Abstract:
Badal Optometer and rotating cylinders are inserted in the AO-OCT to correct large spectacle aberrations such as myopia, hyperopic and astigmatism for ease of clinical use and reduction. Spherical mirrors in the sets of the telescope are rotated orthogonally to reduce aberrations and beam displacement caused by the scanners. This produces greatly reduced AO registration errors and improved AO performance to enable high order aberration correction in a patient eyes.

Description:
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/146,575, filed Jan. 22, 2009, titled: “Compact Adaptive Optic-Optical Coherence Tomography System” incorporated herein by reference. This application is a continuation-in-part of U.S. patent application Ser. No. 11/874,832 titled “High-resolution Adaptive Optics Scanning Laser Opthalmoscope with Multiple Deformable Mirrors,” filed Oct. 18, 2007 now U.S. Pat. No. 7,665,844, incorporated herein by reference. U.S. patent application Ser. No. 11/874,832 claims priority to U.S. Provisional Patent Application Ser. No. 60/852,857, filed Oct. 18, 2006. 
    
    
     The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to optical coherence tomography, and more specifically, it relates to the use of adaptive optics in optical coherence tomography for retinal imaging. 
     2. Description of Related Art 
     Optical coherence tomography (OCT) is a leading modality for non-invasive, in vivo imaging of the human retina, providing high sensitivity, speed and axial resolution in quantifying retinal pathology. The lateral resolution of the OCT image of the retina, however, is poor due to ocular aberrations. 
     Adaptive optics (AO) have been incorporated into OCT systems to increase the lateral resolution by measuring and subsequently compensating for the aberrations in real time. This technology was originally used for correcting image degradation due to atmospheric aberration in astronomy. In recent years, AO technology has been applied to several instruments for retinal imaging, such as flood illumination fundus imaging, scanning laser opthalmoscopy, and most recently, ophthalmic optical coherent tomography. In an AO system, the ocular aberrations of the test subjects are measured by a wavefront sensor. The measured wavefront errors are then used to adjust the shape of a deformable mirror (DM) until the wavefront aberrations are minimized. 
     Population studies have shown that many people have both low-order aberrations with large magnitudes and high-order aberrations with small magnitudes. For these subjects, current technology cannot deliver the phase compensation needed using a single deformable mirror (DM). The use of two deformable mirrors has been investigated. An AO-OCT system incorporating two deformable mirrors was demonstrated by Zawadzki et al. The bimorph DM from AOptix used in that OCT system had a relatively high dynamic range and could correct defocus and astigmatism up to ±3D. This obviated the need for the meticulous use of trial lenses to correct the refractive errors of a subject. The system included a micro-electro-mechanical system (MEMS) DM (from Boston Micro Machine) that had 144 pixels and 1.5 μm stroke, which was used to correct the residual high-order aberrations left by the bimorph DM compensation. Both deformable mirrors were placed in the non-scanning path. Such arrangement, however, generated noticeable beam distortions at the deformable mirrors and the wavefront sensor when large refractive corrections were needed. 
     In an AO-OCT system demonstrated by Zhang et al., the bimorph mirror was placed one relay telescope away from the eye. This minimized the propagation of ocular refractive errors through the system prior to compensation. This arrangement greatly reduced the pupil distortion at the deformable mirrors and wavefront sensor. However, because the bimorph mirror was placed in the scanning path (i.e., between the eye and scanners), the beam at the eye pupil shifted with the changing incidence angles of the light as the beam was steered by the scanners. This would result in degradation of the AO-OCT system. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a compact AO-OCT system that includes a Badal optometer and rotating cylindrical lenses to compensate for defocus and astigmatism, respectively, and a micro-electro-mechanical system (MEMS)-based adaptive optic to compensate for high-order aberrations. 
