“The eyes are the window to the soul” 16th century proverb
The retina gives an unobstructed view of the human vascular.
Also, retinal imaging is important for diagnosing retina diseases.
Fundus camera:
Is a device for photographing the retina.
It is based on the indirect ophthalmoscopy principle, where the Observer’s eye is replaced with a camera.
So, A fundus camera or (retinal camera)
Is a specialized low power Microscope with an attached Camera designed to Photograph the interior surface of the Eye, including the Retina, Optic disc, Macula, and Posterior pole i.e. the Fundus (eye).
Optical principles:
The optical design of fundus cameras is based on the principle of monocular indirect ophthalmoscopy.
A fundus camera provides an upright, magnified view of the fundus. A typical camera views 30 to 50 degrees of retinal area, with a magnification of 2.5x, and allows some modification of this relationship through zoom or auxiliary lenses from 15 degrees which provides 5x magnification to 140 degrees with a wide angle lens which minifies the image by half. The optics of a fundus camera are similar to those of an indirect ophthalmoscope in that the observation and illumination systems follow dissimilar paths.
The observation light is focused via a series of lenses through a doughnut shaped aperture, which then passes through a central aperture to form an annulus, before passing through the camera objective lens and through the cornea onto the retina.The light reflected from the retina passes through the un-illuminated hole in the doughnut formed by the illumination system. As the light paths of the two systems are independent, there are minimal reflections of the light source captured in the formed image. The image forming rays continue towards the low powered telescopic eyepiece.
When the button is pressed to take a picture, a mirror interrupts the path of the illumination system allow the light from the flash bulb to pass into the eye. Simultaneously, a mirror falls in front of the observation telescope, which redirects the light onto the capturing medium, whether it is film or a digital Charge-coupled device.
Because of the eye’s tendency to Accommodation (eye) while looking though
a telescope, it is imperative that the exiting Vergence is parallel in order for an in focus image to be formed on the capturing medium.
Since the instruments are complex in design
and difficult to manufacture to clinical standards, only a few manufacturers exist:
Topcon,
Zeiss, Canon, Nidek, and
Kowa
Applications:
for fundus photography performing the following modes of examination:
1. Color, where the retina is illuminated by white light and examined in full color.
2. Red-free, where the imaging light is filtered to remove red colors, improving contrast of vessels and other structures.
3. Angiography, where the vessels are brought into high contrast by intravenous injection of a fluorescent dye. The retina is illuminated with an excitation color which fluoresces light of another color where the dye is present. By filtering to exclude the excitation color and pass the fluorescent color, a very high-contrast image of the vessels is produced. Shooting a timed sequence of photographs of the progression of the dye into the vessels reveals the flow dynamics and related pathologies. Specific methods include sodium fluorescein angiography.
Pre-examination setting:
1. The examiner
Before attempting photography, must become very familiar with the camera through a training session and by learning the terminology of the camera operation Manual .
2. Examination room
- The room lights are dimmed so that only the lights of the camera and computer monitors remain on.
- The majority of the exam is performed in the dark.
3. The camera
- The camera is mounted on a motorized table for easy Height adjustment, and both the examiner and the SP have an adjustable stool to sit on
- Before each SP is photographed, the objective lens should be checked and cleaned if necessary.
- High humidity or temperatures must be avoided. Dusty conditions mean that the camera will need frequent cleaning.
- It its extremely important that camera lens be inspected and cleaned, if dirty, before each photograph. The retinal camera should remain covered when not in use and the lens should be capped after each SP photograph.
- A checklist itemizing procedures for preparing the camera at the start of each day is posted on the wall near the fundus camera.
- Technicians responsible for the fundus equipment should carefully follow the daily procedures.
4. The patient
- Photography begins with a complete explanation of the procedure to the SP by the examiner.
- It is important to reassure the SP that no retinal damage is caused by this procedure.
- The camera flash is bright and the SP should know when to expect a flash. Since pictures will include the macula (area of central vision), it is normal for the SP to experience a blue or red tint to vision immediately following the flash.
- This totally disappears within two to five minutes. No dilation drops are used for this examination, In non mydriatic procedure and the eyes will not be touched.
BENEFITS OF DIGITAL PHOTOGRAPHY:
- Instant knowledge of image quality
- Networking of images (a must)
- Reduction in administration time
- No film / development costs
- Easy export for presentations / e.mail
- Instantly available for viewing
- Comparison over time on screen
- Digital image manipulation tools
- Linkage to Electronic Patient Records
- Diagnostic Database
Types and manufacturers of fundus camera (retinal angiography machines):
