Abstract:
An apparatus, system and method for measuring oxygen concentration for exciting and detecting oxygen-sensitive fluorescence in biological tissues to detect oxygen levels (e.g., the partial pressure of oxygen).

Description:
RELATED APPLICATION 
       [0001]    The present application is being filed as a non-provisional patent application claiming the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 60/749,698 filed on Dec. 13, 2005. 
     
    
     FIELD 
       [0002]    This application generally relates to the field of tissue oximetry, and more particularly, to tissue oximetry that involves using a fluorescent compound to measure oxygen concentration. 
       BACKGROUND 
       [0003]    Oxygen detection is a critical element of applied wound healing research and clinical wound management and is used for both diagnostic/prognostic and therapeutic purposes. Transcutaneous oximetry (hereinafter, TCOM) is a noninvasive process that directly measures the oxygen level of tissue beneath the skin. In particular, TCOM measures the amount of oxygen that reaches the skin through blood circulation. 
         [0004]    In conventional TCOM, an area to be tested is first prepped (e.g., cleaned, shaved). A gel that conducts electrical impulses is then applied to the area. Adhesive sensors containing electrodes that can sense oxygen are applied to the area over the gel. Electrodes in the sensors heat the area below the skin to dilate the capillaries so oxygen can flow freely to the skin, which improves the reading. The readings are converted to an electrical current and the signal is displayed on a monitor and/or recorded. 
         [0005]    Conventional TCOM, however, have many disadvantages. For example, conventional TCOM is based on electrochemical technology, wherein electrochemical detectors are used that consume oxygen while detecting it, which results in a risk of inaccurate results. Also, oxygen tension is read on the skin at the wound periphery, instead of the more preferable location of the actual wound bed. Furthermore, the electrochemical technology requires a relatively long time (e.g., about 45 minutes) to obtain an accurate oxygen measurement. Further still, unreliable measurements can occur in the presence of lower extremity edema, which is present in all patients with venous stasis ulcers, among other disorders. 
         [0006]    Consequently, there is a need in the art for an improved apparatus, system and method for providing TCOM. 
       SUMMARY 
       [0007]    In view of the above, it is an exemplary aspect to provide an improved apparatus, system and method for measuring oxygen concentration using TCOM. 
         [0008]    It is another exemplary aspect to provide an apparatus, system and method for exciting and detecting oxygen-sensitive fluorescence in biological tissues. 
         [0009]    It is still another exemplary aspect to provide an apparatus, system and method for measuring oxygen-sensitive fluorescence using a frequency domain approach. 
         [0010]    It is an exemplary aspect to provide a wound-implantable oxygen-sensitive fluorescence probe. 
         [0011]    It is another exemplary aspect to provide an oxygen-sensitive fluorescence probe for performing TCOM. 
         [0012]    It is yet another exemplary aspect to use feedback from tissue oximetry to control dosage during oxygen therapy. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The above aspects and additional aspects, features and advantages will become readily apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, wherein like reference numerals denote like elements, and: 
           [0014]      FIG. 1  is a graph illustrating a phase delay between exemplary excitation and emission waveforms. 
           [0015]      FIG. 2  is a graph illustrating phase delay measurements at various modulation frequencies for an exemplary pO 2 -sensitive dye. 
           [0016]      FIG. 3  is a graph illustrating the relationship between phase delay and pO 2  for an exemplary pO 2 -sensitive dye. 
           [0017]      FIG. 4  is a diagram of an exemplary system for measuring oxygen, according to an exemplary embodiment. 
           [0018]      FIG. 5  is a graph illustrating N2-air transitions for an exemplary pO 2 -sensitive dye. 
           [0019]      FIG. 6  is a graph illustrating a typical phase-delay response to N2-air transitions. 
           [0020]      FIG. 7  is a partial diagram of an exemplary device for measuring oxygen, according to an exemplary embodiment. 
           [0021]      FIGS. 8A-8B  are diagrams of an exemplary device for performing TCOM, according to an exemplary embodiment. 
           [0022]      FIG. 9  is a diagram of a variation of the exemplary device of  FIGS. 8A-8B , according to an exemplary embodiment. 
           [0023]      FIG. 10  is a diagram of an exemplary excitation module and an exemplary emission module, according to an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    While the general inventive concept is susceptible of embodiment in many different forms, there are shown in the drawings and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the general inventive concept. Accordingly, the general inventive concept is not intended to be limited to the specific embodiments illustrated herein. 
         [0025]    According to an exemplary embodiment, a system  100  for measuring a partial pressure of oxygen (pO 2 ) is provided. The system  100  is based on oxygen-sensitivity of fluorescence of certain dyes. These dyes undergo modification (i.e., collisional quenching) in their excited state by molecular oxygen. In particular, if the excited dye encounters an oxygen molecule, excess energy is transferred to the oxygen molecule in a non-radiative transfer, thereby decreasing or quenching the fluorescence of the dye. The degree of quenching correlates to the level of oxygen concentration or the pO 2  in the oxygen-containing media (e.g., biological tissue). As a result, an increase in pO 2  decreases fluorescence intensity and lifetime with respect to the dye. Similarly, an increase in fluorescence intensity and lifetime with respect to the dye corresponds to a decrease in pO 2 . 
         [0026]    The emitted fluorescence of the dye is quantitatively related to the pO 2  by the Stern-Volmer equation, i.e., Equation 1. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       F 
                       0 
                     