     This and other objects will be apparent based on the disclosure herein. 
     Embodiments of the present invention include an optical apparatus to correct the spectacle aberrations of patients. Such aberrations include myopia, hyperopia and astigmatism. Residual aberrations are compensated by a MEMS DM, which is placed between the light source, e.g., a superluminescent diode (SLD) light source, and the XY scanners in the AO-OCT system, to minimize the pupil shift at the MEMS. In addition, the compact AO-OCT system is optimized to have minimum system aberrations to reduce AO registration errors and improve AO performance. Unlike the previous large systems which were set-up on a standard lab table, embodiments of the present AO-OCT system are specifically designed for clinical use requiring compact size, low cost, and high reliability. 
     Accordingly, embodiments of compact MEMS-based adaptive optic (AO) optical coherence tomography (OCT) systems with improved AO performance and ease of clinical use are described. Adaptive optic systems often consist of a Shack-Hartmann wavefront sensor and a deformable mirror that measures and corrects ocular and system aberrations. Because of limitations on current deformable mirror technologies, the amount of real-time ocular-aberration compensation is restricted and small in previous AO-OCT instruments. The present invention eliminates the tedious process of using trial lenses in clinical imaging. Different amounts of spectacle aberration compensation are achieved, e.g., by motorized stages, and automated with the AO computer for ease of clinical use. In addition, embodiments of the present invention include a compact AO-OCT system that is optimized to have minimum system aberrations to reduce AO registration errors and improve AO performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and form a part of the disclosure, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is an illustration of a spherical mirror. 
         FIG. 2A  shows the optical layout in the YZ plane of an AO-OCT sample channel. 
         FIG. 2B  shows the optical layout in the XZ plane of an AO-OCT sample channel. 
         FIG. 3A  is an illustration of beam shift for different field angles in an afocal telescope. 
         FIG. 3B  is a magnified view of the region of  FIG. 3A  near the image plane. 
         FIG. 4A  shows large beam shifts when mirrors are rotated in the same planes of various scanning angles at the pupil plane of the eye of the AO-OCT design. 
         FIG. 4B  shows small beam shift when mirrors are rotated orthogonally at various scanning angles at the pupil plane of the eye of the AO-OCT design. 
         FIG. 5  shows the layout of an optical apparatus for spectacle aberration compensation. 
         FIG. 6  shows amount of defocus compensated versus the moving distance of the stage of the Badal optometer. 
         FIG. 7A  shows an embodiment of a present AO-OCT optical system layout for a reference channel and detection channel. 
         FIG. 7B  shows an embodiment of a present AO-OCT optical system layout for a sample channel. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention include an AO-OCT system consisting of three channels: (1) a sample (or signal) channel, equipped with an AO for collecting the retinal image, (2) a reference channel with an optical path length matching that of the sample channel, and (3) detection channel for recording the combined sample and reference signals. The sample channel length is from the beamsplitter (BS) (e.g., from an 80/20 BS) to the target and back to the splitting point of the BS. The reference channel length is from the BS to a mirror at the end of the channel and back. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Summary of exemplary system components 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Light source 
                 Superluminescent diode (SLD) from Superlum 
               
