Patent Publication Number: US-10321822-B1

Title: Non-mydriatic self-imaging fundus camera

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 62/409,528, filed Oct. 18, 2016, which is incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to imaging technologies, and in particular, relates to fundus cameras. 
     BACKGROUND INFORMATION 
     Fundus imaging is a part of basic eye exams, yet the size, cost and complexity of conventional retinal cameras limit the availability of fundus imaging for screening, field diagnosis, and progress monitoring of many retinal diseases. Wide-field fundus imaging is difficult due to the low reflectivity of the fundus, the small eye pupil size, and the high background noise from corneal and iris reflections. Most commercial wide-field fundus cameras employ complex optical designs to image the fundus while avoiding corneal and iris reflections, which require precise lateral and axial alignment of the camera to the patient&#39;s pupil. 
     In both table-top and portable realizations of conventional fundus cameras, fundus imaging usually requires either pupil dilation using dilation agents or a trained operator aided with infra-red imaging for the alignment. There are automated systems that utilize closed-loop optomechanical feedback for camera alignment, but still suffer from large system size and cost, as well as, a slow alignment process. 
     Recently, a self-imaging portable retinal camera has been developed using a separate fixation path with a set of pinhole masks placed near and on the conjugate plane to the retina that confine the ray angles. However, this design suffers from a small imaging field-of-view compared to state-of-the-art commercial systems and the self-alignment scheme is based on pupil-forming pinhole masks, which result in a low imaging yield. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. Not all instances of an element are necessarily labeled so as not to clutter the drawings where appropriate. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles being described. 
         FIG. 1  is an illustration of a fundus camera system for self-imaging of a retina, in accordance with an embodiment of the disclosure. 
         FIG. 2  is an illustration of an alignment target and field stop of the fundus camera system, in accordance with an embodiment of the disclosure. 
         FIG. 3  is an illustration of a fundus camera including a fixation target that aids a user with accommodating to infinity, in accordance with an embodiment of the disclosure. 
         FIG. 4  is a flow chart illustrating a process for self-imaging a retina using a fundus camera, in accordance with an embodiment of the disclosure. 
         FIGS. 5A-5D  illustrate how to use an alignment image output from an alignment target of the fundus camera to self-align a user&#39;s eye to the fundus camera, in accordance with an embodiment of the disclosure. 
         FIG. 6A  illustrates the amount of reflection off of eye structures relative to lateral and axial misalignments, in accordance with an embodiment of the disclosure. 
         FIG. 6B  illustrates how an alignment eyebox is the region of overlap between two eyeboxes, in accordance with an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of a system, apparatus, and method of use of a fundus camera system capable of enabling wide-field, self-imaging of a user&#39;s retina are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     Conventional fundus cameras require an eye care professional for alignment. A self-aligning fundus camera is a fundus camera where a patient can take fundus images by him or herself. Such self-aligning fundus cameras provide some sort of feedback to the patient through an optical path different from the image path from the eye through the eyepiece to the image sensor. Such self-aligning fundus cameras also generally have eye tracking mechanisms to calculate the feedback given to the patient. 
     In various embodiments described herein, a self aligning fundus camera provides an alignment target (which may be different from a fixation target) comprised of at least two illuminated concentric shapes on an image plane between the eyepiece and the image sensor. The concentric shapes can be centered around the image path but outside the field of view of the image sensor so as not to interfere with fundus imaging. 
     An alignment target is used to provide feedback to the patient such that the patient can place his/her head and eye within the intended “eye-box” (i.e., “the volume of space within which an effectively viewable image is formed by a lens system or visual display, representing a combination of exit pupil size and eye relief distance”). In contrast, a fixation target is used to help the patient eye&#39;s focus reach a defined distance (e.g., infinity). In some cases, the brightness and/or contrast of the fixation target can be higher than that of the alignment target such that the alignment target in the peripheral vision of the patient&#39;s field-of-view does not distract the patient from looking at the fixation target. 
