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Overview image
The overview image is generally one that is used to set the scene. In normal practice several images may be required to record faithfully the state of the eye and a simple, low magnification image can be used to orientate the viewers of an image series.
The diffuse illumination is achieved by fitting a diffuser to the slit illuminator and opening the slit to the widest beam. This can be used in combination with the background illuminator to produce an image that has balance and shows good detail. If the sole illuminator is used close to the optical axis the image may lack detail as in this position the lighting will flatten some features in the subject. If the angle between the illumination and slit-lamp objective is increased to around 45 degrees, detail will be enhanced; however, one side of the image will appear dark. Using a second source at 90 degrees to the primary light source will reduce this effect and improve the image (Figures 2a and b).
Using low magnification ensures that a good depth of field is achieved and that, with some slit lamps, cornea, iris and a portion of conjunctiva and sclera will appear in focus. Diffuse illumination is commonly used to image naevi on the iris as this is the simplest form of lighting to reproduce. The eyelids and sclera are also best imaged with diffuse light where reflections and shadows are minimised. Most manufacturers can supply diffusers for their slit lamps but a low cost and readily available alternative can be produce by temporarily attaching a -20D lens in front of the illumination tower mirror.
Narrow slit image - optical section
None of the structures in the eye are absolutely transparent and it is this fact that allows us to view them with the slit lamp. A normal window, much like a cornea, looks clear in diffuse light, but when a narrow beam of intense illumination is introduced at a wide angle to the viewing angle then detail can be observed in the illuminated section. In slit-lamp biomicroscopy this is often referred to as an optical section, as in the cornea and lens it can look similar to a thin slice through the semi-transparent media.
Creating an image of an optical section can be a challenge for some imagining systems. This is because only a relatively small amount of light energy is projected into the eye via the slit. The optical effect can only be observed if the slit width is less than 0.2mm and therefore either very large amounts of energy such as a flash from a photo slit lamp or a highly sensitive camera sensor is required. The first step in creating this image is therefore to turn the flash or lamp energy up to maximum and reduce the slit width to around 0.2mm.
The optical section cannot be observed unless the angle between the incident light and reflected light is large. In the cornea it will be possible to achieve an angle of 90 degrees and with this angle the whole of the corneal section will appear in focus. The maximum angle in the lens is around 45-50 degrees and this can be improved with patient dilation. The maximum detail will be visualised when no fill light is used. However, in some situations a small amount of diffuse light is required to orientate the viewer. Figure 3 shows the set up for a corneal section, Figure 4 for a section of the crystalline lens.
If the slit lamp has an aperture control it is likely that this will have to be at the widest setting for optical section imaging. The magnification of the slit lamp should also be considered as with each increase in power, the depth of field decreases and the effective aperture of the instrument increases and limits the light further. If low light levels are compromising the quality of the image then using a lower magnification may help.
In addition to the optical section the narrow slit can also be used to measure the relative thickness of the cornea, demonstrate anterior chamber depth and define surface topography.
Wide slit beam imaging
A wide slit beam can be classified as all focally illuminating techniques that use a beam width of greater than 0.2mm and they generally fall into two categories. Using a moderate slit width of between 1-2mm can demonstrate separation between corneal layers and, much like a spotlight, can be used to highlight pathology (Figure 5).
A wide beam of 4-8mm projected tangentially across the eye can be extremely effective lighting for some very subtle pathology. The wide angle enhances surface texture and the relatively high light levels allow high magnification and reduced apertures in most systems. Generally in tangentially illuminated images no fill light is used - when a moderate slit width is used a small amount of fill can be used to help orientate the image.
Patients will be less tolerable of wider beam with higher energy, and often you have less time to compose this type of image or excess tears may reduce the quality. Also be aware that the larger patch of light may introduce larger reflections or artefacts that can be managed by altering the angle of the light tower, microscope or even the eye.
Indirect illumination
Indirect illumination can be used to enhance detail in the semi-transparent media of the eye such as the cornea. This very subtle detail can be visualised using light reflected from other surfaces within the eye, as it would otherwise be overpowered by direct focal illumination. In order to achieve this illumination pattern, increased control of the slit illumination is required. In normal use the slit and microscope have a common point of focus. When attempting indirect illumination the slit is defocused from this point and is directed towards the reflecting tissue. The degree of defocus and the background of the subject can have a large effect on the image that is produced. The effect of indirect illumination can be easily observed, but, as the light levels are low, these images can be a challenge to record. This control is best demonstrated where detail is required in the structures that are on the edge of the bright and dark areas of the field and can often include a mix of direct and indirect illumination in the same image (Figure 6).
Many corneal irregularities can be imaged by using a technique of sclerotic scatter as a form of indirect illumination. When a 3-4mm wide beam is fully decentred and directed towards the corneal limbus, the energy reflected from the sclera is returned into the cornea and by total internal reflection is distributed through the whole cornea. This is a similar principle to the function of a fibre optic. A normal cornea will show little detail; however, structures within an abnormal cornea will refract and reflect the light and can be observed and imaged (Figure 7). Best contrast can be achieved without fill illumination and with a dark background such as a dilated pupil can provide.
Proximal illumination is a technique used when detail is required in a subject that is highly reflective. This is commonly the sclera, but can also include conjunctiva and lid margins. The decentred slit beam is positioned adjacent to the region of interest and the light is scattered within the tissue, diffusely illuminating the subject indirectly (Figure 8). The levels of illumination can show large variations and therefore the best results will be achieved from slit-lamp imaging systems with good exposure control.
Retro-Illumination
Retro-illumination is a type of indirect lighting that uses the retina to reflect the energy from the slit illuminator. Abnormalities in the media of the eye can then be observed as this reflected light is either refracted or absorbed by the defect. Iris atrophy and similar defects can also be visualised by retro illumination.
For retro illumination of the lens and cornea the pupil should be dilated, as the light energy has to get in and out of the eye on a different path. With most systems a pupil diameter of at least 4mm will be required, but generally the wider the pupil the better the image. The slit lamp should then be arranged so that the slit illumination and the microscope objective to which the camera is attached are aligned coaxially. The red reflex should be visible but the corneal reflex will compromise the view. Defocusing the slit to the edge of the pupil will move the central reflex and improve the image. Small movements of the slit illuminator can be used to optimise the image brightness and this is best observed monocularly (Figure 9).
The three steps to produce good retro images are:
? Set the slit for coaxial illumination to visualise the red reflex
? Defocus the slit to remove central reflex
? Fine tune the slit (size, width, position) to obtain good reflex with minimal artefact.
Ideally the size and shape of the slit should be adjusted so that the white reflex is minimised and the red retro reflex is bright and even. In some pigmented eyes, through small pupils or where a brighter retinal reflex is required, the patient's fixation can be positioned slightly nasally. This will direct the illumination towards the optic nerve head where the lamina cribrosa will significantly intensify the reflected light.
When attempting to retro-illuminate the iris the slit need not be defocused as the pupil is used for the incident light. Care should be taken to make the light patch slightly smaller than the pupil and thus avoid causing reflections from the iris that may in turn produce a reduction in image contrast. Again a 3-4mm pupil is required to get sufficient light into the eye and therefore a partially dilated pupil will help. Generally two images are captured, one using a small amount of fill illumination to orientate, and a further image with no fill illumination to maximise contrast in the trans-illuminated iris. ?
? Steve Thomson is a former ophthalmic photographer and is an employee of Haag-Streit in Switzerland