NIRS sensor assembly including electrically conductive and optically transparent EMI shielding

A near infrared spectrophotometric sensor assembly for non-invasive monitoring of blood oxygenation levels in a subject's body is provided. The assembly includes at least one light source, at least one light detector operable to detect light emitted by the light source, an electromagnetic interference shielding disposed around at least a portion of the light detector, wherein the electromagnetic interference shielding includes an electrically conductive substrate that is optically transparent, and one or both of a light blocking sheet disposed relative to at least one of the light detectors and an encapsulating material.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to methods and apparatus for non-invasively determining biological tissue oxygenation utilizing near-infrared spectroscopy (NIRS) techniques in general, and to sensors for use with such techniques in particular.

2. Background Information

Near-infrared spectroscopy is an optical spectrophotometric method that can be used to continuously monitor tissue oxygenation. The NIRS method is based on the principle that light in the near-infrared range (700 nm to 1,000 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 varies depending on its oxidation state; i.e., oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) each act as a distinct chromophore. By using light sources that transmit near-infrared light at specific different wavelengths, and measuring changes in transmitted or reflected light attenuation, concentration changes of the oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) 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.

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 a NIRS system that includes a processor and an algorithm for processing signals and the data contained therein. U.S. Pat. No. 7,047,054 and PCT International Application Serial No. PCT US0641268, which are commonly assigned with the present application to CAS Medical Systems, Inc. of Branford, Conn., disclose examples of such a sensor. 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 cooperates with the light source(s) and the light detectors to create, detect, and analyze the signals in terms of their intensity and wave properties. U.S. Pat. Nos. 6,456,862 and 7,072,701, which are commonly assigned to CAS Medical Systems, Inc., of Branford, Conn., disclose a methodology for analyzing such signals. U.S. Pat. Nos. 6,456,862 and 7,072,701, and PCT Application Serial No. PCT US0641268 are hereby incorporated by reference in their entirety.

Meaningful cerebral oxygenation information is collected from light interrogating brain tissue (e.g., passing through, reflecting from, absorbed by, etc.). To non-invasively access the brain tissue, however, the light signal must pass through extra cerebral tissue (e.g., scalp, skull, etc.) before and after interrogating the brain tissue. A light signal traveling within any biological medium (e.g., tissue, fluid, etc.) will attenuate, and the amount of attenuation is a function of the medium. In the case of a mean optical path that non-invasively accesses brain tissue, the attenuation attributable to the extra cerebral tissue does not yield useful information with respect to the cerebral oxygenation. Consequently, it is desirable to account for the signal attenuation attributable to extra cerebral tissue, so that the attenuation attributable to the brain tissue can be distinguished and analyzed.

It is known to use a NIRS sensor having a pair of light detectors specifically spaced apart from a light source as a means to account for extra cerebral tissue. A “near” light detector may be spaced apart from a light source by a first separation distance, and a “far” detector may be spaced apart from the light source by a second separation distance, which is typically greater than first separation distance. The method for spectrophotometric blood oxygenation monitoring disclosed within U.S. Pat. No. 7,072,701 is an example of a method that can be used with two detectors.

A problem common to all NIRS sensors is signal interference from electromagnetic interference (EMI) sources. Mitigating the effect of such interference improves the quality of the signal available, and therefore the patient information available. Another problem common to all NIRS sensors is the cost to manufacture. NIRS sensors are typically disposed of after use, so the cost of the sensor is an important factor in the cost of the monitoring.

What is needed, therefore, is an improved sensor for non-invasively determining the level of oxygen saturation within biological tissue, one that can be configured with one or more detectors, one that mitigates interference, and one that can be readily manufactured.

DISCLOSURE OF THE INVENTION

According to an aspect of the present invention, a NIRS sensor assembly for non-invasive monitoring of blood oxygenation levels in a subject's body is provided. The sensor assembly comprises at least one light source, at least one light detector, electromagnetic interference (EMI) shielding, and a light blocking sheet. The light source is operable to emit light signals of a plurality of different wavelengths, including those in the near-infrared range. The light detector is operable to detect light emitted by the light source and passed through the subject's body tissue. The shielding, which is disposed around at least a portion of the light detector, attenuates local EMI and thereby reduces undesirable noise within the light detector signals. The light blocking sheet is disposed relative to at least one of the light detectors, and includes an aperture sized to mate with the active area of the light detector with which it is disposed.

