Abstract:
A system for imaging of the central and peripheral retina, includes one of a concave mirror and an elliptical mirror having an axis and being configured to rotate around the axis and a scanner configured to using a spectral domain optical coherence tomography system to obtain a non-contact wide angle OCT-image of a large portion of the central and peripheral retina.

Description:
BACKGROUND 
     1. Field of Invention 
     The present invention relation to a system for Non-contact Optical Coherence Tomography (OCT) imaging of the central and peripheral retina. 
     2. Related Art 
     Ophthalmic Optical Coherence Tomography (OCT) of the eye was originally developed by obtaining cross-sectional images of the sensory retina and retinal pigment epithelium. Recently, spectral domain OCT became available, a new technique that allowed major improvements particularly regarding image acquisition speed and image resolution. However, existing instruments do not scan the retinal periphery. The OCT scan is typically restricted to the central&lt;40° of the retina. 
     OCT is presently used (in opthalmology) to evaluate only either the thickness of the central retina in macular diseases or separately the status of the optic nerve head in glaucoma patients. Furthermore obtaining the OCT pictures require dilatation of the pupil prior to taking pictures. OCT can not be performed in patients with a constricted pupil; since presently available optics do not permit it. In convention systems, the existing unbearable reflexes created by their optical elements make it impossible for the operator to simultaneously see the retina and focus on the desired area (e.g., the macula or the Optic Nerve head). In addition, the field of view is very limited; thus, important regions in the peripheral retina can not be visualized with current OCT systems and the system needs a skilled personal to handle the instrument. 
     Present OCT systems generally employ a Fundus Camera Design. A fundus camera is a complex optical system for imaging and illuminating the retina. Due to its location the retina must be imaged and illuminated simultaneously by using optical components common to the imaging and illumination system. 
     Conventional Fundus Cameras used for OCT generally include an objective lens which forms an intermediate image of the retina in front of a zoom lens, designed to accommodate for the refractive error of the patient, which relays the intermediate image to the CCD. Light travels through the objective lens in both directions making the consideration of back reflections important. On the illumination side, the objective lens images an annular ring of light onto the pupil; therefore, the need for dilation of the pupil. This ring of light then disperses to give a near uniform illumination of the interior retinal surface. The objective lens also serves a role in the imaging optics. It captures pencils of light emanating from the eye and forms an intermediate image of the retina. This intermediate image is then relayed by additional optics to a digital imaging sensor or film plane. 
     The objective lens also serves as the limiting factor in the field of view of the camera.  FIG. 1  shows the relationship between the objective lens and the eye. 
     Bundles of rays leaving the periphery of the retina emerge from the emmetropic eye as a roughly collimated bundle of rays. This bundle must pass through the edge of the objective lens in order to become part of the fundus image. Bundles coming from more eccentric points on the retina cannot be captured by the objective and therefore cannot be seen in the fundus image. One method of increasing the field of view in this conventional configuration is to increase the size of the objective lens as seen in  FIG. 2 . 
     However, increasing the diameter of the objective lens causes an increase in the aberration of these lenses with size. Current practical limits taking this approach lead to a roughly 40-degree field of view seen in modern fundus cameras. An alternative method for increasing the field of view is to move the objective lens closer to the eye which has also its own limitations. The extreme case for this technique is where a portion of the objective lens actually comes in contact with the cornea. One drawback to this configuration is patient aversion due to the proximity of the lens, and increased risk of infection and corneal abrasion. 
     SUMMARY OF THE INVENTION 
     The embodiments of the present invention overcome the above problems with the conventional systems. 
     One objective of the present invention is to develop a non-contact wide field OCT system to extend the field of view&gt;200 degree and can be performed in un-dilated pupil. 
