Abstract:
An image is created of blood circulation deep (e.g. a plurality of millimeters) below the surface of living tissue to aid in evaluating a patient. A first beam ( 26 ) of circularly polarized light is directed forwardly (F) against an outer surface ( 14 ) of the tissue. Light that has penetrated to only a shallow depth before moving rearwardly and out of the tissue remains polarized and is blocked by a filter ( 38 ). Light that has penetrated to greater depths ( 12 ), is scattered more and becomes depolarized, and a portion of it passes through the depolarizing filter ( 38 ) and is focused on a photodetector ( 48 ) to create an image. Light spots ( 54 ) on the image that move, represent spaces between blood platelets ( 52 ) that are moving through a capillary, and indicates the velocity of blood through the capillary.

Description:
BACKGROUND OF THE INVENTION 
     There is a great need to determine whether or not there is sufficient perfusion, or blood circulation, in microvessels of a patient. Such microvessels include capillaries, arterioles and venules, which are herein all referred to as capillaries. Adequate microcirculation is vital for the transport of oxygen and other nutrients and the removal of waste. Distinctive microvascular pathologies are associated with different disease states such as in diabetes, hypertension, chronic heart disease, chronic ulcers and sepsis. One promising noninvasive technique is the shining of bright light at tissue and the detection of reflected and scattered light. However, it is found that this technique indicates microcirculation only at shallow depths of no more than about one millimeter below the tissue surface. It is known that circularly polarized light penetrates further into tissue than linearly or unpolarized light, but this has not led to better interrogation of tissue. There is a need for the noninvasive detection of microcirculation at greater depths of a plurality of millimeters, in order to interrogate deeper tissue layers, especially the surface regions of organs. A noninvasive technique that enabled such deeper detection of microcirculation would be of value. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, applicant provides a method and apparatus for use in sensing microcirculation at a depth of a plurality of millimeters below the surface of living tissue. A first beam of circularly polarized light is directed forwardly at an outer surface of the tissue to be interrogated. The light penetrates the tissue and is absorbed by and scattered from the tissue. Much scattering occurs when light reaches a boundary between materials of different indexes of refraction. Some of the light passes deeply into the tissue to reach a depth of a plurality of millimeters, and some of such deep light (light which has reached a large depth of a plurality of millimeters) is scattered in a rearward direction. 
     Some of the deep light which has been scattered travels rearwardly towards the tissue outer surface from a large depth. Such deep light is further scattered by the tissue, and is largely depolarized as it passes rearwardly to constitute a second beam that moves rearwardly out of and then away from the tissue. Light moving out of the tissue along the second beam is filtered to allow only unpolarized light to move along a filtered portion of the second beam. This allows the passage, through the filter, of a higher portion of deep light that has passed rearwardly though the tissue from a large depth. The unpolarized light of the filtered second beam is imaged onto a photodetector such as one with a matrix of photodetecting pixels. The output of the photodetector is used to generate a display which is an image that is largely of tissue at a depth of a plurality of millimeters below the surface of the tissue. The image changes with time and displays moving bright objects which represent spaces between blood platelets moving in one or more capillaries. The rate of such movements represent the velocity of blood in a capillary. 
     It is known that circularly polarized light penetrates living tissue to a greater depth than unpolarized or linearly polarized light. Thus, a considerable portion of the original circularly polarized light penetrates deeply (a plurality of millimeters) into the tissue and some of this deep light then moves rearwardly and reaches the outer surface of the tissue. Much of such deep light passes through a depolarizing filter that passes only depolarized light to become part of the image on the photodetector and therefore on the display. Such light from deep in the tissue, has undergone many scatterings, and in doing so it has become largely depolarized. As a result, a considerable portion of such light passes through the depolarizing filter that passes only unpolarized light. Some of the light in the rearwardly moving second beam is nondeep light that has penetrated only a small depth of the tissue, that has been scattered much less than the deep light, and that therefore contains a higher portion of polarized light that is blocked by the depolarizing filter. Thus, a considerable portion of the unpolarized light reaching the photodetector is from deep in the tissue and therefore represents microcirculation at a depth of a plurality of millimeters below the tissue surface. 
