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Ophthalmic Photography - Documentation versus Diagnostic Information, External Photography, Slit Lamp Biomicrography, Specular Biomicrography, Fundus Photography

eye camera dye cornea

CHRISTYE P. SISSON
Rochester Institute of Technology

Ophthalmic photography is a specific branch of scientific and medical photography that deals exclusively with the eye and its related parts. Ophthalmic photography is a highly specialized but growing photographic field that depends as much on knowledge of anatomy and physiology of the eye as it does on photographic techniques and technologies. This unique combination of art and science allows the ophthalmic photographer to work very closely with physicians, typically ophthalmologists, to optimize the images for best patient care.

Documentation versus Diagnostic Information

There are a number of reasons and methods for the photographic procedures of the eye. As with traditional biomedical photography, a large part of ophthalmic photography is documentation, or the recording of a particular disease or condition at a moment in time. This sort of documentation provides the physician with a photographic record that can be compared to past and future records as patient care continues.

One aspect of ophthalmic photography that makes it particularly unique is the diagnostic role that it plays in ophthalmology. In particular, one type of ophthalmic photography, fluorescein angiography, is the only application of photography that is diagnostic, which means it shows information not otherwise visible by the naked eye. This information is immensely helpful in diagnosing specific eye diseases. Obviously, there are many other forms of diagnostic medical imaging, but fluorescein angiography is the only imaging method that is grounded in traditional photographic methods.

The ability to reveal important diagnostic information from a relatively straightforward, non-invasive procedure is in large part responsible for the continued growth of the field. Ophthalmic imaging is a valuable and inexpensive adjunct to visual examination of the eye that can be used in developed and developing countries to detect the ophthalmologic effects of a number of diseases, such as diabetes, that may otherwise go unnoticed. Fluorescein angiography goes a step further, showing vascular damage and abnormalities, information that is essential for treatment. With the advent of telemedicine applications, ophthalmic photography can now be brought to remote locations to treat populations that may otherwise go without ophthalmologic care. The images can be transferred electronically to centralized reading centers that can screen individuals for signs of retinal disease, such as diabetic retinopathy, age-related macular degeneration, or glaucoma.

External Photography

The region, and the anatomy documented, will traditionally define the categories of ophthalmic photography that are performed. External photography of the eye utilizes a traditional 35 mm SLR or, increasingly, a digital SLR camera equipped with a macro lens to record the external structures of the eye, including the lids, conjunctiva (the vascular membrane covering the globe), cornea (the clear front portion or the eye), pupil, and iris. Lighting is typically from specialized flashes such as ring lights or macro-type, camera-mounted flash setups.

Slit Lamp Biomicrography

To achieve a higher level of magnification on these external structures, a slit lamp biomicroscope is typically used. A slit lamp is commonly used by an ophthalmologist to see if there are defects or anomalies in the anterior segment, which is everything in the eye from the lens forward, including the iris, lens, cornea, and conjunctiva. The slit lamp is mounted to a movable table top that includes an integrated chin rest to keep the patient’s head stationary. The light source produces a beam of light that is projected onto the area of interest, which is in turn viewed by the examiner with the biomicroscope portion of the apparatus. The biomicroscope allows the user to adjust the image magnification from approximately 1.5× up to 4× life size. This equipment is modified for photography by incorporating a beam splitter for the camera and flash system that uses the same light path as the tungsten examination illumination.

The slit lamp biomicroscope earns its name by the type of lighting it produces, which is a very thin, less than 1 mm beam of light that can be raked across the surface of the eye. This is a specialized form of dark-field illumination that might be considered to be similar to the effect of closely drawn curtains on a sunny day where a thin beam of sunlight reveals every speck of dust floating in the air against a dark background. The slit lamp works the same way on the cornea or lens of the eye, which are normally optically clear. The thin slit beam allows the photographer or the physician to visualize any abnormalities or opacities by creating in effect an optical section of the cornea or lens.

