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Slit Lamp Biomicrography
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Slit Lamp Biomicrography
Introduction

James P. Gilman, CRA
Director of Ophthalmic Photography
John Moran Eye Center
Salt Lake City, Utah

Introduction

Slit-lamp biomicrography encompasses a wide spectrum of challenging photographic techniques for imaging structures and diseases of the anterior segment. Most photographic slit-lamps are equipped with two lighting sources, variable flash intensity, changeable magnifications, and changeable angles of illumination. Patients present a wide variety of disease entities that have specific textures, colors, transparency, size and depth. The photographer or clinician examining the patient should employ the slit-lamp lighting technique that will best enhance the characteristics of the lesion being examined/photographed. There is no single definitive photograph that will best describe a patient's condition, therefore a photo essay or series of photographs can often relate the patient's condition more appropriately.

The clinical biomicroscope, or "Slit Lamp", is composed of a binocular viewing system that is copivotal with the illuminator to allow various angles of viewing and angles of lighting to the eye. A severe angle of illumination will enhance the surface details and texture by showing a shadowing on the distal edge of the subject. A direct coaxial angle of illumination will accurately show color size and relative position of the subject in relation to other anatomy and will "flatten" the structures that appeared more three dimensional with the texture lighting. The biomicroscope and illuminator are also parfocal or simultaneously focused at the same point in space at all angles. When the slit beam is focused sharply on the cornea, the beam will be in focus through the biomicroscope and will be isocentric or always centered in the viewing field no matter the angle of illumination or viewing.

The photographic biomicroscope differs slightly from the clinical biomicroscope by having a beam splitter transmit a percentage (50% or 30%) of light to the viewing oculars and reflect the remainder of the light to the camera. Some photo biomicroscopes have the ability to rotate the beam splitter out of the viewing path to allow 100% of the reflected light to be seen by the examiner. This procedure helps to reveal all the subtle details of the patient's eye without interruption of the light. A safety mechanism on a camera of this type, will prevent the camera shutter from opening without the beam splitter rotated in, which would produce a photograph that would be grossly underexposed. An additional feature of the photographic biomicroscope is the synchronized flash illuminator. The camera accurately records the details simultaneously viewed by the examiner/photographer. Another feature specific to the photo biomicroscope is the fill flash. This allows the photographer to fill in the shadow areas opposite the main illuminator and to provide orientation details of the eye when using a fine slit beam illumination. Without the aid of the fill illuminator the slit beam would give the illusion of floating in the middle of a dark void. This can sometimes be desirable if the photographer is interested in details within the slit beam itself to show corneal edema, particles in the stroma or outline topography with the slit.

The slit beam can take on many configurations from a full round circle to a thin beam of light. The illuminator contains controls for beam aperture size which controls the height of the beam and the beam width. The beam can also be decentered with another control to aim the projected beam off axis for creating a series of indirect lighting situation that will be discussed later.

 

Exposure

Photographic exposure is controlled by three separate adjustments. One is the flash power setting on the camera power supply. Another exposure control is an aperture on the elbow of the camera attachment which can vary on the Zeiss Photoslitlamp from f/45 to f/14. The third exposure control is the beam width. A wide beam is effective in higlighting deep endothelial pigmentation. A narrow beam is useful and showing intrastromal defects and topographic features.

Other factors that affect the exposure of the photograph are the tissue being illuminated, the sensitivity of the film or ISO setting on the digital camera, and the magnification of the microscope. If the tissue is very opaque and highly reflective, the aperture needs to be smaller or reduce the flash power appropriately. Less light will be required to photograph this subject as opposed to a clear cornea. Blue irides will reflect more light than brown irides and therefore exposure is affected by iris color. Some slitlamps contain a rotating filter wheel. If a photoslitlamp does not have an aperture diaphragm it will have a colorless gray filter called a neutral density filter. This filter is an exposure adjustment control that lowers the amount of flash hitting the eye by one stop (0.3ND = 1 stop) Bracketing exposures above and below the manufacturer's published tables, will allow you to get properly exposed results with the narrow exposure ranges that slide film allows. ( Plus or minus ½ stop) Not all patients with corneal lesions will sit quietly through a rigorous photo session that requires multiple aperture settings for each type of illumination or magnification change. I recommend producing an exposure table based on photographing a blue and brown iris with multiple settings and various beam widths. Record the parameters for each exposure and base subsequent clinical photographs on the settings used for the well exposed slides.

