Abstract:
An eye examination system is presented that obtains several parameters of the eye. A system according to some embodiments of the present invention include a keratometry system, a low coherence reflectometry system, and a low coherence interferometry system co-coupled to the eye. In some embodiments, the low coherence interferometry system can provide interferometric tomography data. A processor can be coupled to receive data from the keratometry system, the low coherence reflectometry system, and the low coherence interferometry system and calculate at least one parameter of the eye from that data.

Description:
RELATED APPLICATIONS 
   The present application claims priority to Provisional Application No. 60/543,862, “Optical Apparatus and Methods for Eye Examine, ” by Jay Wei, filed on Feb. 10, 2004, which is herein incorporated by reference in its entirety. 

   BACKGROUND 
   1. Field of the Invention 
   The present invention is related to optical apparatus and methods for obtaining parameters in an eye examination and, in particular, to optical interferometric apparatus for performing eye examinations. 
   2. Discussion of Related Art 
   Refractive surgery to correct refraction error in the human eye has been widely accepted. Several different types of surgical methods have been explored for this purpose. PRK and LASIK are surgical methods that use laser radiation to ablate cornea tissue in order to change the refractive power of the cornea. Phakic IOL surgery implants intra-ocular lenses in either the anterior or posterior chamber of the eye to compensate for refraction error in the eye. To achieve good clinical outcomes from any of these surgeries, physical dimensions of the eye such as cornea thickness, anterior chamber depth, angle-to-angle width, and sucus-to-sucus width, for example, need to be accurately measured pre-operatively. In some cases, post surgery diagnosis, which is important for patient follow up, also requires good measurements of these physical parameters of the eye. 
   In another common surgery on the eye, cataract surgery has been performed for many years on cataract patients. To achieve an accurate refraction power as a result of the surgery, parameters of the eye such as the axial length, cornea power, anterior chamber depth, and the equatorial plane of the crystalline lens should be accurately measured in order to calculate the power of the intra-ocular lens to be implanted in a Phakic IOL surgery. The true cornea power is especially important when the cataract surgery is performed on a post-Lasik patient. 
   There are various existing devices that can be used to measure one or two of these parameters, but not all of them in a single apparatus. To acquire all required parameters for surgical preparation, the measurement of various physical parameters of the eye needs to be performed by different instruments. Sometimes, inconsistency in measured results will occur due to discrepancies between the instruments and discrepancies of alignment of the eye with the various instruments. For example, the cornea power can be measured with an Orbscan (by Orbtek, Bausch &amp; Lomb, Rochester, N.Y.) by combination of Pacido ring and slit projection methods. The Pacido ring method uses multiple concentric ring-shaped light sources to illuminate the eye. The cornea is like a mirror and reflects the illumination from the light sources into a CCD camera. The image size and shape of the rings formed by the cornea can then be used to analyze the contour of the cornea. The slit projection method illuminates the cornea with a thin slit of light. The scattering caused by the illuminated cornea tissue can then be imaged in a CCD camera. The cornea thickness and curvature can be calculated from the image of the illuminated cornea. 
   Due to the slow scan speed, eye motion effects can cause the results of such a test to be highly inaccurate. The anterior chamber depth can be measured either by the slit projection method (IOL Master (by Carl Zeiss, Jena, Germany), Orbscan (by Orbtek, Bausch &amp; Lomb, Rochester, N.Y.), Ultrasound Microscope (UBM by Paradiam Medical, Salt Lake City, Utah), Artemis (by Ultralink, LLC, St. Petersburg, Fla.), B-scan (by Ophthalmic Technologies, Inc., Toronto, Canada), or Optical Coherence Tomography (OCT) (Case Western Reserve University and Cleveland Clinic Foundation). None of these devices is capable of measuring all required parameters in a single compact apparatus. 
   Because the refractive surgery and cataract surgery can be performed by the same surgeon, it is desired to have a single compact apparatus to measure all of the parameters required by both refractive and cataract surgeries. Therefore, there is a need to provide a single instrument that provides measurements of groups of parameters in order to eliminate inaccuracies due to utilization of several instruments for measurements of these parameters. 
