Abstract:
Devices, systems, and methods for measuring tissue oxygen saturation are disclosed. An illustrative spectrometer for interfacing an optical sensor with a display unit includes a number of measurement radiation sources, a number of radiation source fibers each optically coupled to one of the measurement radiation sources, a reflected radiation fiber optically coupled to the optical sensor, a measurement radiation output fiber including an image fiber, and a radiation mixing bar intermediate the radiation source fibers and the measurement radiation output fiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 to U.S. Provisional Application No. 61/256,356, filed on Oct. 30, 2009, entitled “Radiation Resistant Spectrometer Interface,” which is incorporated herein by reference in its entirety for all purposes. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to a near infrared spectrometer for measuring tissue oxygen saturation. 
     BACKGROUND 
     Tissue oxygenation (StO2) is a proven indicator of perfusion status in patients experiencing undifferentiated shock. High-risk patients who receive continuous monitoring of StO2 from the trauma bay through ionizing radiation, such as X-ray and CT imaging, and other procedures to the operating room have been shown to receive effective interventions sooner, resulting in significant reductions in ICU admission, length of stay, morbidity and mortality. 
     Near infrared spectrometer systems are known and reported in, for example, U.S. Pat. No. 5,411,023 to Morris, Sr. et al. and U.S. Pat. No. 6,377,840 to Gritsenko et al. U.S. Pat. No. 5,411,023 discloses an optical sensor system for use on a patient in an MRI or other electrically isolated environment. Control and display modules transmit and receive electrical signals to a remotely located light source and light detector, respectively. Fiber optic cables transmit and receive analog optical signals between the light source/detector and the patient within the electrically isolated environment. Electrical signals from the light detector are transmitted over an electrical cable for analysis by the control unit to determine the patient&#39;s heart rate and oxygen saturation. To insure accurate analysis, the control unit is typically customized to work specifically with the remotely located light source and light detector. The fiber optic cables extend from the patient to outside the electrical field. In this way, attenuation of the analog optical and electrical signals is minimized. The fiber optic cables are susceptible to damage from exposure to radiation procedures such as X-ray and CT imaging. To prevent damage to the fiber optic cables, the patient interface is typically removed from the patient during radiation procedures. 
     U.S. Pat. No. 6,377,840 discloses a spectrophotometric instrument utilizing multiple LED&#39;s to provide measurement radiation at discreet wavelengths. The spectrometer includes an electronics package, a remotely located optical probe for interfacing with the patient measurement site and a probe connector for coupling the optical probe to the electronics package. The electronics package includes a processor/controller and an optical bench for detecting and processing radiation that has been reflected from the measurement site. The probe connector includes the measurement source and reference LED&#39;s; an electrical connector for connecting the LED&#39;s to the electronics package; optical fibers for transmitting measurement and reflected radiation to and from the optical probe; and optical connector ferrules for connecting reference and reflected radiation to the optical bench. The optical bench comprises a series of mirrors, band pass filters and photomultiplier tube sensors. The optical probe which interfaces with the patient measurement site is connected to the probe connector by an optical fiber bundle comprising a single fiber for each of the measurement radiation LED&#39;s and a single fiber for transmitting reflected radiation. To insure accurate control of the measurement radiation LED&#39;s and accurate analysis of the transmitted reflected radiation, the electronics package can be customized to work specifically with the probe connector and optical probe. The optical fiber bundle is susceptible to damage from exposure to radiation procedures such as X-ray and CT imaging. To prevent damage to the fiber bundle, the patient interface is typically removed from the patient during radiation procedures. 
     There remains a need for a spectrometer that is robust to ionizing radiation and provides continuous StO2 monitoring during radiation procedures. To further enhance the usefulness of the spectrometer, any such spectrometer could be compatible with various, generic display units, easy to use, compact, light weight and cost effective to manufacture. 
     SUMMARY 
     The present invention is a compact, StO2 spectrometer for interfacing an optical sensor with a display unit. The spectrometer may be remote from and intermediate the display unit and patient interface. The spectrometer interface may be releasably, electrically connected to the display unit and may be releasably, optically connected to the patient interface. In some embodiments, the spectrometer interface includes a plurality of measurement radiation sources; optics and light guides for conditioning and directing measurement, reference and reflected radiation; photodiodes for receiving reference and reflected radiation; and a processor for controlling the measurement radiation sources and converting reflected radiation signals into StO2 data. 
