Patent Publication Number: US-2013253332-A1

Title: Tissue interface systems 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 systems 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. 
     However, consistent application and detection of the light or other optical signals into the tissue of the patient can be difficult to achieve. For example, conventional devices typically include a hinged spring mechanism to clamp over a finger of a patient. These spring clamp devices are subject to patient-specific noise and inconsistencies which limits the accuracy of such devices. For example, the size of the tissue under measurement can vary from one patient to another, such as in examples of finger-based measurements. Clamp-style devices are thus typically limited in the ranges of patient tissue sizes, and thus cannot provide a consistent application of the optical signals into the tissue due to these varying tissue sizes. 
     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 by using a flexible fiber optic cable between the equipment. However, having a long cable can introduce errors into the measurement and subsequent processing of the optical signals due to various mechanical stresses and tensions due to the long cables. 
     Overview 
     Systems and methods for applying optical signals into tissue of a patient are provided herein. In one example, a tissue interface system for applying optical signals to tissue of a patient is provided. The tissue interface system includes a tissue interface pad configured to apply the optical signals carried by at least one optical source into the tissue, and a pressurized volume configured to apply pressure to the tissue interface pad to couple a portion of the tissue interface pad to the tissue. 
     In a second example, a method for applying optical signals to tissue of a patient is provided. The method includes applying the optical signals carried by at least one optical fiber into the tissue by a tissue interface pad, and applying a pressure from a pressurized volume to the tissue interface pad to couple a portion of the tissue interface pad to the tissue. 
     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. In one 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 (SpO2), heart rate, lipid concentrations, among other parameters, including combinations thereof. 
     As a first example of a system for applying optical signals to tissue of a patient,  FIG. 1  is presented.  FIG. 1  illustrates system  100 , which includes tissue interface pad  110 , optical cable  120 , tissue  130 , and pressurized volume  140 . A top view, end view, and side view of system  100  are included in  FIG. 1  to highlight the various elements of system  100 . The end view is sectioned at section cut  135 . It should be understood the dashed features of  FIG. 1  are merely intended to highlight various elements of system  100 , and are not intended to be exact wireframe representations of the elements of system  100 ; variations are possible. 
     In operation, optical signals generated by measurement system  180  are applied to tissue  130  for measurement of a physiological parameter, as indicated by optical signals  125 . In this example, optical signals  125  are applied to tissue  130  via input optical fiber  121  terminated at location  111  of tissue interface pad  110 , and optical signals  125  are detected through tissue  130  via output optical fiber  122  terminated at location  112  of tissue interface pad  110 . Pressurized volume  140  is configured to apply a pressure to tissue interface pad  110  to couple at least a portion of tissue interface pad  110  to tissue  130 . 
       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  applies ( 201 ) optical signals  125  carried by at least one optical source into tissue  130 . Tissue interface pad  110  couples to biological tissue, namely tissue  130 , to allow for introduction of optical signals  125  into tissue  130 . Tissue interface pad  110  also allows for receipt of optical signals propagated through tissue  130 . Tissue interface pad  110  routes input optical fiber  121  carrying input optical signals  125  to first location  111  in tissue interface pad  110  via a first guide channel disposed within tissue interface pad  110 . Tissue interface pad  110  routes output optical fiber  122  carrying received optical signals  125  to second location  112  in tissue interface pad  110  via a second guide channel disposed within tissue interface pad  110 . 
     Pressurized volume  140  applies ( 202 ) a pressure to tissue interface pad  110  to couple a portion of tissue interface pad  110  to tissue  130 . Although not required,  FIG. 1  shows pressurized volume  140  receiving the pressure over pressure link  141 . The pressure applied to tissue interface pad  110  allows for repeatable and controlled coupling of optical signals  125  to and from tissue  130  via tissue interface pad  110 . The portion of tissue interface pad  110  which is coupled to tissue  130  includes at least locations  111 - 112  of tissue interface pad  110 , which allows for optical signals carried by input optical fiber  121  to be introduced into tissue  130  and for optical signals propagated through tissue  130  to be received by output optical fiber  122 . Although pressurized volume  140  is shown as an encircling pressure volume resembling a cuff in  FIG. 1 , other configurations can be employed, such as those illustrated in  FIGS. 3-6 . 