   SUMMARY 
   The current invention is related to an optical apparatus and method for examining an eye in order to obtain a plurality of optical parameters relevant to an optical surgery on the eye. The optical apparatus is associated with a low coherence interferometer that can be used for non-invasive optical imaging and measurement. 
   An eye examination system according to the present invention can include a low coherence reflectometer coupled to illuminate an eye; a low coherence interferometer coupled to illuminate the eye; an LED and camera system coupled to measure a virtual image of the LEDs reflected from the eye; and a processor coupled to receive data from the low coherence reflectometer, the low coherence interferometer, and the camera system and to calculate at least one parameter of the eye. In some embodiments, the system can further include a visual target coupled to provide an image to the eye. 
   A method of obtaining eye parameters according to the present invention can include receiving reflectometry data from a low coherence reflectometer coupled to an eye; receiving interferometry data from a low coherence interferometer coupled to the eye; receiving keratometry data from a camera coupled to receive a virtual image of a plurality of LEDs from the eye; and calculating at least one parameter of the eye. 
   These and other embodiments are further discussed below with respect to the following figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  illustrate measurements of the cornea power and cornea thickness of an eye. 
       FIG. 2  illustrates a system for conventionally measuring the Sim-K, anterior chamber depth, and the eye axial length. 
       FIG. 3  illustrates a system for conventionally measuring the angle-to-angle width, anterior chamber depth, and cornea thickness of an eye. 
       FIG. 4  illustrates an eye measurement system according to some embodiments of the present invention. 
       FIG. 5  illustrates another embodiment of an eye measurement system according to some embodiments of the present invention. 
   

   In the figures, elements having the same designation have the same or similar functions. 
   DETAILED DESCRIPTION 
     FIGS. 1A and 1B  illustrate a current state-of-the-art slit projection method to measure cornea thickness. As shown in  FIG. 1A , a slit of light  101 , also referred to herein as a slit beam, is projected on to a cornea  100 . Light  110  is reflected from a front surface  102  of cornea  100  and light  104  is reflected from a back surface  103  of cornea  100 . Also, because the cornea will scatter light, the segment of cornea stoma  105  will be seen by an observer or CCD camera through an imaging system. A corresponding reflection image  108  from eye  111  of slit beam  101  that is scanned across cornea  100  has a shape of an early moon on the cornea. From an incident angle of slit beam  101  and imaging system parameters, the thickness of cornea  100  can be calculated from the width of a moon-like reflection image  108 . In principle, the cornea curvature can also be calculated from reflected light  110  and  104 . But limited by the scan speed of the slit beam  101  (in the 1 to 2 seconds range), eye motion can cause the measurement to be inaccurate. As a result, cornea power has not been measured by the slit projection principle alone. Orbscan (by B&amp;L) add a Placido ring illumination system to map the front surface cornea curvature and use the thickness to estimate the back cornea surface curvature. The cornea power can then be calculated from these data associated with index of refraction of cornea tissue, which is essentially a constant. The accuracy of this approach suffers due to the inaccuracy of cornea thickness measurement. 
     FIG. 2  illustrates an examination system  200  for performing optical coherence tomography (OCT) on an eye  208 . Optical coherence tomography (OCT) presents a better method of measuring the cornea thickness due to its superior optical resolution. Cornea imaging by OCT with a 10 to 20 μm optical resolution has been demonstrated by Cleveland Clinic Foundation (Arch. of Ophtal. Vol. 119, No. 8, 1179-1185, August 2001). The accuracy is a couple of orders of magnitude better than the slit projection method discussed above with respect to  FIG. 1 . 
