Abstract:
A method and apparatus for measuring the intraocular pressure of a cornea includes an interferometer directing a beam of a coherent light along a path to the cornea, a sensor for sensing the reflected light from the cornea, an air supply device for directing puffs of air to the cornea in alignment with the beam to cause the surface of the cornea to be artificially displaced and means for measuring variations of light intensity reaching the sensor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed toward an improved instrument for use in providing an accurate measurement of the intraocular pressure (IOP) of an eye without making any physical contact with the eye and without need for eye drops or anesthetic. The instrument of the present invention achieves these measurements through non-invasive and non-contact techniques, thereby providing an improved method for use in the early detection of glaucoma. 
     Glaucoma is an eye disease which can damage the optic nerve and which is one of the leading causes of blindness in the U.S. and throughout the world. Two out of every one hundred persons over age 35 have vision threatened by glaucoma. 
     When an object is viewed, the image is carried from the retina of the eye to the brain by the optic nerve. The optic nerve is an accumulation of over one million individual transmitters, each carrying a message to the brain. The individual messages all join together to provide side vision or peripheral vision as well as sharp central vision. Glaucoma can permanently damage the optic nerve, causing blind spots in areas of vision to develop. If glaucoma is undiagnosed, the optic nerve may sustain considerable irreversible damage and may even be destroyed, resulting in blindness. 
     Glaucoma is detectable by measuring the intraocular fluid pressure at the front surface or cornea of the eye. Intraocular fluid flows through the inner eye continuously to maintain the structure of the eye, in particular, the cornea. If the outflow or drainage system within the eye becomes blocked for any reason, the fluid backs up within the inner eye causing the intraocular fluid pressure to increase, thereby increasing the potential for damage to the optic nerve. The primary preventative measure which can be taken is the early detection of glaucoma by periodic testing of the intraocular pressure (IOP) since an elevated intraocular pressure (IOP) is clearly basic to the whole concept of glaucoma. 
     A variety of devices have been devised to facilitate the measurements of the intraocular pressure. The most common is a tonometer which measures the force necessary to applanate or flatten a given area of the cornea. An adjustable known force is applied to flatten a predetermined area of the cornea. This permits a direct measure of pressure to be made because the force and the area are directly known. The most common unit of this type is the Goldmann tonometer. While accurate in its measurements, the Goldman tonometer is an undesirable tool for many reasons. It is designed to provide a single time-segregated measurement of intraocular pressure. The application-type tonometer must be used with a topical anesthetic and a fluorescein dye. It includes physical touching of the eye, which many patients find objectionable. There is also an inherent risk of abrasion, injury, or infection to the eye as a result of contact. 
     Another common tonometer apparatus is the Schiotz or plunger-type tonometer. The Schiotz tonometer is placed before the eye along the optical axis and a plunger is released which flattens the cornea to a specified diameter and measures the forces applied. The Schiotz tonometer has the same undesirable qualities as the Goldman tonometer. It has been found that the patient usually has a somewhat high level of fear and physical discomfort as a result of such eye contact. Thus, the patient will tend to avoid the procedure, if possible. 
     A new generation of tonometers have been designed in an effort to limit physical contact with the eye which utilize a very strong air puff that impacts the eye. The air puff impinges on the cornea causing a sudden curvature reduction, applanation, and finally a slight concavity before restoration. Patient objections are still encountered when using the air puff system due to the discomfort caused by the force of the air puff on the eye and the accompanying audible explosion of the air puff as it is generated. Other disadvantages include the fact that an air puff measurement is a one-time occurrence and may, therefore, be offset from the actual average pressure value. 
     Other types of non-contact tonometers are disclosed in U.S. Pat. Nos. 4,928,697 and 5,148,807 both of which are assigned to the assignee of the present invention. The tonometers disclosed in those patents utilize the principles of induced phase modulation and/or frequency modulation of optical or acoustic waves which are directed toward the cornea as a diagnostic beam. The high frequency diagnostic waves are transmitted either as high frequency sonic waves or visible or invisible light waves. 