According to an aspect of the present invention, a NIRS sensor assembly for non-invasive monitoring of blood oxygenation levels in a subject's body is provided. The sensor assembly comprises a pad, at least one light source, at least one light detector, and electromagnetic interference (EMI) shielding. The light source is operable to emit light signals of a plurality of different wavelengths, including those in the near-infrared range. The light detector is operable to detect light emitted by the light source and passed through the subject's body tissue. The shielding, which is disposed around at least a portion of the light detector, attenuates local EMI and thereby reduces undesirable noise within the light detector signals.

According to another aspect of the present invention a near infrared spectrophotometric sensor assembly for non-invasive monitoring of blood oxygenation levels in a subject's body tissue is provided. The assembly includes a flexible circuit, at least one light detector, at least one light source, an electrical connector, EMI shielding, a light blocking sheet, and a pad. The at least one light detector is in electrical communication with the flexible circuit. The light detector has an active area for receiving light signals. The at least one light source is in electrical communication with the flexible circuit. The electrical connector is in electrical communication with the flexible circuit. The EMI shielding is disposed relative to the at least one light detector. The light blocking sheet is disposed relative to at least one of the light detectors, and includes an aperture sized to mate with the active area of the light detector with which it is disposed. The pad has a detector aperture and a light source aperture, and the pad is positioned within the assembly to contact the subject during operation of the sensor.

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.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, a near infrared spectroscopy (NIRS) system10includes one or more NIRS sensor assemblies12connected to a base unit11. The base unit11includes a display13, operator controls, and a processor14for providing signals to and/or receiving signals from the NIRS sensor assembly(ies)12. The processor14is adapted (e.g., programmed) to selectively perform the functions necessary to operate the sensor(s). It should be noted that the functionality of the processor14may be implemented using hardware, software, firmware, or a combination thereof. A person skilled in the art would be able to program the processor14to perform the functionality described herein without undue experimentation. For purposes of providing a detailed description of the present NIRS sensor assembly12, the sensor assembly12will be described herein as being used in connection with the NIRS system described in U.S. Pat. Nos. 6,456,862 and 7,072,701, which are examples of acceptable NIRS systems. The NIRS sensor assembly12is not, however, limited to use with any particular NIRS system.

An embodiment of a NIRS sensor assembly12is shown inFIGS. 2-8. The NIRS sensor assembly12includes a pad16, at least one light source18, at least one light detector20, a detector housing22, electromagnetic interference (EMI) shielding24, and a cover26. In those embodiments of the present sensor assembly12that include more than one light detector20, the present invention may include a plurality of detector housings22. The present invention sensor is not limited to this particular NIRS sensor assembly, which is described herein for illustrative purposes. Specifically, the present invention includes novel, unobvious, and advantageous EMI shielding configurations (described below) that may be used with a variety of different NIRS sensors.

Now referring to the embodiment shown inFIGS. 2 and 3, the pad16has a width28, a length30, a substantially uniform thickness32, a patient side surface34, a hardware side surface36, at least one source aperture38, and at least one detector aperture40. The width28and length30are preferably contoured around one or both of the source aperture38and the detector aperture40. In the embodiment shown inFIGS. 2 and 3, the pad16includes a pair of detector apertures40and a source aperture38. The detector apertures40are each shaped to receive a portion of a detector housing22, and the source aperture38is shaped to receive a portion of the light source18. The detector and source apertures40,38are typically aligned along a center line42of the pad16. In some embodiments an adhesive17is applied to the patient side surface34for attaching the pad16to the patient (seeFIG. 8). A removable protective layer44may be mounted on the adhesive covered patient side surface34to protect the adhesive until use. In some embodiments an adhesive is applied to the hardware side surface36.

The pad16is preferably made from a flexible material (e.g., foam) that substantially or completely blocks the transmission of light energy through the pad16. Poron® cellular urethane foam, a product of Rogers Corporation of Woodstock, Conn. USA, is an example of an acceptable pad16material.

In the embodiment shown inFIG. 2, the cover26has a geometry that matches the geometry of the pad16. The cover26has a width46, a length48, a thickness50, a hardware side surface52, and an outer surface53. The cover26may be made of a number of different materials, including the foam such as Poron® cellular urethane foam.

Referring toFIG. 8, one or more support layers54may be attached to one or both of the pad16and the cover26. The support layer54is flexible, and may be described as having a width and a length oriented in similar direction as the width and length of the pad16and the cover26. The support layer54resists stretching in the widthwise and/or lengthwise directions. Once a support layer54is attached to the pad16or to the cover26, therefore, the support layer54resists stretching of the individual pad16or cover26, and collectively the entire sensor assembly12. An example of an acceptable support layer54material is Reemay® brand spunbonded polyester media, style no. 2006, offered by Reemay, Inc, of Charleston, S.C., USA. The present invention is not limited, however, to support layers54consisting of Reemay® brand spunbonded polyester media. In the embodiment shown inFIG. 8, a first support layer54is adhered to the hardware side surface52of the cover26, and a second support layer54is adhered to the hardware side surface36of the pad16.