     The tissue can be interrogated by directing a wide circularly polarized first beam at the tissue, wherein the first beam converges at the desired depth such as a plurality of millimeters within the tissue. Such beam also is blocked from moving in a direct forward direction into the tissue. As a result, the largely forward-moving first beam illuminates only a deep portion of the tissue. All of the scattered light that moves directly rearwardly though the tissue and out of its surface along a second beam is therefore light that originated from the deep tissue. A result is that more of the unpolarized light that emerges from the tissue along the second beam is light that represents a deep portion of the tissue. 
     The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation view of a system for interrogating patient tissue that lies a plurality of millimeters below the tissue surface in order to evaluate blood microcirculation therein. 
         FIG. 2  is an enlarged view of a capillary of a patient, showing what the system of  FIG. 1  can detect. 
         FIG. 3  is a view of a portion of an image on a display screen of the system of  FIG. 1 . 
         FIG. 4  is a view similar to that of  FIG. 3  but at a slightly later time. 
         FIG. 5  is a side elevation view of a system similar to that of  FIG. 1 , but which uses converging beam portions to better interrogate the tissue. 
         FIG. 6  is an isometric view of a portion of the system of  FIG. 5 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a system  10  for indicating microcirculation in a region  12  that lies deeply (a plurality of millimeters) below an outer surface  14  of tissue of a patient. The system includes a source  20  of light that is directed though a circular polarizer  28 . The circular polarizer  28  passes only light that is circularly polarized in a first direction, such as a clockwise direction when the beam is viewed facing along the direction of beam movement. The circularly polarized light passes through a beam splitter  22  that directs the light forwardly F as a first light beam  26  into the surface, or outer surface  14 , of living tissue  32  Because of the fact that light in the first beam as it approaches the tissue is circularly polarized, a considerable portion of it reaches the deep region  12  of the tissue, which lies a distance D of a plurality of millimeters below the surface. The beam width is on the order of three millimeters, both along the first light beam  26  and the second light beam at  36  and at  44 . 
     Microvessels, including capillaries, arterioles, and venules, all of which are referred to herein as capillaries, lie in living tissue. The flow, or perfusion, of blood though capillaries lying a plurality of millimeters below the surface of a patient&#39;s tissue, such as in the deep region  12 , is an important indicator of the patient&#39;s condition. The present invention provides a system for indicating microperfusion in such deep regions. 
     The circularly polarized light that has reached the deep region  12  has been scattered (reflected and refracted) at least about 10 times, and as a result becomes depolarized. Some of that light moves rearwardly R from the deep region and forms part of a second beam  36  that moves rearwardly though the tissue and out of the tissue. As the second beam moves rearwardly through the tissue it also picks up light from a shallow portion  42  of the tissue that lies less than a plurality of millimeters below the tissue surface. Much of this shallow light has been scattered only a few times and is not depolarized. As a result, only a small portion of this light is unpolarized. 
       FIG. 1  shows that light moving rearwardly along the second beam encounters a depolarizing filter  38  that rejects circularly polarized light and passes primarily only unpolarized light. As mentioned above, the depolarization of deep light results in a greater percent of it being depolarized than shallow light. The unpolarized portion of the second beam becomes an unpolarized second beam portion  44 . The unpolarized second beam portion passes through a converging lens  46  that focuses the beam onto a photodetector  48  that usually includes an array of photosensitive pixels, such as a linear array or, more commonly a two-dimensional array. The output of the photodetector is used to create an image on a display screen  49 . 
     Applicant notes that the circular polarizer  28  and the depolarizing filter  38  should be maintained with their faces precisely perpendicular to each other. Applicant assure this by mounting each of them on a corresponding face of the glass beam splitter formed by a pair of prisms with a pair of contacting faces. 
       FIG. 2  shows a capillary  50 , and shows blood platelets  52  and hemoglobin  54  that are passing though the capillary. The blood platelets  52  create dark images at  60  on the display screen  49  of  FIG. 3 .  FIG. 4  shows the display  49 A a short time (e.g. a half second) later, showing that the dark images  60  have moved. The fact that the dark images, each of the expected size of a blood platelet, have moved indicates that the image represents blood flow in a capillary. The distance on the screen that the images have moved in a given time period indicates the velocity of blood flow. 
