Patent Publication Number: US-2013253333-A1

Title: Tissue interface elements for application of optical signals into tissue of a patient

Description:
TECHNICAL FIELD 
     Aspects of the disclosure are related to the field of medical devices, and in particular, tissue interface elements for application of optical signals into tissue of a patient and optical measurement of physiological parameters of blood and tissue. 
     TECHNICAL BACKGROUND 
     Various devices, such as pulse oximetry devices or photon density wave (PDW) devices, can measure parameters of blood or tissue in a patient, such as heart rate and oxygen saturation of hemoglobin, among other parameters. These devices are non-invasive measurement devices, typically employing solid-state lighting elements, such as light-emitting diodes (LEDs) or solid state lasers, to introduce light into the tissue of a patient. The light is then detected and analyzed to determine the parameters of the blood flow in the patient. 
     In some examples, optical fibers are employed to transfer optical signals between processing and signaling equipment and the tissue of a patient. These optical fibers can often deliver higher quality signals to the tissue than directly introducing optical signals from a light source onto the tissue, in part because optical fibers allow the placement of the optical source (or multiple sources) away from the tissue, enabling the use of higher-quality sources without substantially impacting cost. However, consistent application and detection of the light or other optical signals into the tissue of the patient can be difficult to achieve using optical fibers, especially in examples where long optical fibers are employed. For example, challenges are encountered with introducing optical signals into tissue from an optical fiber routed parallel to the tissue, due in part to the large minimum bend radius of optical fibers. These challenges are accentuated when using optical fibers with a large core radius. Such large core fibers are capable of collecting more light from tissue, the fiber light collection capacity being approximately proportional to the area of the fiber core. However, the larger the core diameter of the fiber requires a larger bend radius. 
     In further examples, measurement and processing systems are located remotely from various optical elements used for interfacing optical signals with the tissue of the patient. This configuration can provide some patient mobility and ease of use for the clinical staff by using a flexible fiber optic cable between the equipment. 
     OVERVIEW 
     Systems and methods for applying optical signals into tissue of a patient are provided herein. In a first example, a tissue interface pad for applying an optical signal to tissue of a patient is provided. The tissue interface pad includes a first surface configured to interface with the tissue of the patient, at least one guide channel disposed within the tissue interface pad and configured to route an input optical fiber carrying the optical signal to a first location in the tissue interface pad, and a second surface at the first location configured to direct the optical signal from the input optical fiber into the tissue through the first surface. 
     In a second example, a tissue interface pad for applying an optical signal to tissue of a patient is provided. The tissue interface pad includes a first surface configured to interface with the tissue of the patient, at least one guide channel disposed within the tissue interface pad and configured to route an input optical fiber carrying the optical signal to a first location in the tissue interface pad, and a second surface configured to reflect the optical signal received from the input optical fiber through an optically transmissive portion of the tissue interface pad to direct the optical signal into the tissue through the optically transmissive portion of the tissue interface pad and the first surface. 
     In a third example, a tissue interface pad for applying an optical signal to tissue of a patient is provided. The tissue interface pad includes a first surface configured to interface with the tissue of the patient, at least one guide channel disposed within the tissue interface pad and configured to route an input optical fiber carrying the optical signal to a first location in the tissue interface pad, and an optical interface element coupled to one end of the input optical fiber at the first location and configured to direct the optical signal received from the input optical fiber toward a second surface. The second surface is configured to reflect the optical signal to direct the optical signal into the tissue through the optical interface element and the first surface. 
     This Overview is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. It should be understood that this Overview is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a system diagram illustrating a system for applying optical signals to tissue of a patient. 
         FIG. 2  is a flow diagram illustrating a method of operation of a system for applying optical signals to tissue of a patient. 
         FIG. 3  is a system diagram illustrating a system for applying optical signals to tissue of a patient. 
         FIG. 4  is a system diagram illustrating a system for applying optical signals to tissue of a patient. 
