Fundus Photography Overview

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Excerpted from:
Ophthalmic Photography: Retinal Photography, Angiography, and Electronic Imaging, 2^nd Edition
Patrick J. Saine and Marshall E. Tyler
Butterworth-Heinemann Medical; ISBN: 0750673729

Introduction

Fundus photography documents the retina, the neurosensory tissue in our eyes which translates the optical images we see into the electrical impulses our brain understands. The retina can be photographed directly as the pupil is used as both an entrance and exit for the fundus camera's illuminating and imaging light rays. The patient sits at the fundus camera with their chin in a chin rest and their forehead against the bar. An ophthalmic photographer focuses and aligns the fundus camera. A flash fires as the photographer presses the shutter release, creating a fundus photograph like the picture above. Ophthalmologists use these retinal photographs to follow, diagnose, and treat eye diseases.

Fundus photography can be performed with colored filters, or with specialized dyes including fluorescein and indocyanine green.

What is a Fundus Camera?

A fundus camera is a specialized low power microscope with an attached camera. Its optical design is based on the indirect ophthalmoscope. Fundus cameras are described by the angle of view - the optical angle of acceptance of the lens. An angle of 30°, considered the normal angle of view, creates a film image 2.5 times larger than life. Wide angle fundus cameras capture images between 45° and 140° and provide proportionately less retinal magnification. A narrow angle fundus camera has an angle of view of 20° or less.

Simultaneous stereo fundus cameras use one exposure to place two images side by side on a single 35mm frame.

Fundus Camera Optics

Light generated from either viewing lamp or the electronic flash is projected through a set of filters and onto a round mirror.



Filters and Mirror	Flash and Viewing Lamp

This mirror reflects the light up into a series of lenses which focus the light. A mask on the uppermost lens shapes the light into a doughnut. The doughnut shaped light is reflected onto a round mirror with a central aperture, exits the camera through the objective lens, and proceeds into the eye through the cornea.

Mirror with Central Aperture

Assuming that both the illumination system and the image are correctly aligned and focused, the resulting retinal image exits the cornea through the central, un-illuminated portion of the doughnut.

The light continues through the central aperture of the previously described mirror, through the astigmatic correction device and the diopter compensation lenses, and then back to the single lens reflex camera system.

Focusing the Fundus Camera

Using the Reticle to Focus

This figure illustrates the relationship between sharpness in the reticle, your fundus view, and the sharpness in the final fundus photograph. When you focus the image without regard to the sharpness of the focusing reticle, you will perceive the fundus image as sharp while the reticle image appears blurry. The image you see is focused by your eyes above the focusing screen, closer than infinity. Even though you see a sharp image in the viewfinder, the exposed photograph will be blurry.

When you view through a correctly adjusted eyepiece before the fundus has been focused on, then the fundus appears blurry while the reticle is distinctly sharp. You have correctly focused on the reticle, however the camera's image has not yet been adjusted to coincide with the receiving plane. If a picture is taken, the resulting image will be as unsharp as you see it in the viewfinder.

After correctly adjusting the eyepiece and the fundus camera's focusing mechanism, both the fundus image and the reticle appear sharp and clear. Your eyes are focused on the reticle, and the image from the fundus camera corresponds with the film plane. Only this final combination will yield a sharply focused fundus photograph.

Setting Your Eyepiece

In order to obtain a sharp view of the reticle, your eyepiece must be correctly set. Here is a step by step procedure for eyepiece adjustment:

Have your eyes corrected for their best visual acuity. You may choose to photograph with or without glasses or contact lenses if your correction is spherical. Remember however, that the correct eyepiece setting will be different depending on whether you do or do not wear your corrective lenses. If you have astigmatism, you should wear your corrective lenses while adjusting the eyepiece. Alternately, correct the eyepiece itself by securing the appropriate trial lens directly over the eyepiece. Do not wear reading glasses.
Eliminate any subject matter from the camera's field of view. Set your camera to the farthest focusing extreme and hold or tape a piece of white cardboard in front of the objective lens. When you look through the viewfinder, you should see a blurry reticle on a white field. The illumination level should be adjusted to a medium or low setting (approximately the same intensity as when viewing the retina). If the picture is too bright, it may cause your pupil to constrict, bringing depth of field into play and making an accurate setting difficult.
Set the eyepiece adjustment to the maximum plus diopter setting. This will blur any image of the focusing reticle. When looking through the viewfinder, you should see an evenly illuminated white blur.
Relax your eyes. Blink and look at something far away: perhaps down a long hall or at a reflection. It is extremely important that your eyes be focused on infinity to obtain the correct setting. Keeping both eyes open may help keep you from accommodating.
Peer through the viewfinder and begin turning the eyepiece ring towards the zero setting. Smoothly rotate the ring. Rotate at a slow enough rate that you perceive the reticle becoming sharper, but not so slowly that your eyes accommodate for the change. Remember to keep both eyes open and focused at infinity.
STOP rotating when the reticle is JUST in sharp focus. If you continue to rotate the reticle after you achieve sharp focus, or if you begin to search for sharpness by rotating the reticle back and forth, then accommodation may be influencing your decision. If your eyes are at their best correction, your setting will probably be within a diopter or so of the zero mark. If your diopter setting is a high minus, chances are that you have accommodated. If you are wearing no correction, the setting should be within a diopter of your best correction.
Repeat, Repeat, Repeat. Continue setting the eyepiece until three successive, near-normal settings agree. Note the most consistent position of the eyepiece ring. Remember to correctly set the eyepiece before each patient session. New photographers should repeat this exercise daily until they are confident that their eyepiece setting is accurate. Each fundus camera should be set individually by each photographer. The correct setting for each camera (especially if brands vary) may be slightly different for the same photographer, and the correct setting for a single camera will vary between photographers in the same office. If an instrument has more than one user, a signal can be developed to alert alternate users of possible eyepiece setting changes (perhaps an empty film box placed over the eyepiece).
Check the eyepiece setting and reticle before each patient. It cannot be over emphasized that once your personal setting has been ascertained, it is imperative to check it before each patient. Be aware of the focusing screen's reticle throughout each photographic procedure.

Only conscientious use of the eyepiece/reticle focusing system will assure consistently sharp fundus photographs.

Retinal Fundus Photographs

Normal Fundus Photograph

Fundus photographs are visual records which document the current ophthalmoscopic appearance of a patient's retina. One picture is worth, in this instance, a thousand words in the physician's notes. They allow the physician to further study a patient's retina, to identify retinal changes on follow-up, or to review a patient's retinal findings with a colleague.

Fundus photographs are routinely ordered in a wide variety of ophthalmic conditions. For example, glaucoma (increased pressure in the eye) can damage the optic nerve over time. Using serial photographs, the physician studies subtle changes in the optic nerve and then recommends the appropriate therapy. (Ref: Armaly, MF. Optic cup in normal and glaucomatous eyes. Invest. Ophth. 9(6):425-429)

Glaucoma

Fundus photography is also used to document the characteristics of diabetic retinopathy (damage to the retina from diabetes) such as macular edema and microaneurysms. This is because retinal details may be easier to visualize in stereoscopic fundus photographs as opposed to with direct examination. (Ref: Kinyoun, JL, et al, 1992. Ophthalmoscopy versus fundus photographs for detecting and grading diabetic retinopathy. Invest. Opthalmol. & Vis. Science. 33:888-93.)

Diabetic Retinopathy

Fundus photography is also used to help interpret fluorescein angiography because certain retinal landmarks visible in fundus photography are not visible on a fluorescein angiogram.

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