     While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an StO2 monitoring system incorporating a spectrometer in accordance with an embodiment the present invention. 
         FIG. 2  is cross-sectional view, taken along line  2 - 2  in  FIG. 1 , of an optical cable according to the present invention. 
         FIG. 3  is an exploded perspective view of a spectrometer interface according to an embodiment of the present invention. 
         FIG. 4  is an exploded perspective view of the optics of the spectrometer interface shown in  FIG. 3 . 
         FIG. 5  is a perspective view of the radiation source fiber housing of  FIG. 4 . 
         FIG. 6  is a cross-sectional view, taken along line  6 - 6  in  FIG. 3 , of the radiation mixing housing in accordance with an embodiment of the present invention. 
         FIGS. 7   a  and  7   b  are cross-sectional views, taken along line  7   a - 7   a ,  7   b - 7   b , respectively, in  FIG. 3 , of the fiber holder, photo diode housing, radiation mixing housing and processor board according to an embodiment of the present invention. 
         FIG. 8  is an end view of the attenuator mount opening according to one embodiment of the present invention. 
         FIGS. 9   a ,  9   b  and  9   c  are exploded perspective views of the mixer bar and film couplers according to an embodiment of the present invention. 
         FIG. 10  is a cross-sectional view, taken along line  10 - 10  in  FIG. 7   b , of the conductive gasket according to an embodiment of the present invention. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
       FIG. 1  shows a tissue oxygenation or StO2 monitoring system  8  according to one embodiment of the present invention. The StO2 monitoring system  8  includes display unit  40 , optical sensor  50  and spectrometer interface  10 . Electrical connector  42  releasably connects spectrometer interface  10  to display unit  40  via electrical cable  44 . Optical connector  52  releasably connects spectrometer interface  10  with optical sensor  50  via optical cable  54 . In operation, StO2 monitoring system  8  directs measurement radiation from spectrometer  10  to a patient measurement site via optical cable  54  and optical sensor  50 . Radiation reflected from the patient measurement site is then directed, via optical sensor  50  and optical cable  54 , to spectrometer  10  where it is converted to StO2 data. The StO2 data is then sent, via electrical cable  44 , to display unit  40 . 
     Display unit  40  provides a power source for spectrometer interface  10  and displays StO2 measurement data in various user-defined formats. As shown, display unit  40  includes an LCD screen  41  and user interface touch pads  43 . Display unit  40  may further include an AC power cord, backup battery power source and computer interface ports. An exemplary display unit is the Model 650 Monitor available from Hutchinson Technology of Hutchinson, Minn. 
     In the embodiment shown, optical sensor  50  includes an adhesive coated, fabric skirt or light shield  51  for attaching the sensor to a measurement site of a patient. A cap  53  attaches the sensor head to the skirt  51 . Exemplary optical sensors are disclosed in, for example, U.S. Pat. No. 7,460,897 and U.S. Pat. No. 6,839,583, both of which are herein incorporated by reference in their entirety for all purposes. 
       FIG. 2 , which is a cross section taken along line  2 - 2  in  FIG. 1 , shows an exemplary optical cable  54  comprising at least one send fiber  55  and at least one receive fiber  56 . Send and receive fibers  55 , 56  transmit measurement and reflected radiation, respectively and are optically coupled, via connector  52 , to spectrometer interface  10  at one end and optically coupled to the patient measurement site via sensor  50  at another end. Optical connector  52  includes alignment disk  45  ( FIG. 3 ) and overmold nut assembly  46  which remain with spectrometer  10  upon detachment of sensor  50 . One example of optical connector  52  is disclosed in, for example, U.S. Pat. No. 7,165,893, which is incorporated herein by reference in its entirety for all purposes. An aesthetic flexible covering  57  such as, for example, a thermoplastic polyurethane elastomer surrounds a radiation blocking or radiation shielding covering  58 . Radiation shielding covering  58  surrounds and protects the send and receive fibers  55 . 56  from the damaging effects of external radiation, such as from x-ray and CT imaging procedures. In one embodiment, radiation shielding covering  58  is comprised of a tungsten-filled thermoplastic urethane. Other radiation shielding coverings may comprise, for example, lead, gold, platinum or bismuth. Radiation shielding covering  58  enables optical sensor  50  to be continuously attached to the patient during radiation procedures without compromising the optical properties of the send and receive fibers over the useful life of the sensor. 