     For axial movement alignment, the patient can be instructed to adjust his/her position (while maintaining focus on the fixation target) until at least one of the concentric shapes in his/her peripheral vision is visible and at least one of the concentric shapes is not visible. For lateral movement alignment, the patient can also be instructed to adjust his/her position until the visible concentric shape&#39;s border is visible. In one example, the illuminated concentric shapes are circles. In one example, the illuminated concentric shapes are displayed by a microdisplay, a ring of light emitting diodes, or any other light source, light deflectors, optical guide, or otherwise. 
     When utilizing the concentric shapes as an alignment target, the accuracy of the alignment may be affected by the pupil size of the patient&#39;s eye. Accordingly, in some embodiments, the image sensor can detect the pupil size and adjust the size of the concentric shapes based on the pupil size. In some embodiments, the alignment target includes multiple concentric shapes, and the patient can self-select which of the concentric shapes to use for alignment. In some embodiments, the fundus camera can shine light on the patient&#39;s eye in order to decrease pupil size prior to alignment. 
     Embodiments disclosed herein describe a wide-field, non-mydriatic fundus camera that enables self-imaging of the fundus or retina. The fundus is the rear interior surface of the eye that includes the retina, optic disc, macula, fovea, and posterior pole. The design enables a compact and low-cost realization of a fundus camera that guides the user to self-align their eye to the camera and thus can remove the need for a trained camera operator for precise alignment. Embodiments of this self-imaging fundus camera can be useful for screening, early diagnosis, and long-term monitoring of various retinal diseases. In some embodiments, the fundus camera system may be packaged as a portable handheld camera that the user holds up to their eye (monocular implementations) or eyes (binocular implementations). In other embodiments, the fundus camera system may be packaged as a tabletop system, a desktop system, or a wall mounted system that incorporates chin and forehead rests for added stability. 
       FIG. 1  is an illustration of a fundus camera system  100  for self-imaging of a retina  105  of an eye  110 , in accordance with an embodiment of the disclosure. Fundus camera system  100  illustrates a single optical path for implementing a monocular fundus camera; however, it should be appreciated that the components along the optical path may be replicated for two eyes in binocular implementations. The illustrated embodiment of fundus camera system  100  includes a controller  115 , an image sensor  120 , one or more focusing lenses  125  (two are illustrated), a polarization plate  130 , an illumination ring  135 , a polarizing ring  140 , a field stop  145  having an inner edge  150 , an alignment target  155 , and an eyepiece lens  160 . In the illustrated embodiment, eye  110  includes a pupil  165 , which resides along an entrance pupil plane  170  as perceived by fundus camera system  100 . When fundus camera system  100  is correctly aligned to eye  110  and focused for imaging retina  105 , field stop  145  and an emissive aperture of alignment target  155  are positioned along an eyepiece image plane  175  (which is also a conjugate plane to retina  105 ) while illumination ring  135  is positioned along a conjugate plane  180  to entrance pupil plane  170  (also referred to as an entrance pupil conjugate). 
     Retinal cameras can include three optical paths: an illumination path, an imaging path, and an eye fixation path. In conventional retinal cameras, the three paths are often combined using beam splitters or holed mirrors to project the fixation path into the user&#39;s field of view (FOV). In contrast, the illustrated embodiments of fundus camera system  100  combines the illumination path of illumination light with the imaging path of image light and an alignment path of alignment light by concentrically aligning alignment target  155 , illumination ring  135 , and image sensor  120  about a common center optical axis  101 . While in some embodiments image sensor  120  may be repositioned off center optical axis  101  using various optical elements, by aligning all three components (alignment target  155 , illumination ring  135 , and image sensor  120 ) about center optical axis  101 , a compact form factor is achieved that reduces the overall number of optical components (e.g., mirrors, beam splitters, lenses, etc.). 
