the price has dropped considerably on these instruments, the cost for the disposable and the discomfort and health risk associated with having open bleeding is undesirable.
Accordingly, a number of groups have recently tried to make non-invasive instruments for testing a variety of analytes, particularly glucose. A recent trend in non-invasive testing has been to explore the use of the near infrared spectral region, primarily 700-1100 nm because this is the spectral response range of the silicon detectors typically used in the prior art. A wider wavelength range to ~1800 nm can be accessed by the addition of germanium and/or InGaAs detectors, and useful measurements can be made into the 2500 nm range with InSb or other detectors. The region below ~1400 nm is the most useful in transmission, as tissue is transparent enough there to allow high enough photon flux for accurate detection. Above 1400 nm, the strong absorption of water limits the penetration depth of tissue, so that useful measurements are typically made in reflectance geometry. Below 1100 nm, the penetration of the light is sufficient that the signal modulation during the arterial pulse can be measured comfortably in both transmission and reflectance geometries. Above 1400 nm, such pulsatile measurements are extremely difficult in transmission due to low intensity, and similarly difficult in reflectance because the light does not penetrate deeply enough to sample the pulsatile capillary beds.
Most of the non-invasive testing work has been carried out using classic spectrophotometric methods, such as a set of narrow wavelengths sources, or scanning spectrophotometers which scan wavelength by wavelength across a broad spectrum. The data obtained from these methods are spectra which then require substantial data processing to eliminate background; accordingly, the papers are replete with data analysis techniques utilized to glean the pertinent information. Examples of this type of testing includes the work by Clarke, see U.S. Pat. No. 5,054,487; and primarily the work by Rosenthal et al., see e.g., U.S. Pat. No. 5,028,787. Although the Clarke work uses reflectance spectra and the Rosenthal work uses primarily transmission spectra, both rely on obtaining near infrared spectrophotometric data.
The major successful application of non-invasive testing is the measurement of hemoglobin oxygen saturation with pulse oximetry. The most common method compares the percentage modulation of the intensity of light traversing a body part at two wavelengths chosen so that the ratio of their respective modulations is a relatively strong function of oxygen saturation. The observed change in this ratio is relatively large because the two hemoglobin species involved have both high enough concentrations and specific absorptions that they dominate the creation of the pulsatile signal components at the wavelengths of interest. As a result, the ratio of modulations can be attributed substantially to the two hemoglobins alone, and only needs to be measured to the order of 0.1% in order to achieve clinically significant detection limits with acceptable universality of calibration.
The optical system in typical pulse oximeters have two or more LED emitters placed side-by-side on one side of a finger, and a single detector receiving the radiation on the other side of the finger. Some more recent systems have the detector on the same side of the tissue as the emitters, with baffles preventing the direct illumination of the detector by the sources. As the sources are physically small and optically displaced from each other and the detector, the light from each detector enters the tissue at slightly different locations, and therefore travel different paths through tissue to the detector.
Despite its relatively large signal levels, pulse oximetry has well-known difficulties such as the selection of an adequately vascular sampling site on each individual and variability of the results with motion of the site and breathing by the patient, as well as sensitivity to changes in blood pressure, heart rate, temperature, and tissue hydration. Disturbances such as motion and breathing artifacts typically appear as statistical outriders, i.e., as measurements which fall well off the “average” calibration curve of the instrument obtained from a group of individuals breathing controlled gas mixtures to vary their oxygen saturation.
The calibration of a pulse oximeter is subject to these same error sources; it is not uncommon to find site-to-site variations on the same individual, with results that suggest that the calibration curve even varies, for example, with the absolute magnitude of the pulsatile signal modulation. The effort to obtain a meaningful universal calibration is clearly at odds with intra- and inter-individual physiological variations.
Despite recent efforts to improve the measurement S/N by increasing source intensities and lowering detector noise, as well as increasing the number of detectors, the frequency of outriders and the universality of calibration have not improved substantially. Thus it is clear that while the light traversing the tissue is being measured more precisely, the site- and physiologically-induced variability has not been improved significantly below the 0.1% level needed for the measurement of oxygen saturation.
While these physical and physiological interferences are marginally acceptable for oxygen saturation measurements, they set a lower limit of detectivity that is too high for other clinical analytes such as glucose and cholesterol for which the combination of concentration and specific absorption requires optical measurements to be made 100-1000 times more precise than for the hemoglobins used in pulse oximetry. The hemoglobins, which in themselves are difficult to calibrate in the presence of these site- and physiologically specific limitations comprise a major background interference for the measurement of such trace constituents as glucose.
The optical systems employed for these lower concentration analytes naturally drew on the experience of pulse oximetry, and typically employ similar arrangements of a plurality of slightly displaced LED's to extend the wavelengths sampled, or which use fiber optics to carry light to and from the sources and/or spectrometers which perform the separation of the signal into the different wavelengths employed. Displacement of the sources and wide nume
Block Myron J.
Sodickson Lester
Epps Georgia
Hanig Richard
Lahive & Cockfield LLP
Optix LP
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