                     F 
                   
                   = 
                   
                     1 
                     + 
                     
                       
                         K 
                         SV 
                       
                        
                       
                         
                           p 
                            
                           O 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0027]    where F 0  is the fluorescence when the pO 2 =0, where F is the measured fluorescence at pO 2 , and where K SV  is the Stern-Volmer constant. Thus, F 0  is the unquenched fluorescence intensity and F is the fluorescence intensity for the pO 2 . Accordingly, if F 0  and F are known, the pO 2  can be determined. 
         [0028]    Since the steady state fluorescence of the dye is dependent on its concentration, measuring an intrinsic parameter of the dye such as its fluorescence lifetime is useful. The fluorescence lifetime of the dye is quantitatively related to the pO 2  by an alternative form of the Stern-Volmer equation, i.e., Equation 2. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       F 
                       0 
                     
                     F 
                   
                   = 
                   
                     
                       
                         τ 
                         0 
                       
                       τ 
                     
                     = 
                     
                       1 
                       + 
                       
                         
                           K 
                           SV 
                         
                          
                         
                           pO 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
         [0029]    where τ 0  is the lifetime when pO 2 =0, where τ is the measured lifetime at pO 2 , and where K SV  is the Stern-Volmer constant. Thus, τ 0  is the unquenched lifetime and τ is the lifetime for the pO 2 . Accordingly, if τ 0  and τ are known, the pO 2  can be determined. 
         [0030]    A direct approach for measuring the lifetime of the oxygen-sensitive dyes is to follow the rate of fluorescence decay in response to a pulse excitation. This time-domain approach, however, does not result in faster acquisition of pO 2  samples. 
         [0031]    This problem of slow acquisition times is avoided by the frequency-domain approach of the system  100 . Accordingly, in the system  100 , changes in fluorescence lifetimes appear as changes in the phase delay of an emission wave when the excitation is via an intensity modulated sine wave, as shown in  FIG. 1 . The phase delay is related to the fluorescence lifetime of the dye by Equations 3-5. 
         [0000]      tan Φ=ωτ  (Equation 3)
 
         [0032]    where Φ is the phase delay, where ω is the angular frequency (expressed in radians in per second), and where τ is the fluorescence lifetime of the dye for the pO 2 . 
         [0000]      ω=2πf  (Equation 4)
 