The light source18is selectively operable to guide or emit infrared light (i.e., light in wavelength range of about 700 nm to about 1,000 nm). As stated above, infrared light provides particular utility in determining tissue oxygenation because hemoglobin exposed to light in the near-infrared range has specific absorption spectra that varies depending on its oxidation state; i.e., oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) each act as a distinct chromophore. In alternative embodiments, however, there may be utility in examining blood metabolites that are best examined with a light outside the infrared range; e.g., in the visible light range between 400 nm and 700 nm, such as red light at 650 nm, or green light at 510 nm, or both visible and infrared light combinations, etc. In those applications, a light source may be utilized that emits or guides light outside the infrared range. In some embodiments, the light source18is an assembly that includes a fiber optic light guide58and a light redirecting prism60. One end of the fiber optic guide58is optically connected to the prism60. The other end of the fiber optic guide58is typically disposed within a connector62(seeFIG. 1) that permits the fiber optic guide58to be optically coupled to fiber optic guide connected to the NIRS system10. Other embodiments may have an optical fiber that is bent to create the proper alignment (e.g., bent ninety degrees (90°). Examples of acceptable connector62embodiments are disclosed below. The fiber optic light guide58is diagrammatically shown inFIGS. 4 and 7as a single fiber light guide. The fiber optic light guide58is not limited to a single fiber embodiment, and may comprise a plurality of fibers in alternative embodiments. In alternative embodiments, the light source18includes one or more light emitting diodes (LEDs) mounted within the sensor assembly12in place of, or in combination with, the fiber optic light guide58and prism60. The one or more LEDs are electrically connected to and operationally controlled by elements (e.g., processor14) disposed within the base unit11.

In the fiber optic light source embodiments described above, the light source18does not create a light signal itself. Rather, a light signal or signals (collectively referred to hereinafter as a light signal) are introduced into the fiber optic guide58at a position external of the NIRS sensor assembly12, and are guided into the sensor assembly12via the fiber optic guide58. The present invention NIRS sensor assembly12is not limited to use with any particular method and/or apparatus for introducing a light signal into the fiber optic guide58. U.S. Pat. No. 7,047,054, incorporated by reference hereinabove, discloses an acceptable example of an apparatus for introducing light energy into the fiber optic guide58that includes the use of laser diodes.

A light signal exits the fiber optic guide58and enters the prism60through an entrance face64and is redirected out of the prism60through an exit face66. The fiber optic guide58can be connected to the entrance face64of the prism60in a variety of different ways. For example, the fiber optic guide58can be butted against the entrance face64of the prism60and held in place by a layer of clear epoxy disposed between the prism60and the fiber optic guide58. In some embodiments, the prism60may be disposed within the NIRS sensor assembly12so that it will contact the patient's skin during use of the NIRS sensor assembly12. The prism60is rigid so that when it is pressed against the patient's skin during the monitoring of blood oxygen, the surface of the skin is flattened, and the distance between the fiber optic guide58output and the skin surface via the prism60is constant across the entire illuminated area of the skin. This configuration controls the input light intensity and light illumination spot size on the skin, which is important in making accurate measurements. In the embodiments wherein the light source18includes one or more LEDs mounted within the sensor assembly12, light signals emitted from the LED(s) impinges on the subject's skin.

In the embodiment shown inFIG. 7, optical shielding67operable to at least partially impede the passage of light into or out of the prism60from a surface other than the entrance face64and exit face66is disposed around at least a portion of the prism60. An example of an acceptable optical shielding67is a metal (e.g., copper) foil tape.

The light detector(s)20includes a light responsive transducer such as a photodiode that is operable to sense light intensity derived from light emitted by the light source18after such light passes through a portion of the subject's body. The light detectors20are electrically connected to the NIRS system to enable the output of the light detectors be communicated to the NIRS system10. In a preferred embodiment, one or more EMI shielded cables68connect the light detectors20to the NIRS system10. In the sensor embodiments having two light detectors20, the light detector disposed closest to the light source18may be referred to as the “near detector20a” and the other detector20disposed further away from the light source18as the “far detector20b”.

In the sensor embodiment shown inFIGS. 2-6, the far detector20bincludes a pair of photodiodes69mounted on a substrate70that includes a printed circuit board71. The photodiodes69in the far detector20bare electrically connected to the printed circuit board71, and the printed circuit board, in turn, is electrically connected to the shielded cable68. The near detector20aincludes a single photodiode69connected to the shielded cable68. The number of photodiodes in one or both of the near and far detectors20a,20bmay change to suit a particular application. In alternative embodiments, the substrate70may include a flexible circuit.