     An exploded perspective view of spectrometer interface  10  is shown in  FIG. 3 . In the illustrated embodiment, spectrometer interface  10  includes measurement radiation source board  12 , source optics housing  14 , radiation source fiber housing  16 , processor board  18 , reflected radiation photo diode  17 , reference radiation photo diode  19 , radiation mixing housing  20 , photo diode housing  22  and fiber holder or housing  24 . In one embodiment, radiation source board  12 , housing  14 ,  16 ,  20 ,  22  and fiber holder  24  are essentially serially arranged or located. Alignment pins  25  extend the length of spectrometer  10  and provide for precise alignment of source board  12 , optics, fiber and mixing housings  14 ,  16 ,  20  and fiber holder  24 . In this way, precise alignment and coupling of the measurement radiation optics, reference radiation optics and reflected radiation optics is achieved. 
     Measurement radiation optics ( FIG. 4 ) includes lenses  11   a ,  11   b ,  11   c ,  11   d ,  15   a ,  15   b ,  15   c ,  15   d ; filters  13   a ,  13   b ,  13   c ,  13   d ; fibers  21   a ,  21   b ,  21   c ,  21   d ; mixing bar  30  and output fiber  60 . Reference radiation optics ( FIG. 7 ) includes reference radiation fiber  62  and attenuator  70 . Reflected radiation optics ( FIG. 4 ) includes reflected radiation fiber  76  and ambient light filter  77 . An electrically conductive shield housing  26 , comprised of material providing electromagnetic interference shielding and high yield strength, for example, C1008 ASTM steel, contains and protects the internal components of spectrometer interface  10 . Retainer  27  guides and supports electrical cable  44  and encloses the proximal end of spectrometer  10 . Alignment pins  25  include threaded ends  37  that protrude from the proximal end of retainer  27 . Upon assembly, nuts (not shown) are fastened on to threaded ends  37  of alignment pins  25  compressing the spectrometer components together to insure optical coupling between measurement radiation, reference radiation and reflected radiation optics. A strain relief boot  28  provides additional support for electrical cable  44  and seals proximal end of spectrometer  10 . 
     While the illustrated embodiment shows two pins aligning source board  12 , housings  14 ,  16 ,  20  and fiber holder  24 , other embodiments with a different number and/or pin configuration are also contemplated. For example, 3 or more alignment pins may be used. In still other embodiments, not shown, 2 or more alignment pins may be used to align fewer components. For example, pins  25  may align only the optics housing  14 , fiber housing  16 , mixing housings  20  and fiber holder  24 . In the illustrated embodiment, alignment pins  25  have a circular cross-section though other cross-sectional shapes such as, for example, square, hexagonal, triangular or octagonal are contemplated as well. 
     As shown, shield housing  26  is open at its distal end  38  and substantially closed at its proximal end  39 . In the embodiment shown, proximal end  39  includes openings for electrical cable  44  and alignment pins  25 . Shield housing may have a circular cross section and, in one embodiment, is approximately 20-25 mm in diameter and approximately 50-60 mm long. Shield housing  26  may also have a generally square, rectangular, triangular or other cross-sectional geometry. Though shield housing  26  is shown to have an essentially constant cross sectional shape and size along its length, the cross sectional shape and size may vary between the housing&#39;s proximal end  39  and distal end  38 . For example, housing  26  may have a smaller diameter at its distal end  38  and a larger diameter at its proximal end  39 , or housing  26  may have a cross sectional shape that transitions from rectangular at its proximal end to circular at its distal end. In the embodiment shown, source board  12 , source optics housing  14 , radiation source fiber housing  16 , processor board  18 , radiation mixing housing  20  and photo diode housing  22  are sized and shaped to fit within shield housing  26 . 
