Abstract:
A method and apparatus for calibrating an NIRS system which includes a sensor portion ( 42 ) and for evaluating an NIRS system for proper functioning is provided that includes an enclosure ( 40 ) with at least two windows ( 16,18 ) disposed in a wall of the enclosure ( 40 ) The windows ( 16,18 ) allow the light source ( 22 ) and one or more detectors ( 24 ) of an NIRS system sensor ( 42 ) to interface with the enclosure ( 40 ) One window ( 16 ) is dedicated to the light source ( 22 ) while each light detector ( 24 ) has a window ( 18 ) dedicate thereto Thus, the enclosure ( 40 ) includes a number of windows ( 16,18 ) equal to the number of light detectors ( 24 ) in the NIRS system sensor plus one The inner surface of the wall(s) of the enclosure ( 40  is of a light-absorbing color, e.g., black A diffuse reflectance member ( 28 ) of a light-reflecting color, e.g., white, is disposed in the enclosure ( 40 ) spaced apart from the surface with the windows ( 16,18 ) disposed therein

Description:
[0001]    Applicant hereby claims priority benefits under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 60/735,244 filed Nov. 9, 2005, the disclosure of which is herein incorporated by reference. 
     
    
       [0002]    This invention was made with Government support under Contract No. 2R44NS45488-01 awarded by the Department of Health and Human Services. The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Technical Field 
         [0004]    This invention relates in general to spectrophotometric systems including sensors for non-invasively determining biological tissue oxygenation utilizing near-infrared spectroscopy techniques, and in particular to a device for calibrating such systems and for evaluating the proper functioning of such systems. 
         [0005]    2. Background Information 
         [0006]    Near-infrared spectroscopy (NIRS) is an optical spectrophotometric method that can be used to continuously monitor tissue oxygenation levels. The NIRS method is based on the principle that light in the near-infrared range (700-1000 nm) can pass easily through skin, bone and other tissues where it encounters hemoglobin located mainly within micro-circulation passages; e.g., capillaries, arterioles, and venuoles. Hemoglobin exposed to light in the near-infrared range has specific absorption spectra that vary depending on its oxygenation state; i.e., oxyhemoglobin (HbO 2 ) and deoxyhemoglobin (Hb) each act as a distinct chromophore. By using light sources that transmit near-infrared light at specific different wavelengths, and by measuring changes in transmitted or reflected light attenuation, concentration changes of the oxyhemoglobin and deoxyhemoglobin can be monitored. The ability to continually monitor cerebral oxygenation levels, for example, is particularly valuable for those patients subject to a condition in which oxygenation levels in the brain may be compromised, leading to brain damage or death. 
         [0007]    NIRS-type sensors typically include at least one light source and one or more light detectors for detecting reflected or transmitted light. The light signal is created and sensed in cooperation with an overall NIRS system that includes a monitor portion having a computer or processor that runs an algorithm for processing signals and the data contained therein. Typically the monitor portion is separate from the sensor portion. Thus; the sensor and monitor portions comprise the overall NIRS system. Light sources such as light emitting diodes (LEDs) or laser diodes that produce light emissions in the wavelength range of 700-1000 nm are typically used. A photodiode or other light detector is used to detect light reflected from or passed through the tissue being examined. The NIRS system processor cooperates with the light source and detector to create, detect and analyze the signals in terms of their intensity and wave properties. U.S. Pat. No. 6,456,862, and U.S. Pat. No. 7,092,701, both of which are hereby incorporated by reference in their entirety and are commonly assigned to CAS Medical Systems, Inc., of Branford, Conn., the assignee of the present patent application, disclose an NIRS system (e.g., a cerebral oximeter) and a methodology for analyzing the signals within the NIRS system. 
