Abstract:
An instrument has been designed to study the pulsatile motion of the eye by analysis of a beam of light reflected from the corneal surface. A laser light beam probe of small spot size and low divergence strikes the cornea apex and the reflected movement is recorded by a sensor. Analysis of the beam movement reveals the energy in the eye pulse without the necessity of physically touching the eye. The value of the intraocular pressure is determined from the calculated power spectrum. The sensitivity, accuracy and efficiency of the light beam makes possible studying both eyes concurrently and comparison of the pulse parameters of onset, amplitude and duration reveals any delay in circulation to an eye.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to U.S. Provisional Patent Application Ser. No. 60/920,209 filed on Mar. 26, 2007, the entire disclosure of which is incorporated by reference as if set forth in its entirety herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to ophthalmic clinical screening for the detection of eye and cerebrovascular disease by means of non-contact measurement of the ocular pulse amplitude of a subject&#39;s eyes. 
       BACKGROUND OF THE INVENTION 
       [0003]    Determination of the intraocular pressure (IOP) is a component of the standard complete eye examination, a necessary test in the diagnosis and management of glaucoma and an indicator of underlying cerebrovascular diseases. A direct measure of the IOP requires surgically opening the eye and placing a device within it. This is not clinically useful on a large scale. Clinically, indirect methods are required to determine the IOP. The instruments used for this determination are called tonometers. Tonometers evaluate the IOP by seeing how much force it takes to distort the eye and inferring what intraocular pressure is required to resist the applied force, and declaring that to be the IOP. 
         [0004]    The force is applied to the eye by a plunger, an illuminated prism, a blast of air, a pneumatic pressure probe or piezoelectric crystals. All of these methods suffer from multiple problems. First, distorting the shape of the eye changes the pressure in the eye, reducing the accuracy of the test. Second, all of these instruments have a large mechanical component, introducing friction into the measurement. The damping effect of friction decreases the sensitivity of the test. Third, the force required for a tonometer to indent or flatten the eye surface in measuring the IOP is large compared to the energy contained within the ocular pulse wave. Therefore, the information carried in the ocular pulse wave and created in the eye following each heartbeat remains hidden. Fourth, the lack of ability to produce detailed ocular pulse wave measurements eliminates the opportunity to test both eyes concurrently and compare this data between the eyes. Fifth, applying force to the eye subjects the cornea to trauma. 
       SUMMARY OF THE INVENTION 
       [0005]    Our invention relates generally to ophthalmic clinical screening for the detection of eye and cerebrovascular disease by a non-contact measurement of the ocular pulse amplitude waveform 
         [0006]    The invention, an ocular pulse analyzer, probes the eye with a light beam and evaluates the eye&#39;s expansile response to an ocular pulse by analyzing movement of the reflected light beam. Valuable information about the ocular pulse is recorded in real time. The eye is not touched. The eye is not distorted. There is no friction interfering with the data and there is no trauma to the eye. The time of onset of the ocular pulse after the heart beat is easily measured. The amplitude of the ocular pulse, the duration of the ocular pulse and the shape of the ocular pulse is recorded to be compared with normal for each parameter. Since both eyes are tested concurrently, they are compared with each other. 
         [0007]    An embodiment of the ocular pulse analyzer consists of a pulsed laser diode followed by a laser beam expander with an internal spatial filter and a laser beam shaper which forms a low divergence small spot incident beam at the power required to safely strike the eye surface at a large incident angle on the apex of the cornea. The reflected beam emerges at an equal and opposite incident angle striking, the analyzer CCD camera array forming a pixelated image of the beam spot. During a single ocular pulse up to 128 points are obtained by calculating the center of gravity of each reflected beam spot profile. The distance of each data point from the CCD systolic reference point is proportional to the distance the eye front surface has moved. The collection of these points defines the ocular pulse amplitude waveform for each heart pulse. Sensors are used to generate triggers that pulse the laser and synchronize the analyzer control and data acquisition. Fast Fourier transforms (FFT) are used to analyze the data producing both the ocular pulse waveform and its power spectrum. The zero frequency amplitude of the power spectrum is a measure of the energy in the mean intraocular pressure (IOP) Non-zero frequency amplitudes measure energy that may be related to pressure changes during the ocular pulse and may be indicators of eye disease. Left-right eye data comparison may allow rapid detection of eye and cerebrovascular disease. Two of these optical analyzers may be mounted so that both of the subject&#39;s eyes can be examined simultaneously. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0008]    Understanding of the present invention will be facilitated by consideration of the following detailed description of an embodiment of the present invention taken in conjunction with the accompanying drawings, in which like numerals refer to like parts and in which: 
           [0009]      FIG. 1  illustrates a functional block diagram of the device according to an aspect of the present invention; and 
           [0010]      FIG. 2  illustrates a diagram of a folded optical system for the left eye for measuring the movement of the eye front surface during the ocular pulse for the apparatus of  FIG. 1 ; and 
           [0011]      FIG. 3  is a cross section of the light beam shaping, collimator, and beam dump module  203  of  FIG. 2  for the apparatus of  FIG. 1 ; and 
           [0012]      FIG. 4  is a layout of the patient and operator optics  107  and  108  of  FIG. 2  for the apparatus of  FIG. 1 ; and 
           [0013]      FIG. 5  illustrates how the light beam responds to the movement of the eye front surface in response to the ocular pulse; and 
           [0014]      FIG. 6  is a plot of the light beam optical magnification vs. the incident angle of the input laser beam illustrating the variation of magnification; and 
           [0015]      FIG. 7  is a plot of the displacement of the light beam vs. the displacement of the eye surface in response to the ocular pulse; and 
           [0016]      FIG. 8  is a plot of the reflected light beam energy at the optical detector as a function of the light beam incident angle; and 
           [0017]      FIG. 9  is a plot of a simulated power spectrum illustrating the IOP as the zero frequency component with three higher frequency components. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for the purpose of clarity, many other elements found in typical optical systems and methods of manufacturing the same. Those of ordinary skill in the art will recognize that other elements and/or steps are desirable and/or required in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. The disclosure herein is directed to all such variations and modifications to such elements and methods known to those skilled in the art. 