               
                   
                 λ = 842 nm, δλ = 50 nm 
               
               
                 Deformable mirror 
                 MEMS from Boston Michomachines Corp. 
               
               
                   
                 3.3 mm × 3.3 mm optical aperture, 
               
               
                   
                 12 × 12 actuators 
               
               
                   
                 1.5 μm stroke 
               
               
                 Horizontal/vertical 
                 Cambridge Technology; 6220M40 galvanometric 
               
               
                 scanners 
                 scanner, ±20° 
               
               
                 Wavefront sensor 
                 Shack-Hartmann wavefront sensor 
               
               
                   
                 Lens array from Adaptive Optics Associates 
               
               
                   
                 20 × 20 elements, 500 μm pitch, 30 mm focal 
               
               
                   
                 length and CCD camera from Dalsa 
               
               
                 Diffraction grating 
                 Holographic transmitting diffraction grating 
               
               
                   
                 from Wasatch Photonics 
               
               
                   
                 1200 lines/mm 
               
               
                 CCD camera 
                 Line-scan camera from Atmel, 
               
               
                   
                 12 bit, 2048 pixels 
               
               
                   
               
             
          
         
       
     
     In an embodiment, in the sample channel, the light from a broadband superluminescent diode (SLD) is coupled into a single mode fiber. The light is then collimated and relayed by mirror telescopes to a deformable mirror, the horizontal and vertical scanners, the spherical-cylindrical correction apparatus and finally to the eye. Each component is placed at the image plane of an afocal relay telescope. Spherical mirrors, instead of lenses, are used in the afocal telescope design to reduce back-reflections and minimize chromatic aberrations in the system. This is important because the spectrum of the light source must be very broad to achieve high axial resolution. The off-axis configuration of the reflective spherical mirrors, however, creates substantial system aberrations. It also results in beam displacement at the pupil of the eye due to scanning in both X-Y directions. Both aberrations and beam displacements result in performance degradation of the AO-OCT system. However, the sample channel in the present invention includes a means to compensate aberrations in real-time to achieve substantial higher lateral resolution. 
     In the reference channel, the optical path length is matched to that of the sample channel by folding the optical path with several spherical mirrors. In the detection channel, the light from the sample and reference arms is combined by the fiber coupler and sent to a spectrometer-based detector, which in some embodiments is a holographic transmitting diffraction grating focused onto a line-scan charge coupled device (CCD). 
     As mentioned above, the off-axis configuration of the reflective spherical mirrors creates substantial system aberrations. The deformable mirror is able to compensate a portion of both the optical system aberrations and the ocular aberrations of patients. However, if part of the stroke of the deformable mirror is used to compensate the system aberrations, the magnitude of the compensation of the ocular aberrations is reduced. Furthermore, the aberrations will introduce pupil aberration and distortion, which cause performance degradation of the AO compensation. The present invention minimizes the aberrations from the optical system itself. 
     Important guidelines in the optical design of embodiments of the present invention can be derived from the magnitudes of aberration produced by a spherical mirror.  FIG. 1  is an illustration of a spherical mirror  10  having a radius of curvature (R). The entrance pupil  12  is at distance L from the vertex  14  of the mirror. The incidence angle of the beam is θ. For a single reflective spherical mirror such as the one shown in  FIG. 1 , the magnitude of aberrations is given in the second column of Table 2. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Aberration type 
                 Magnitude 
                 
                   
                     
                       
                         
                           When 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           L 
                         
                         = 
                         
                           R 
                           2 
                         
                       
                     
                   
                 
               
               
                   
                   
               
             
             
               
                   
                 Spherical abberation 
                 
                   
                     
                       
                         1 
                         
                           128 
                           ⁢ 
                           
                             F 
                             3 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         1 
                         
                           128 
                           ⁢ 
                           
                             F 
                             3 
                           
                         
                       
                     
                   
                 
               
               
                   
                   
               
               
                   
                 Coma 
                 
                   
                     
                       
                         
                           
                             
                               ( 
                               
                                 L 
                                 - 
                                 R 
                               
                               ) 
                             
                             ⁢ 
                             θ 
                           
                           ) 
                         
                         
                           16 
                           ⁢ 
                           
                             RF 
                             2 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         θ 
                         
                           32 
                           ⁢ 
                           
                             F 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                   
                   
               
               
                   
                 Astigmatism 
                 
                   
                     
                       
                         
                           
                             
                               ( 
                               
                                 L 
                                 - 
                                 R 
                               
                               ) 
                             
                             2 
                           
                           ⁢ 
                           
                             θ 
                             2 
                           
                         
                         
                           2 
                           ⁢ 
                           
                             R 
                             2 
                           
                           ⁢ 
                           F 
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         
                           θ 
                           2 
                         
                         
                           8 
                           ⁢ 
                           F 
                         
                       
                     
                   
                 
               
               
                   
                   
               
             
          
         
       
     