     Application of the pressure to pressurized volume  140  can occur through pressure link  141 , although other configurations can be employed. Pressure link  141  can couple a pressure application device, such as a piston, syringe, pump, or other pressure application device to pressurized volume  140  via a tube or piping. Pressure sensors can be coupled to pressurized volume  140  to relay a presently applied pressure to a pressure control system or an operator for modification or monitoring of the pressure. In yet further examples, a quality of optical signals  125  is monitored, such as a magnitude of a pulsatile signal component detected over output fiber  122 . The applied pressure may then be modified to ensure a desired quality of optical signals  125  in tissue  130 . Upon receiving optical signals over optical fiber  122  after propagation through tissue  130 , measurement system  180  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 surface that couples to biological tissue, namely tissue  130 . The surface includes at least one optical signal emission point  111  and may include at least one optical signal detection point  112 . Tissue interface pad  110  includes a mechanical arrangement to position and hold optical fibers  121 - 122  in a generally parallel arrangement to tissue  130 . These mechanical arrangements can include grooves, channels, holes, snap-fit features, or other elements to route optical fibers  121 - 122  to a desired position in tissue interface pad  110 . 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 plastic, glass, acrylic glass, polymethyl methacrylate (PMMA), or other materials, including combinations thereof. Optically transmissive adhesives can also be employed in tissue interface pad  110 , such as to mate optical fibers  121 - 122  to optical interface elements of tissue interface pad  110 . These optical adhesives 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. In yet further examples, tissue interface pad  110  is thermally welded or otherwise adhered or attached to pressurized volume  140  to form an integrated tissue interface pad with pressurized volume. 
     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, or other tissue and blood components of tissue  130 . 
     Pressurized volume  140  comprises an inflatable vessel for containing and applying a pressure to an external component, such as tissue interface pad  110 . Examples of pressurized volume  140  include pressure cuffs, pressure pads, balloons, pistons, chambers, or other volumes which can receive and maintain a pressure via application of a pressurized fluid such as air to the volume. In some examples, pressurized volume  140  is integrated with a bandage configured to couple to tissue  130 . The bandage can be made out of plastic, vinyl, PVC, plastic resin-filled paper, or other materials which allow the bandage to be flexible enough to conform to wrapping around tissue  130 , such as a finger. The bandage is typically compatible to thermal welding of a thin and flexible cuff material. The cuff material can be in sheet form and thermally welded to the bandage such that the welding edges make an air-tight or pressure seal to form pressurized volume  140 . It should be understood that although a wrapped or circular volume is discussed, other volumes styles can be employed. However, due to the pressure applied to tissue interface pad  110  and tissue  130 , an equal and opposite force is typically also applied to an opposing side of tissue  130 , such as by using a bandage/cuff that wraps around tissue  130  and applies the pressure around tissue  130  (as shown in  FIG. 1 ), or by a structural or casing element coupled to tissue  130  and pressurized volume  140  which can include preloading elements. Further examples are discussed herein for  FIGS. 3-6 . Sizing of pressurized volume  140  or an associated bandage/cuff is typically driven by a size of tissue interface pad  110  to minimize overlap and movement of tissue  130  on application of the pressure and measurement. Velcro or adhesive can be placed on the bandage/cuff so pressurized volume  140  can be wrapped around tissue  130  and securely held from slippage or changing in orientation, as well as to allow for adjustment for different sizes of tissue  130 . In yet further examples, tissue interface pad  110  is thermally welded or otherwise adhered or attached to pressurized volume  140  to form an integrated tissue interface pad with pressurized volume. 
     In alternative configurations, pressurized volume  140  comprises a static pressure element. The static pressure element can include a spring, foam pad, rubber pad, or other static pressure element which does not receive a pressure from an external source. Variations are possible, such as combination pressurized volumes which include both static pressure elements and dynamic pressure elements, such as an inflatable volume with a foam pad coupled thereto. 