     This invention can provide access to the retinal periphery and simplifies taking images. These sections of the retinal periphery may provide important information about the diseased areas of the peripheral retina permitting evaluation and potential treatment. Additionally, the present invention enables ophthalmologists to visualize and diagnose a variety of ailments including glaucoma, macular degeneration, retinal detachment, and diabetic retinopathy and all peripheral retinal diseases. The OCT system of the present invention can utilize low coherent visible or preferably infra red wave lengths of up to 10640 nm. The OCT system of the present invention can also makes a patient&#39;s examination easier by eliminating the time spent to dilate the patients pupil and reducing the focusing time, which improves the patient&#39;s tolerance of the procedure since a wide-field fundus photograph can be generated with the same infrared light without blinding the patient with visible light. 
     In conventional systems no information can be obtained from the vitreoretinal interface in the retinal periphery which is main cause of retinal tear and retinal detachment which is one of the main cause of blindness more frequently in near sighted patients. What is needed is a wide-field retinal imaging OCT system that extends the fields of view&gt;200 degree field. 
     Additional features and advantages are described herein, and will be apparent from, the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a convention objective lens positioned adjacent an eye; 
         FIG. 2  illustrates an objective lens having an increased diameter; 
         FIG. 3  illustrates an embodiment of the present invention in which the mirror is configured, if needed, to wobble or rotate around the visual axis; 
         FIG. 4  illustrates an embodiment of the present invention in which a central part of the mirror can be switched automatically to achieve the beam diversion see attached. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention may include imaging modalities, such as optical coherence tomography, snapshot imaging Polarimetry and computed tomography imaging spectrometer. Additionally, White Light LED, infra red diodes, Nd-YAG as an illumination source, can each maintain excellent image quality across the field of view. 
     Spectral Domain Optical Coherence Tomography, OCT, operates in substantially the same manner as a low coherence Michelson interferometer. That is, light from a broadband source is separated into a reference and sample arm. Reflected light from both arms is recombined in the detection arm to form interference fringes when the optical path difference between sample and reference arms is within the coherence length of the source. The signal produced by the detection of interference fringes is proportional to the backscatter from the depth structure of the sample. Spectral components of the light in the detector arm are separated by a spectrometer allowing for the detection of interference fringes at different wavelengths corresponding to different optical path differences representing depth information. Structural depth information from the sample is recovered by a Fourier Transform of the spectral data allowing for optical sectioning of the sample. Swept Source OCT operates by replacing a broadband source with a rapidly scanning source that sweeps across a large range of wavelengths. The detector records the backscattered signal for each wavelength and position. By utilizing this methodology the recovered signal is equivalent to Spectral Domain OCT. Swept Source OCT allows for faster axial scanning than conventional Spectral Domain OCT and eliminates the need for a high performance spectrometer. Light from the swept source is coupled into a fiber coupler that separates the light into the reference and sample arms of the OCT system. The reference arm of the OCT system consists of a collimating lens, a block of material for compensating the material dispersion of the source and fixed mirror. Light entering the reference arm is collimated, passes through the dispersion compensating material is reflected by the fixed mirror back through the dispersion compensating material and is coupled into the fiber coupler. The sample arm of the system, responsible for imaging the retina, consists of scan mirrors, an afocal relay, a wide field of view through a concave and or elliptic mirror and subject&#39;s eye. The Oct images can be also converted into two dimensional of the retina by the computer soft ware. 
     The embodiments of the present invention may utilize a concave mirror  10  (circular or elliptical) that focuses a beam of light  12  toward its focal point located inside the patient&#39;s pupil (of 2-6 mm or larger diameter). The beam scanner and receivers are attached to the mirror. As shown in  FIG. 3 , the mirror can, if needed wobble, (or oscillate) slightly. This wobbling moves the focal point of the mirror slightly from one side of the pupil to the other side permitting the scanning light (low coherent wave lengths etc.) inside the eye to cover a larger peripheral field than possible without oscillation. Because the oscillation of the beam can be &gt;1000× faster than the oscillation of the mirror, an expanded retinal field may be achieved, even if some of the beam is clipped by the pupil. 