     Another non-contact instrument for measuring displacement of the cornea is disclosed in a thesis of Theodore Trost entitled Laser Interferometer Having Multiple Sensors which was published in 1995 and is available at The Ohio State University Library. As disclosed in that thesis, there is provided an interferometric displacement measurement apparatus having a coherent laser beam incident upon a partially reflective mirror, forming a measurement beam which is reflected back onto a sensor field. The incident beam also forms a reference beam incident upon the sensor field. The sensor field comprises at least two and preferably three or more photodetecting sensors spaced radially of the measurement beam axis arriving at the sensor field. 
     The interferometer disclosed is theoretically designed to measure the relative displacement of a target surface such as the surface of the cornea. 
     The Trost interferometer has many deficiencies and was not successfully reduced to practice. Problems were encountered in translating the relative displacement of the surface of the cornea to provide meaningful measure of intraocular pressure. There is ambiguity in determining the absolute direction of movement of the cornea surface and problems in eliminating excessive ambient noise received and measured by the system. Finally, measurements taken by the Trost apparatus are found to bear no statistical relationship to like measurements taken by a Goldman apparatus. 
     Most recently, an interferometer utilizing optical modulation to measure optical displacement has been patented to Gust (U.S. Pat. No. 5,828,454). Gust teaches the measurement of the static and dynamic displacement of a cornea by measuring the phase shift of an optical pathway. While the Gust patent is predicated on the theory that the measured phase shift is linearly proportional to deflection of the cornea, recent research has established that such a direct correlation is not necessarily as simple and accurate as Gust presents. 
     For instance, the eye has a multiplicity of reflective surfaces such as the lens, iris, front surface of the tear layer and the corneal surface. If the optical beam is not properly focused, it cannot be accurately predicted which surface is reflecting the beam, thus reducing the dependability and reliability of the instrument. 
     These and other non-contact tonometer attempts to make use of light waves and sound waves to measure corneal displacement have all suffered from two major deficiencies: the inability to accurately focus the measurement beam on the cornea and align the measurement beam with the sensor. Many complicated physical and mathematical techniques designed to meet and overcome these techniques have contravened the goal of simplicity in obtaining an accurate measurement of intraocular pressure by means of a non-contract tonometer. 
     Therefore, it is an object of the invention to provide an accurate non-invasive method and apparatus for performing the method of measuring the intraocular pressure of an eye. 
     A further object of the invention is to provide a method and apparatus for performing the measurement of the intraocular pressure of an eye continuously for a selected period of time in order to view variations in the pressure over time. 
     Yet another object of the present invention is to apply diagnostic energy to the cornea in a controlled, non-invasive, direct manner to accurately focus the energy onto a desired surface of the eye. 
     Yet another object of the invention is to provide a non-contact tonometer that can be adopted for use in an office or hospital on a fixed stand or, alternatively, be provided as a portable unit for use by health care professionals working, for instance, in nursing homes and assisted living homes, or be provided as a portable home unit simple enough to be used by individuals with little or no health care training. 
     It is a final object of the present invention to provide an instrument which accurately measures the intraocular pressure through non-invasive and non-contact type techniques. 
    
    
     IN THE DRAWINGS 
     FIG. 1 is a schematic view showing apparatus of the present invention. 
     FIG. 2 is a schematic showing a pattern of ring signals created by the interferometer of the present invention and a centered one dimensional linear array of optical detectors serving as the sensor of the present invention and a representation of a single optical detector sensor or a sensor having a small number of juxtaposed optical detectors. 
     FIG. 3 is a graph showing the output received from two sensors caused by motion of the cornea as tested by the non-contact tonometer of the present invention. 
     FIG. 4 is a schematic showing the linear array sensor of FIG. 2 in a non-aligned position. 