         [0008]    Oftentimes an NIRS system typically includes a sensor portion having a plurality of discrete light sources located together in one location along with one or more light detectors disposed at predetermined distances from each other and from the light source. Each light source produces an infrared light signal at a particular wavelength at which a known absorption response is produced depending on the amount of oxygen concentration in the hemoglobin. Several different wavelengths are typically employed, for example, at 780 nm, 805 nm, and 850 nm. Ideally, the light sources would operate at specific wavelengths that do not vary at all from one NIRS system to another, nor would the wavelength of light output from an individual light source vary over time. However, in practice the light output from a discrete light source varies from device to device due to, e.g., manufacturability and material constraints and variables, and also over time due to operating variables such as, e.g., temperature. 
         [0009]    What is needed, therefore, is a device that can be used to accurately and quickly calibrate an NIRS system and to evaluate the proper functionality of the NIRS system. 
       SUMMARY OF THE INVENTION 
       [0010]    A device that may be used both for calibrating an NIRS system and for evaluating an NIRS system for proper functioning includes an enclosure with at least two windows disposed in a wall of the enclosure. Depending on the configuration of the enclosure, the enclosure may have more than one wall. The windows allow the light source and one or more detectors of an NIRS system sensor to interface with the enclosure. One window is dedicated to the light source while each light detector has a window dedicated thereto. Thus, the enclosure includes a number of windows equal to the number of light detectors in the NIRS system sensor plus one. In some embodiments, each window is covered by a thin, light-diffusive material. The inner surface of the wall(s) of the enclosure is of a light-absorbing color; e.g., black. A diffuse reflectance member of a light-reflecting color, e.g., white, is disposed in the enclosure spaced apart from the surface with the windows disposed therein. 
         [0011]    In operation of the calibration device, a NIRS system sensor is positioned relative to the calibration device so that the light source aligns with the one window dedicated thereto, and the one or more light detectors each align with the corresponding one or more windows dedicated thereto. When the NIRS system is activated, light from the light source enters the enclosure. A portion of the light is absorbed by the light-absorbing walls. Another portion of the light is diffusively reflected by the reflectance member. The diffusively reflected light will reach the one or more light detectors of the NIRS system sensor and be detected. In some applications, a previously calibrated NIRS system sensor is interfaced with the calibration device and operated to generate a measurement of light that can be used as a reference value. The reference value may then be used to subsequently calibrate an uncalibrated NIRS system that is interfaced to the calibration device. If the sensor for each NIRS system is equivalent to one another, the reference value is used to calibrate the monitor portion of each of those NIRS systems, to thereby calibrate the entire NIRS system. In other instances where the sensors are not equivalent, the calibration device may be utilized to calibrate only the sensor, or calibrate the sensor and monitor portions combined. 
         [0012]    The reflectance member reflects light in all directions (i.e., diffusive) and in a wavelength-independent manner (i.e., reflects equally over the wavelengths of interest). That is, the response of the reflectance member to the various wavelengths of light of interest is relatively flat or constant, thereby providing for relatively constant light characteristics across all wavelengths of interest. To simulate a desired attenuation of light, the surface area of the reflectance member may be made larger or smaller relative to the interior surface area of the light absorbing wall(s) of the enclosure. In other words, the interior surface of the walls acts as a spectrally constant or flat light absorber. The separation distance between the reflectance member and the windows may be changed to alter the light reflectance characteristics of the calibration device. 
         [0013]    The calibration device is versatile in that various configurations may be utilized to accommodate different optical light source and detector arrangements. Also, the calibration device uses no solid substances or wavelength-dependent optical filters in the light pathway between the NIRS system sensor light source and detector which could undesirably absorb a relatively large proportion of the light and tend to degrade the desired characteristics of the calibration device. 
         [0014]    The calibration device may be configured for use with NIRS systems that utilize sensors having one or more light detectors. For those sensors that include more than one detector, the calibration device can accommodate light detectors disposed at equal or unequal distances from the light source. NIRS system sensors used in cerebral applications, for example, typically have light detectors placed at different distances from the light source; e.g., a “far” detector and a “near” detector. The level of light received by the “far” detector is typically much lower in magnitude as compared to the magnitude of light received by the “near” detector because of the amount of tissue traversed. The calibration device simulates the differences in light magnitude by sizing the reflectance member and positioning the same relative to the windows for the light source and the detectors. For example, positioning the reflectance member further away from the far detector window than the near detector window causes the far detector to receive a magnitude of light lower than that received by the near detector. 