     Measurement system  180  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  180  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  180  also includes laser elements such as a laser diode, solid-state laser, or other laser device, along with associated driving circuitry. Optical couplers, cabling, or attachments can be included to optically mate laser or detector elements to optical fibers  121 - 122 . 
     Optical fibers  121 - 122  each comprise an optical waveguide, and each use glass, polymer, air, space, or some other material as the transport media for transmission of light, and can each include multimode fiber (MMF) or single mode fiber (SMF) materials. A sheath or loom can be employed to bundle optical fibers  121 - 122  together with further optical links for convenience, as indicated by optical cable  120 . One end of each of optical fibers  121 - 122  mates with an associated optical driver or detector component of measurement system  180 , and the other end of each of optical fibers  121 - 122  is configured to terminate in tissue interface pad  110  for optically interfacing with tissue  130 . Various optical interfacing elements can be employed to optically couple optical signals carried by optical fibers  121 - 122  to tissue  130 , such as prisms, reflective surfaces, refractive materials, or the like. Each of optical fibers  121 - 122  may include many different signals sharing the same associated link, as represented by the associated lines in  FIG. 1 , comprising channels, forward links, reverse links, user communications, overhead communications, frequencies, wavelengths, modulation frequencies, carriers, timeslots, spreading codes, logical transportation links, packets, or communication directions. 
     Also, although  FIG. 1  illustrates two optical fibers  121 - 122 , 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. Also, although the optical source of  FIG. 1  is shown as optical fiber  121  in this example, in further examples a direct light source can be included in tissue interface  110  and applied to tissue  130 . Such direct light sources can include light-emitting diodes (LED), laser sources, or other signal sources, including combinations thereof. 
     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 system  300  for applying optical signals to tissue  330  of a patient. System  300  includes tissue interface pad  310 , optical cable  320 , tissue  330 , pressure cuff  340 , casing  350 , measurement system  380 , and pressure system  390 . A top view and a side view of system  300  are included in  FIG. 3  to highlight the various elements of system  300 . The side view is sectioned at section cut  335 . It should be understood the dashed features of  FIG. 3  are merely intended to highlight various elements of system  300 , and are not intended to be exact wireframe representations of the elements of system  300 ; variations are possible. 
     Tissue  330  is shown as a finger of a patient undergoing measurement in this example. Other tissue portions of a patient may instead be included. In operation, a tip portion of the finger is inserted into casing  350  to undergo measurement. Once the finger is inserted into casing  350 , optical signal  325  generated by measurement system  380  is applied to tissue  330  for measurement of a physiological parameter. In this example, optical signal  325  is applied to tissue  330  via an input optical fiber associated with optical cable  320 , and optical signal  325  is detected through tissue  330  via an output optical fiber associated with optical cable  320 . Optical signal  325  is coupled between the associated optical fibers and tissue  330  by tissue interface pad  310 . Upon receiving optical signal  325  over the output optical fiber after propagation through tissue  330 , measurement system  380  may detect and process the optical signal to determine various characteristics of the detected optical signal. Physiological parameters of the tissue and patient can then be identified based on the various characteristics of the detected optical signals. 
     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 a generally planar surface configured to interface with tissue  330  to allow for introduction of optical signals into tissue  330  and for receipt of optical signals from tissue  330 . Tissue interface pad  310  also may include elements as discussed above for tissue interface pad  110 , although these elements can use different configurations. 
     Pressure cuff  340  is configured to apply a pressure received over pressure link  341  from pressure system  390  to tissue interface pad  310  to couple at least a portion of tissue interface pad  310  to tissue  330 . Pressure cuff  340  also may include elements as discussed above for pressurized volume  140 , although these elements can use different configurations. The size, shape, and configuration of pressure cuff  340  can vary according to many factors. For example, properties of casing  350 , tissue  330 , and tissue interface pad  310  each can influence the size, shape, and configuration of pressure cuff  340 , among other factors including desired pressure. Although pressure cuff  340  is shown as an inflatable pad or balloon-style pressurized volume in  FIG. 3 , it should be understood that a wrap-around cuff as shown in  FIG. 1  may instead be employed. Pressure link  341  can comprise tubing, piping, or other pressure conduits for transferring a pressure generated by pressure system  390  to pressure cuff  340 . Various pressure control and coupling elements can also be included, such as valves, pistons, couplers, thermally welded elements, or friction-fit elements. 