         FIG. 5  is a system diagram illustrating a system for applying optical signals to tissue of a patient. 
         FIG. 6  is a system diagram illustrating a system for applying optical signals to tissue of a patient. 
     
    
    
     DETAILED DESCRIPTION 
     Various physiological parameters of tissue and blood of a patient can be determined non-invasively, such as optically (e.g., optical spectroscopy). A first example is the use of light at dual wavelengths to perform non-invasive arterial blood oxygen saturation measurements, as is done in pulse oximetry. In another example, optical signals introduced into the tissue of the patient are modulated according to a high-frequency modulation signal to create a photon density wave (PDW) optical signal in the tissue undergoing measurement. Due to the interaction between the tissue or blood and the PDW optical signal, various characteristics of the PDW optical signal can be affected, such as through scattering or propagation by various components of the tissue and blood. The various physiological parameters can include any parameter associated with the blood or tissue of the patient, such as regional oxygen saturation (rSO2), arterial oxygen saturation (Sp02), heart rate, lipid concentrations, among other parameters, including combinations thereof. Another type of optical tissue interaction is a photoacoustic-based interaction, where the light launched into the tissue is absorbed by an appropriate absorber, which causes localized heating and expansion of the absorber and results in acoustic waves that can be detected by an appropriate acoustic sensor. 
     As a first example of a system for measuring a physiological parameter of blood in a patient,  FIG. 1  is presented.  FIG. 1  illustrates system  100 , which includes tissue interface pad  110 , input optical fiber  120 , tissue  130 , and measurement system  140 . Tissue interface pad  110  includes first surface  111  and second surface  112 . In operation, optical signals generated by measurement system  140  are applied to tissue  130  for measurement of a physiological parameter, as indicated by optical signal  125 . In this example, optical signal  125  is applied to tissue  130  via input optical fiber  120  and tissue interface pad  110 . Only the tissue interface pad is shown in the top view in  FIG. 1  to highlight the tissue interface pad, it should be understood the tissue  130  may have been included. Also, although only one optical fiber  120  is shown in  FIG. 1 , in typical examples more than one optical fiber is employed in a parallel configuration to optical fiber  120 . However, the examples shown herein focus on the tissue interface pad and simplify the optical fiber quantity for clarity. 
       FIG. 2  is a flow diagram illustrating a method of operation of system  100  for applying optical signals to tissue of a patient. The operations of  FIG. 2  are referenced herein parenthetically. In  FIG. 2 , tissue interface pad  110  interfaces ( 201 ) with tissue  130  of a patient at first surface  111  of tissue interface pad  110 . First surface  111  couples to biological tissue, namely tissue  130 , to allow for introduction of optical signals into tissue  130 . In further examples, first surface  111  also allows for detection of optical signals propagated through tissue  130 . Tissue interface pad  110  routes ( 202 ) input optical fiber  120  carrying optical signal  125  to first location  113  in tissue interface pad  110  via a guide channel disposed within tissue interface pad  110 . The guide channel can include a groove or channel which holds input optical fiber  120  and terminates an end of input optical fiber  120  at first location  113 . 
     Tissue interface pad  110  directs ( 203 ) optical signal  125  from input optical fiber  120  into tissue  130  through first surface  111  via second surface  112  at first location  113 . Optical signal  125  is received from input optical fiber  120  and reflected or refracted by second surface  112  for eventual introduction into tissue  130 . Different reflection or refraction configurations can be employed for second surface  112 , as discussed below. Advantageously, optical fiber  120  can be placed horizontally along tissue  130  while optical signal  125  can be introduced vertically into tissue  130  via tissue interface pad  110 . 