     Alignment target  155  outputs an alignment image that serves as a visual cue in the user&#39;s/patient&#39;s peripheral vision to perform a self-alignment between fundus camera system  100  and their eye within a small margin of error required for wide-field fundus imaging with reduced background noise from corneal and iris reflections. In some embodiments, alignment target  155  further operates as an accommodation target in the user&#39;s peripheral vision for aiding eye accommodation to infinity. 
     During operation, controller  115  controls and orchestrates the operation and timing of the other electronic components of fundus camera system  100 . In particular, controller  115  can activate/deactivate alignment target  155  for eye alignment. Additionally, controller  115  can synchronize the flashing of illumination ring  135  with a shutter signal to image sensor  120  to acquire an image of retina  105  at the time of illumination. In one embodiment, controller  115  is a microcontroller executing software/firmware instructions. In another embodiment, controller  115  is an application specific integrated circuitry (ASIC), field programmable gate array (FPGA), or other hardware logic. Controller  115  may further include memory for storing retinal images output from image sensor  120 . Image sensor  120  may be implemented using a variety of technologies including a charged coupled device (CCD) image sensor, a complementary metal-oxide-semiconductor (CMOS) image sensor, or otherwise. 
     Illumination ring  135  is a ring shaped (e.g., circular, elliptical, square, hexagonal, etc.) light emitter with an aperture  137  disposed in its center to pass the image light received through eyepiece lens  160  to image sensor  120 . A ring shaped illuminator provides high contrast retinal images by rejecting corneal reflections through angular separation. In one embodiment, illumination ring  135  is a circular array of light emitting diodes (LEDs) mounted along an annular shaped substrate. In another embodiment, illumination ring  135  is a series of optic fibers having emission apertures embedded around an annular shaped substrate and input apertures coupled to one or more light sources. In an alternative embodiment, illumination ring  135  is a ring-shaped reflector that is illuminated by an off-axis lamp source. The light sources may emit visible wavelengths and/or near-infrared wavelengths. For example, infrared emitters may be interspersed with visible light emitters to aid in autofocusing. In the illustrated embodiment, illumination ring  135  is axially aligned about center optical axis  101  and resides on conjugate plane  180  to entrance pupil plane  170  when fundus camera system  100  is aligned. However, it should be appreciated that illumination ring  135  need not exactly reside on conjugate plane  180 , but rather may reside adjacent to conjugate plane  180  within tolerances permitted by alignment eyebox  505  (see  FIG. 505C ). 
     The diffuse reflections from retina  105  are collected through pupil  165  and aperture  137  in illumination ring  135  by image sensor  120 . In one embodiment, aperture  137  through illumination ring  135  is the limiting aperture for image sensor  120  and fundus camera  100 . Image sensor  120  is positioned on a conjugate plane to eyepiece image plane  175 . In the illustrated embodiment, the back reflections of the illumination light off of eyepiece lens  160  are rejected using a cross-polarization scheme. For example, ring polarizer  140  is positioned in front of illumination ring  135  to polarize the output illumination light along a first polarization axis and polarizing plate  130  is placed behind aperture  137  to polarize the reflected light along a second polarization axis orthogonal to the first polarization axis, thereby passing only cross-polarized diffuse reflectance from retina  105 . In one embodiment, the ring polarizer  140  and plate polarizer  130  are linear polarizers with orthogonal orientations. To further reduce deleterious back reflections, the surfaces of eyepiece lens  160  may be coated with anti-reflection (AR) films. 
       FIG. 2  is an illustration of alignment target  155  and field stop  145 , in accordance with an embodiment of the disclosure. The illustrated embodiment of alignment target  155  outputs an alignment image  205  that includes an outer shape  210  and an inner shape  215  concentrically aligned around aperture  147 . The inner edge  150  of field stop  145  and/or alignment target  155  define aperture  147 . In some embodiments, the opaque portions of alignment target  155  external to outer shape  210  and inner shape  215  operate as the field stop  145 . Alignment target  155  and field stop  145  are positioned between eyepiece lens  160  and illumination ring  135  along a common plane. As illustrated in  FIG. 1 , the common plane is eyepiece image plane  175 , which is also a conjugate plane to retina  105  when eye  110  is aligned to fundus camera system  100 . 