     Casing  350  is a rigid housing which seats tissue  330  for measurement. Casing  350  includes preload tension elements  352  for applying a preload pressure to tissue  330  to initially align tissue  330  in casing  350  to tissue interface pad  310  and likewise to pressure cuff  340 . Three pairs of preload studs  351  are included in this example to attach each preload tension element  352  to casing  350 . Preload tension elements  352  can include elastic bands, rubber cords, shock cords, fabric sleeves, springs, or other tension element to place a preload pressure on tissue  330  in casing  350 . By employing preload tension elements  352 , casing  350  initially adapts to different finger or toe sizes and shapes before application of a pressure by pressure cuff  340 . Casing  350  also includes an angled tip to accommodate a tip of a finger or toe of a patient. As pressure cuff  340  applies a pressure, such as due to inflation, tissue  330  can move against preload tension elements  352  while maintaining alignment and contact with tissue interface pad  310 . Example preloading pressure exerted on tissue  330  by preload tension elements  352  include 5-10 mmHg, to ensure adequate pressure and contact between tissue  330  and tissue interface pad  310  once pressure cuff  340  is inflated. The preload pressure typically is configured to ensure slight engagement of tissue interface pad  310  on tissue  330 . An example over-pressure of the preload is 20 mmHg or greater. Other preload pressures or tensions can be applied, and preload pressure or tension can be determined based on a size of the tissue of the patient, such as a finger size. For example, a larger diameter finger may use a smaller preload tension while a smaller diameter finger may use a larger preload tension, or vice versa, to maintain a desired preload pressure of tissue  330  on tissue interface pad  310 . This example illustrates casing  350  suited for a tip of a finger or toe of a patient and a smaller casing is thus employed. A larger casing is discussed in  FIG. 4 . 
     Measurement system  380  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  380  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  380  also includes laser elements such as a laser diode, solid-state laser, or other laser device, along with associated driving circuitry. Optical couplers, cabling, or attachments can be included to optically mate laser or detector elements to optical fibers of optical cable  320 . 
     Pressure system  390  includes pressure generation, control, and monitoring equipment. Pressure system  390  can include pumps, pistons, syringes, pressure gauges, pressure sensors, user control and monitoring interfaces. Pressure system  390  can also comprise tubing, piping, or other pressure conduits for transferring a pressure generated by pressure system  390  to pressure cuff  340 . Various pressure control and coupling elements can also be included, such as valves, pistons, couplers, thermally welded elements, or friction-fit elements. 
       FIG. 4  is a system diagram illustrating system  400  for applying optical signals to tissue  430  of a patient. System  400  includes tissue interface pad  410 , optical cable  420 , tissue  430 , pressure cuff  440 , casing  450 , composite cable  460 , measurement system  480 , and pressure system  490 . A top view and a side view of system  400  are included in  FIG. 4  to highlight the various elements of system  400 . It should be understood the dashed features of  FIG. 4  are merely intended to highlight various elements of system  400 , and are not intended to be exact wireframe representations of the elements of system  400 ; variations are possible. 
     Tissue  430  is shown as a finger of a patient undergoing measurement in this example. Other tissue portions of a patient may instead be included. In operation, a portion of the finger is inserted into casing  450  to undergo measurement. Once the finger is inserted into casing  450 , optical signal  425  generated by measurement system  480  is applied to tissue  430  for measurement of a physiological parameter. In this example, optical signal  425  is applied to tissue  430  via an input optical fiber associated with optical cable  420 , and optical signal  425  is detected through tissue  430  via an output optical fiber associated with optical cable  420 . Optical signal  425  is coupled between the associated optical fibers and tissue  430  by tissue interface pad  410 . Upon receiving optical signal  425  over the output optical fiber after propagation through tissue  430 , measurement system  480  may detect and process the optical signal to determine various characteristics of the detected optical signal. Physiological parameters of the tissue and patient can then be identified based on the various characteristics of the detected optical signals. 