FIGS. 5 and 6illustrate EMI shielding24disposed relative to the near and far light detectors20a,20b. The shielding24embodiment for the far detector20bshown inFIG. 5includes an optically transparent portion72and a non-transparent portion74. In this embodiment, the non-transparent portion74of the EMI shielding24is disposed around substantially the entirety of the far detector20b, and the transparent portion72is disposed in alignment with the light detection surfaces of the sensor20b. For purposes of this description, the term “optically transparent” may be defined as follows: an optically transparent medium is one through which an amount of light may pass through, which amount is adequate for purposes of a NIRS evaluation. Conversely, an “optically non-transparent” medium is one that prevents passage of substantially all of light there through, which light would otherwise be available for a NIRS evaluation. In other embodiments, the EMI shielding24embodiment may consist entirely of an optically transparent portion72, or it may include portions that are less than optically non-transparent; e.g., disposed in areas outside of alignment with the light detection surfaces of the sensor20b. The EMI shielding24is preferably directly or indirectly connected to ground, but not necessarily.

As indicated above, a percentage of the light signal produced by the light source18passes through the biological tissue of the patient, then through the optically transparent portion72of the EMI shielding24, after which it is sensed by the far light detector20b. At the same time, the optically transparent portion72attenuates local EMI that may be present.

In some embodiments, the optically transparent portion72includes a structure that is operable to isotropically distribute EMI. For example, the optically transparent portion72may include an electrically conductive wire mesh (e.g., copper wire mesh). As another example, the optically transparent portion72may include a thin electrically conductive substrate that is optically transparent as described above. An example of such an electrically conductive substrate is a fiber-filled, conductive adhesive tape such as XYZ-Axis Electrically Conductive Tape 9713, offered by 3M Company of St. Paul, Minn., USA. The electrically conductive substrate (e.g., 9713 Tape) may contain electrically conductive fibers that allow for isotropic distribution of EMI. The electrically conductive substrate can come in a double sided form that has adhesive on both sides. An electrically conductive substrate (e.g., the 9713 Tape) provides several advantages, including: (1) it is relatively inexpensive; (2) it does not require a soldered connection to ground; (3) it is available in roll form; (4) it has a low profile; and (5) it is flexible. In embodiments in which the sensor includes a flexible circuit, use of an electrically conductive substrate like the 9713 Tape on, in, or around the flexible circuit is particularly advantageous because the electrically conductive substrate in combination with the flexible circuit can improve the flexibility of the sensor while still providing adequate EMI shielding for the sensor. The ability to utilize the electrically conductive substrate on, in, or around the flexible circuit provides considerable utility. The present invention is not limited to using 9713 Tape as an electrically conductive substrate that is optically transparent, and other similar products may be used alternatively.

In those embodiments of the EMI shielding24that utilize an optically non-transparent portion74, that portion74may include an electrically conductive metal foil, such as a copper metal foil. In those embodiments in which the optically transparent portion72includes an electrically conductive substrate like the 9713 Tape, the non-transparent portion74may be adhered to the transparent portion72in those areas of the detector20a,20bnot utilized to sense light. An electrically conductive gasket, such as silicone paste, adhesive, foam, or other similar material, may be used to create an electrical interface between the optically transparent and optically non-transparent portions72,74of the shielding24, particularly in those embodiments that utilize a wire mesh as the optically transparent portion72.

The EMI shielding24for the near detector20aincludes an arrangement similar to those described above for the far detector20b. The EMI shielding embodiment used on the near detector20amay be different from that used on the far detector20b. For example, in the embodiment shown inFIG. 6, optically transparent shielding24is disposed around substantially the entirety of the near light detector20a.

The above-described shielding24reduces undesirable EMI generated noise, and improves the signal to noise ratio of the light detectors20(e.g., photodiodes). For example, the optically transparent shielding24creates a Faraday Cage around the light detector20, while allowing light to reach the light-sensitive surface of the light detector20. In fact, the EMI shielding of the present invention can be implemented to create as few as one Faraday Cage operable to provide the requisite EMI shielding, or alternatively can be implemented to create more than one Faraday Cage relative to the sensor to provide the requisite EMI shielding. For example, in one embodiment, a detector20may be protected from interference by creating two Faraday Cages; e.g., a first Faraday cage surrounding the electronic circuitry of the sensor and a second Faraday Cage around the light detector20itself. The present invention is not limited to any particular Faraday Cage embodiment. In those instances where the present invention shielding24is used within the exemplary sensor embodiment described herein, the detector housings22also create further EMI attenuation by increasing the light detector-to-biological tissue separation distance. The optically transparent spacer created by the detector housing22reduces the capacitance between the light detector20light sensitive surface and the biological tissue, such as human skin, resulting in an increased reduction in electromagnetic coupling and generated noise currents when compared to a sensor application that does not include such an optical spacer.