         [0015]    One of the advantages of the calibration device is that it can be made in an inexpensive disposable form. The calibration device may be fabricated from relatively inexpensive materials that allow it to be used once and discarded. A disposable calibration device provides a distinct advantage, since it can be used with NIRS systems that are typically employed in a healthcare environment where a relatively high degree of cleanliness is desired. 
         [0016]    These and other features and advantages of the present invention will become apparent in light of the drawings and detailed description of the present invention provided below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a diagrammatic perspective view of a device for calibrating a spectrophotometric system. 
           [0018]      FIG. 2  is a diagrammatic side view of the device of  FIG. 1 . 
           [0019]      FIG. 3  is a diagrammatic side view of a spectrophotometric sensor for use with the device of  FIGS. 1 and 2 . 
           [0020]      FIG. 4  is a diagrammatic perspective view of another embodiment of a device for calibrating a spectrophotometric system. 
           [0021]      FIG. 5  is a diagrammatic side view of the device of  FIG. 4 . 
           [0022]      FIG. 6  is a diagrammatic side view of a spectrophotometric sensor for use with the device of  FIGS. 4 and 5 . 
           [0023]      FIG. 7  is a diagrammatic perspective view of a device for calibrating and evaluating the functionality of a disposable spectrophotometric sensor. 
           [0024]      FIG. 8  is a diagrammatic side view of the device of  FIG. 7  together with the disposable spectrophotometric sensor attached thereto. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Referring to  FIGS. 1-3 , a device  10  for calibrating an NIRS spectrophotometric system (e.g., a cerebral oximeter), which includes a sensor portion  12 , has an enclosure  14 . In  FIGS. 1-2 , the enclosure  14  is a rectangular-shaped box having a plurality of walls. However, the enclosure  14  is not limited to a box. Instead, the enclosure  14  may take on other shapes or forms, depending, for example, on the configuration of the sensor  12 . An example of an NIRS spectrophotometric system is described and illustrated in the aforementioned U.S. Pat. No. 6,456,862, which is hereby incorporated by reference in its entirety. The present calibration device is not, however, limited to use with the device disclosed within U.S. Pat. No. 6,456,862. The walls of the box  14  have their inside surfaces of a color (e.g., black) for the purpose of absorbing light. The walls of the box  14  may comprise a plastic, cardboard or other type of suitable material. Two windows  16 ,  18  are disposed in the top wall  20  of the box  14 . The windows  16 ,  18  may be covered by a thin, light-diffusive material that allows for diffusion of light into the box  14  while sealing the box from contaminants. The windows  16 ,  18  are positioned to align with a light source  22  and a light detector  24 , respectively, of the sensor  12 . 
         [0026]    A diffuse reflectance member  28  that is of a color (e.g., white) that reflects light is disposed inside the box  14 . The member  28  may be made from roughened plastic, glass filled PTFE, ceramic or tile, paper, cardboard, flat/matte paint, or relatively higher optical grade plastics such as Spectralon from Labsphere, Inc. of North Sutton, N.H. In general, any material of a color that reflects light in a diffusive manner (i.e., in all directions) and that has a roughened surface instead of a relatively shiny surface can be used as the reflectance member  28 . The member  28  may be positioned anywhere within the enclosure  14  to achieve the desired reflectance of light. In the embodiments illustrated in  FIGS. 1-3 , the member  28  is attached to a surface of the wall  26  that is opposite the top wall  20 . In alternative embodiments, the member  28  can be positioned anywhere within the enclosure  14  where the desired amount of light reflectance can be achieved. In addition, the member  28  can be mounted within the box  14  in a variety of different ways, rather than being disposed on a surface of a wall  26  (e.g., mounted on a pedestal attached to a wall). 