     The entrance pupil is at distance L from the vertex of the mirror. F stands for the F-number. For a set of afocal telescopes where L is half of R, the magnitude of the aberrations is simplified in the third column. Examination of the magnitude in detail is provided below. 
     1) Spherical/Coma/Astigmatism ˜1/F 3  to 1/F 2  to 1/F. Aberrations decrease as the F-number increases. Thus, afocal telescopes with longer focal lengths have smaller aberrations. 
     2) Spherical/Coma/Astigmatism ˜1 to θ to θ 2 . Spherical aberration is independent of the tilt angle of the reflective mirror. Both coma and astigmatism increase as the tilt angle increases, so afocal telescopes having a smaller tilt angle will have smaller aberrations. 
     3) Spherical/Coma/Astigmatism=1 to 4θF to (4θF) 2 . For a mirror with a radius of curvature of 0.5 m, an entrance pupil diameter of 10 mm and a tilt angle of 5°, the ratio of Spherical/Coma/Astigmatism=1 to 8.7 to 75.7. In most cases, astigmatism is the dominating aberration in the system. 
     In addition, most telescopes typically form images over a curved surface instead of a flat surface. The aberration of the field curvature can be expressed as a defocus term θ 2 /16R p F 2  where R p  is the radius of Petzval field curvature. A system with small tilt angle and large focal length thus has smaller aberration due to field curvature. 
       FIG. 2A  shows the optical layout in the YZ plane of an embodiment of an AO-OCT sample channel. The present invention removes system aberration. The references numbers are identical to those used in  FIGS. 7A and 7B  because the elements and configuration are identical. Astigmatism is removed in the following configuration. If the optical layout is made on a table top, the view of  FIG. 2A  would be a top view.  FIG. 2B  shows the optical layout of  FIG. 2A  in the XZ plane. Again, if the optical layout is made on a table top, the view of  FIG. 2B  would be a side view from the bottom edge of  FIG. 2A . The two mirrors ( 98 ,  100 ) in the relay telescope between horizontal scanner and vertical scanner are tilted orthogonally. The amount of tilt is determined by the amount of astigmatism produced from the reflections by the spherical mirrors. The amount of tilt therefore is set to remove the astigmatism. 
     The above description presents some challenges for the design of a compact AO-OCT system. For example, long focal lengths result in a large footprint and small tilt angles cause physical conflicts between components. Embodiments of the present invention minimize the total aberrations by targeting astigmatism because it is the dominating aberration in the afocal telescope. By tilting the second spherical mirror  100  of the afocal telescope in the orthogonal plane, the two spherical mirrors canceled the astigmatism that would be introduced by a single mirror. An additional benefit is derived because the mirrors are placed at different heights, so the physical conflict is less an issue for the smaller tilt angles. The two mirrors ( 98 ,  100 ) in the relay telescope between horizontal scanner and vertical scanner are tilted orthogonally. The Y-scanning mirror  102  is placed in the same line between the two mirrors ( 98 ,  100 ) in the YZ plane, but at a different height. This leads to a compact design and can be used to correct for system aberrations as discussed below. 
     In one embodiment, light is scanned on the retina in a raster pattern with a horizontal scanner and a vertical scanner (mirrors  96  and  102  in  FIGS. 2A and 2B ). These mirrors can each be mounted on a galvanometric scanner (e.g., Cambridge Technology; 6220M40 galvanometric scanner, ±20°). The two scanners are separated by a relay telescope (mirrors  98  and  100  in  FIGS. 2A and 2B ) designed to make them optically conjugate to each other and to the entrance pupil of the eye. This minimizes the movement of the scanning beam at the pupil. 
       FIG. 3A  is an illustration of a beam shift for different field angles in an afocal telescope. A beam  30  is focused at  30 ′ by spherical mirror  31  and is then reflected by mirror  33  to image plane  40 . Another beam  32  is directed at spherical mirror  31  at a different angle than that of beam  30 . Beam  32  is focused at  32 ′ and then reflected from spherical mirror  33  to image plane  40 .  FIG. 3B  is a magnified view of the region of  FIG. 3A  near the image plane  40 . Notice the slight difference in position of beams  30  and  32  on image plane  40 . In operation, the chief rays from various field angles do not all pass through the center of the pupil and thus, the beam is shifted as a function of the field angle. This will degrade the system performance in several ways. First, this will cause beam clipping at the pupil, which causes intensity fluctuation as a function of scanning angle. Second, the phase correction the DM would apply would depend on the scanning angle, thereby resulting in the AO system suffering a kind of anisoplanatism. Third, the speed of the scanners is much faster than the response time of the wavefront sensor and the DM. Hence, the Shack-Hartmann wavefront sensor would measure the averaged wavefront and high spatial frequency aberrations would be averaged out and uncompensated. Both horizontal and vertical scanners produce beam shifts. The shifts have to be minimized for the optimal performance. It is desirable that the shift is less than half of the sub-aperture at the Shack-Hartman wavefront sensor. 