     In a first example configuration of second surface  112 , tissue interface pad  110  reflects ( 204 ) optical signal  125  using second surface  112  to direct optical signal  125  through first surface  111  and into tissue  130 . Further optical interface elements can be employed to optically couple input optical fiber  120  to second surface  112  and to tissue  130 . For example, a prism can be configured to mate with an end of input optical fiber  120  and couple optical signal  125  into tissue  130 . This prism may be composed of an optically transparent material, such as optical adhesive, plastic, or glass, and may be optically index matched to a material of input optical fiber  120  to allow propagation of optical signal  125  from input optical fiber  120  into tissue  130 . 
     In a second example configuration of second surface  112 , tissue interface pad  110  reflects ( 205 ) optical signal  125  through tissue interface pad  110  itself to direct optical signal through first surface  111  and into tissue  130 . In this second example configuration, at least a portion of tissue interface pad  110  is composed of optically transmissive material and optical signal  125  is directed through this optically transmissive material after being reflected by second surface  112 . Tissue interface pad  110  can be made of a clear or optically transparent material in this second example configuration. This optically transparent material can be optically index matched to the material of input optical fiber  120  to allow propagation of optical signal  125  from input optical fiber  120  through tissue interface pad  110  and into tissue  130 . 
     Although only input optical fiber  120  is shown for simplicity in  FIG. 1 , further optical fibers or links may be included to receive optical signals which have been propagated, reflected, or scattered by tissue  130 . Upon receiving optical signals after propagation through tissue  130 , measurement system  140  may process the detected optical signals to determine various characteristics of the detected optical signals. Physiological parameters of the tissue and patient can then be identified based on the various characteristics of the detected optical signals. 
     Referring back to  FIG. 1 , tissue interface pad  110  comprises a physical structure having a first surface that couples to biological tissue, namely tissue  130 . The first surface includes at least one optical signal emission point and may include at least one optical signal detection point. Tissue interface pad  110  includes a mechanical arrangement to position and hold optical fiber  120  in a generally parallel arrangement to tissue  130 . These mechanical arrangements can include grooves, c-grooves, v-grooves, channels, holes, snap-fit features, or other elements to route input optical fiber  120  to a desired position in tissue interface pad  110 . As shown in  FIG. 1 , tissue interface pad  110  positions an end of input optical fiber  120  at location  113 . Tissue interface pad  110  may be comprised of plastic, foam, rubber, glass, metal, adhesive, or some other material, including combinations thereof. In some examples, tissue interface pad  110  is comprised of optically transmissive materials, such as optically transmissive adhesive, optically transmissive plastic, glass, acrylic glass, polymethyl methacrylate (PMMA), or other materials, including combinations thereof. 
     Measurement system  140  includes optical interfaces, digital processors, computer systems, microprocessors, circuitry, non-transient computer-readable media, user interfaces, or other processing devices or software systems, and may be distributed among multiple processing devices. Measurement system  140  may also include photon density wave (PDW) generation and measurement equipment, electrical to optical conversion circuitry and equipment, optical modulation equipment, and optical waveguide interface equipment. Measurement system  140  also includes light emitting elements, such as LEDs, laser diodes, solid-state lasers, or other light emitting devices and combinations thereof, along with associated driving circuitry. Optical couplers, cabling, or attachments can be included to optically mate light emitting elements to input optical fiber  120 . 
     Tissue  130  is shown in  FIG. 1  as a finger of a patient. It should be understood that tissue  130  can be any tissue portion of a patient, such as a finger, toe, arm, leg, earlobe, forehead, or other tissue portion of a patient. In this example, tissue  130  is a portion of the tissue of a patient undergoing measurement of a physiological blood parameter. The wavelength of signals applied to the tissue can be selected based on many factors, such as optimized to a wavelength strongly absorbed by hemoglobin, lipids, proteins, water, or other tissue and blood components of tissue  130 . 