         [0027]    The portion of the light emanating from the light source  22  of the sensor  12  and coming through the light source window  16  which encounters the member  28  is reflectively scattered throughout the inside of the box  14 . The remainder of the light from the light source  22  that enters the box  14  is absorbed by the inner walls of the box  14 . A portion of the input light that is reflected by the member  28  is reflected back through the light detector window  18  to the light detector  24  of the sensor  12 . The amount of surface area of the reflectance member  28 , the positioning of the reflectance member  28  relative to the windows  16 ,  18 , the amount of surface area on the inside panels of the box  14 , and the dimensions of the box  14 , collectively determine the amount of light received by the light detector  24  from the light source  22 , and the amount of light that is absorbed within the box  14 . The calibration device  10  can therefore be configured to simulate the amount of light attenuation that a spectrophotometric sensor  12  would normally encounter during examination of biological tissue or other medium when the sensor is place in contact therewith, in a wavelength independent manner throughout the wavelengths of interest. 
         [0028]      FIGS. 4-6  illustrate an embodiment of a device  30  for calibrating an NIRS spectrophotometric system that is similar to the calibration device  10  illustrated in  FIGS. 1-2  and described above. In the embodiment shown in  FIGS. 4-6 , three windows  16 ,  18 ,  32  are disposed in the top wall  20  of the box  14 . One window  16  is aligned with the light source  22  of a sensor  34 , while the other two windows  18 ,  32  are aligned with the two separate light detectors  24 ,  36  of the sensor  34  (i.e., a “neat” light detector  24  and a “far” light detector  36 ). An example of an NIRS system, including such a sensor, is described and illustrated in the aforementioned U.S. Pat. No. 7,092,701, which is hereby incorporated by reference in its entirety. The reflectance member  28  is sized and be positioned relative to the windows  16 ,  18  so that the near light detector  24  and the far light detector  36  receive different magnitudes of light emanating from the light source  22 . That is, the far light detector  36  receives a relatively smaller amount of light as compared to the amount of light received by the near light detector  24 . This is done to simulate typical light measurement conditions in biological tissue encountered by the cerebral oximeter in actual use on a human subject. 
         [0029]    Operation of the calibration devices  10 ,  30  of  FIGS. 1-2  and  4 - 5  for calibrating a spectrophotometric system may occur as follows. First, a spectrophotometric system that includes a sensor, such as the cerebral oximeter of the aforementioned U.S. Pat. No. 6,456,862, or U.S. Pat. No. 7,092,701, is calibrated apart from and without use of the respective calibration device  10 ,  30 . That is, the spectrophotometric system may be calibrated, for example and without limitation, through use of empirical data collected from a subject under study, as described in more detail in the aforementioned U.S. Pat. No. 6,456,862. As discussed in that patent, the calibrating of a spectrophotometric system such as the cerebral oximeter disclosed therein provides a relatively accurate indication of the total oxygen saturation level in human tissue by removing extraneous information relating to certain types of undesirable light attenuation from the determination. Once that particular spectrophotometric system is calibrated, the sensor  12 ,  34  of that calibrated system is interfaced with the box  14 . Using predetermined fixed wavelengths of infrared light, for example, 780 nM, 805 nm, and 850 nm, to determine fixed absorption coefficients that are characteristic of the particular type of spectrophotometric system (each wavelength, for example, having a ±5 nm tolerance), with the infrared light emanating from the light source  22  of the sensor  12 ,  34 , a measurement is made of the amount of light that is received by each of the one or more light detectors  24 ,  36  of the sensor,  12 ,  34 . This measurement of light corresponds to a reference value. Since the calibration device  10 ,  30  has the desired spectrally constant or flat light absorption characteristic, individual wavelength tolerances do not alter the amount of the measured value of light received by the detectors  24 ,  36 . When a second, uncalibrated similar type of spectrophotometric system has its sensor portion interfaced to the calibration device  10 ,  30 , the internal calibration parameters of this uncalibrated spectrophotometric system may be adjusted until the calculated amount of measured light received by the light detectors  24 ,  36  agrees with the reference value measured earlier in conjunction with the first calibrated spectrophotometric system. As described in detail in the aforementioned U.S. Pat. No. 6,456,862, the internal calibration parameters account for the effects of undesirable light attenuation due to various sources, such as scattering within tissue, thereby allowing for a more accurate determination of total oxygen saturation. Once calibrated in this manner, the second spectrophotometric system can be used with its own particular wavelengths to measure absolute cerebral blood oxygenation. Thus, once the first or reference spectrophotometric system is calibrated, other similar spectrophotometric systems can be calibrated with the need for any type of human invasive procedure. 