       FIGS. 4A and 4B  illustrate the beam shifts of various scanning angles at the pupil plane of the eye of the AO-OCT design.  FIG. 4A  shows large beam shifts when mirrors are rotated in the same planes of various scanning angles at the pupil plane of the eye of the AO-OCT design.  FIG. 4B  shows small beam shifts when mirrors are rotated orthogonally at various scanning angles at the pupil plane of the eye of the AO-OCT design. The beam shift is proportional to the tilt angle of the reflective mirror and the F-number. So a smaller radius of curvature of the reflective mirror is preferred to minimize the beam displacement due to the scanning. This is beneficial for a compact design. However, a small radius of curvature of the mirrors would increase aberrations. Radii of curvatures were compromised to meet the design specification of both aberrations and beam displacements. By rotating the second mirror orthogonally to the first mirror in the afocal telescope, both aberrations and beam displacements are reduced. 
     In an exemplary layout of an optical apparatus designed for spectacle aberration compensation, defocus is compensated by a Badal optometer and astigmatism is compensated by rotating cylinders. Badal lenses and rotating cylinders are used to compensate the large spectacle aberrations in the design of the AO-OCT for clinical use because of the limitations of the deformable mirrors. Current deformable mirror technology has limited stroke, and many of them have not been yet proven sufficiently reliable for long-term clinical testing. 
     In an embodiment shown in  FIG. 5 , the Badal optometer includes two achromatic transmissive lenses ( 50 ,  52 ) with focal lengths of 100 mm, and further includes two folding reflective mirrors ( 54 ,  56 ). The two folding mirrors are located on and controlled by one motorized translation stage  58 . The two rotating cylinders  59  are placed at the conjugate plane of the eye pupil to compensate for the astigmatism in the eye.  FIG. 6  shows the amount of defocus compensated versus the moving distance of the stage. The amount of defocus the apparatus could compensate was linear to the moving distance of the two mirrors. 
       FIG. 7A  shows an optical system layout for a reference channel and detection channel for an embodiment of the present AO-OCT system.  FIG. 7B  shows a sample channel for an embodiment of the present AO-OCT system. To make the AO-OCT system compact, the sample channel is set-up on one optical breadboard and the detection channel/reference channel is set up on a second optical breadboard. The two breadboards are stacked together by supporting posts. 
     Referring to  FIG. 7A , superluminescent laser diode  60  produces laser light that passes through an optical isolator  62  and is coupled into 80/20 fiber beamsplitter (BS)  64 . One arm of BS  64  begins the path of the sample channel, and produces a diverging output beam that is collimated by lens  66 , the output of which is passes through aperture  68 , and beamsplitter  70  to be reflected from spherical mirror  72 . (Another arm of BS  64  is passed through a reference channel as discussed below.) The beam is then directed to a first telescope mirror  74  (which can be a spherical mirror) that directs the beam upwards to a second telescope mirror  76  (which can be a spherical mirror), which is shown on  FIG. 7B . Notice that the beam reflected from spherical mirror  72  goes through a number of foci as it traverses the sample channel to the target  86 . Thus, from telescope mirror  76  to the target  86  the beam traverses a path consecutively through optics as follows: spherical mirror  88 , MEMS adaptive optic  90 , spherical mirror  92 , spherical mirror  94 , scanning mirror  96  (flat), spherical mirror  98 , spherical mirror  100 , scanning mirror  102  (flat), spherical mirror  104 , spherical mirror  106 , spherical mirror  108 , reflective optic  110 , cylindrical lenses  112 , achromatic transmissive lens  114 , mirror  116 , mirror  118 , achromatic transmissive lens  120 , mirror  122  and beamsplitter  124 . The target  86  is intended to be a human eye the gaze of which looks through beamsplitter  125  and is fixed on point  126 . The pupil image is directed from beamsplitter  125  to mirror  128  and finally to pupil camera  130 . 
     Light reflected from target  86  travels in reverse back to spherical mirror  72  ( FIG. 7A ) which reflects the light to beamsplitter  70  and then to SHWS  132 . Light traversing beamsplitter  70  is focused back into BS  64 , the output of which is collimated with lens  134 , dispersed with grating  136 , and imaged with lens  138  onto CCD camera  140 . Optical isolator  62  prevents the light returning from target  86  from entering the light source  60 . 
     As mentioned above, a second arm of BS  64  ( FIG. 7A ) is sent to a reference channel where it is collimated with lens  142  and is reflected by mirrors  151 - 156  to be focused by lens  158  onto mirror  160 . Lens  158  and mirror  160  are mounted on a movable stage  162  that is used to change the path length of the reference channel. Light reflected from mirror  160  travels back through BS  64  and onto CCD camera  140  to produce an interference pattern upon interaction with light from target  86 . 
     Alternate components may be substituted for the components described in the embodiment of  FIGS. 7A and 7B . Such alternates will be apparent to those skilled in the art based on this description. For example, the light source may be any appropriate light source having a large optical bandwidth and a short coherence length. The system can be configured without the use of a fiber optic beamsplitter, using, e.g., standard beamsplitters, lenses and mirrors. 
     The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments disclosed were meant only to explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is to be defined by the following claims.