     By grounding distal end  38  of shield housing  26 , a Faraday cage or Faraday shield is created which protects the internal components of spectrometer  10  from electromagnetic interference. In the embodiment shown, distal end  38  of shield housing  26  is grounded via conductive gasket  48  electrically coupling fiber holder  24  and shield housing  26 . In one embodiment, shown in  FIG. 10 , conductive gasket  48  is a length of conductor-filled elastomer such as, for example, silver/copper-filled silicone from Laird Technologies of Chesterfield, Mo., joined at the ends by a conductive pin  48   a . In the illustrated embodiment, pin  48   a  is a gold plated brass pin such as, for example, part number YPN005-001H from Hypertronics Corporation of Hudson, Mass., though any suitable conductive material such as, for example, copper, aluminum, silver or stainless steel plated with nickel and gold may be used. In other embodiments, conductive gasket may be a preformed ring or otherwise joined at the ends by, for example, adhesive or thermal fusing. To facilitate sealing and electrical connection at distal end  38 , gasket may have a generally circular, “D” shaped, rectangular, ovoid or other cross-sectional shape. Other ground connections, for example, an electrical connector tab or a conductive plate or washer between the photo diode housing  22  and fiber holder  24 , are also contemplated. 
       FIG. 4  is an exploded perspective view showing details of the optics of the spectrometer interface  10 . In the embodiment shown, source board  12  includes four light emitting diodes  31   a ,  31   b ,  31   c ,  31   d  emitting near-infrared measurement radiation generally centered at wavelengths of 680 nm, 720 nm, 760 nm and 800 nm respectively. The emitted radiation from each LED  31   a ,  31   b ,  31   c ,  31   d  passes through source optics to collimate and direct the radiation from each LED onto a radiation source fiber optic  21   a ,  21   b ,  21   c ,  21   d  within radiation source fiber housing  16 . The source optics, mounted within source optics housing  14 , include, for each LED, a collimating lens  11   a ,  11   b ,  11   c ,  11   d , a band-pass filter  13   a ,  13   b ,  13   c ,  13   d  and a focusing lens  15   a ,  15   b ,  15   c ,  15   d . The band-pass filters control the emitted radiation to 10 nm full width-half max. Optics housing  14  may be made from, for example, 6061-T6 aluminum to provide a light weight, rigid structure. 
     As shown in  FIG. 5 , source radiation fiber housing  16  is comprised of proximal source fiber terminator plate  23  and distal source fiber terminator plate  29 . Terminator plates  23 ,  29  may be made from, for example, 6061-T6 aluminum to allow for precise machining of critical features while providing a light weight, rigid structure. An input end of each radiation source fiber  21   a ,  21   b ,  21   c ,  21   d  is optically coupled to a focusing lens  15   a ,  15   b ,  15   c ,  15   d  via fiber ferrules or terminators  23   a ,  23   b ,  23   c ,  23   d . To enable the compact size of spectrometer  10 , optical coupling between the LED sources  31   a ,  31   b ,  31   c ,  31   d  and radiation mixing bar  30  ( FIG. 4 ) is provided by routing the output end of each source fiber bundle in a relatively short “S” shaped path to fiber ferrule or terminator  35 . Fiber terminators  23   a ,  23   b ,  23   c ,  23   d ,  35  may include grooves (not shown) to provide a mechanical interlock and improve the adhesive retention between source fibers  21   a ,  21   b ,  21   c ,  21   d  and fiber terminators  23 ,  29 . In some embodiments, source fibers  21   a ,  21   b ,  21   c ,  21   d  are 0.75 mm diameter image fibers, part number MBI-750 from Asahi Kasei Corporation of Tokyo, Japan. Image fibers, fused bundles of 7400 individual fibers, provide optimal bend radius to fiber diameter ratio, minimize radiation leakage at the bend locations and provide sufficient angular distribution and spatial uniformity at the output ends. In other embodiments, source fibers may be clad solid core fibers such as PGR-FB750 from Toray Industries of Tokyo, Japan. 
     As further shown in  FIGS. 4 ,  9   a ,  9   b  and  9   c , the output end of each source fiber  21   a ,  21   b ,  21   c ,  21   d  is optically coupled to the input end  30   a  of a mixing bar  30  via a thin film coupler  32 ,  32   a ,  32   b . The output end  30   b  of mixing bar  30  is optically coupled to a measurement radiation output fiber  60  and a reference fiber  62  via a thin film coupler  34 ,  34   a ,  34   b . Film couplers prevent instabilities in optical transmission that may be caused by interference effects. 