     FIG. 5 is a schematic view of a system for aligning the interferometer of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, there is shown in FIG. 1 an interferometer generally designated by the numeral  10  comprising a laser  12 , a beam expander  14  and a beam splitter  16  mounted on the housing  11 . The laser  12  may be one of a number of well known types such as, for example, a helium-neon laser or a diode laser. The laser envisioned for use with the invention is classified as either a Class 1 or Class 2A laser under the American National Standards Institute&#39;s (ANSI) Z136.1 Safety Use of Lasers standard. This “no hazard” classification allows a patient to be safely viewed by the laser for up to 1000 seconds or at least 15 minutes in a single setting without risk of ocular damage. The laser  12  emits a beam  13  which passes through the beam expander  14  comprising first and second lens  15 A and  15 B to form the expanded beam  13 A. The expanded beam  13 A passes through a beam splitter  16  which divides it into two beams, namely, a reference beam  23  and a measurement beam  24 . The reference beam  23  is reflected from a mirror  18  back to the beam splitter  16  which in turn reflects the reference beam back to a sensor system  20  as a reflected beam  23 A. Preferably the sensor system  20  is a one dimensional linear array of pixels as shown in FIG.  2 . 
     The measurement beam  24  passes through a lens  21  which causes it to converge upon the surface of the cornea  26  of the eye  27  being tested. The measurement beam  24  is reflected back from the surface of the cornea  26 , through the beam splitter  16  to the sensor system  20 . Interference between the measurement beam  24  and the reflected beam  23 A creates an interference fringe pattern of rings on the surface of the sensor system  20 . As the surface of the cornea  26  is displaced toward or away from the sensors of the sensor system  20 , the interference rings expand outwardly from a ring center or contract inwardly toward a ring center. The propagating rings pass over the sensors which convert the variations in the light intensity to proportional variations in voltage, thus causing an output, as shown in FIG. 3, for two sensors that varies from high to low as the illumination varies from bright to dark. 
     The output signal of the sensor system  20  generates a voltage that is proportional to the varying light intensity as the rings pass over it. A transition from dark to light and back to dark corresponds to a target surface displacement of one-half optical wavelength along the direction of the beam between the surface of the cornea  26  and the sensors. Therefore, the number of rings passing over the sensor system  20  is a function of the displacement, which permits calculation of the displacement of the cornea  26  relative to the sensor  20 . Viewing FIG. 2, a horizontal one dimensional linear array is shown in comparison with prior art single point sensors or sensor clusters. It has been found that a horizontal one dimensional linear sensor array serves to represent the movement and pattern of the fringe rings more completely than prior art single sensors and sensor clusters. For instance, if the sensor array is not in direct alignment with the interferometer ring patterns, as shown in FIG. 4, accurate processing of the resultant shifted output pattern is still possible because the misalignment only causes a position shift of the sensor output pattern. A single sensor or sensor cluster is more easily misaligned with the ring pattern often falling entirely away from the sensor. Thus, the use of a horizontal linear array of sensors greatly simplifies the alignment issues for the tonometer of this invention. 
     Extending from the housing  11  of the interferometer system  10  is a nozzle  30  having an outlet orifice  31  intended to be aligned with and spaced from the surface of the cornea  26  of the patient being examined. The nozzle  30  has an inlet passageway  32  which is connected to an air puff supply system  33  by means of tube  34 . The air puff supply system  33  pulses the air directed to the nozzle such that it exits the outlet orifice  31  onto the surface of the cornea  26  with a periodic rhythm of between 5 to 100 Hertz. The nozzle  30  emits the puffs coaxially with the measurement beam  24  from the interferometer  10  so that the cornea is struck with a periodic sequence of air puffs along the same axis of alignment as the measurement beam  24 . The measurement beam  24  is thus monitoring the region on the cornea  26  which is directly deflected by the air puff force. The periodic string of puff pulses is preferably created by a reciprocating pump operating between 5 and 100 Hertz and preferably in the range of 30 to 60 Hertz. It has been found that, if the puff rate is less than 10 Hertz, normal human motion which commonly ranges from 2-20 Hertz will cause the surface of the cornea to move in the same periodic domain as the puff rate, thereby overlapping the puff rate period and making post processing of the sensor output difficult. On the other hand, if the puff rate is greater than 100 Hertz, the physical dynamics of the cornea will resist oscillation and inhibit it from responding to the puffs, thereby inhibiting accurate deflection and measurement of the corneal surface. 
     It is important that the measurement beam  24  be properly aligned with the cornea  26 . If it is not properly aligned, the sensor system  20  cannot accurately provide fringe signals for processing. The alignment accuracy is preferably within a tolerance of +0.5 mm. 