In those sensor embodiments that utilize a detector housing22, the detector housing22includes a base76and a cap78that together define an internal cavity80, which cavity80is sized to enclose a light detector20at least partially covered with shielding24(and other materials as applicable). The base76and the cap78may be hinged together or they may be separable. The cap78includes a port88(seeFIG. 2) for receiving a shielded cable68. Alternatively, the port88may be partially disposed in both the base76and the cap78, or solely in the base76. The base76includes a well82sized to receive at least a portion of the light detector20, and the cap78is sized to receive the remainder of the light detector20not received within the base well82. The detector housing embodiments shown inFIGS. 2-6, for example, include a base76having a well82and a cap78having a well84, which wells82,84are sized to receive the light detector20in combination. In other embodiments, the base well82may be sized to receive the entire light detector20and shielding24, and the cap78may be planar across the base well82, or shaped to extend into the base well82.

In some embodiments, the dimensions of the internal cavity80(i.e., height, width, depth) are such that there is a slight press fit between the light detector20, shielding24, etc. and the housing22. As a result, the light detector20is positionally located and maintained within the housing22. In alternative embodiments, the light detector20and shielding24may be positionally located and maintained within the internal cavity80by features (e.g., stems, ribs, etc.) integrally formed with the base76or cap78, or elements disposed within the internal cavity80(e.g., a spacer, or biasing element), or by other means.

The base well82includes a window panel86that consists of an optically transparent material that allows light to pass there through and be sensed by the light detector20. The window panel86may be an optically flat surface or it may be modified to focus or defocus detected light. The window panel86may also be treated to act as a filter with respect to chromaticity, wavelength, etc.

The base76and the cap78may be made out of the same material or different materials. In a preferred embodiment, the base76and cap78are formed from a material that has favorable dielectric characteristics (e.g., electrically insulative). The particular material, and therefore the dielectric characteristics, can be chosen to suit the application at hand. In addition, the amount of dielectric strength can also be chosen to affect the capacitance of a fixed geometry capacitor such as the window panel86. As indicated above, the window panel86portion of the detector housing22increases the light detector-to-biological tissue separation distance, thereby reducing the capacitance between the light detector light sensitive surface and the biological tissue.

The base76and cap78engage and may attach to one another using adhesives, mechanical features (e.g., mating male/female pairs), welding, or the like. In the embodiment shown inFIGS. 2-6, the base76and cap78include mating flanges90,92that attach to one another by an adhesive disposed on one or both of the flanges90,92. Each detector housing22is positioned so that at least a portion of the base well82is received within the respective detector aperture40of the pad16. In the embodiment shown inFIGS. 2-6, the near light detector20aand far light detector20bare disposed within independent detector housings22. In alternative embodiments, the bases76of the detector housings22may be connected to one another and/or the caps78of the detector housings22may be connected to one another.

The spacing between and the relative positioning of the near light detector20aand the far light detector20bwithin the sensor assembly12is preferably chosen so that: 1) the light source18, near light detector20aand the far light detector20bare substantially linearly aligned with one another; and 2) the separation distance between the far light detector20band the near light detector20ais greater than the separation distance between the light source18and the near light detector20a. A greater distance between the near light detector20aand the far light detector20b(as compared to the separation distance between the light source18and the near light detector20a) creates a significant difference between the region defined by the mean optical path extending between the light source18and near light detector20a, and the region defined by the mean optical path extending between the light source18and the far light detector20b. As a result, the information representing the contrast of the two signals is greater than it would be if the two mean optical paths were closer to one another.

The following examples illustrate light source18/light detector20spacing for neonate, small adult/pediatric, and adult embodiments of the present sensor assembly12. In an adult NIRS sensor assembly12, the light source18may be positioned in the range of approximately forty-seven to fifty millimeters (47 mm to 50 mm) from the far light detector20band approximately fifteen millimeters (15 mm) from the near light detector20a. In a small adult/pediatric embodiment of the NIRS sensor assembly12, the light source18may be positioned in the range of approximately forty to forty-three millimeters (40 mm to 43 mm) from the far light detector20band approximately twelve millimeters (12 mm) from the near light detector20a. In a neonate embodiment of the NIRS sensor assembly12, the light source18is positioned in the range of approximately twenty-five to thirty millimeters (25 mm to 30 mm) from the far light detector20band approximately ten millimeters (10 mm) from the near light detector20a. In an alternative neonate embodiment of the NIRS sensor assembly12, the light source18may be positioned in the range of approximately twenty-five to thirty millimeters (25 mm to 30 mm) from a single light detector20. The light source18/detector20spacings described above represent examples and the present invention should not be construed to be limited to these examples.

