Abstract:
An optical coherence tomography scanning system includes a lens divided into two halves: one optimized to achieve normal incidence angles on the posterior surface of the eye&#39;s lens, the other optimized for the anterior surface of the eye&#39;s lens.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application No. 61/414,088, filed Nov. 16, 2010, whose disclosure is hereby incorporated by reference in its entirety into the present disclosure. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed to a scanning optical system and more particularly to such a system having asymmetric focusing optics. 
       DESCRIPTION OF RELATED ART 
       [0003]    Large-scan-depth anterior segment OCT is important in understanding accommodation to measure the following: lens shape change with accommodation, age-related optical changes in the lens, and the efficacy of accommodative intraocular lenses (IOLs). However, the limitations of current anterior segment OCTs include a limited depth penetration, as shown in  FIG. 1A  and the need for pupil dilation, as shown in  FIG. 1B . 
         [0004]    More specifically, the total axial length of the anterior segment from the apex of the anterior cornea to the back surface of the lens is approximately 7.7 mm (10.2 mm optical path length at n=1.33). That poses a challenge, since sensitivity of an OCT system decreases with depth. Thus, the signal from the deeper ocular surfaces, especially the posterior surface of the lens, becomes weak. Most of the commercial anterior segment OCT systems use telecentric scan geometry in which the beam is shined straight into the eye parallel to the optical axis. The angle of incidence on the ocular surfaces, and especially the posterior surface of the crystalline lens, increases as the regions farther away from the optical axis of the eye are scanned. That leads to a significant reduction in the amount of backscattered light collected by the OCT system. The other limitation with traditional telecentric scan is the fact that pupil dilation is necessary to image a large diameter of the lens. The pupil dilation may cause some differences in accommodative response from naturally stimulated accommodation in which pupil size actually decreases with accommodation (known as pupil miosis). 
       SUMMARY OF THE INVENTION 
       [0005]    It is therefore an object of the invention to overcome those limitations. 
         [0006]    To achieve the above and other objects, the present invention is directed to an asymmetric focusing optics system which provides an angle of incidence of each scan beam normal to the ocular surfaces. That system includes a lens divided into two halves: one optimized to achieve normal incidence angles on the posterior surface of the lens, the other optimized for the anterior surface of the lens. Two semicircular halves from each system were glued together to form a single optical system. That configuration results in a substantial increase in light intensity reflected from each surface. 
         [0007]    The inventor&#39;s work is described in Yadav et al, “Scanning system design for large scan depth anterior segment optical coherence tomography,”  Optics Letters , Vol. 35, no. 11, Jun. 1, 2010, whose disclosure is hereby incorporated by reference in its entirety into the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    A preferred embodiment of the present invention will be set forth in detail with reference to the drawings, in which: 
           [0009]      FIGS. 1A and 1B  are a diagram and a photograph that show drawbacks of present optical coherence tomography; 
           [0010]      FIGS. 2A-2C  are diagrams that show a first design principle of the present invention; 
           [0011]      FIGS. 3A-3D  and  3 F are drawings, and  FIG. 3E  is a photograph, that show a second design principle of the present invention; 
           [0012]      FIGS. 4A and 4B  are diagrams that show image distortion; 
           [0013]      FIG. 5  is a set of scans that show imaging performance; 
           [0014]      FIG. 6  is a graph that shows the improvement in signal-to-noise ratio; and 
           [0015]      FIG. 7  is a graph that shows distortion-corrected surface profiles. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]    A preferred embodiment of the present invention will be set forth in detail with reference to the drawings, in which like reference numerals refer to like elements throughout. 
         [0017]    The first design principle will be described with reference to  FIGS. 2A-2C . The scanning system  200  includes telecentric optics  202  configured for normal incidence on the ocular surfaces of the eye E. The telecentric optics  202  include optics  204  configured for normal incidence on the anterior surface A of the lens L, as shown in  FIG. 2B , and optics  206  configured for normal incidence on the posterior surface P of the lens L, as shown in  FIG. 2C . Thus, the scanning system  200  is asymmetric. 