Film sensitivity is carefully chosen to allow the dimmest form of indirect lighting to be properly exposed. Most photobiomicroscopy is done with a film sensitivity of ISO 200. There are situations where a faster film emulsion such as ISO 400 is required. Films with a low ISO number have lower sensitivity but can render fine details better because the emulsion contains a finer grain. (ISO stands for International Standards Organization which assigns the tested emulsions their proven sensitivity classification numbers).

Digital Single-Lens Reflex (SLR) cameras are rapidly replacing 35mm film cameras in slit lamp biomicrography. The sensitivity of the sensor can simply be adjusted on the camera, selecting an ISO. Because higher ISO settings on digital cameras create "noise" in the image which may impair its ability to render fine detail, the same settings of ISO 200 or ISO 400 should be used with digital SLR systems as well.

Microscope magnification will have an affect on the exposure of the film when taking a picture. The higher the magnification the more light needed. If a photograph is taken of the patient's eye at 6X magnification and a small lesion is photographed at 40X magnification the exposure difference will be significantly changed and the photographer will have to either open the aperture diaphragm further or increase the flash power to compensate for the illumination decrease. Some biomicroscopes will show no change in exposure vs. magnification.

 

Direct Illumination

Illumination forms fit into two main categories of direct and indirect lighting. If the lesion is opaque, crystalline, or opalescent, a direct form of illumination delineates the areas of interest better than an indirect form of illumination. If the lesion on the eye is transparent, refractile, and almost invisible, indirect forms of illumination are more successful at enhancing fine details contained in subtle lesions that direct lighting would overpower. First, we will explore the types of Direct Illumination.

Dual Diffuse


Dual diffuse illumination is achieved when the main light has a diffusion filter in front of the beam and the fill illuminator/flash is on, directed opposite the main light illuminator, to fill in the shadows. This photograph is taken in all cases to allow orientation of the eye being examined, and is typically taken at low magnification (1OX) and at (16X).

Fine Slit Beam

Fine slit beam illumination is created by removing the diffusing filter from the main light illuminator and reducing the beam width to a fine slit. The fill light is also employed to provide the photograph with surrounding details of the slit's position. The camera or observer oculars should be positioned at approximately 60 degrees to the fine slit beam's direction. Since microscopes will generally have a narrow depth of focus, the 60 degree angle will allow more of the beam depth or optical section to be in focus to the observer and the photograph. The fine slit illumination can show corneal thickening with corneal edema or corneal thinning due to erosion. The fine slit beam can also show topographic information in cases of iris lesions, corneal or conjunctival masses, cataracts and fluid-filled cysts.

Broad Tangential

Broad tangential Illumination is made possible by increasing the beam width and increasing the angle of incidence for the beam to spread across the cornea. This type of illumination is more effective without the fill illuminator. Broad tangential illumination helps to increase contrast and show varying degrees of texture in the ocular tissue. This lighting is especially helpful for showing slight corneal scarring, pseudoexfoliation of the lens, and small bullae in a bullous keratopathy.


 

 

Indirect Illumination

Sclerotic scatter

Sclerotic scatter is an indirect illumination that is created by decentering the slit beam and directing a broad beam to the temporal sclera. With the fill illuminator off, the bright light directed to the sclera transmits across the cornea like a fiber-optic light pipe. The photographer can see a gentle 'glow' from the distal sclera when the illumination is properly aligned. The beam will always be severely overexposed in the photograph because of the broad beam reflecting off the white sclera. The soft indirect light highlights very subtle corneal changes such as corneal scarring, vortex dystrophies, and crystalline keratopathy.