In some embodiments, an EMI shielding material may be applied directly to the interior and/or exterior surfaces of the detector housing walls (e.g., wells, etc.) in addition to or in place of the EMI shielding24that is disposed around the light detectors20. The EMI shielding may be applied to only some detector wall surface, or a sufficient amount of detector wall surface so as to create a Faraday Cage around the light detector20disposed within the housing. The shielding applied to the detector housing walls may include any of the materials (e.g., electrically conductive wire mesh, electrically conductive substrate, etc.) discussed above with regard to the EMI shielding24that is disposed around the light detectors20. The shielding may be applied by processes including printing, adhering, spraying, etc.

The cover26is shaped and positioned on the NIRS sensor assembly12so that the light source18, the detector housings22containing the near and far light detectors20a,20b, and the shielded cable68are disposed between the cover26and the pad16. The cover26preferably consists of a soft pliable material that can be used in a patient environment. Examples of acceptable cover materials include, but are not limited to, vinyl materials, plastic materials and foam materials (e.g., Poron®). The cover26may be attached to the NIRS sensor assembly12in a variety of different ways; e.g., by adhesive, mechanical features, etc. The cover26material preferably blocks light from entering the NIRS sensor assembly12. The cover26may be molded, cast or formed in place over the sensor elements to create a tailored fit.

In preferred embodiments, the NIRS sensor assembly12includes a connector62that allows for attachment and removal of the sensor assembly12from the NIRS system10. The connector62includes a fiber optic coupler and a shielded cable coupler. The fiber optic coupler provides an interface for optically connecting the fiber optics of the NIRS sensor assembly12to the NIRS system10. Similarly, the shielded cable coupler provides an interface for connecting the photodiode output of the NIRS sensor assembly12to the NIRS system10. In some embodiments, the connector62is a hybrid connector that incorporates the fiber optic coupler and the shielded cable coupler together into a single unit. In other embodiments, the connector62includes a fiber optic coupler and a shielded cable coupler that are independent of one another. In those embodiments where the fiber optic coupler and the shielded cable coupler are independent of one another, the two couplers may be located apart from one another; e.g., the fiber optic coupler at the sensor and the shielded cable coupler at a mid point.

In some embodiments, a multi-fiber optic combiner may be used that allows for multiple laser light sources18of different wavelengths to be coupled into a small diameter core fiber optic output leading to the NIRS sensor assembly12. The present invention sensor assembly12does not require the use of a multi-fiber optic coupler, and if one is used the present NIRS sensor assembly12is not limited to using any particular type or make of multi-fiber optic coupler. U.S. Pat. No. 7,047,054, which was earlier incorporated by reference into the present application, discloses an example of an acceptable multi-fiber optic coupler.

The connector62can also include sensor identification encoding means so that the NIRS system10can identify the type of NIRS sensor assembly12connected; i.e., an adult, pediatric, neonate, and other configured sensor. Once the type of sensor12is identified, the NIRS system10can then select appropriate information for use with that sensor12; e.g., calibration information for a specific sensor configuration. Methods of encoding include but are not limited to: 1) setting different resistor values for each differently configured sensor12in which the NIRS system10can measure the resistance value though a voltage divider circuit; 2) incorporating a small memory device, such as a serial PROM, which has sensor identification information stored to be read by the NIRS system10; and 3) including an RF identification device.

According to another aspect of the present invention, a NIRS sensor assembly112as shown inFIGS. 10 and 11may be used with a NIRS system10. The NIRS sensor assembly112may include some or all of the following elements: a flexible electrical circuit114, at least one light detector116, at least one light source118, a connector120, EMI shielding122, a light blocking sheet124, a pad126, a bottom housing128, and a top housing130.

Each light detector116includes a light responsive transducer (e.g., a photodiode) that is operable to sense the intensity of light emitted by the light source118after such light passes through a portion of the subject's body. Each light detector116includes an active region through which impinging light can be sensed. The light detectors116are electrically connected to the NIRS base unit11to enable processing of the output of the light detectors116. In a preferred embodiment, the light detectors116are mounted on a flexible electrical circuit114(as will be described below), which circuit provides the electrical connections between the detectors116and the base unit11. In the sensor embodiments having two or more light detectors116spaced apart from one another and light source118, the light detectors may be referred to as a “near detector116a” and a “far detector116b” (relative to the light source118) as described above.