     It is generally not possible to wobble (i.e., oscillate) rapidly a heavy fundus camera, nor would such wobbling significantly move the camera&#39;s view inside the eye. Additionally, a strip of an elliptical can be rotated along its axis if an emitter is attached to the mirror. This rotation may eliminate the need of an emitter oscillating in a circular fashion, since the elliptical mirror can rotate 360 degree thus covering the entire field of fundus. 
     Embodiments of the present invention also can have two emitters  14  and  16  and receiver arms  18  and  20  positioned in different areas with respect to the above mentioned concave mirror ( FIG. 3 ). In these embodiments, a central OCT may be easily achieved, and then can be rapidly switched to a larger peripheral one. Both are preferably attached to the concave mirror  10 , one central in front of the pupil for central scanning alone and one peripheral where the beam is reflected off the concave mirror before entering in the eye. These two arms  18  and  20  can be switched from one to the other electronically or manually, if desired. As illustrated in  FIG. 4 , a small central part  22  of the mirror  10  can be switched automatically to achieve the beam diversion. However, if desired the peripheral arm can be used alone. 
     Light entering the sample arm can be collimated and directed to a scan mirror responsible for sweeping the beam along the x axis. An afocal relay can direct the beam deflected by the x axis scan mirror to another scan mirror responsible for sweeping the beam along y axis. The y axis scan mirror can direct the beam to a wide field of view of a concave mirror or an elliptical mirror positioned in front of the subject&#39;s eye. This combination of scan mirrors, afocal relay and a wobbling-rotateable concave mirror or an elliptical mirror placed in front of the patients eye and an OCT system allows for three dimensional volumetric image acquisition over 200 degree field of the retina. Light incident to the retina is back scattered by retinal structures and propagates back through the scanning system and coupled into the fiber coupler. Light from the reference and sample arms is combined in the detection arm of the system producing interference fringes at the detector. Detection of the interference fringes is synchronized with the propagation of the specific wavelength coupled in the system by the swept source and the position of the x and y axis scan mirrors provides the desired wide angle OCT of the, central and peripheral retina. This method provides not only information on the retinal structure (thickness, degeneration, erosion, holes etc) but also the vitreoretinal interface, such as persistent vitreoretinal attachments and tractions on the retina. The use of a circular concave mirror, or preferably an elliptical mirror, permits us to place the focal point (or the second focal point of an elliptical mirror) at the pupil or further in a posterior plane inside the eye permitting the scan to pass with ease through a small pupil while larger area of the retina are being scanned. The central part of these mirrors can also be replaced by a transparent glass so that a central OCT scan can be obtained initially from limited central area. ( FIG. 3  A, B,C). As soon as an image is obtained from the retina the central part of the mirror can be replaced automatically by the second scanner reflecting the light off the concave mirror and scanning and acquisition is obtained from this separate line which gives a wide angle OCT, imaging including the entire retina. 
     After positioning the patient&#39;s head in front of a head holder, used for fundus photography, the OCT system can be moved toward the eye. Once the retina is visualized with the infrared beam through the central (transparent) part of the concave mirror, the central part can be switched with equal size mirror and the other arm of the OCT system can be activated which records rapidly a circular field of 120-200 degree, depending on the mirror used. Alternatively, the central part can also be made with a partial reflecting mirror which would eliminate actual physical exchange of the central part. The data collected can be analyzed and compared with opthalmoscopic examinations and fundus photography done on each patient. Embodiments of the present invention are also capable of providing a black and white fundus picture. The scan can be modified to provide a false color image of the retina and its interface. The OCT according to embodiments of the present invention is not only capable of providing a cross sectional view of the entire area including the Optic Nerve head, Vitreoretinal adhesions, but also can provide an elevation map of the scanned field. The information gained is invaluable for decision making prior to vitreoretinal surgery, and laser application in patients with diabetic retinopathy, macular degeneration, glaucoma, inherited retinal degeneration, retinal diseases predisposing to a retinal detachment etc. 
     The same system can be used for obtaining wide angle OCT of the cornea and the anterior segment of the eye including the lens by moving the mirror away from the eye. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.