     In one embodiment, film couplers  32 ,  34  may be approximately 0.125 mm thick and may be stamped or otherwise formed from a silicone rubber sheet having a durometer of 50 Shore A such as part number 87315K61 from McMaster-Carr of Elmhurst, Ill. The semi-rigid film couplers  32 ,  34  provide a number of advantages over commonly known adhesive coupling materials. Semi-rigid film couplers provide an optical coupling surface free from air bubbles (surface wetting) and are not subject to squeeze-out during assembly, shrinkage during curing, creep deformation or cracking over time. 
     In another embodiment, film couplers  32   a ,  34   a  may be formed from cast-in-place silicone paste such as, for example, TSE 392C from Momentive Performance Materials of Albany, N.Y. Cast-in-place film couplers  32   a ,  34   a  may be surrounded by optical shims  49 ,  59 , respectively to control the thickness and location of couplers  32   a ,  34   a . Shim  49  may be sized and shaped to at least partially surround the optical interfaces of mixer bar  30  and source fibers  21   a ,  21   b ,  21   c ,  21   d . Shim  59  may be sized and shaped to at least partially surround the optical interface of mixer bar  30  and measurement radiation output fiber  60  and a reference fiber  62 . Shims  49 ,  59  may be made from 300 series stainless steel having a thickness of 0.125 mm. The cast-in-place couplers may be formed by applying silicone paste at the optical interfaces of mixer bar input and output ends  30   a ,  30   b  such that, during assembly of spectrometer  10 , couplers  32   a ,  34   a  solidify to a thickness defined by shims  49 ,  59 . 
     In yet another embodiment, air-film couplers  32   b ,  34   b  are formed at the optical interfaces of mixer bar input and output ends  30   a ,  30   b  by optical shims  49   a ,  59   a  via openings  104 ,  106 . Shims  49   a ,  59   a  may be made from 300 series stainless steel having a thickness of 0.125 mm and may be sized and shaped to approximately match and provide even clamping for the end faces of mixing housing  20 . 
     Mixer  30  is made from, for example, SF 11 glass, Schott North America Inc. of Duryea, Pa., and is sized and shaped to equally distribute the intensity of the measurement radiation output. Schott SF 11 glass, or equivalent, is used because, among other things, it does not degrade in medical x-ray environments and it provides desirable optical (transmittance, refractive index, dispersion), mechanical and thermal properties for the range of wavelengths employed. In the embodiment shown, mixer  30  has an essentially constant, rectangular cross-section along its length. Other polygonal cross-sectional shapes such as, for example, triangular, circular, oval, trapezoidal and/or octagonal may also be used. The cross-sectional shape of mixer  30  may also vary between input and output ends  30   a ,  30   b , respectively. 
     Mixer  30  is enclosed and approximately centered within a radiation mixing housing  20 . In some embodiments, mixing housing  20  is made of Kovar® alloy, which has a coefficient of thermal expansion matched to the mixer bar glass. As shown in  FIG. 6 , taken along line  6 - 6  in  FIG. 3 , mixer bar  30  is mounted in the housing  20  with a sphere or bubble-filled epoxy  36 . The epoxy, for example, Epo-Tek 301, Epoxy Technology of Billerica, Mass., has a lower numerical aperture than mixer bar  30  to maintain total internal reflection of the measurement radiation within the mixer bar. The bubbles, for example, 3MTM iM30K Hi-Strength Glass Bubbles, 3M energy and Advanced Materials Division of St. Paul, Minn., have an average diameter of 18 um each and provide a relatively solid, thin, uniform epoxy bond line to prevent the mixer bar  30  from contacting the inner walls  33  of the mixing housing  20  and to maintain alignment (maximize radiation transfer) between the mixer bar  30  and adjacent optics. Other materials such as, for example, plastic spheres or bubbles can be incorporated into the epoxy as well. The thin, uniform epoxy bond line provides for a stronger, essentially tension-free bond between the square bar mixer  30  and the housing walls  33 . A thicker, non-uniform bond line may be more susceptible to failure and experience more shrinkage during curing. In some cases, a failed bond line could cause shifting of the mixer bar off-optical axis or create air gaps resulting in undesirable Fresnel effects at the optical interfaces. Tension in the mixer bar caused from increased shrinkage of a thicker bond line could result in stress cracks in the mixer bar and undesirable effects on the optical throughput. 