     Optical fiber  120  comprises an optical waveguide, and uses glass, polymer, air, space, or some other material as the transport media for transmission of light, and can include multimode fiber (MMF) or single mode fiber (SMF) materials. A sheath or loom can be employed to bundle optical fiber  120  together with further optical links for convenience. One end of optical fiber  120  mates with an associated optical driver component of measurement system  140 , and the other end of optical fiber  120  is configured to terminate in tissue interface pad  110  for optically interfacing with tissue  130 . Various optical interfacing elements discussed herein can be employed to optically couple optical signals carried by optical fiber  120  to tissue  130 . Optical fiber  120  may include many different signals sharing the same associated link, as represented by the associated line in  FIG. 1 , comprising channels, forward links, reverse links, user communications, overhead communications, frequencies, wavelengths, phases, modulation frequencies, modulation depths, carriers, timeslots, spreading codes, logical transportation links, packets, or communication directions. 
     Also, although  FIG. 1  illustrates only a single optical fiber  120 , it should be understood that any number of input links and measurement links can be included, as well as any associated optical source and detector equipment. For example, tissue interface pad  110  may route many optical fibers to different physical locations on tissue  130 , and these optical fibers can carry optical signals of different wavelengths. Alternatively, or in addition, tissue interface pad  110  may have measurement links positioned at different distances from input links or positioned over different anatomical structures. 
     The term ‘optical’ or ‘light’ is used herein for convenience. It should be understood that the applied and detected signals are not limited to visible light, and can comprise any photonic, electromagnetic, or energy signals, such as visible, infrared, ultraviolet, radio, x-ray, gamma, or other signals. Additionally, the use of optical fibers or optical cables herein is merely representative of a waveguide used for propagating signals between a transceiver and tissue of a patient. Suitable waveguides would be employed for different electromagnetic signal types. 
       FIG. 3  is a system diagram illustrating an oblique view of system  300  for applying optical signals to tissue of a patient. System  300  is an example of system  100 , although system  100  may use different configurations. System  300  includes tissue interface pad  310 , optical fiber  320 , and optical interface element  322 . A detailed view of the assembly comprising optical fiber  320  and optical interface element  322  is shown in view  301 . Another embodiment of elements of system  300  is shown in view  302 , namely modified optical fiber  325 . Although not required, modified optical fiber  302  can be employed in system  300  instead of the assembly shown in view  301 . 
     Tissue interface pad  310  may be composed of plastic, foam, rubber, glass, metal, adhesive, or some other material, including combinations thereof. Tissue interface pad  310  includes first surface  311 , second surface  312 , and channel  313 . First surface  311  forms a generally planar surface as shown on the ‘top’ of tissue interface pad  310  in  FIG. 3 . First surface  311  is configured to interface with tissue of a patient, not shown in  FIG. 3  for clarity, to allow for introduction of optical signals into the tissue. 
     Second surface  312  comprises an angled surface formed from the material of tissue interface pad  310  at one end of channel  313 . In some examples, second surface  312  is coated with a reflective coating. In other examples, such as when tissue interface pad  310  is composed of a metallic material, second surface  312  is polished to create a reflective surface. The angle of second surface  312  is typically 45 degrees with respect to first surface  311 , or  135  degrees with respect to the longitudinal axis shared by optical fiber  320  and channel  313 . Thus, when optical signals  321  carried by optical fiber  320  are incident upon second surface  312 , then optical signals  321  are directed generally perpendicular to the longitudinal axis shared by optical fiber  320  and channel  313  and through the plane of first surface  311  into the tissue under measurement. In further examples, different angles of second surface  312  can be employed based on the refractive index of the tissue interface pad elements, such as second surface  312  or optical interface element  322 , or to increase reflective efficiency when using higher numerical aperture types of optical fibers. 
     Channel  313  is formed into tissue interface pad  310  to hold optical fiber  320 . In this example, channel  313  cuts through the plane formed by first surface  311 . Other configurations of channel  313  can be employed such as a bore-hole that does not break the plane formed by first surface  311  and sized to hold optical fiber  320 . Also, channel  313  is shown as a generally square groove (in cross-section). Other groove cross-sectional styles can be employed, such as c-grooves, v-grooves, or snap-fit features, among other styles. Channel  313  is typically sized to fit optical fiber  320  securely or tightly. In further configurations, channel  313  can be coated or painted with an optically absorbent or opaque material to prevent optical signals escaping from the length of optical fiber  320  from entering the tissue or other optical fibers if included. 