     Referring to FIG. 5, there is shown schematically a system for aligning the interferometer  10  such that the measurement beam  24  and the nozzle  30  are properly aligned with the cornea  26  during the measurement process. There is provided a horizontal support  40  having mounted thereon a combined chin rest and head rest  42  for supporting the head of the subject S being tested. The interferometer system  10  is supported on the support  40  in a position such that the nozzle  30  will be generally aligned with the cornea  26  of the subject S whose head is supported in the chin/head rest  42 . In order to move the interferometer system  10  and the nozzle  30  extending from its housing  11 , there is provided a conventional knob  44  for effecting vertical adjustments and a controller  45  which may be moved left or right to effect, in cooperation with the knob  44 , for alignment of the measurement beam  24  of the interferometer system  10  with the cornea  26 . The controller  45  or other conventional adjustment mechanism may also effect movement of the interferometer system  10  toward and away from the cornea  26  to ensure proper spacing of the outlet orifice  31  from the surface of the cornea  26 . 
     In order to determine when the interferometer system is properly aligned with the cornea  26 , there may be provided a system of amplifiers and speakers or, preferably, a computer. In the simplest embodiments utilizing the speakers, the speakers are attached to an amplifier such that the input to the amplifier is a signal proportional to the brightness of the reflected measurement beam  24 . Variations in such reflected measurement beam  24  due to normal uncontrolled motion are on the order of 500 to 3000 cycles per second. This is a Doppler effect and, in this case, the oscillations are in the normal range of human hearing. When the measurement beam  24  is properly aligned with the cornea  26 , the sensor system  20  and amplifier will cause the speakers to give off a warbling tone thereby indicating that interferometer  10  and the sensor system  20  are in proper alignment. Normally it is possible to achieve this alignment indication in 10 to 20 seconds and to hold the system in alignment for nearly a minute. 
     The measured raw data is the light intensity of the reflected measurement beam illuminating the sensors of the sensor system  20 . The light intensity on the sensor varies rapidly when the cornea relative velocity is large, and varies slowly when the cornea relative velocity is small. Once the measurements have been taken on a person, the raw data consists of a set of rapidly oscillating voltages. 
     If a single sensor or a small number of sensors are used, then the signal processing usually involves taking a running spectral estimation (as a function of time). The frequency of oscillation is directly proportional to the relative speed of the cornea. There is a directional ambiguity in these results, however. 
     If an array sensor, such as that shown in FIGS. 2 and 4, is used, then a pattern of moving light and dark lines emerges from the data. These light and dark moving patterns provide information about the motion of the interferometer rings even if the sensor is offset as in FIG.  4 . The in and out motion of the interferometer rings is directly proportional to the axial deflection of the cornea. 
     The time history of the displacement of the cornea permits the displacement of the cornea due to the periodic air puff to be separated from the displacement due to random human motion. It is the induced displacement of the cornea resulting from periodic air puffs that is related to the intraocular pressure. 
     The present invention permits the measurement of cornea deflection as a function of time. This is done by extracting the beam interference frequency which is the periodic oscillation of the optical brightness caused by the interferometric interaction between the reference beam and the beam reflected from the cornea. A plot of this value as a function of time provides the speed of the cornea as a function of time. If the speed is then integrated, it is possible to obtain a measure of the cornea deflection as a function of time. The signal may then be filtered in such a way as to separate the background human motion induced signals from the periodic puff induced signals. From the puff induced cornea displacement, intraocular pressure may be computed based upon calibration experiments that establish the relationship between intraocular pressure and the cornea displacement. 
     The present invention provides a non-contact instrument as a small unit which can be mounted on a support which can be easily adjusted for alignment using a joy stick or other convenient positioning system. Additionally, it may be constructed in a sufficiently small package that it can be attached to a slit-lamp eye inspection unit for convenient use during a conventional eye exam. The method of operation of the instrument of the present invention permits accurate alignment of the unit with the cornea. 
     Many modifications will become readily apparent to those skilled in the art. Accordingly, the scope of the present invention should be determined only by the scope of the claims appended thereto.