     Cross-sectional views, taken along line  7   a - 7   a  and  7   b - 7   b  in  FIG. 3 , of the fiber holder or housing  24 , photo diode housing  22 , radiation mixing housing  20  and processor board  18  are shown in  FIGS. 7   a  and  7   b . As shown, fiber holder  24  is comprised of proximal fiber block  61  and distal fiber block  63 . Distal fiber block  63  includes output terminator or ferrule  68  and input terminator or ferrule  73 . Ferrules  68 ,  73  optically interface, via connector  52 , with send and receive fibers  55  and  56 , respectively, of sensor  50 . Reference radiation attenuator  70  and reference radiation photo diode  19  are supported within photo diode housing  22 . Measurement radiation output fiber  60  and reference radiation fiber  62  are optically coupled to mixing bar  30  via film coupler  34  and straight wall guiding aperture  64 . Reference radiation fiber  62  is optically coupled to attenuator  70  via feedback aperture  67  and straight wall guiding aperture  66 . 
     Output fiber is guided to output ferrule  68  via straight wall aperture  64 , grooved aperture  65  and output aperture  92 . Reference fiber  62  is guided to attenuator  70  via straight wall aperture  64 , grooved aperture  65 , feedback aperture  67 , grooved aperture  69  and straight wall aperture  66 . Straight wall aperture  64  serves to maintain perpendicularity (parallelism to optical axis) of measurement radiation output fiber  60  and reference radiation fiber  62  to mixing bar  30 . Apertures  64 ,  65  may have an approximately tear-dropped shaped cross-section to accommodate both fiber bundle  60  and fiber  62 . Straight wall guiding aperture  66  serves to maintain perpendicularity (parallelism to optical axis) of reference radiation fiber  62  to attenuator  70 . Grooved apertures  65  and  69  provide a mechanical interlock to improve the adhesive retention of fibers  60 ,  62  within proximal fiber block  61 . In the illustrated embodiment, grooved apertures  65 ,  69  have generally annular grooves  65   a ,  69   a , respectively. In other embodiments (not shown), grooved apertures may have, for example, generally helical or linear grooves. To provide rigidity and support during assembly, precise machining of critical features and an electrically conductive path to shield housing  26 , proximal fiber block  61  may be made from, for example, 6061-T6 aluminum. To provide an optically flat-black, electrically insulating component while allowing precise machining of critical features, distal fiber block  63  may be made from, for example, RS500 Radel® polyethersulfone. Making distal fiber block  63  out of an optically flat-black material minimizes stray light interference with measurement, reference and reflected radiation optics. 
     In some embodiments, output fiber  60  is a 1.5 mm diameter image fiber, part number MBI-1500 from Asahi Kasei Corporation of Tokyo, Japan and reference fiber  62  is a 0.25 mm diameter solid core fiber. In certain embodiments, output fiber  60  is a fused bundle of 7400 individual fibers and preserves the spatial uniformity of the radiation output from mixing bar  30  while providing room for reference fiber  62  within aperture  64 . In other embodiments, not shown, output fiber  60  may be a clad solid core fiber such as, for example, PGR-FB1500 from Toray Industries of Tokyo, Japan. 
     Reference fiber  62  directs approximately 3% (reference radiation) of the output radiation from mixing bar  30  to reference photo diode  19 . So that the intensity of the reference radiation is comparable with the intensity of the radiation reflected from the patient measurement site, the reference radiation is attenuated by attenuator  70 . As shown, attenuator  70 , within photo diode housing  22 , includes scattering media  71  and attenuating pin-hole aperture  72 . Scattering media  71  is mounted in attenuator mount opening  91  and includes radiation input end  71   a  and radiation output end  71   b . Scattering media should be thermally stable, hygrothermally stable and insensitive to differences in angular distribution from the various wavelengths of measurement radiation. Scattering media may be, for example, an optical grade, diffuse reflectance, thermoplastic resin having a relatively flat spectral distribution such as, for example, Spectralon® reflectance material from Labsphere of North Sutton, N.H. Other materials such as, for example, a silicon dioxide-epoxy material may also be used for scattering media. 