     Alignment target  155  may be implemented as an alignment target display using a variety of technologies, such as, an array of LED lights, one or two annulus shaped light guides each with a backlight, an light emitter with a center hole and blackout regions to define the outer shape  205  and inner shape  215 , or other display technologies. In another embodiment, alignment target  155  is formed from two concentric arrays of optic fibers having emission apertures mounted in concentric annular shapes to a substrate and input apertures positioned to collect the alignment light from one or more light sources. In one embodiment, alignment image  205  is monochromatic. In another embodiment, alignment image  205  is multi-color with inner shape  215  having a first color (e.g., green) or first pattern (e.g., dots, dashes, etc.) that is different from a second color (e.g., red) or second pattern of outer shape  210 . The differing colors or patterns can be helpful to the user to differentiate inner shape  215  from outer shape  210 , since these shapes are positioned in their peripheral vision. Although  FIG. 2  illustrates outer shape  210  and inner shape  215  as being concentric, solid, circles, other shapes or patterns may be used. In fact, a number of patterns (e.g., colors, dots, dashes, etc.) and shapes (e.g., square, etc.) that are concentric about center optical axis  101  may be used. In additional to color, shape, and pattern variations, temporal variations may also be used to distinguish the inner and outer shapes. For example, temporal variations such as blinking, variable brightness/color, and shape moving (e.g., rotating) may be used. 
     Returning to  FIG. 2 , alignment image  205 , including outer shape  210  and inner shape  215 , serves as a visual cue to the user for self-alignment of fundus camera system  100  to eye  110  and in particular to pupil  165  (discussed in detail in connection with  FIGS. 4 and 5A-5D ). Alignment image  205  is emitted towards eye  110  from a location that is peripheral of inner edge  150  of field stop  145 . As such, alignment image  205  is outside the FOV of image sensor  120  and seen by the user in their peripheral vision when fundus camera system  100  is aligned. Alignment target  155  is designed such that the proper alignment image  205  is seen by the user when eye  110  is aligned within an eyebox region (e.g., see alignment eyebox  505  in  FIG. 5C ) that ensures acceptable retinal imaging quality. The size (or diameter) of inner edge  150  may be selected to be equal to or larger than the size of the image of retina  105  (for a selected field of view) formed at eyepiece image plane  175 . In one embodiment, axial offset alignment is achieved when the user can see inner shape  215  (i.e., inner shape  215  is within the user&#39;s FOV), but cannot see outer shape  210  (i.e., outer shape  210  is outside the user&#39;s FOV). In one embodiment, lateral alignment is achieved by centering alignment target  155  in the user&#39;s FOV. 
       FIG. 3  illustrates a fundus camera system  300 , in accordance with an embodiment of the disclosure. Fundus camera system  300  is similar to fundus camera system  100 , except for the addition of a fixation target  305  that operates as a visual aid for eye  110  to accommodate to infinity. In the illustrated embodiment, fixation target  305  is positioned along eyepiece image plane  175  within the user&#39;s center of vision, or foveal vision, and provides an image or target for fixation. Fixation target  305  may be a simple dot, pinhole, crosshair, or otherwise. In one embodiment, fixation target  305  is etched or painted on a transparent substrate positioned across aperture  147 . In yet another embodiment, fixation target  305  is painted on or formed by a shutter that moves in and out of the user&#39;s foveal vision along eyepiece image plane  175 . For example, fixation target  305  may be painted on or formed by a shutter that is normally closed, but which rapidly opens by moving out of the imaging path when the user triggers a picture, causing illumination ring  135  to flash illumination light and image sensor  120  to acquire the retinal image. In other embodiments, fixation target  305  is a small, subtle target (e.g., dot or crosshair) that remains stationary. Fixation target  305  may further include its own illumination so that it is viewable by the user. In some embodiments, fixation target  305  may include as a small light source (e.g., LED, fiber tip, etc.) that is placed on a moving target, such as a shutter. In other embodiments, fixation target  305  is a light paint pattern (or etching) on a transparent substrate and is illuminated from the periphery so that the light paint pattern or etching scatters light towards eye  110 . 