Specular Biomicrography

Another form of anterior segment ophthalmic photography is called specular biomicrography and is used to visualize a very specific portion of the cornea known as the corneal endothelium. The single-cell layer corneal endothelium acts as a metabolic pump for the cornea, keeping this structure dehydrated and therefore optically clear. When these cells fail, adjacent cells swell to take up the space left by the dead cell. Over time, this endothelial cell death results in the cornea becoming hydrated, causing opacities. Visualizing this layer helps to provide the corneal ophthalmologist with cell count information used to determine overall corneal health, usually as it relates to corneal transplant. While traditional specular biomicroscopes used film and required direct contact with the cornea for imaging, modern instruments are digital and highly automated, with many of them being noncontact.

Fundus Photography

The posterior segment, or all regions behind the iris, including the vitreous, a clear gel that fills the globe and the retina, is referred to as the fundus. The retina is made of up several layers including the photoreceptors, which are responsible for the eye’s ability to distinguish light and dark, color, and acuity. The retina lines the rear surface of the globe, and in ophthalmology, is an area of primary concern if visual loss is an issue not traceable to refractive error or problems in the cornea. To photograph the retina, it is usually required that the iris be dilated with pharmaceuticals (to prevent its closure when a light source is used) and photographed with a mydriatic fundus camera (mydriatic means the iris must be dilated to form an image).

A fundus camera is an integrated camera system with a circular, axial flash lighting setup integrated directly into the lens. This donut-shaped light source is designed specifically to fit inside the dilated pupil when the camera is placed at the correct working distance. Like the slit lamp, a fundus camera is mounted to a movable tabletop with integrated chin rest for the patient. There are some specialized hand-held fundus cameras available, primarily for use with bed-ridden or otherwise immobilized patients. There are also non-mydriatic fundus cameras, highly automated cameras of limited capability that are designed primarily for screening purposes.

While fundus cameras are traditionally film-based, the recent trend has been digital imaging fundus camera systems, which allow the photographer and the physician to view the images as they are taken on a computer monitor. The images are then transferred to a specially designed patient database
for later local or remote viewing and/or integration into an electronic medical records system.

Angiography of the Eye

In addition to documentation of the retina, the fundus camera is also used for different forms of diagnostic dye tests, in which a dye is injected into the body and its progress is tracked through the retinal vessels. The most common of these, fluorescein angiography, uses fluorescein sodium dye. This dye fluoresces under a very specific wavelength of blue light near UV, provided by the fundus camera through a set of filters. This fluorescence is then recorded on monochromatic film or digital system as white.

The dye itself is injected into a vein in the arm or the hand. The timing and speed of this injection are critical, as the dye only takes about eight seconds post-injection to reach the eye in a healthy individual. The photographer must anticipate the arrival of the dye in the eye, as the first split seconds of the angiogram are considered the most critical. Once this critical “early” phase passes, the patient and photographer then wait five or more minutes before taking more photographs, known as “lates.”

The appearance of the retina changes rapidly over the course of the angiogram, requiring frequent photographs as the dye enters the arteries and veins. A timer is imprinted on the film or digital file as each photograph is taken to indicate the exact timing of the entire test.

Since this fluorescent dye travels everywhere the blood does, this test shows areas of leakage or blockage in the retinal vessels not otherwise visible. This allows the physician to treat these areas, if required, by using the fluorescein angiogram as a “road map” for laser photocoagulation of the affected areas.

Another dye, Indocyanine Green (ICG), is occasionally used when it is important to visualize a portion of the retina that is deeper than the retinal vasculature, known as the choroid. ICG also fluoresces but only in the infrared region, which allows for the visualization through the retinal pigment. Because of the inherent sensitivity of digital cameras to infrared, ICG is only done with an integrated digital system. If an ICG test is administered, it is usually done in conjunction with a fluorescein angiogram as a complement to the information it provides, not a replacement.

Professional Certifications and Organizations

Ophthalmic photography is very specialized and its practitioners usually work very closely with ophthalmologists. There are national and international professional societies for ophthalmic photography, a few of which grant recognized professional levels of certification. The only current professional certification, known as CRA., or Certified Retinal Angiographer, awarded by the Ophthalmic Photographer’s Society, is considered to be a professional achievement, but is not required as a license to practice. The certification requires a minimum of 1-2 years in practice, depending on education, as well as the successful completion of a professional portfolio, written test, and practical exam. The CRA requires continuing education in the field in order to maintain the certification.

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