     Tissue interface pad  410  may be composed of plastic, foam, rubber, glass, metal, adhesive, or some other material, including combinations thereof. Tissue interface pad  410  includes a generally planar surface configured to interface with tissue  430  to allow for introduction of optical signals into tissue  430  and for receipt of optical signals from tissue  430 . Tissue interface pad  410  also may include elements as discussed above for tissue interface pads  110  or  310 , although these elements can use different configurations. 
     Pressure cuff  440  is configured to apply a pressure received over pressure link  441  from pressure system  490  to tissue interface pad  410  to couple at least a portion of tissue interface pad  410  to tissue  430 . Pressure cuff  440  also may include elements as discussed above for pressurized volume  140  or pressure cuff  340 , although these elements can use different configurations. Although pressure cuff  440  is shown as an inflatable pad or balloon-style pressurized volume in  FIG. 4 , it should be understood that a wrap-around cuff as shown in  FIG. 1  may instead be employed. Pressure link  441  can comprise tubing, piping, or other pressure conduits for transferring a pressure generated by pressure system  490  to pressure cuff  440 . Various pressure control and coupling elements can also be included, such as valves, pistons, couplers, thermally welded elements, or friction-fit elements. 
     Casing  450  is a rigid housing which seats tissue  430  for measurement. Casing  450  includes preload tension elements  452  for applying a preload pressure to tissue  430  to initially align tissue  430  in casing  450  to tissue interface pad  410  and likewise to pressure cuff  440 . Six pairs of preload studs  451  are included in this example to attach each preload tension element  452  to casing  450 . Preload tension elements  452  can include elastic bands, rubber cords, shock cords, fabric sleeves, springs, or other stretchable element to place a preload pressure on tissue  430  in casing  450 . By employing preload tension elements  452 , casing  450  initially adapts to different finger or toe sizes and shapes before application of a pressure by pressure cuff  440 . Casing  450  includes an angled tip to accommodate a tip of a finger or toe of a patient, as well as an angled casing portion to accommodate the length of a finger or toe to allow for a more uniform preload along the length of tissue  430 . Casing  450  also includes sensor box  453  to hold tissue interface pad  410  and pressure cuff  440  in casing  450  while still allowing for tissue  430  to slide into casing  450 . As pressure cuff  440  applies a pressure, such as due to inflation, tissue  430  can move against preload tension elements  452  while maintaining alignment and contact with tissue interface pad  410 . This example illustrates casing  450  suited for a full length of a finger or toe of a patient and a larger casing is thus employed. A smaller casing is discussed in  FIG. 3 . 
     Measurement system  480  and pressure system  490  can include similar elements as discussed above for measurement system  380  and pressure system  390  of  FIG. 3 . In this example, optical cable  420  and pressure link  441  are also included in a composite cable  460 . Since measurement system  480  and pressure system  490  may be located remotely from tissue  430 , the optical and pressure links are bundled in a loom, casing, sheath, or the like, which allows for bundling and common routing of the associated links between systems  480 - 490  and casing  450 . Composite cable  460  can be routed generally parallel to the tissue under measurement, such as a finger of a patient. 
       FIG. 5  is a system diagram illustrating system  500  for applying optical signals to tissue  530  of a patient. System  500  includes tissue interface pad  510 , optical links  520 , tissue  530 , pressure pad  540 , and boot  550 . Associated measurement and pressure systems are omitted from  FIG. 5  for clarity, but similar measurement and pressure systems as found in  FIGS. 1-4  may be included. Also, optical signals propagated within tissue  530  are omitted in  FIG. 5  for clarity and to highlight mechanical features of system  500 . A side view of system  500  is included in  FIG. 5  to highlight the various elements of system  500 . It should be understood the dashed features of  FIG. 5  are merely intended to highlight various elements of system  500 , and are not intended to be exact wireframe representations of the elements of system  500 ; variations are possible. 