         [0019]      FIG. 1  is a block diagram of an embodiment of the ocular pulse analyzer. A solid state laser  101  produces a pulsed axially symmetric Gaussian beam of 670 nm frequency, and 1 milliwatt power which is injected along the incident-beam-line IO into the beam forming optics module  102 , which forms a small low divergence exit beam of up to 500 micro meters diameter and less than 1.0 milliradians angle which strikes the apex of the cornea O at a 65 to 75 degree angle with respect to the eye visual axis. The incident beam is reflected onto the reflected-beam-line OR at an angle of −65 to −75 degrees where it strikes a CCD camera array  105  which forms a pixelated image of the reflected beam spot width. The motion of the beam spot pixelated image on CCD camera array  105  is proportional to the movement of the eye front surface  104  as it responds to the ocular pulse. The eye orbit cup  109  is a compliant cup that forms an air tight seal on the eye orbit so that the equilibrium pressure at the eye front surface can be varied. Cup  109  also is the interface to the patient&#39;s face. The optics target module  107  projects an infinity image of an eye fixation target on which the patient focuses his vision thus minimizing random eye movement. Observation module  108  is used by the operator to align and observe the measurement. Both the fixation module  107  and the observation module  108  are aligned along the eye visual axis. The ocular pulse analyzer beam forming optics is contained in beam tube  103 , and the reflected beam components  106  are contained in beam tube  106 . The fixation module  107 , the observation module  108  and the eye orbit cup  109  are attached to an instrument stand  110 . 
         [0020]    The laser pulser is synchronized to the heart pulse by delayed triggers derived from the blood pulse sensors BPS, during each heart pulse up to 128 pixelated beam spot widths are digitized by the analog-to-digital-converter ADC and processed to provide measurements of the eye surface movement. The computer/display CPU/Disp system processes these data providing the operator with the IOP and other parameters. These data are stored in the patient data base DB. The CP/disp receives the pulse and timing data and sends control data to the DAC which then controls the laser pulser and the cup control module. By using the real time control module RTCntrl the operator controls the measurement and data processing parameters. 
         [0021]      FIG. 2  illustrates an embodiment of a compact left eye optic head  200  with mirror folded beam lines that allows high angle light reflection at the apex of the cornea. The incident laser beam from laser  101  is transformed by the beam forming module  102  which contains a 2.5× inverted Keplerian telescope that expands and collimates the beam. The beam expander comprises a diffraction limited entrance lens  201  such as an infinity corrected, 5× microscope objective of 36 mm focal length and numerical aperture 0.1, which forms a beam waist of about 14 micrometers diameter, followed by an exit lens  202  such as a 90 mm focal length doublet located one focal length from plane  201   a  which forms a well collimated exit beam with at least a 10 mm diameter and a total divergence of less than 0.5 mrad at the entrance of beam shaping, collimator and dump module  203  which attenuates and collimates the beam using apertures of decreasing diameter. At the beam shaping and dump module  203  exit, the beam has a diameter no larger than 500 micrometers, a divergence of less than 1.0 mrad, and a power of 1 microwatt at turning mirror  204 , which deflects the incident beam so that it strikes the apex of the cornea at an incident angle of from 65 to 75 degrees to the visual axis of the eye where it is reflected onto turning mirror  205 , which reflects the beam onto the normal to CCD camera array  105 , the beam then passes through the quadrature error detectors  206  which detect out of range reflected beam pulses, finally impinging onto CCD camera array  105  which forms a pixelated image of the reflected beam spot. To provide noise and stray light shielding; the beam forming module  102  is mounted in beam tube  207 , the reflected beam components  206  and  105  in beam tube  208  and the compact optic head  200  in instrument case  209 , which also mounts the orbit cup  109  and mounts the fixation module  107  and the observation module  108 . As may be understood by those possessing an ordinary skill in the pertinent arts instrument case  209  may be mounted on a standard slit lamp bench or other ophthalmic examination stand. 