     As shown in  FIG. 8 , which is an end view of opening  91  and input end  71   a  of scattering media  71 , opening  91  may have an approximately circular cross-section and includes relief areas  100 ,  102 ,  104  such that cylindrically shaped scattering media  71  is frictionally supported along side surface locations  93 ,  95 ,  97  and surrounded by air gaps  99 ,  101 ,  103 . By using a line-on-line friction or press fit to mount scattering media  71 , the need for other mounting means such as epoxy is eliminated. During heating and cooling cycles, epoxy can tend to shrink causing an inconsistent optical interface between scattering media  71  and the walls of opening  91 . Air gaps  99 ,  101 ,  103  create a stable, consistent optical interface between scattering media  71  and the walls of opening  91  and thus more consistency in the reference radiation directed to photo diode  19 . 
     While opening  91  is shown to have an approximately circular cross-section with relief areas  100 ,  102 ,  104  and scattering media  71  is shown to have a substantially circular cross-section, other sizes and shapes that provide a line-on-line or reduced-contact-area press fit mounting and air gaps are contemplated. For example, scattering media  71  and opening  91  may both have substantially rectangular or circular cross-sections with small, equidistantly-located protrusions providing the reduced-contact-area press fit mounting and air gaps. Alternatively, opening  91  may have a substantially rectangular or triangular cross-section and scattering media  71  may have a substantially circular cross-section or opening  91  may have a substantially circular cross-section and scattering media  71  may have a substantially rectangular or triangular cross-section. 
     Attenuating pin-hole aperture  72  serves to further attenuate the intensity of the reference radiation delivered to reference photo diode  19 . Aperture  72  may have a cross-sectional area that is less than the cross-sectional area of scattering media  71  and less than the active area of photo diode  19 . In the embodiment shown, pin-hole aperture  72  is approximately 0.75 mm, though larger or smaller apertures are contemplated. Photo diode housing  22  is made from, for example, R5500 Radel® polyethersulfone, which provides an electrically insulating, optically flat-black component that allows for precise machining of critical features. 
     As shown in  FIG. 3 , reflected radiation photo diode  17  and reference radiation photo diode  19  are mounted on a distal end edge  74  of processor board  18 . Measurement radiation transmitted into and reflected from the patient measurement site (reflected radiation) is directed to reflected radiation photo diode  17  via receive fiber  56 , reflected radiation fiber  76  and ambient light filter  77  ( FIG. 4 ). Reflected radiation fiber  76  is, for example, a 1.5 mm diameter solid core fiber and is guided to photo diode  17  via input aperture  94 , grooved aperture  96  and straight wall aperture  98  ( FIG. 7   a ). Grooved and straight walled apertures  96 ,  98  are similar to apertures  65 ,  69  and  64 ,  66 , respectively, described above with respect to output fiber  60  and reference fiber  62 . Ambient light filter  77  filters radiation outside of the measurement radiation range between about 680 nm and 800 nm and also prevents saturation of photo diode  17 . 
     Photo diodes  17 ,  19  convert reflected and reference radiation to electrical current signals. Processor board  18  converts the reflected radiation electrical signals into StO2 data and uses the reference radiation electrical signal to compensate for variation in measurement radiation due to, for example, degradation of the source LED&#39;s  31   a ,  31   b ,  31   c ,  31   d . StO2 data can be stored on processor board  18  or directly displayed on display unit  40 . The electrical signals from the reflected radiation may be converted to StO2 data using the algorithm disclosed in U.S. Pat. No. 5,879,294, which is incorporated herein by reference in its entirety for all purposes. Additionally, the electrical signals from the reflected radiation may be converted to a tissue hemoglobin index (THI) measurement using the algorithm disclosed, for example, in U.S. Pat. No. 6,473,632, herein incorporated by reference in its entirety for all purposes. 
     Spectrometer  10  further includes sensor contact sockets  78 ,  80  protruding through openings  86 ,  87  in fiber block  63  and electrically connected to board  18  via power and ground wires  79 ,  81 . Contact sockets  78 ,  80 , such as part number YSK006-010AH from Hypertronics Corporation of Hudsen, Mass., are accessible through opening  47  of alignment disk  45  to engage with sensor pins connected to an integrated circuit chip (not shown) within the sensor-end of optical connector  52 . The integrated circuit chip can be preprogrammed with, for example, calibration data, encryption information and/or a sensor-use timing counter. Processor board  18  reads preprogrammed information from and writes probe use time to sensor circuit chip. In one embodiment the sensor circuit chip and processor board  18  may be programmed to send a sensor-use time signal to display unit  40 . To provide for multiple, reliable connection and disconnection between sensor  50  and spectrometer  10  without damaging the sensor pins, contact sockets  78 ,  80  must freely float within opening  47 . 