       FIG. 4  is a flow chart illustrating a process  400  for self-imaging a retina using fundus camera systems  100  or  300 , in accordance with an embodiment of the disclosure. Process  400  is described with reference to  FIGS. 5A-5D . The order in which some or all of the process blocks appear in process  400  should not be deemed limiting. Rather, one of ordinary skill in the art having the benefit of the present disclosure will understand that some of the process blocks may be executed in a variety of orders not illustrated, or even in parallel. 
     In a process block  405 , controller  115  activates alignment target  155  to output alignment image  205  through eyepiece lens  160  towards eye  110 . Alignment image  205  is a peripheral image that is outside the FOV of image sensor  120  and therefore not imaged by image sensor  120 . With alignment target  155  (and/or fixation target  305 ) activated and the user looking through eyepiece lens  160  (process block  410 ), the user can commence self-alignment and adjust focus. In process block  415 , focus can be adjusted in a number of ways to account for different user prescriptions. In a manual focus embodiment, eyepiece lens  160  has a manually adjustable position, which the user can change until alignment image  205  and/or fixation target  305  are in focus. In an autofocus embodiment, an infrared light illuminates retina  105  allowing image sensor  120  to perform an autofocus routine that adjusts the focus of focusing lens(es)  125 . In this autofocus embodiment, the user&#39;s vision of alignment image  205  or fixation target  305  is not auto-adjusted, though the user can still adjust the relative position of the fundus camera to bring alignment image  205  (and/or fixation target  305 ) into focus. 
     In decision block  420  and process block  425 , lateral misalignments between pupil  165  and eyepiece lens  160  are corrected until an acceptable lateral alignment (e.g., vertical and horizontal alignment) is achieved. Lateral alignment of pupil  165  is important in order to reduce background noise from iris reflections. If eyepiece lens  160  images the illumination light from ring illuminator  135  offset from pupil  165 , this lateral misalignment results in the illumination being focused on the surface of the iris, which causes the diffuse back reflections of the illumination light to enter the imaging path, resulting in high background noise in the retinal image. Thus, it is advantageous to accurately center pupil  165  to eyepiece lens  160  to increase the illumination light entering pupil  165  while reducing deleterious reflections from the iris. One example of achieving this centering is by using an alignment object (e.g., alignment image  205 ) that is symmetrical about center optical axis  101  to ensure that all sides (or the entire circumference if the alignment target is circular) are seen equally at the periphery of the visual field (e.g., see  FIG. 5A or 5D ). In this scheme, if the eye is laterally misaligned, the user will see an asymmetric alignment image (e.g., see  FIG. 5B ). Accordingly, in process block  425 , the lateral position between pupil  165  and eyepiece lens  160  is adjusted based upon how the shapes generated by alignment target  155  are centered in the user&#39;s vision. In one embodiment, the lateral position is adjusted by the user until the user sees at least one (if not both) of the shapes centered in their vision. 