   The measurement system shown in  FIG. 2  illustrates a common optical examination system  200  for a cornea scan OCT system. The light from a low coherence interferometer  201  is collimated by a lens  202 . Mirrors  203  and  204  can be mounted relative to each other to intercept and direct an optical beam from collimating lens  202 . Mirrors  203  and  204  can be driven by a scanning mechanism that can scan the beam in two dimensions. A scanning lens  205  can be positioned to intercept the beam from mirror  204  and focus a beam  207  onto the anterior chamber of eye  208 . Scanning mirrors  203  and  204  can be located proximately to the back focal plane of lens  205  so that scanning beam  207  is parallel. A beamsplitter  206  can be positioned to receive beam  207  from lens  205 . Beamsplitter  206  is a dichroic beamsplitter that reflects beam  207  into eye  208  but transmits a video image from eye  208 , illuminated with different wavelengths other than the wavelength of the light source in interferometer  201 , to a CCD camera system  210 . Enough light is reflected back into interferometer  201  by beam splitter  206  for interferometer  201  to operate appropriately. In some embodiments, an imaging lens  209  can be placed between beamsplitter  206  and CCD camera system  210 . Again, due to scan time (current state of art is about 125 ms for one single scan), the desired accuracy of a calculated cornea curvature parameter based on examination system  200  is still not feasible with the OCT scan method because of the inability to immobilize eye  208  during the test. 
   Lenses and other optical systems shown in this disclosure (e.g., lenses  205  and  209 ) can include any number of optical components to accomplish the described function. The lenses and other optical components illustrated here are utilized to demonstrate the overall function. 
     FIG. 3  illustrates another conventional examination system  300  that provides a combination of three different methods to measure parameters required for calculating the power of the intra-ocular lens for cataract surgery. System  300  is commercially available from Carl Zeiss Meditec, Germany. The front surface cornea power is measured with a Keratometer. The anterior chamber depth is measured by a projection slit of light. The axial length of the eye is measured by low coherence reflectometry. 
   As shown in  FIG. 3 , the Keratometer is configured with three or more LEDs  301  to illuminate a cornea  302 . Light from LEDs  301  is reflected from cornea  302  into a CCD camera  305 . Therefore, the virtual image of the LEDs is measured in CCD camera system  305 . The cornea curvature structure can be calculated from the virtual image size of the LEDs reflected from a front surface of cornea  302 . 
   A low coherence light source from the sample arm of a low coherence reflectometer  304  is directed into cornea  302  by a beamsplitter  303 . A single eye axial length (from cornea to retina) measurement can be performed. In some embodiments, reflectometer  304  can be an OCT interferometer without transverse scan. 
   The slit projection light source is not shown in system  300  of  FIG. 3 , but the slit image is imaged on CCD camera  305  for analysis. The true cornea power can not be calculated, because the back cornea surface curvature and cornea thickness are not available utilizing system  300 . 
     FIG. 4  shows an examination system  400  according to some embodiments of the current invention. Some embodiments of system  400  are capable of measuring most, if not all, of the important parameters that are needed in the performance of refractive and cataract surgeries. The anterior chamber eye image is acquired by scanning a near infrared beam on the anterior chamber of an eye  401 . A sample beam from a low coherence interferometer  408  is collimated by a lens  407  and spatially scanned by scanning mirrors  406  and  405 . Scanning mirrors  406  and  405  can be driven by motors (not shown) in order to scan the beam from interferometer  408  across eye  401 . The beam from scanning mirrors  406  and  405  is focused onto the anterior chamber of eye  401  by a scanning lens  404 . In some embodiments, beamsplitters  403  and  402  are positioned to direct light between lens  404  and eye  401 . Scanning mirrors  406  and  405  are proximately located at the back focal plane of lens  404  in order that the scanning beam from low coherence interferometer  408  is parallel in front of eye  401 . The light reflected from the tissue of the anterior chamber of eye  401  will be propagated back into low coherence interferometer  408 . The interference signal from low coherence interferometer  408  then will be processed by a central processing unit  440  to form an optical coherence tomography. Low coherence interferometer  408  and a low coherence reflectometer  420  can be one of a number of low coherence interferometer arrangements, including the low coherence interferometers described in U.S. application Ser. No. 11/055,900, filed concurrently with the present application by Jay Wei, herein incorporated by reference in its entirety. 
   In some embodiments, light from three or more LEDs  411  is reflected from the cornea of eye  401  onto a CCD or CMOS camera  415  in order to measure front surface cornea power. The virtual image of LEDs  411  reflected from the cornea of eye  401  can, in some embodiments, be imaged by lenses  413  and  414  camera  415 . The LED image reflected by the cornea of eye  401  onto camera  415  can be acquired in central processing unit  440  for calculating the front surface cornea curvature. 