     To prevent fluid ingress and debris from damaging the sensitive electrical and optical components within spectrometer  10 , contact sockets  78 ,  80  and opening  47  are sealed by boot  82 . Seal boot  82  includes a base  84  and socket boots  81 ,  83 . Seal boot  82  is sandwiched between alignment disk  45  and fiber holder  24  such that base  84  seats within a recess  85  of distal fiber block  63  and socket boots  81 ,  83  fit over and seal contact sockets  78 ,  80 . As shown, socket boots  81 ,  83  are generally conically shaped and have curved, tapering side walls. Socket boot shapes such as cylindrical, rectangular or conical with straight side walls could also be used. Boot  82  can be made from, for example, silicone having a durometer of 45 Shore A, and may provide a flexible, impermeable seal that allows contacts  78 ,  80  to float within opening  47 . 
     To enable the compact size and shape of spectrometer  10 , source radiation fiber housing  16 , processor board  18 , mixing housing  20  and photo diode housing  22  are designed to be nested together. Proximal source fiber terminator  23  of fiber housing  16  includes tabs  88  ( FIG. 5 ) for engaging and supporting proximal end  75  of processor board  18 . In this way, fiber housing  16  overlaps approximately one-half to two-thirds of board  18 . Mixing housing  20  overlaps the remaining one-third to one-half of board  18  and is supported by alignment pins  25  via alignment apertures  89  ( FIG. 6 ). Photo diode housing  22  includes aperture  90  ( FIG. 3 ) through which mixing housing  20  slidably passes to provide optical coupling, via coupler  34 , between mixing bar  30  and measurement radiation output fiber  60  and reference radiation fiber  62 . 
     In one example of the operation of StO2 monitoring system  8 , the LED&#39;s  31   a ,  31   b ,  31   c ,  31   d  are sequentially energized to transmit measurement radiation from source board  12 , one wavelength at a time, through spectrometer optics and send fiber  55  to a patient measurement site. Exemplary measurement sites may include, for example, the thenar eminence and/or the deltoid muscle. Radiation reflected from the tissue within the measurement site is transmitted back through receive fiber  56  and reflected radiation fiber  76  to photo diode  17  and processor board  18  where an absorbance value is calculated. The foregoing send-receive-absorbance calculation process is repeated for each of the four measurement radiation wavelengths. A ratioed second derivative absorbance value is calculated from the measured absorbance values and compared with predetermined stored data correlating ratioed second derivative absorbance values with StO2 values. In this manner, StO2 values are calculated and either stored on processor board  18  or displayed by display unit  40  every two seconds. 
     Advantages provided by this invention include providing physicians with a portable StO2 spectrometer that remains attached to the patient and provides continuous monitoring of StO2 during transport to various treatment locations within the hospital. Spectrometer interface  10  and optical cable  54 , being robust to ionizing radiation, allow for continuous monitoring of StO2 during radiation procedures such as X-ray and CT imaging. Because spectrometer  10  is essentially self-contained, it can be used with different display units throughout the hospital. For example, display unit  40 , such as the Model 650 InSpectra StO2 Monitor from Hutchinson Technology, Hutchinson, Minn. may be connected to spectrometer  10  during X-ray while another display unit such as the IntelliVue MP90 from Phillips Healthcare, Andover, Mass. may be connected to spectrometer  10  in a recovery room. 
     Although the present invention is described and shown with reference to the illustrated embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention. For example, fewer than or greater than four wavelengths of measurement radiation may be employed such as disclosed in, for example, published U.S. Patent Publication No. 2005/0277818, which is incorporated herein by reference in its entirety for all purposes. Other patient interface optical sensor and/or display unit designs may also be used with the spectrometer interface. For example, a clip-on or spot check-type optical sensor and/or a portable or hand held display unit may be used.