     In decision block  430  and process block  435 , axial offset (i.e., eye relief) misalignments between pupil  165  and eyepiece lens  160  are corrected until an acceptable axial alignment (e.g., eye relief or axial offset) is achieved. The axial alignment is important to achieve a full illumination/imaging FOV, while avoiding corneal reflections. When eye  110  is too far from the desired eye relief, the illumination and imaging FOVs are decreased as they are obstructed by the iris. When eye  110  is too close, corneal and iris reflections contribute significant background noise in the retinal image. For precise axial alignment, the imaging FOV of eye  110  is used when eye  110  looks at eyepiece image plane  175  through eyepiece lens  160 . In a non-pupil forming system, such as the alignment path of fundus camera  100 , the overall imaging FOV monotonically increases as the eye moves closer to the eyepiece. Thus, alignment target  155  outputs two shapes (e.g., two concentric rings such as outer shape  210  and inner shape  215 ) with different sizes and the ring diameters are selected such that only the inner ring can be seen when eye  110  is at the right axial location (e.g., see  FIG. 5A ). When eye  110  is too far, none of the alignment shapes will be seen in user&#39;s FOV (e.g., see  FIG. 5C ) and when eye  110  is too close, both alignment shapes will be seen in the user&#39;s FOV (e.g., see  FIG. 5D ). Accordingly, in process block  435 , the axial offset position between pupil  165  and eyepiece lens  160  is adjusted based upon how many of the shapes of the alignment image are in the user&#39;s vision at a given time. For example, the axial offset position is adjusted until the user sees one of the two alignment shapes but not both of the two alignment shapes. If both of the alignment shapes are within the user&#39;s FOV (e.g., see  FIG. 5D ), the axial offset position is increased. If none of the alignment shapes are within the user&#39;s FOV (e.g., see  FIG. 5C ), the axial offset position is decreased by the user. 
     Accordingly, the user will see only a single alignment shape (e.g., inner shape  215 ) that is symmetrically centered in their FOV when the center of pupil  165  falls within alignment eyebox  505 , illustrated in  FIG. 5C . The alignment margins can be determined by the size, width, and separation of these alignment shapes output by alignment target  155 . Different colors and/or spatial and temporal patterns can be used to distinguish between the inner and outer alignment shapes. Small LEDs, display panels, optical fibers, mask patterns with pinholes, or otherwise can be used to implement alignment target  155  for outputting the alignment image. 
     Returning to  FIG. 4 , once the pupil  165  is laterally and axially aligned within alignment eyebox  505 , the user can press a shutter button that causes ring illuminator  135  to flash (process block  440 ) and image sensor  120  to acquire the retinal image (process block  445 ). In some embodiments, a fixation target is moved out of the imaging path when the user triggers the image acquisition. In a process block  450 , the user deactivates the fundus camera and removes their eye from eyepiece lens  160 . 
     Alignment image  205  is designed to match alignment eyebox  505  with a retinal imaging eyebox, such that when the user sees the correct pattern (e.g., only inner shape  215 ), image sensor  120  also sees the full FOV of retina  105 . In order to match these two eyeboxes, the retinal imaging eyebox is designed and measured, and then an alignment target is selected to match the size and the location of alignment eyebox  505  to the retinal imaging eyebox. 
     The quality of the retinal images is sensitive to the camera-to-eye alignment. Alignment eyebox  505  is defined as a volume of eye alignment margin in which the retinal image quality is acceptable. The size and the location of alignment eyebox  505  is determined by the retinal illumination intensity, retinal reflection collection efficiency, iris reflection, and the corneal reflection at the eye&#39;s position relative to image sensor  120 .  FIG. 6A  illustrates the total amount of retinal, iris and corneal reflections collected by image sensor  120  as a function of the eye&#39;s lateral and axial position. The retinal imaging eyebox can be found as the highlighted region where the retinal signal collection efficiency is highest and the corneal reflections and the iris reflections are sufficiently rejected. 
     In an alignment scheme using eyepiece imaging plane  175 , the imaging FOV increases as eye  105  moves closer to eyepiece lens  160 . Alignment eyebox  505  can be determined by an overlapped region of two eyeboxes where: 1) the entire circumference of inner shape  215  is seen ( FIG. 6B , graph  605 ) and 2) none of outer shape  210  is seen ( FIG. 6B , graph  610 ). The overlapped region of these two eyeboxes ( FIG. 6B , graph  615 ) is matched to the retinal imaging eyebox by changing the sizes of the two alignment shapes (e.g., outer shape  210  and inner shape  215 ). 
     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.