     As shown in  FIG. 1 , the light source  20  is collimated and passes though a tuneable spectral filter  70 . The light source  20  generates light of a wide range of wavelengths, such as white light. The tuneable spectral filter  70  passes only a limited range of light wavelengths. The collimated beam  72  passes through the beam splitter  22 , which directs about half of the light forwardly to become the first beam  26  that moves in the forward direction F into the tissue. The tunable spectral filter allows different wavelengths of light to pass and become part of the first and second beams. For example, light of a range from 420 nanometers (blue-purple light) to 810 nanometers (red to near infrared) may be emitted from the light source. The filter may be tuned to different frequency bands such as to pass only a band from 525 to 575 nanometers (light green) and later a band of 550 to 600 nanometers to see which band produces the clearest image of blood platelets. Applicant notes that oxy-hemoglobin is maximally absorbed at about 420 nanometers (blue purple) while deoxyhemoglobin exhibits multiple scatters in deeper regions at 810 nanometers (red to infrared). A compromise is green light at about 548 nanometers (a majority of light between 500 nanometers and 600 nanometers). The wavelength directed forwardly into the tissue can be a wavelength that is tuned to the absorption bands of a particular blood component, such as oxygen, nitrous dioxide and/or nitrous oxide. A majority of light in the directed beam has a wavelength that is absorbed by blood platelets more than any other frequency, and a wavelength band of 525 to 575 nanometers is preferred. 
       FIG. 5  illustrates another system  100  wherein a pair of light beams  102 ,  104  are directed at angles A of 30° to a line  106  that is perpendicular to the tissue surface  110 . The light beams pass though tissue regions  112 ,  114  and illuminate a lower portion of a cylindrical volume  122  of the tissue, with a highest level of illumination being in a deep volume  124  of the tissue. Intense light reaching the volume  124  is largely depolarized and some of it moves rearward along the cylindrical volume  122  to create a second light beam  130  that is large depolarized light. The second light beam passes through a depolarizing filter  132  to create an unpolarized second beam  134  that is imaged onto a photodetector such as  44  in  FIG. 1  and the resulting image is displayed. The advantage of the system  100  is that more of the light of the second beam  130  originates from the deep volume  124  of tissue that is to be analyzed, and less of the light originates from more shallow regions below the tissue surface. Also, light directed at the surface of the tissue is diffused over a larger area so more light can be directed at the region  124  of interest without damage to the tissue. The light would appear to intensify as it approaches the deep volume, but the light also diffuses as it approaches the deep volume region  124 . Light moving 30° to a line perpendicular to the tissue surface must travel 15% further (1/cos 30°=1.15) to reach the deep region. This is a disadvantage, but is more than compensated for by the much higher light intensity in the deep region than in a shallow region rearward of the deep region. 
       FIG. 6  shows that the two beams  102 ,  104  of  FIG. 5  are preferably portions of a conical beam  140  with a conical hole  142  along its axis. The conical beam  140  is readily generated by directing a wider circularly polarized and collimated first beam similar to beam  26  of  FIG. 1 , through a converging lens, with the middle of the beam blocked. 
     Thus, the invention provides a method and apparatus for noninvasively interrogating tissue lying more than a minimum distance below the surface of living tissue, such as tissue lying a plurality of millimeters below the tissue surface. This is accomplished by directing circularly polarized light towards the deep tissue, rejecting circularly polarized light that emerges from the deep tissue and detecting only unpolarized light. The detected unpolarized light is focused on a photodetector and the image is displayed. The image can display blood platelets (actually, spaces between platelets) moving though a capillary to help in the diagnosis of a patient. Applicant prefers to direct a beam of a diameter on the order of 3 millimeters at the tissue, or that converges on a deep region of a diameter on the order of 3 millimeters and located a plurality of millimeters forward of the tissue surface such as three millimeters. 
     Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.