         [0018]    The second design principle will be described with reference to  FIGS. 3A-3D . As shown in  FIG. 3A , when the scanning light is made normally incident on the anterior surface A of the lens L, the scanning plane is a first scanning plane SP 1  behind the lens. However, when the scanning light is made normally incident on the posterior surface P of the lens L, the scanning plane is a second scanning plane SP 2  between the cornea C and the lens L. 
         [0019]      FIG. 3B  shows a ray diagram of the asymmetric scanning system of the preferred embodiment. The optical elements include a scanning mirror  302 , a first concave mirror  304 , and a second concave mirror  306 .  FIG. 3C  adds a focusing lens  308  and shows the position of the above elements relative to the eye E. As explained above, the focusing lens  308  includes two half lenses  308   a ,  308   b  glued or otherwise fastened together to achieve the two types of scanning noted above. The first concave mirror  304  is asymmetric and includes two components corresponding to the two half lenses; the second concave mirror  306  is symmetric.  FIG. 3D  shows the above and also optics including an amplified frequency swept laser  310 , a 10/90 beamsplitter  312 , a circulator  314 , a 50/50 beamsplitter  316 , collimators  318  and  320 , a reference mirror  322 , a dual balanced detector  324 , and a Mach-Zehnder interferometer  326 .  FIGS. 3E and 3F  are a photograph of a test system and a diagram of a test eye used in the system, respectively. 
         [0020]    Two optical systems are used in the preferred embodiment. One is the asymmetric relay optics with one asymmetric concave mirror and symmetric concave mirror with different radii of curvatures. The other is the asymmetric focusing lens in front of the eye. These two systems are dependent each other to achieve what is needed; if the design of one of the two systems is changed, the other has to be redesigned as well. 
         [0021]      FIGS. 4A and 4B  show image distortion in telemetric versus asymmetric scans. Owing to the nontelecentric nature of the present scanning method, large amounts of refractive distortion are expected in the images even for the first surface. The expected distortion was theoretically evaluated by optical ray tracing software (CodeV, Optical Research Associates).  FIG. 4A  shows the expected raw images from anterior and posterior lens surface scan geometries. Note that these images represent only half of the anterior segment. To correct for these distortions, a flat surface, placed at the same axial location as the first surface of the model eye, was imaged, as shown in  FIG. 4B . The distortion obtained for the flat surface was then used as a reference to construct the true first surface. Subsequent surface distortions were then compensated by using ray tracing. 
         [0022]      FIG. 5  shows imaging performance in traditional versus asymmetric scans.  FIG. 6  shows the improvement in the signal-to-noise ratio.  FIG. 7  shows distortion-corrected surface profiles. 
         [0023]    More specifically, the images obtained by a swept source OCT system (λ=1300 nm, δλ=100 nm) for the model eye by using a conventional telecentric scan ( FIG. 5 , left) and the scanning system according to the preferred embodiment ( FIG. 5 , right) are shown. The distortion observed in  FIG. 5  has a good agreement with the distortion theoretically predicted. Each image consists of 400 A scans. The SNR for each interface was obtained by dividing the peak amplitude corresponding to a particular interface by the average of a portion of the noise floor. The region used for averaging the noise floor was kept consistent for each A scan. The SNR values for each interface were then averaged over all A scans. An average SNR improvement of 5 and 12 dB was obtained for the anterior and posterior lens surfaces, respectively. For the posterior corneal surface SNR improvement of 4 dB was observed, while the SNR improvement for the anterior corneal surface was 0:7 dB.  FIG. 7  shows the distortion-corrected surface profiles along with the fitted circular curves. The fitted values are nearly identical to the actual values with a relatively small discrepancy. These errors might arise because of slight tilt and decentration of the artificial cornea and lens in the model eye and the flat surface to be used as the reference. 
         [0024]    From the above, the following may be concluded. The proposed scanning method significantly increases scan-depth and SNR. A larger diameter of the posterior surface of the lens is thus imaged without pupil dilation. Anterior segment OCT with the proposed scanning system can be a powerful tool for understanding the mechanism of accommodation and presbyopia. 
         [0025]    While a preferred embodiment has been set forth in detail above, those skilled in the art who have reviewed the present disclosure will readily appreciate that other embodiments can be realized within the scope of the invention. For example, the specifics of the system shown in  FIGS. 3A-3F  can be varied in accordance with any desired optical techniques. Therefore the present invention should be construed as limited only by the appended claims.