     Also included in system  300  is optical interface element  322 . Optical interface element  322  is configured to optically couple optical signals  321  carried by optical fiber  320  to second surface  312  and into the tissue under measurement. In this example, one end of optical fiber  320  is mated to optical interface element  322  by abutting optical interface element  322  with a generally flat end. More specifically, this end of optical fiber  320  is typically cut and polished perpendicular to the longitudinal axis of optical fiber  320 . Optical interface element  322  is typically composed of a material that is index matched to the material of optical fiber  320 , such as being composed of a material with a similar index of refraction as optical fiber  320 . Thus, optical signals  321  will generally not be refracted when exiting optical fiber  320  until emerging off of second surface  312 . Once optical signals  321  are refracted by second surface  312 , optical signals  321  are directed into the tissue under measurement through first surface  311 . 
     Optical interface element  322  can comprise a prism composed of glass, plastic, or other optically transmissive material. In some examples, optical interface element  322  is composed of an optical adhesive which is introduced into a void or chamber adjacent to second surface  312 . This optical adhesive, once cured, will then form an index matched transition prism between optical fiber  320 , second surface  312 , and the tissue under measurement. As shown in view  301 , optical interface element  322  has an angled surface  323  which mates with second surface  312 . In some examples, such as when optical interface element  322  comprises a glass or plastic prism, angled surface  323  is coated with a reflective material and forms second surface  312  for reflecting optical signals  321  into the tissue. However, in examples where optical interface element  322  comprises optical adhesive introduced in a liquid state and cured into a void adjacent to second surface  312 , then second surface  312  is typically made reflective by the introduction of optical adhesive and thus angled surface  323  would merely be the surface of optical interface element  322  which is mated to second surface  312 . Surface  323  can be made reflective by using a metallic mirror (e.g. protected silver), which is less reflective but operates over a wide range of wavelengths, or using a dielectric mirror (e.g. a stack of dielectric layers of controlled indices of refraction and thicknesses) which is more reflective at specific wavelengths, among other techniques and materials. 
     In an alternate embodiment, modified optical fiber  325  is shown in view  302 . Modified optical fiber  325  can be composed of a similar material as optical fiber  320 . However, modified optical fiber  325  is shown with an obtusely angled surface  327  located at one end of optical fiber  325  to form a hypotenuse surface. Surface  327  is then polished and typically coated with an optically reflective material. When optical signals  326  are carried by optical fiber  325  and are incident upon surface  327 , optical signals  326  are reflected by surface  327  and directed generally perpendicular to the longitudinal axis of optical fiber  325 . If optical fiber  325  is placed into channel  313  of tissue interface pad  310 , then the angled surface  327  can act as second surface  312  to direct optical signals  326  into tissue under measurement. It should be noted that surface  327  does not have to be coated in all examples. For example, total internal reflection (TIR) techniques can direct the optical signals into the tissue up to a specific critical angle. Most glasses and polymers of indexes around n=1.4-1.5 can properly direct the optical signals using 45 degree cuts in the optical fiber (respect to an associated surface of a tissue interface pad) without coating. TIR can fail if foreign material, such as dirt, oil, or moisture, touches the TIR interface, and thus a reflective coating can be employed to make the system more robust. Other optical beam turners can be employed, such as diffraction elements. 
     The reflective coating employed by either second surface  312  or angled surface  327  can comprise aluminum, silver, dichroic stack, or other materials, including combinations thereof. The optical adhesive employed to form optical interface element  322  or to bond optical interface element  322  to an end of optical fiber  320  can comprise Loctite 3321 or Norland 68 compositions which are cured using ultraviolet (UV) light. Other optically transmissive adhesives can be employed, including combinations thereof. 