 

 

Retroillumination from the Iris

Retroillumination from the iris is created by making a moderately thin slit beam and directing the beam to the iris at a 45 degree angle and keeping the plane of focus on the cornea. The soft reflected beam from the iris will enhance transparent corneal irregularities that are too subtle to be observed with other lighting situations. Details of corneal changes are best observed in the line between light and dark coming from the unfocused iris surface.

 

 

Retroillumination from the retina

Retroillumination from the retina can be set up by decentering the broad beam and rotating in the beam aperture that most resembles the patient's dilated pupil diameter. (The patient must be dilated for this type of illumination.) The next step is to create a 'half moon' shape by rotating a half aperture position out of the beam path. The decentered 'half moon' shape is then positioned just inside the dilated pupil. The observer must then look for a bright orange reflex that appears when the light is just axial enough to reflect off the patient's retina and glows from behind the lens and cornea. The position for the light and camera should be very close to axial but the retroillumination effect does not appear in both oculars in the binocular biomicroscope. The observer should close one eye and concentrate to see the retro illumination effect in the ocular that the camera shares. The retroillumination effect can be further enhanced or brightened by orienting the patient's eye (have the patient look slightly nasal) to make the optic nerve reflect the beam. This lighting situation enhances the observation of cataracts, corneal scars, transparent cysts, and refractile bodies.

 

 

Iris Transillumination

Iris transillumination requires an undilated pupil and is created by making an axial light beam shine into the small pupil and reflect off the retina. The beam aperture should match the pupil size or be smaller than the pupil to avoid iris reflections. The beam direction is again critical to proper alignment and the observer should again use only the ocular shared by the camera to properly align the light beam. This lighting situation is used to illustrate the degree of iris thinning either due to ocular albinism or pigmentary glaucoma. The light transmitted through the iris is the dimmest of all lighting techniques outlined. The flash power must be at its maximum level, or the aperture for the camera should be at its widest opening to make the photograph. Some photographers have more sensitive film on hand just for this occasion. (Typical films for biomicrography are ISO 200 -- a more sensitive emulsion would be ISO 400.)

 

 

Summary

Patients present with corneal and anterior segment problems that contain obvious details and subtle details. In our quest for documenting the disease process with best images possible, we must employ the lighting situation and magnification that best describes the problem. An obvious lesion of opaque, or crystalline nature will best be described with the direct lighting techniques outlined. A lesion that is transparent, opalescent, or refractile in nature, will be better imaged using an indirect form of illumination so that subtle details will not be washed out in a harsh light. If a combination of lighting techniques and various magnifications are used in this pursuit, a well documented disease with potential for publication quality will be the result.

Exposure Tables

Exposure Parameters Table
Zeiss 40 SLP Photo Slitlamp - ISO 400

Illumination

Flash Power

Aperture

Magnification

Dual Diffuse

2

f/28

10x

Dual Diffuse

2

f/22

16x

Fine Slit
(Full Fill)

4

f/45

10x

Broad Tangential

4

f/32-45

16x

Broad Tangential

4

f/32

10x

Retroillumination
(No Fill)

4

f/22

16x

Retroillumination
(No Fill)

4

f/22

10x

 

 

Exposure Parameters Table
Zeiss Photo Slitlamp - ISO 200

Illumination

Flash Power

Aperture

Magnification

Dual Diffuse

4

f/45

10x

Dual Diffuse

4

f/45

16x

Fine Slit
(Full Fill)

4

f/16

10x

Broad Tangential

4

f/22

16x

Broad Tangential

4

f/22

10x

Retroillumination
(No Fill)

4

f/14

16x

Retroillumination
(No Fill)

4

f/14

10x

 

References

Coppinger, M., Maio, M., Miller,K.: Ophthalmic Photography. Slack Incorporated, New Jersey

Justice, J.: Ophthalmic Photography. Little Brown and Company. Boston. 1982.

Martonyi, C.,Bahn, C., Myer, R.: Clinical Slit Lamp Biomicroscopy and Photo Slit Lamp Biomicrography. Time One Ink. Michigan.1985.

Wong, D.: Textbook of Ophthalmic Photography. Inter-Optics Publications. New York. 1982.

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ALL RIGHTS RESERVED.