The light source118is selectively operable to produce infrared light (i.e., light in wavelength range of about 700 nm to about 1,000 nm), and in some embodiments may also produce light in the visible range. In preferred embodiments, the light source118is an assembly that includes a plurality of light-emitting diodes (LEDs), each selected to produce light at a predetermined wavelength. The present invention NIRS sensor assembly112is not limited to use with LEDs, however. As will be described below, the light source118is preferably mounted on a flexible electrical circuit114for electrical communication with the NIRS base unit11.

The detector(s)116and the light source118are preferably mounted on a flexible electrical circuit114(i.e., a “flex circuit”). The flex circuit114may be described as a patterned arrangement of printed wiring (i.e., electrically conductive paths that may, for example, be formed by printing or etching conductive material) mounted relative to a flexible base material. The wiring of the flex circuit114electrically connects the detectors116and the light source118to the connector120. The connector120, in turn, provides the structure that allows the sensor assembly112to be electrically connected to the base unit11; e.g., in signal communication with the base unit11. In the embodiment shown inFIGS. 10 and 11, the flex circuit114is configured so the far detector116bis positioned proximate one end of the flex circuit114, and the near detector116ais spaced apart from the far detector116band is disposed between the far detector116band the light source118. Adjacent the light source118, a lead portion132of the flex circuit114extends outwardly, terminating at the connector120. Examples of acceptable configurations of the relative positioning of the detectors116and the light source118are described above; e.g., the light source118may be positioned approximately forty to forty-three millimeters (40 mm to 43 mm) from the far light detector116band approximately twelve millimeters (12 mm) from the near light detector116a. The sensor assembly112is not limited to any particular detector/light source118spacing configuration, however.

In the embodiment shown inFIGS. 10 and 11, an encapsulation material134is disposed in contact with each detector116. The encapsulation material134encapsulates and protects the detector116, and the connection between the detector116and the flex circuit114. The encapsulating material also provides a dielectric barrier between the patient and the electrical circuit of the sensor. In preferred embodiments, the encapsulating material also encapsulates one or both of the EMI shielding122and light blocking sheet124proximate the respective detector116. The encapsulating material134also acts as an optical interface between the detector116and the subject (when mounted on a subject), and depending upon the type of encapsulating material134used, can also be operable to electrically insulate the subject from the detector116and the flex circuit114. An example of an encapsulating material134is an ultraviolet curable epoxy; e.g., 3525 epoxy made by Loctite or a dielectric film such as FEP tape made by Dupont. In some sensor applications, it may be desirable to use encapsulating material134relative to only one detector116, or not at all.

The sensor assembly112includes EMI shielding122disposed relative to the light detectors116. An optically transparent portion of the EMI shielding122is disposed in alignment with the active regions of the light detectors116. The EMI shielding122may also include a portion disposed around the periphery of one or more of the detectors116, and that portion may or may not be optically transparent. For purposes of this description, the term “optically transparent” may be defined as follows: an optically transparent medium is one through which an amount of light may pass through, which amount is adequate for purposes of a NIRS evaluation under normal operating circumstances for the sensor assembly. The EMI shielding122is preferably directly or indirectly connected to ground, but not necessarily.

As described above, the optically transparent portion of the EMI shielding122may comprise an electrically conductive wire mesh (e.g., copper wire mesh) or it may comprise a thin electrically conductive substrate such as a fiber-filled, conductive adhesive tape; e.g., XYZ-Axis Electrically Conductive Tape 9713 (“9713 Tape”), offered by 3M Company of St. Paul, Minn., USA. The present invention is not limited to using 9713 Tape as an electrically conductive substrate that is optically transparent, and other similar products may be used alternatively. For those embodiments that utilize non-transparent portions of the EMI shielding122, those portions may include an electrically conductive metal foil, such as a copper metal foil. The EMI shielding122may be integrated into the encapsulating material134, or attached to an exposed surface of the encapsulating material134. A significant advantage of the fiber-filled, conductive adhesive tape is that it can be adhered in place during assembly, which greatly facilitates assembly.