       FIGS. 4 and 5  are system diagrams illustrating oblique views of system  400  for applying optical signals to tissue of a patient. System  400  is an example of system  100 , although system  100  may use different configurations. System  400  includes tissue interface pad  410  and optical fiber  420 . A first view  401  of system  400  shows optical fiber  420  inserted in tissue interface pad  410 . A second view  402  of system  400  omits optical fiber  420  for clarity. The hidden lines forming elements of tissue interface pad  410 , such as those in  FIG. 4  defining portions of second surface  412 , transmissive material  415 , or optical index gap  414  are included to highlight some portions of the hidden structure due to the limitations of the oblique view. It is not intended that these lines are exact mechanical representations of the internal wireframe structure, nor do the internal structures necessarily terminate as the dashed lines indicate. 
     Tissue interface pad  410  includes first surface  411 , second surface  412 , groove  413 , optical index gap  414 , transmissive material  415 , and back surface  416 . Tissue interface pad  410  may be composed of an optically transmissive material, and is typically an index matched material to that of optical fiber  420  to prevent reflection or refraction of optical signal  421  at an optical interface of optical fiber  420  and tissue interface  410 . In some examples, tissue interface pad  410  is comprised of optically transmissive adhesive, optically transmissive plastic, glass, acrylic glass, polymethyl methacrylate (PMMA), or other materials, including combinations thereof. Thus, optical signals  321  will generally not be refracted when exiting optical fiber  420  until reflecting off of second surface  412 . In some examples, only a portion of tissue interface pad  410  is composed of an optically transmissive material, such as elements  412  and  415  and any further portions intended to carry optical signal  421 . First surface  411  forms a generally planar surface as shown on the ‘top’ of tissue interface pad  410  in  FIGS. 4 and 5 . First surface  411  is configured to interface with tissue of a patient, not shown in  FIGS. 4 and 5  for clarity, to allow for introduction of optical signals into the tissue. 
     Second surface  412  comprises an angled surface formed from the material of tissue interface pad  410  near one end of groove  413 . In this example, second surface  412  is separated from an end of optical fiber  420  by a small amount of optically transmissive material  415  of tissue interface pad  410 . Second surface  412  is also adjacent to optical index gap  414 . Optical index gap  414  provides a change in optical index to refract optical signal  421  once optical signal  421  is carried through material  415  of tissue interface pad  410 . Thus, when optical signal  421  is transferred from optical fiber  420  through material  415 , second surface  412  will refract optical signal  421  through further optically transmissive material of tissue interface pad  410 . This refraction is shown in  FIG. 4  as optical signal  421  directed ‘upward’ by second surface  412  through tissue interface pad  410  and first surface  411 . 
     Optical index gap  414  can comprise an air-material interface between material of tissue interface pad  410  and the surrounding air. It may be desirable to ensure moisture or other foreign material is not introduced into optical index gap  414 , and protective cladding material can be coated onto surfaces of optical index gap  414  or filled into optical index gap  414  to ensure a desired index of refraction for second surface  412  is maintained. In some examples, a cladding material of lower refractive index is employed in optical index gap  414  to alter the angle required for second surface  412 . In typical examples, the angle of second surface  412  is 45 degrees with respect to first surface  411  of tissue interface pad  410 , or 135 degrees with respect to the longitudinal axis of optical fiber  420 . Thus, when optical signal  421  carried by optical fiber  420  are incident upon second surface  412 , then optical signal  421  is directed generally perpendicular to the longitudinal axis shared by optical fiber  420  and groove  413  and through the plane of first surface  411  into the tissue under measurement. If a lower optical index cladding material is used in optical index gap  414  than that of the material of tissue interface pad  410 , a different angle of second surface  412  can be employed to direct optical signal  421  generally perpendicular to the longitudinal axis of optical fiber  420 . 