         [0033]    where ω is the angular frequency (expressed in radians in per second), and where f is the frequency (expressed in cycles per second). 
         [0000]    
       
         
           
             
               
                 
                   M 
                   = 
                   
                     1 
                     
                       
                         ( 
                         
                           1 
                           + 
                           
                             
                               ω 
                               2 
                             
                              
                             
                               τ 
                               2 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     5 
                   
                   ) 
                 
               
             
           
         
       
     
         [0034]    where M is Amplitude modulation, where ω is the angular frequency (expressed in radians in per second), and where τ is the fluorescence lifetime of the dye for the pO 2 . 
         [0035]    It will be appreciated that any suitable oxygen-sensitive (e.g., pO 2 -sensitive) dyes can be used. For example, Tris(1,10 phenatroline)ruthenium (II) (hereinafter, Ru[Phen]) is one such dye. Ru[Phen] is a fluorescent dye with an excitation wavelength (λ ex ) of 460 nm and an emission wavelength (λ em ) greater than 600 nm. Several phase delay measurements were obtained using a commercial lifetime fluorometer at various modulation frequencies for Ru[Phen], as shown in  FIG. 2 . 
         [0036]    Pd-meso-tetra (4-carboxyphenyl)porphyrin (hereinafter, Pd-porphyrin), which has been used in human studies, is another exemplary dye. Pd-porphyrin is a phosphorescent dye with an excitation wavelength (λ ex ) of 523 nm and an emission wavelength (λ em ) greater than 600 nm. A phase-delay vs. pO 2  plot for Pd-porphyrin, which has a long lifetime, is shown in  FIG. 3 . The plot was simulated assuming K SV =300 mmHg-1 sec −1  and τ 0 =640 ms. As can be seen in  FIG. 3 , the Pd-porphyrin exhibits a high sensitivity for pO 2  in the range of 0-60 mmHg. 
         [0037]    The exemplary system  100  is shown in  FIG. 4 . The system  100  includes, for example, an excitation source  102  (e.g., a light source) and a function generator  104 . In one exemplary embodiment, the excitation source  102  is a blue LED. In another exemplary embodiment, the excitation source  102  is a green LED. Light from the excitation source  102  is intensity modulated as a sine wave by the function generator  104 . In an exemplary embodiment, the sine wave is 6 volts peak-to-peak. In an exemplary embodiment, the light from the excitation source  102  is intensity modulated at 1 KHz. In another exemplary embodiment, the light from the excitation source  102  is intensity modulated at 100 KHz. 
         [0038]    The modulated output of the excitation source  102  (i.e., an excitation wave) is directed to the surface or other area of a media  106  to be measured. In an exemplary embodiment, the media  106  is a polymeric film containing a pO 2 -sensitive dye. The dye can be Ru[Phen], Pd-porphyrin or any other suitable dye. In another exemplary embodiment, the media  106  is a probe with a portion (e.g., a tip) of the probe containing the dye. 
         [0039]    A filter  108  is disposed between the excitation source  102  and the media  106  to limit the excitation wavelength of the modulated output of the excitation source  102 . In an exemplary embodiment, the peak excitation wavelength is 460 nm. In another exemplary embodiment, the peak excitation wavelength is 530±40 nm. 
         [0040]    A fluorescence emission (i.e., an emission wave) leaves the media  106  at an angle (e.g., of about 60 degrees) relative to an excitation axis. A detector  110  detects the fluorescence emission from the media  106 . In an exemplary embodiment, the detector  110  is a high speed avalanche photodiode. 
         [0041]    Another filter  112  is disposed between the media  106  and the detector  110  to limit the emission wavelength. In an exemplary embodiment, the peak emission wavelength is greater than 600 nm. 
         [0042]    A phase delay  114  between the excitation and emission waves is measured by a phase detector  116 . In an exemplary embodiment, the phase detector  116  is a lock-in amplifier having a bandwidth of 120 KHz. The phase delay  114  is then transmitted to a computer  118 , for example, at 1 KHz and at a resolution of 16 bits. 