         [0030]    Referring to  FIGS. 7-8 , there illustrated is a device  40  for both calibrating and evaluating a spectrophotometric system, for example, having a disposable spectrophotometric sensor  42 . The device  40  itself may be disposable and as such is suitable for one time use. To facilitate the disposability and one time usage of the device  40  and the sensor  42 , the disposable device  40  may be packaged together with the sensor  42 , where an adhesive release liner  44  is disposed between the device  40  and the sensor  42 . The disposable device  40  may be similar to the calibration devices  10 ,  30  described hereinabove and illustrated in  FIGS. 1-2  and  4 - 5 . The disposable device  40  may have its box  14  constructed of relatively inexpensive materials such as cardboard paper, plastic, or other suitable material, with inner walls of a color, e.g., black, that absorbs light. The windows  16 ,  18  in the box  14  may be covered by a thin diffusive tape or membrane. When an unused sensor  42  is connected to the disposable device  40 , the device  40  can evaluate the proper functioning of the sensor  42  and corresponding system before the sensor  42  is attached to the subject being monitored. Once the functionality of the sensor  42  is determined to be acceptable, the disposable device  40  and the sensor  42  may be separated by removing the sensor  42  from the adhesive release liner  44 , thereby exposing adhesive on a patient-contacting side of the sensor  42  for attachment to the skin of the subject to be tested. 
         [0031]    With the unused sensor  42  still attached to the device  40 , the sensor  42  can be calibrated and its functionality evaluated relatively easy and quickly. Regarding calibration, similar to the procedure described above with respect to the device  10 ,  30  of  FIGS. 1-2  and  4 - 5 , if the actual or measured calibration values agree with the expected calibration values, then the sensor  42  may be determined to be functioning properly and is ready to be attached to the subject after the disposable device  40  is removed. If this procedure is used, some portion or all sensors  42  of a given type may have individual calibration information and do not need to be individually calibrated to any one particular sensor  42 . Regarding the evaluation of proper functionality, a determination may be made as to whether the amount of light received by the light detector  24  of the sensor  42  is within an acceptable predetermined range. If the received light is within range, then the sensor  42  may be determined to be functioning properly and is ready for use on a subject. If an out of range reading (e.g., a low light level) is measured, then the connection of the sensor  42  to the device  40  monitor may need to be cleaned, or the sensor  42  may not be properly interfaced to the device  40 , or possibly some other problem with the cerebral oximeter exists, for example, the sensor light source optical fiber may be fractured. If no light is detected, then the sensor  42  may have a broken electrical lead, a broken optical fiber, some type of misconnection, or possibly the cerebral oximeter itself has a problem. 
         [0032]    The enclosure has been described and illustrated herein as being a box  14  having a generally rectangular shape. However, the enclosure is not limited as such. The enclosure may take on any shape, with planar walls and with non-planar (e.g., curved) walls. It suffices that the enclosure be of a shape such that at least two windows can be formed in the enclosure to allow light to enter the enclosure through one window and exit the enclosure through the other window, and that a reflectance member be placed within the enclosure such that light from the source is reflected off the member and back to the light detector in a desired amount, and further that the inner wall surfaces of the enclosure be of a color that absorbs light in a desired amount. 
         [0033]    Although the present invention has been illustrated and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.