     Optical fiber  420  mates with material  415  at a generally flat surface of material  415 . Thus, the end of optical fiber  420  which mates with tissue interface pad  410  is cut and polished generally perpendicular to the longitudinal axis of optical fiber  420 . An optical adhesive can be employed to optically mate the one end of optical fiber  420  to the flat surface of material  415 . Groove  413  is formed into tissue interface pad  410  to hold optical fiber  420 . In this example, groove  413  cuts through the plane formed by first surface  411 . Other configurations of channel  413  can be employed such as a bore-hole that does not break the plane formed by first surface  411  and sized to hold optical fiber  420 . Also, groove  413  is shown as a generally v-shaped groove (in cross-section). The v-groove configuration of groove  413  ensures side-to-side self-alignment of optical fiber  420  within groove  413 . Other groove cross-sectional styles can be employed, such as c-grooves, square grooves, or snap-fit features, among other styles. Groove  413  is typically sized to fit optical fiber  420  securely or tightly. In further configurations, groove  413  can be coated or painted with an optically absorbent or opaque material to prevent optical signals leaking from the length of optical fiber  420  from entering the tissue or other optical fibers if included. Optical fiber  420  can be fastened into groove  413  by an adhesive along the length of optical fiber  420  or by a cover piece or jacket coupled over first surface  411  of tissue interface pad  410 . If a cover piece is employed, and optically opaque material can be used to prevent stray signals from leaving tissue interface pad  410  or from entering into optical fiber  420 . 
     Although this example illustrates second surface  412  directing optical signal  421  ‘upward’ through material  415  of tissue interface pad  410 , other configurations can be employed. For example, second surface  412  can be angled oppositely than as shown in  FIGS. 4 and 5 , thus making the angle of second surface  412  as 135 degrees with respect to first surface  411 , or 45 degrees with respect to the longitudinal axis of optical fiber  420 . Thus, when optical signal  421  carried by optical fiber  420  are incident upon second surface  412 , then optical signal  421  is directed generally perpendicular and to the longitudinal axis shared by optical fiber  420  and groove  413  and ‘downward’ through the plane of back surface  416 . Tissue under measurement can then be coupled to back surface  416  for measurement and application of optical signal  421  which now passes through the body of tissue interface pad  410 . In these further examples, optical fiber  420  is located on an opposite side of tissue interface pad  410  than the surface which is coupled to the tissue, allowing for distance and protection for optical fiber  420  from contact with any tissue. Also, in these further examples, portions of tissue interface pad  410  which optical signals  421  pass through can be composed of an optically transmissive material, and is typically an index matched material to that of optical fiber  420  to prevent reflection or refraction of optical signal  421  at an optical interface of optical fiber  420  and tissue interface  410 . 
       FIG. 6  is a system diagram illustrating tissue interface assembly  600 . Tissue interface assembly  600  includes kayak  610  and optical cable  640 . Kayak  610  is an example of tissue interface pad  110 , tissue interface pad  310 , or tissue interface pad  410 , although these may use different configurations. Kayak  610  is coupled to tissue  630  in this example. Tissue  630  can comprise any tissue described herein, such as a finger of a patient. Optical cable  640  comprises several optical fibers, namely optical fibers  621 - 622 , for carrying optical signals to and from kayak  610 . In  FIG. 6 , several axes are shown for reference purposes. For the top view, a ‘y’ axis is shown relative to the ‘up-down’ page orientation and an ‘x’ axis is shown relative to the ‘left-right’ page orientation. For the end view, a axis is shown in the side view as a thickness of kayak  610 . The end view is sectioned at section cut  635  from the side view. It should be understood the dashed features of  FIG. 6  are merely intended to highlight various elements of system  600 , and are not intended to be exact wireframe representations of the elements of system  600 ; variations are possible. 
