By Lee Pepper
Optometrist
Optical Manufacturers

There have been few quantum leaps in the examination of the posterior pole of our patients’ eyes. The direct ophthalmoscope gave a magnified albeit restricted view of the back of the eye. The binocular indirect ophthalmoscope provided a larger field of view in three dimensions with limited magnification. The non-mydriatic fundus camera afforded the practitioner unprecedented observation and digital recording of the retina. However the drawback of all of these methods was the reliance on reflection of visible light. Each instrument had a system of illuminating the back of the eye and the light reflected from the inner layer of the posterior pole was captured and observed as it left the eye. Thus the result was a two dimensional view of the back of the eye with more detail than previously achievable but insufficient means to effectively discriminate between the sub-layers of the patient’s retina.

Contemporary practitioners seeking information about the back of the eye are now standing on the precipice of a diagnostic revolution with the advent of Ocular Coherence Tomography, or OCT as it is more commonly known. This exciting new technology provides the practitioner with information that previous method that simply employed visible light could not.
So how does OCT achieve this? The secret is the magic of low-coherence interferometry. Detailed explanations of the science of this technology are available elsewhere but a simple explanation is as follows: Long wavelength (near infrared) light is given off by a superluminescent diode and is split into two separate beams. One beam is passed over the retina and is then compared with another beam reflected from a mirror with a similar optical path length.
The earliest OCTs adjusted the position of the mirror to obtain a comparison of the two beams. This method became known as “Time Domain OCT”; its limitation being the time to mechanically adjust the mirror. Vastly quicker scanning speeds are achievable by 3D OCTs, which employ a “Fourier”, or “Spectral Domain” technique through introduction of a diffraction grating in the path of the reflected beams. This improves analysis of differences between the two beams immeasurably, making many more scans possible in a shorter period of time. Speeds of up to 40,000 A-scans per second are possible, however at higher scanning rates there is a trade-off between image quality and scanning speed. The Topcon 3D OCT uses an optimal scan-rate of 18,000 A-scans per second, which ensures excellence of capture in patients with poor fixation or with corneal disease or cataract without compromising the quality of the sequential B-scan.

An area as large as 6 mm x 6 mm can now be scanned with extreme accuracy. 3D OCT and fundus photography are carried out in one sitting at the same instrument: after OCT scanning is complete, Topcon’s in-built fundus camera captures a hi-res photograph of the fundus. The beauty of this system is that the retinal photograph is not only diagnostically useful in its own right but it also provides a reference for the practitioner to identify the exact location of any underlying lesions.
3D OCT technology clearly assists the practitioner with the diagnosis of any condition that affects the retina. The section of the retina to be scanned is determined by the optometrist for each particular patient. The scanning area can be centred on the macula, on the optic nerve head or elsewhere on the retina in accordance with the individual patient’s needs through preset fixation targets that make exact positioning possible.
The real diagnostic value of this instrument however is through the compare function and “PinPoint Registration”; the ability to scan an area of the retina on two separate occasions and to precisely overlay the two scans to achieve an exact assessment of any differences in the two retinal states. The software will not only precisely align the two images but with a point-and-click on one image, the identical scan from the two visits can be visually and mathematically compared (see graphic “Compare Function”). Furthermore, the import function of the OCT will allow comparison of a current scan with previous images taken on an older instrument.
The Topcon 3D OCT makes it much easier to detect and diagnose many retinal conditions that are less obvious with fundus photography or direct and indirect ophthalmoscopy. Subtle retinal changes that may or may not be apparent via direct observation can be investigated and evaluated with exceptional ease, eliminating the guesswork from identification and monitoring of epiretinal membranes, macular holes or oedema, papilledema and several other conditions. Scans of glaucoma suspects will also reveal a nerve fibre layer (RNFL) profile, which is digitally compared to a multi-ethnic normative data distribution within seconds. Best of all, visible manifestation of these results in colour and as a retinal “slab” which can be rotated and analysed slice-by-slice or layer-by-layer has immense value for practitioner and patient alike.
For optometrists who seek superior information about the retina, the emergence of the Topcon 3D OCT as the gold standard in retinal care makes it hard to imagine practising without this instrument. Purely and simply, it takes the guesswork out of assessment of the retinas of our patients.