   Once the curvature of the front surface cornea is known, the shape of the back surface of the cornea can be obtained by adding the thickness of the cornea, acquired by the optical coherence tomography of the cornea from low coherence interferometer  408 , to the front surface shape. With the known average index of refraction of the human cornea, which is 1.38 at an OCT scan wavelength of about 1300 nm, the cornea power can be calculated with a simple well-known optical equation. Central processing unit  440 , then, receives the image from camera  415 , tomography data from interferometer  408 , and reflectometer data from low coherence reflectometer  420  to determine all of the parameters needed to characterize eye  401 . 
   A light beam  421  from low coherence reflectometer  420  can be utilized to measure an eye axial length  423 . As shown in  FIG. 4 , light beam  421  can be projected onto eye  401  by a lens  422  through beamsplitters  403  and  402 . More particularly, light beam  421  can be transmitted through beam splitter  403  and reflected into eye  401  by beamsplitter  402 . Optical beams reflected from the cornea and retina of eye  401  will propagate back to low coherence reflectometer  420 . The distance between the retina and the cornea can then be calculated by central processing unit  440  from the interference signal generated in low coherence reflectometer  420 . 
   In some embodiments, a visual target  430  can be seen by eye  401  of the patient, through lenses  431  and  432  and beamsplitters  412  and  402 . The target can move back and forth in a fashion controlled by central processing unit  440  to compensate for the patient&#39;s refraction error. Visual target  430  can serve at least three purposes. First, visual target  430  provides a reference for the patient to fixate on during the examine. Second, visual target  430  can force the patient to focus to the desired accommodation distance. Third, visual target  430  can be a visual acuity target for subjective refraction tests on the patient. The second purpose is important for Phakic IOL implant and Presbyopia implant surgery preparation and post surgery diagnosis. The equatorial plane of the crystalline lens to be inserted by these surgeries can either be visualized on a light pigmented iris eye or estimated by the front and back surface of the crystalline lens. The cornea power, anterior chamber depth, equatorial plane of the crystalline lens, and the eye axial length contain all the optical information required by a cataract surgeon to calculate the IOL power for the implant. The angle-to-angle width measured by low coherence interferometer  408  is used for fitting angle supported anterior chamber Phakic IOL, and the sucus-to-sucus width is used for fitting a posterior chamber Phakic IOL. 
   As shown in  FIG. 4 , each of LEDs  411 , low coherence reflectometer  420 , low coherence interferometer  408 , and visual target  430  operate at sufficiently different wavelengths that light is routed correctly through system  400 . For example, beam splitter  402  reflects light from low coherence reflectometer  420  and low coherence interferometer  408  and passes light from LEDs  411  and from visual target  430 . Additionaly, beam splitter  403  transmits light to low coherence reflectometer  420  and reflects light to low coherence interferometer  408 . Further, beam splitter  412  transmits light from LEDs  411  and reflects light from visual target  430 . In some embodiments of the invention, low coherence reflectometer  420 , low coherence interferometer  408 , and diodes  411  can be operated simultaneously so that all of the data is taken simultaneously. In some embodiments, however, sequential operation of low coherence reflectometer  420 , low coherence interferometer  408 , and diodes  411  may be utilized. 
     FIG. 5  shows an embodiment of measurement system  500 , which is another embodiment of the current invention. Because cornea  420  should be positioned at the front focal plane of scanning lens  404 , as shown in  FIG. 4 , and scanning mirrors  405  and  406 . should be close to back focal plane of lens  404 , beamspitters  402  and  403  increase the total optical path of low coherence interferometer  408 . The diameter of imaging lens  432  and  413  also should be increased due to the increase distance between cornea  420  and lens  432 . These aspects of system  400  can result in increased size and cost. 
   In system  500  of  FIG. 5 , a lens  501  can be placed between eye  401  and beamsplitter  402 . It is a common component of all optical paths from eye  401 . Such an arrangement significantly reduces the size and cost of system  500  because a lens  512 , a lens  506  and a lens  508  can be smaller than lens  407 , lenses  432  and  431 , and lenses  413  and  414 , respectively. Another advantage of system  500  is that a beamsplitter  519  and a window  520  can provide a see-through scene which eliminates the instrument&#39;s myopia effect. 
   Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.