         [0043]    Exposure of the media  106  to an oxygen-deprived environment (e.g., by subjecting the media  106  to an N 2  stream) leads to a rapid increase in both the phase delay  114  and an intensity of fluorescence consistent with a decrease in the extent of quenching by the loss of the oxygen. The transitions between the media  106 , which contains the Ru[Phen] dye, being exposed to air (containing oxygen) and N 2  (without oxygen) are illustrated in  FIG. 5 . 
         [0044]    Each time the N 2  stream ends, the diffusion of oxygen into the media  106  begins immediately and results in the phase delay  114  and the intensity of fluorescence returning to their original values, which is consistent with an increase in quenching owing to the elevated oxygen levels in the media  106 . 
         [0045]    A typical phase-delay response resulting from N 2 -air transitions is illustrated in  FIG. 6 . The changes in the phase delay  114  and demodulation can be correlated to the pO 2  in the N 2 -air mixture levels using, for example, the Stern-Volmer equations described above. 
         [0046]    In view of the exemplary system  100  described above, various apparatuses and methods can also be used for measuring pO 2  based on oxygen-sensitive dyes. An exemplary device  120  (e.g., a probe) for measuring pO 2 , according to an exemplary embodiment, is shown in  FIG. 7 . 
         [0047]    The device  120  includes, for example, a tip  122  or other portion that contains a pO 2 -sensitive fluorescence dye (e.g., in film or tablet form). In an exemplary embodiment, a sensor film  124  containing the dye is located in the tip  122 . In the sensor film  124  the dye is bound to silica microparticles in silicone rubber. The device  120  also includes, for example, a bifurcated fiber optic bundle forming a Y-end (not shown). One arm of the Y-end is connected to an excitation module which is described below. The other arm of the Y-end is connected to an emission module which is described below. 
         [0048]    The position of the tip  122  of the device  120  determines the locale from which the pO 2  is sensed. The device  120  can be implanted into the actual wound bed for more accurate readings. 
         [0049]    A Silastic (a registered trademark of Dow Corning Corp.) tubing  126  surrounds the tip  122  and the fiber optic bundle. The use of the Silastic tubing  126  permits facile oxygen flux into the embedded oxygen-sensitive dye at the tip  122  of the device  120 . 
         [0050]    The bifurcated fiber optic bundle has an excitation fiber  128  at its core. Several emission fibers  130  encircle the excitation fiber  128 . 
         [0051]    Because the device  120  is intended for localization in the wound bed, the sensor film  124  is likely to undergo fouling. Accordingly, periodic replacement of the sensor film  124  may be necessary. To facilitate the replacement of the sensor film  124 , it is easy to disconnect the tip  122  from the device  120  and remove the sensor film  124  at the end of the fiber optic bundle. 
         [0052]    An exemplary device  132  (e.g., a probe) for performing TCOM, according to an exemplary embodiment, is shown in  FIGS. 8A-8B . The device  132  includes a heating element  134  (e.g., a platinum electrode) for raising the temperature of the skin  136  under a sensor film  138  of the device  132 . In an exemplary embodiment, the skin  136  under the sensor film  138  is raised to 44° C. by the heating element  134 . The increased skin temperature results in elevated perfusion to the area under the sensor film  138 . As this hyperfusion overwhelms the local demand, oxygen in the blood diffuses into a sampling volume  140  under the device  132 . 
         [0053]    A change in the pO 2  in the sampling volume  140  is then sensed through changes in fluorescence lifetime of an oxygen-sensitive dye embedded in the sensor film  138 . Such changes are measured by using an excitation source  142  (e.g., a blue LED) and detecting an emission using a detector  144 , wherein the excitation source  142  and the detector  144  are held together by a detector plate  146 . In an exemplary embodiment, the detector  144  is an avalanche photodiode, as shown in  FIGS. 8A-8B . In another exemplary embodiment, a device  132   a  includes the detector  144  is a head-on photomultiplier tube  148 , and includes a filter  150  and a fiber optic plate  152 , as shown in  FIG. 9 . 
