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When held at its correct working distance from the eye, the condensing lens produces a magnified image of the fundus approximately the same distance in front of the lens. The field size and magnification are dictated by the design of the lens and the image magnification can be further adjusted by the slit-lamp magnification control (Figures 1a and 1b).
Some clinical examinations can be carried out on undilated eyes but imaging through a small pupil is extremely difficult. In addition, observing the image binocularly can mask some reflection and artefacts and therefore it is recommended that imaging should be attempted monocularly through dilated pupils whenever possible. Attention should also be paid to the condition of the condensing lens as scratches, fingerprints or other marks will significantly reduce the image quality.
For those without practice in retinal imaging on the slit lamp the following guidelines will help, although there is no real substitute for practical experience:
? Darken the room lighting as much as possible. Stray light can cause unwanted reflections from the lens surface
? Set the slit lamp magnification to 16X, slit height to 8mm and width of 2mm-3mm. The illumination should be almost but not exactly coaxial with the ocular path to which the camera is attached. Reflections from coaxial illumination are more challenging to manage
? The slit lamp brightness should be set mid range to avoid dazzling the patient and if the slit lamp has aperture or sensitivity control this will help to obtain the optimum image exposure
? Background illumination should be switched off
? Focus the slit in the plane of the patient's pupil
? Holding the lens between thumb and forefinger (use right hand for patient's left eye vice-versa) and position it around 1cm from the corneal apex. Looking around the slit lamp microscope and ensuring that the slit is still directed through the pupil will help
? While holding the lens steady and observing the view through the eyepieces, slowly move the slit lamp cross slide directly backwards until the retina comes into focus. The lens surface and then a red blurred reflex will be seen before the sharp retina can be visualised. The backward movement should be between 2cm and 2.5cm, depending on the lens used
? The image can be improved by making small movements of the lens in x, y and z planes and it is at this stage that the image should be observed through the imaging eyepiece only. Tilting the lens slightly can remove unwanted reflections and small vertical and lateral lens movements can be made to locate the region of interest
? If possible, review the images as they are captured and make any adjustments that may be required after each image capture. Remember that the dynamic range of the camera sensor is different to the eye and therefore this is an important step.
Novice users may find difficulty holding the lens steady and will find it helpful to use either the slit-lamp headrest or the patient's cheek for some support. The use of an elbow rest can also help keep the lens still. Slit-lamp imaging systems that use flash or have the benefit of a history trigger can assist all users create high quality images.
The use of a lens that is held in contact with the eye such as the Goldmann three mirror can improve image quality as reflexes can be reduced further and stability is increased. The central optical zone of the three mirror is used for visualisation of the posterior pole and peripheral retinal can be observed via either of the two largest mirrors. Adversely the cornea requires to be anaesthetised and a coupling gel is required when a contact glass is used (Figures 2a and 2b).
Fluorescence imaging
Sodium fluorescein has a number of uses in ophthalmology and it is frequently used with the aid of a slit lamp to visualise regions where the corneal epithelium has become eroded or damaged (Figure 3). A further application of topically applied fluorescein is to mix with the tear film in order to facilitate tonometry, assess the tear film break up time and improve the fitting of contact lenses.
A small amount of sodium fluorescein is sufficient to temporarily stain damaged epithelial cells and de-epithelialised regions of the corneal surface. The dilute sodium fluorescein (orange) will give off energy at a higher wavelength (yellow-green) when stimulated by an exciter (blue) light source and thus by using the cobalt blue filter built into most slit lamps, the biomicroscope can be used to observe the stained tissue. It is possible to document the fluorescence stain but it should be considered that the light levels are very low and either flash or a high sensitivity camera sensor is required.
The key to obtaining good quality fluorescein images is to use only a small amount of dye and then, after a few blinks by the patient, rinse the excess from their eye using a sterile solution. Imaging performed quickly after this stage will show good detail without being over-saturated by fluorescein pooled in the tears.
The slit-lamp magnification should be set to 16X with the slit illumination fully open. Using the cobalt blue (exciter) filter in place and the instrument viewing bulb at full intensity will ensure that maximum light is available. Even illumination is desirable and therefore an additional light source should be used and should be close to the microscope (Figure 4). Disturbing reflexes can be managed by slight positional changes to this arrangement. The fluorescent stain can be observed against the blue background of the cobalt blue light and this is often sufficient to provide some background or orientation information. If more detail of the stained region is required a yellow-green barrier filter can be used. This filter blocks the blue excitation light and therefore increases contrast in the areas stained with fluorescein but it will also further reduce the light level. A matched filter from the slit-lamp manufacturer is best but a Kodak Wratten No 12 is a good, lower cost substitute (Figures 5a, 5b and 5c).
Preparing images
Slit-lamp images may be required for several reasons. Routine documentation as part of the clinical exam is the main use, but an increasing number of images are being used for publication and teaching purposes. Generally, routine clinical images need no further manipulation but those images intended for wider audiences could be improved by using image-editing software.
Clinicians are obliged to treat medical images in the same confidential manner that they would with patient medical records. Furthermore, significantly altering clinical images using digital manipulation is not recommended. Consequently, when preparing images for other uses it is suggested that a copy of the original image is used and that the original remains unaltered.
There are a number of image editing applications available that have a wide range of functions that could be applied to slit-lamp images. Cost is a major factor and choice is generally a balance between suitability and available budget. A common program used by medical photographers is Adobe Photoshop CS Extended (www.adobe.com/products/photoshop/index). This has long been considered the standard by imaging professionals and this latest version has been developed with input from medical specialists, supports nearly all file formats including DICOM but is at the higher end of the cost scale. Adobe Photoshop Elements www.adobe.com/products/photoshopel/main) is targeted more at the home/digital camera user but includes many of the most common features of the full version at a fraction of the cost. Paint Shop Pro (www.corel.com) is another useful application available at a reasonable price.
Functions of image editing software that are most likely to be used are as follows:
? Image crop and resize (Figures 6a and 6b) - Cropping an image to show only the area of interest or to remove unwanted reflections is a simple but effective method to improve the image. Re-sizing may be required especially if an image is to be reduced in size to make presentations run more smoothly
? Image optimisation (Figure 6c, 6d and 6e) - Adjustment to the luminosity levels as well as brightness/contrast and colour adjustment are the main features that can be applied to significantly improve the image
? File format - Most image-capture systems will store images as JPEGs, but continued changes and re-saves in JPEG can result in compression artefacts becoming visible. If possible use a Bitmap or TIFF copy of the original and carry out any editing. Publishers generally require these higher quality images, but for web or presentation use, a compressed JPEG copy can easily be made using the editor
? Watermark (Figure 7) - Inserting watermarks on medical images has become common practice to protect owner's copyright.
Summary
Producing high quality slit-lamp images can be extremely rewarding and, by investing some time in practice, almost everyone can improve their technique. Attempt to appreciate the different types of illumination and try to observe the very subtle patterns that small changes in illumination technique produce. Attempt to remember the differences between the clinical exam and imaging and procedure. Finally, learning to understand the abilities or limitations of your slit lamp will also be of benefit. ?
? Steve Thomson is a former ophthalmic photographer and is an employee of Haag-Streit in Switzerland.
Further resources
Slit Lamp: Examination and Photography, Csaba L Mártonyi, Charles F Bahn & Roger F Meyer. ISBN 13:978-0-615-16519.
www.twinchimney.com
www.opsweb.org
www.oia.org.uk