The sensor assembly112may further include a light blocking sheet124positioned below, or, on top of the encapsulating material134and the EMI shielding122. The light blocking sheet124includes an aperture136sized to mate with the active region of the detector116with which it is positioned. In the embodiment shown inFIGS. 10 and 11, a light blocking sheet124is positioned relative to the near detector116a. In alternative embodiments, the light blocking sheet124may be disposed relative to either the near or far detector, or both. A preferred embodiment of the light blocking sheet124is a thin flexible black material that has an adhesive backing that facilitates positioning and securing of the sheet124relative to the detector116. A particularly useful light blocking sheet124is one that is also electrically conductive. An example of an acceptable light blocking sheet124is an electrically conductive transfer tape, model ARcare® 90366, manufactured by Adhesives Research, Inc. of Glen Rock, Pa., U.S.A. The light blocking sheet124is not limited to this specific product, however. For those embodiments that utilize an electrically conductive light blocking sheet124, the conductive property of the light blocking sheet124facilitates the effectiveness of the EMI shielding122. A significant advantage of adhesive backed light blocking sheet124is that it can be adhered in place during assembly, which greatly facilitates assembly.

The above-described structure (e.g., the stack up of flex circuit114and detector116, encapsulating material, EMI shielding122, and light blocking sheet124) provides a structure that allows light signals to be sensed, and at the same time reduces undesirable EMI generated noise and improves the signal to noise ratio of the light detectors116. The optically transparent shielding122may be described as providing a Faraday Cage around the light detector116, while allowing light to reach the light-sensitive surface of the light detector116. By reducing the aperture for the photodetector, light shunting from the emitter through the tissue is reduced and the path length for the transmitted light is refined.

The pad126has a patient side surface138, a component side surface140, at least one source aperture142, and at least one detector aperture144. Each detector aperture144is shaped to surround the respective detector116, and the light source aperture142is shaped to surround the light source118. In some embodiments an adhesive is applied to the patient side surface138for attaching the pad126to the subject. The pad126is preferably comprises a material such as that described above (e.g., Poron® cellular urethane foam).

The bottom housing128is disposed on the side of the sensor assembly112that is placed in contact with the subject. The top housing130is positioned on the opposite side of the sensor assembly112. Both housings128,130are preferably flexible and operable to protect the flex circuit114disposed between the housings128,130. The housings128,130are attached to one another to enclose a portion of the flex circuit114. A portion of the top housing130is attached to the pad126to enclose the portion of the sensor assembly112containing the detectors116and the light source118. The housings128,130may be made of a number of different materials, including the foam such as Poron® cellular urethane foam. In preferred embodiments, the housings comprise a synthetic material consisting of high-density polyethylene fibers which is tear resistant, but breathable (e.g., water vapor permeable). Tyvek® brand material, produced by the DuPont company, is an example of a synthetic material consisting of high-density polyethylene fibers that can be used for the housings128,130. The bottom housing128is not adhered to the component side surface140of the pad126, creating a tab. The tab better adheres the sensor assembly112to the skin of the patient.

The connector120is configured to provide electrical/signal communication directly, or indirectly, between the sensor assembly112and the base unit11. In some NIRS systems10, a base unit cable extends out from the base unit11for connection with the sensor assembly112. The base unit cable may include a photodiode preamplifier operable to amplify the signals from the sensor assembly112. In the embodiment shown inFIGS. 10 and 11, the connector120includes a printed circuit board card (“PCB card”) that mates with the base unit cable to form a shielded connection; e.g., a Hirose model LX40-16P, display port, or minidisplay port. The connector120is not, however, limited to a PCB card, however. An example of an alternative type of connector120is an I/O connector. An alternate embodiment for the sensor incorporates a cable extension that is attached between the flex circuit tail and the PCB card or I/O connector. This construction allows the sensor connector to be located remotely from the patient. The sensor assembly embodiment shown inFIGS. 10 and 11shows the sensor112extending along a substantially straight line between the detector/emitter region and the connector120. The present sensor is not limited to a straight configuration. In alternative embodiments, the region shown as straight can include one or more deviations (e.g., bends, or jogs—shown in phantom) that facilitate flexing of that portion of the sensor assembly. The additional flexibility helps to prevent inadvertent detachment of the sensor from the subject during operation of the sensor.

In the operation of the present invention, once the NIRS sensor assembly12,112is positioned relative to the subject's skin, the sensor may be actuated and near infrared light signals introduced into the subject's body tissue. The light introduced into the subject's body tissue is subsequently detected using the near and far light detectors, producing signals representative of such detected light. The signals are relayed back to the NIRS base unit11, where they are processed to obtain data relating to the blood oxygenation level of the subject's body tissue. As stated above, the present invention NIRS sensor assemblies described above are not limited to use with any particular NIRS system10.

Since many changes and variations of the disclosed embodiment of the invention may be made without departing from the inventive concept, it is not intended to limit the invention otherwise than as required by the appended claims. For example, the present invention is disclosed in the context of a cerebral application. The present invention is not limited to cerebral oximetry applications and can be used for non-invasive monitoring of blood oxygenation levels in other body tissues and fluids.