     Tissue  530  is shown as a finger of a patient undergoing measurement in this example. Other tissue portions of a patient may instead be included. In operation, a portion of the finger is inserted into boot  550  to undergo measurement. Once the finger is inserted into boot  550 , optical signals are applied to tissue  530  for measurement of a physiological parameter. Optical signals are coupled between tissue  530  by tissue interface pad  510 . As discussed herein, optical signals may be detected and processed 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. 
     Tissue interface pad  510  may be composed of plastic, foam, rubber, glass, metal, adhesive, or some other material, including combinations thereof. Tissue interface pad  510  includes a generally planar surface configured to interface with tissue  530  to allow for introduction of optical signals into tissue  530  and for receipt of optical signals from tissue  530 . Tissue interface pad  510  also may include elements as discussed above for tissue interface pads  110 ,  310 , or  410 , although these elements can use different configurations. 
     Pressure pad  540  is configured to apply a pressure received over pressure link  541  from pressure system  590  to tissue interface pad  510  to couple at least a portion of tissue interface pad  510  to tissue  530 . Pressure pad  540  also may include elements as discussed above for pressurized volume  140  or pressure cuffs  340  or  440 , although these elements can use different configurations. Although pressure pad  540  is shown as an inflatable pad or balloon-style pressurized volume in  FIG. 5 , it should be understood that a wrap-around cuff as shown in  FIG. 1  may instead be employed. Pressure link  541  can comprise tubing, piping, or other pressure conduits for transferring a pressure generated by a pressure system to pressure pad  540 . Various pressure control and coupling elements can also be included, such as valves, pistons, couplers, thermally welded elements, or friction-fit elements. 
     Boot  550  is a rigid housing which seats tissue  530  for measurement. Boot  550  includes notch element  551  for allowing for various sizes of tissue  530 , such as different finger or toe sizes. Notch element  551  may be a v-groove, square notch, rounded notch, or the like. By using notch element  551 , boot  550  initially flexes and adapts to different finger to toe sizes and shapes before application of a pressure by pressure pad  540 . Boot  550  includes a rounded tip to accommodate a tip of a finger or toe of a patient. As pressure pad  540  applies a pressure, such as due to inflation, boot  550  maintains alignment and contact between tissue  530  and tissue interface pad  510 . 
       FIG. 6  is a system diagram illustrating system  600  for applying optical signals to tissue  630  of a patient. System  600  includes tissue interface pad  610 , tissue  630 , pressure pad  640 , and boot  650 . Associated measurement and pressure systems are omitted from  FIG. 6  for clarity, but similar measurement and pressure systems as found in  FIGS. 1-4  may be included. Also, pressure links, optical links and optical signals propagated within tissue  630  are omitted in  FIG. 6  for clarity and to highlight mechanical features of system  600 . An end view and a side view of system  600  are included in  FIG. 6  to highlight the various elements of system  600 . The side view is sectioned approximately at section cut  635 . 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. 
     Tissue  630  is shown as a finger of a patient undergoing measurement in this example. Other tissue portions of a patient may instead be included. In operation, a portion of the finger is inserted into casing  650  to undergo measurement. Once the finger is inserted into casing  650 , optical signals are applied to tissue  630  for measurement of a physiological parameter. Optical signals are coupled between tissue  630  by tissue interface pad  610 . As discussed herein, optical signals may be detected and processed 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. 
     Tissue interface pad  610  may be composed of plastic, foam, rubber, glass, metal, adhesive, or some other material, including combinations thereof. Tissue interface pad  610  includes a generally planar surface configured to interface with tissue  630  to allow for introduction of optical signals into tissue  630  and for receipt of optical signals from tissue  630 . Tissue interface pad  610  also may include elements as discussed above for tissue interface pads  110 ,  310 ,  410 , or  510 , although these elements can use different configurations. 
     Pressure pad  640  is configured to apply a pressure to tissue interface pad  610  to couple at least a portion of tissue interface pad  610  to tissue  630 . Pressure pad  640  also may include elements as discussed above for pressurized volume  140  or pressure cuffs or pads  340 ,  440 , or  540 , although these elements can use different configurations. Although pressure pad  640  is shown as an inflatable pad or balloon-style pressurized volume in  FIG. 6 , it should be understood that a wrap-around cuff as shown in  FIG. 1  may instead be employed. 