         [0022]      FIG. 3  illustrates an embodiment of a beam shaping, collimator and beam dump  203  of  FIG. 2  for the apparatus of  FIG. 1 . The expanded laser beam enters beam dump  302  where the laser beam diameter is reduced by striking the walls which may be made of graphite, the beam passes through four alternating radial light traps  306  and four apertures  304  further reducing the beam diameter and divergence, light traps  306  collect large angle light rays, and scattered rays from wall scattering by multiple reflections in the radial traps further reducing stray light after passing through a beam defining exit aperture  308  the beam power may be reduced by a factor greater than 1000 may have a reduction of beam divergence of less than 1 milliradian and a diameter of less than 500 micrometers. As may be understood by those possessing an ordinary skill in the pertinent arts the apertures may be produced by laser or other machining means and may be coated with an optically absorbent coating by sputtering or other means. The internal parts may be assembled with spacers into a stack that is self aligning. 
         [0023]      FIG. 4  illustrates the eye fixation module  107  and operator module  108  pertaining to  FIG. 1  and compact optic head  200  of  FIG. 2 . An incandescent light source  402  which is aligned on the extension of the eye visual axis, illuminates a grounded glass  404 , which then illuminates an optical fixation target  406 . Light from target  406  passes through beam splitter  408 , onto lens  410  which forms an image of the fixation target at infinity, which the patient fixes his eye on minimizing random eye motion. Light from eye surface  412  is reflected 90 degrees by beam splitter  408 , onto lens  414 , which forms an image of eye surface  412  on CCD camera array  416 , which provides the operator with a video observation of the measurement. 
         [0024]      FIG. 5  shows a geometric model of how the reflective head functions. Initially the optic head is aligned to the eye surface at the time of the systolic ocular pulse point when eye surface  104  has moved to its maximum outward extent  104 . At this time the optics projects the apertured spot image onto the corneal apex and reflects it onto the CCD array  105  which collects the reflected beam data point. As the ocular pulse amplitude reduces the eye surface  104  moves inward. The projection of the incident beam is not changed, but the angle of incidence to the perturbed surface is changed (Snell&#39;s law applies). The reflected light centroid is shifted on the CCD camera array  105 , and the relative position of the beam on the detector increases. A similar situation occurs when the ocular pulse pressure is raised, except that the reflected light shift moves in the opposite direction. The curvature of the corneal apex and the change in the location of the eye surface normal with respect to the incident beam provide a 50 times magnification of the eye surface movement at an incidence angle of 75 degrees with an equal reflected beam drift of 50 mm. The measurement of the eye surface motion can be explained from the single ray trace deflection diagram given in  FIG. 5  which shows two positions of the eye surface  104  during a single ocular pulse. The origin of the diagram is at the point (0,0) at a time t s  the time of the systolic peak of the ocular pulse. As the pulse amplitude decreases the eye front surface  104  moves radially inward along the eye visual axis VA, moving to the second eye surface  104  position  104 P 2 . To measure this movement we place a high brightness light source at the point (X0, Y0) aligned so the center of the output beam represented by a single ray intersects the initial eye surface  104 P 1  at (0,0) the corneal apex. The angle AO between the input ray and the eye crown visual axis is the angle of incidence of the input beam. Physically the laws of reflection apply and the incident light ray is reflected along line (0,0)-(−X0,Y0) where it strikes the detector at the pixel located at point (−X0,Y0) where the beam intensity profile is collected and the profile center of gravity calculated. At time (t s +δt) the eye surface  104  moves towards the eye center along the eye visual axis until it reaches the second eye surface position  104  where the incident beam intersects the surface at the point (XN,YN) and is reflected to the array detector at point (XS,YS) along a new normal and a different angle of incidence and another data point is collected. The distance between the two reflected beam detector profiles is related to the displacement of the two eye surface locations. 
         [0025]      FIGS. 6 through 8  show the dependence of magnification on the angle of incidence, the pixelated signal displacement as a function of the eye surface motion and the fraction of beam reflected power as a function of angle of incidence.  FIG. 6  shows the dependence of the light sensor magnification on the angle of incidence AO. Facial obstructions may limit the maximum angle of incidence to about 75 degrees with a useful range of about 65 to 75 degrees with a magnification range of about 30 to 50 times.  FIG. 7  is a plot of a model of the pixelated beam spot image displacement versus the eye surface movement for a 12.5 mm radius spheroidal eyeball with an 8.5 mm corneal radius and a magnification of 50 times.  FIG. 8  shows the optical beam energy reflected from the corneal apex vs the incident angle AO at an eyeball index of refraction of 1.356. The model results show that a coherent beam reflects more beam energy than an incoherent beam. 
         [0026]      FIG. 9  is a plot of a power spectrum from a simulated ocular pulse amplitude wave train showing the zero frequency power intensity plus three higher frequency components. The zero frequency power is the value of the mean energy in the IOP. This power spectrum is an example of the output obtained by using a commercial DSP program as may be understood by those possessing an ordinary skill in the pertinent arts 
         [0027]    Those of ordinary skill in the art will recognize that many modifications and variations of the present invention may be implemented without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modification and variations of this invention provided they come within the scope of the appended claims and their equivalents.