         [0054]    The components of the device  132 ,  132   a  are held hermetically sealed in an enclosure  154 . In an exemplary embodiment, the enclosure  154  is formed so as to facilitate replacement of the sensor film  138 . The enclosure  154  can be light-proof and/or made of a polymeric material. The enclosure  154  can include an insulator  156  that thermally and/or electrically insulates the device  133  and  132 . 
         [0055]    The devices (e.g., devices  120 ,  132 ,  132   a ) are connected to an excitation module  158  and an emission module  160  to record the pO 2 . See  FIG. 10 . The structure of the excitation module  158  is similar for both the device  120  and the device  132 / 132   a . For the wound implantable device (i.e., device  120 ), the excitation module  158  produces the intensity-modulated excitation light output which is connected to the excitation arm of the tip  122  of the device  120 . The excitation light can be, for example, a blue or green LED. The modulation is produced by a sine-wave generator  162  (i.e., function generator) and frequencies between 4-200 KHz. The output of the function generator  162  is connected to the LED through a bias-tee  164 . Power to the LED injected through the bias-tee  164  is derived from a stable and precise current source  166 . The current source  166  and the function generator  162  can be controlled through a radio telemetric receiver and transmitter (not shown) in the excitation module  158 . In the case of the TCOM devices (i.e., devices  132  and  132   a ), the output of the bias-tee  164  is fed to the LEDs on the detector plate  146 . 
         [0056]    The structure of the emission module  160  is similar for both the device  120  and the device  132 / 132   a . In the case of the wound implantable device (i.e., the device  120 ), the emission module  160  receives the fluorescence emission through one of the arms of the fiber optic bundle. This emission can be detected by a photomultiplier  170  with a built-in high-voltage source  172  and trans-impedance amplifier  174 . The phase delay in the emission relative to the excitation can be detected by the dual phase lock-in amplifier  174 . The reference for the lock-in is synched to the sine wave generator  162  of the excitation module  158 . 
         [0057]    The analog outputs of the lock-in phase delay and magnitude are sampled at a resolution of 16 bits and 1 sample per second. The digital output can then be sent to a remote computer via an embedded radio-telemetric receiver and transmitter  176 . For the TCOM device, the trans-impedance amplifier  174  will be held close to the photomultiplier tube  148  or the avalanche photodiode  144 , which will be part of the sensor package itself, to prevent contamination of low-level signals. The excitation module  158  and the emission module  160  facilitate high speed wound/bed oximetry. 
         [0058]    In one exemplary embodiment, software monitors the outputs of the lock-in amplifier  174  and provides feedback control signals to a control unit of a hyperbaric chamber. In this manner, the oximetric feedback is used so that the hyperbaric chamber is automatically pressurized to the prescribed pO 2 . Accordingly, the oximetric feedback allows the oxygen therapy to be much more personalized. 
         [0059]    Other exemplary functions of the software include: (1) telemetric setting of the function generator  162  and the current source  166 ; (2) telemetric setting of the lock-in amplifier  174  in real time; (3) providing a user interface for parameter settings and remote monitoring of pO 2  and skin temperature; and (4) providing a database for archiving patient-dependent information in a secure manner. 
         [0060]    The above description of specific embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the general inventive concept and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. It is sought, therefore, to cover all such changes and modifications as fall within the spirit and scope of the general inventive concept, as defined herein, and equivalents thereof. Thus, the embodiments described in the specification are only exemplary or preferred and are not intended to limit the terms of the claims in any way. The terms in the claims have all of their broad ordinary meanings and are not limited in any way or by any descriptions of these exemplary embodiments.