     Boot  650  is a rigid housing which seats tissue  630  for measurement. Boot  650  includes adjustable preload elements  651 - 654 . Tissue interface pad  610  is coupled to shaft  653  which fits into slots  651 . The ends of shaft  653  include rivets  654  to allow shaft  653  to slide within slots  651  while preventing escape of shaft  653  and providing side-to-side alignment of tissue interface pad  610  within casing  650 . Coupled to rivets  654  is optional tension member  652 . Tension member  652  can include elastic bands, rubber cords, shock cords, fabric sleeves, springs, or other stretchable element to place a preload pressure  615  on tissue interface pad  610  and likewise onto tissue  630  held in casing  650 . Since shaft  653  is coupled to tissue interface pad  610 , the tension provided by tension member  652  to rivets  654  will cause shaft  653  to slide in slots  651  and automatically adjust to different sizes of tissue  630 . After alignment and adjustment via adjustable preload elements  651 - 654  a pressure can be applied via pressure pad  640 . The various elements of adjustable preload elements  651 - 654  can be composed of metal, plastic, wood, composite material, rubber, or other material, including combinations thereof. 
     In the examples discussed herein, the pressurized volumes are typically configured to apply a pressure on a tissue interface pad to maintain a desired contact pressure of a tissue interface pad on tissue undergoing measurement. This pressure is adjusted to maintain a desired pressure and optical signal quality in the tissue due to the effect of varying tissue sizes, skin conditions, ambient conditions, or other factors which may affect the measurement process. To ensure the desired contact pressure is maintained, a threshold signal quality of optical signals in the tissue of a patient can be monitored, such as by measurement systems  180 ,  380 , or  480 . For example, a detected portion of the optical signals can be monitored to determine if a threshold signal quality is met. The signal quality can include a signal-to-noise ratio, magnitude of a signal component, or other signal quality metric. In some examples, the optical signals can include detected tissue parameters, such as pulsatile signal components due to the pulse of the patient, or other tissue parameters identified after propagating the optical signals through tissue. The threshold signal quality can thus include a threshold level of the pulsatile signal detected from the optical signals through the tissue. The threshold signal quality can include any threshold tissue parameter. A measurement system can also be configured to identify the detected signal quality of the propagated optical signals and determine a target pressure for the pressurized volume based on the detected signal quality and a desired signal quality. For example, if the measurement system determines that the detected signal quality is too poor or below a low threshold level, then the pressure applied by a pressure system to the pressurized volume can be increased. This increase in pressure forces the associated tissue interface pad more tightly against the tissue and can provide for a higher intensity of the propagated signal in the tissue. Likewise, if the measurement system determines that the detected signal quality is above a high threshold level, then the pressure applied by a pressure system to the pressurized volume can be decreased. This decrease in pressure forces the associated tissue interface pad less tightly against the tissue and can provide for a lower intensity of the propagated signal in the tissue. A pressure may be too high if the optical signal indicates clipping from too high of detected signal intensity or if the signal exceeds a dynamic range of signal processing circuitry. It should be understood that the measurement system and associated pressure system can be configured to communicate to adjust the applied pressure based on the optical signal quality. 
     In order to determine the desired pressure to be applied to the pressurized volume, a pressure system can be configured to apply a range of pressures to the pressurized volume. A target pressure can be identified when monitored optical signal quality factors fall within a threshold range, as discussed above. The pressure system or measurement system can then identify the target pressure for the pressurized volume to obtain the desired signal quality and apply the target pressure as the pressure to the pressurized volume. 
     Although a pressurized volume, such as a pressure cuff or pad, has been discussed herein, other configurations can be employed. For example, a thermally sensitive spring element can be employed to apply an adjustable pressure to a tissue interface pad. An electronic heating element can be coupled to the thermally sensitive spring to modify a spring constant, and likewise an applied pressure, based on a heat applied by the heating element to the spring. Thus, an adjustable pressure can be applied by the adjustable spring element instead of an inflatable volume. Another configuration includes a servo-gear mechanism using a lever to apply an adjustable pressure to a tissue interface pad. 
     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.