     Kayak  610  comprises a surface for contacting tissue  630 . In operation, kayak  610  will lay coincident on tissue  630 . Kayak  610  also comprises two v-channels  611 ,  612  for routing optical fibers  621 - 622  to the locations shown. Each channel is positioned at a specific channel location in the ‘y’ direction, and each channel is routed to a certain length within kayak  610  in the ‘x’ direction. The depth of each channel  611 - 612  in the ‘z’ direction is determined by the thickness of kayak  610 , and the size of each optical fiber or optical interface elements  615 , among other considerations. Surfaces of kayak  610  can be colored dark to minimize optical reflection and stray light. In some examples, kayak  610  is coated or anodized to a dark color, while in other examples kayak  610  is composed of a dark material such as plastic with injected dark pigment. In yet other examples, optically transmissive portions of kayak  610  are not coated dark or composed of dark material. 
     In this example, optical fiber  622  is an input optical fiber for introducing optical signals into tissue  630 . Output optical fiber  621  terminates at a location relative to the input optical fiber  622 . Specifically, the termination point of output optical fiber  621  is located a first distance from the termination point of input optical fiber  622 . Typical spacing between the input optical fiber termination point and the output optical fiber termination points are 5-10 mm for arterial-based tissue measurements, and 30-40 mm for cerebral-based tissue measurements. Advantageously, this spacing arrangement allows the optical fibers to be aligned generally parallel within kayak  610  and thus optical cable  640  is aligned along the length of tissue  630 . This parallel configuration allows for greater repeatability in measurement and consistent coupling of kayak  610  to tissue  630  by reducing perpendicular or normal stresses and forces on the optical fibers and kayak  610 . Although specific spacing and location dimensions are given herein, it should be understood that the dimensions may vary. Also, although tissue interface assembly  600  includes two optical fibers, a different number of optical signals and associated optical fibers can be employed. 
     Kayak  610  also includes optical interface elements  615 . Since the optical fibers transport optical signals parallel to the surface of tissue  630 , a 90 degree optical turn must be established to properly introduce the optical signals into tissue  630  or to properly detect optical signals from tissue  630 . Each optical interface element  615  can comprise a prism, optical adhesive, lens, mirror, diffuser, and the like, to optically couple the associated optical fibers to the tissue under measurement. In some examples, optical interface elements  615  are formed from the material of kayak  610 . The optical interface elements  615  can each be adhered to the associated optical fiber end, such as with optically transmissive glue or other adhesive. 
     Also included in kayak  610  is fin  613 . Fin  613  is an optically opaque separation member disposed between channels  611 - 612  and configured to inhibit optical coupling through kayak  610  between optical fibers  621 - 622 . Fin  613  is disposed between and generally parallel to each of channels  611 - 612 . The material and configuration of fin  613  prevents cross-talk or inadvertent optical coupling between optical signals carried by optical fibers  621 - 622  as well as to prevent cross-talk or inadvertent optical coupling between optical signals introduced into and detected from tissue  630 . In examples of an optically transmissive kayak, such as found in  FIGS. 4-5 , fin  613  can prevent interference and noise associated with multiple optical fibers carried by a single optically transmissive tissue interface pad or kayak. In examples of kayaks with even further optical fibers, further fin elements can be employed between each channel carrying an optical fiber. In further examples, a black insert is placed in the kayak between individual optical fibers, where the black insert is placed into a slit cut into the kayak between ones of the optical fibers. In yet further examples, the slit can be filled with a black epoxy. 
     The included descriptions and drawings depict specific embodiments to teach those skilled in the art how to make and use the best mode. For the purpose of teaching inventive principles, some conventional aspects have been simplified or omitted. Those skilled in the art will appreciate variations from these embodiments that fall within the scope of the invention. Those skilled in the art will also appreciate that the features described above can be combined in various ways to form multiple embodiments. As a result, the invention is not limited to the specific embodiments described above, but only by the claims and their equivalents.