Monitoring constituents of an animal organ using discrete...

Details Monitoring constituents of an animal organ using discrete... Monitoring constituents of an animal organ using discrete...

1999-08-28

2001-02-27

Winakur, Eric F. (Department: 3736)

Surgery

Diagnostic testing

Measuring or detecting nonradioactive constituent of body...

C600S310000

Reexamination Certificate

active

06195574

ABSTRACT:

BACKGROUND
There has been a desire for an instrument to monitor constituents in an animal or human organ non-invasively. A particular example is monitoring of oxygen level in the brain which is particularly important, for example during surgery, where a significant number of patients come out of the anesthesia with various degrees of and sometimes permanent brain function deficiency. It is believed that in a significant portion of such cases, lack of sufficient oxygen to the brain is the cause of such deficiencies. Thus, the ability to accurately monitor oxygen level in the brain directly, rather than through indirect methods such as a pulse oximeter placed on another portion of the body, would have obvious advantages including non-invasiveness, immediate and timely results, and relative simplicity. Techniques to achieve such monitoring have involved passing near-infrared radiation through a cranium and analyzing the modified output radiation.
One known method is to pass radiation having several discrete wavelengths from laser diodes equal in number to the number of constituents to be measured, for example two wavelengths for oxygenated and deoxygenated hemoglobin. The radiation is modulated with radio frequency. The output modified by the brain is used to calculate changes in amplitude and phase which lead to determination of absorption coefficients at the different wavelengths. Simultaneous equations with these coefficients determine concentrations of the constituents of interest and the oxygen saturation which is the percentage of oxygenated to total hemoglobin.
Another method is to utilize continuous-wave radiation, in which output from a detector on a cranium is spectrally analyzed to yield oxygen saturation. Although a full spectrum is used, the analysis is based on modeling with either a small number of wavelengths or a few known constituents such as the oxy and deoxy hemoglobin and water.
Any such monitoring encounters difficulties resulting from the biological complexities of an organ such as a brain, compared with spectrometric instrumentation that ordinarily analyzes fluids that are readily probed, contained or flowing in a tube suitable for the instrument. Geometries of different subjects vary considerably and variations occur even within an individual. Further, tissues are not uniform. The radiation is scattered so that a path is not well defined. Signal to noise ratios for infrared radiation through solid material are generally low. Current methods for monitoring of craniums depend on theoretical or mathematical models that may be oversimplified or inaccurate. Thus there is a need for better accuracy and reproducibility.
Consequently, an object of the invention is to provide a novel method and means for monitoring constituents in an animal organ non-invasively, particularly oxygenated and deoxygenated hemoglobin in a brain.
SUMMARY
The foregoing and other objects are achieved, at least in part, by monitoring one or more selected constituents in an animal organ with a spectrometric instrument that includes a source of an input beam of discrete radiation and a radiation detector receptive of such radiation to generate representative signal data. The discrete radiation is formed of a plurality of discrete wavelength components in an infrared spectral range that includes absorbance wavelengths of the selected constituents and one or more additional constituents in the organ. Each wavelength component has a predetermined input amplitude and is modulated with a radio frequency signal having a predetermined input phase. The plurality of wavelength components is at least equal in number to the total number of constituents.
The input beam is directed into an animal organ such that the radiation is modified by the constituents. The detector is positioned so as to be receptive of the modified radiation from an exit site from the organ, and generates a corresponding output signal for each wavelength component. From each output signal, an output amplitude and an output phase are determined for each wavelength component. An absorption coefficient is computed for each wavelength component from the input amplitude, the output amplitude, the input phase and the output phase, utilizing respective equations relating phase, amplitude, absorption coefficient and scattering coefficient. Concentration of each of the selected constituents is calculated from a plurality of simultaneous equations at least equal to the total number of constituents, each equation being for a wavelength component relating absorption coefficient to concentrations of all of the constituents proportionately with respective predetermined extinction coefficients. These procedures are particularly advantageous for selected constituents comprising oxygenated hemoglobin and deoxygenated hemoglobin, with the additional constituents being water, protein and lipid. Oxygen saturation in the blood may be calculated as a ratio of concentration of oxygenated hemoglobin to a total of concentrations of oxygenated and deoxygenated hemoglobin.
In a further aspect, to predetermine each input amplitude and input phase, the input beam is passed through a neutral density filter to the radiation detector so as to generate a corresponding reference signal for each wavelength component, the filter having a predetermined optical density. From each reference signal, a reference amplitude and a reference phase are determined for each wavelength component, the input phase being equal to the reference phase. The input amplitude is calculated from the reference amplitude and the optical density.
Accuracy may be improved by a correction procedure. An absorption relationship is provided relating total absorption coefficient for the organ to oxygen saturation, specific absorption coefficients and volume fractions respectively for water, tissue matrix, blood, and for constituents of the tissue matrix and the blood. The specific absorption coefficients are predetermined for each wavelength component, and the volume fractions can vary. At least one set of values is selected for the oxygen saturation and the volume fractions. For each set, from these values, and from the specific absorption coefficients, a corresponding organ absorption coefficient is calculated for a selected wavelength component. The calculated organ absorption coefficients are utilized for computing concentrations of oxygenated hemoglobin and deoxygenated hemoglobin from the plurality of simultaneous equations. Oxygen saturations are calculated from the concentrations. The calculated oxygen saturations are compared to the selected values for oxygen saturation to obtain a correction factor which is stored for future application to the measured oxygen saturation to obtain a corrected oxygen saturation.
In another aspect, calibration factors are generated. At least one standardized medium of a non-absorbing material containing scattering matter is provided, the medium having a predetermined scattering coefficient and nil absorption coefficient. Also a standard sample of each of the selected and additional constituents is provided, the standard samples having predetermined concentrations and predetermined extinction coefficients.
The input beam is directed through the medium in a manner similar to an organ to generate an output signal for the medium. From each output signal, a measured phase is determined for the medium for each wavelength. Additionally, a computed phase for each wavelength is computed from the predetermined scattering coefficient using a model equation relating phase to scattering coefficient.
The input beam is further directed into each standard sample so as to generate an output signal for each sample. From each output signal, a measured amplitude is determined for each sample for selected wavelengths. A set of hypothetical concentrations of the sample constituents is derived for hypothetical organs. From these and the predetermined concentrations, a hypothetical absorption coefficient is computed for each wavelength from an absorption equation relating absorp

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