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This series is written with 18 months' experience gained from acquiring, using and marketing the Topcon 3D-OCT in two high street optometric practices. After being enticed with the prospect of the imminent arrival of 3D Fourier domain OCT at the 2006 Sight Care International meeting in Seville, we had a few months before product launch (at Optrafair 2007) to evaluate our respective equipment and come up with a viable business plan to bring 3D-OCT to high street independent practice.
Why? Well once you put aside the unashamed gadgety aspect that keeps us both awake at night with anticipation we firmly believe that primary care optometric practice is a natural home for what we both feel is the greatest single advance in eye care since digital retinal imaging. Like power steering and satellite navigation in cars, Ocular Coherence Tomography is the technology that, once experienced, will not allow you to find your way back.
The science of OCT dates back to the latter days of WW1 when the submarine menace of the U-boat was being fought. The committee known as ASDIC (reputedly but not actually named after the 'Allied Submarine Detection Investigation Committee') developed the acoustic based method later known as Sonar (sound navigation and ranging) to detect submarines. By utilising a sound source and a water microphone (hydrophone) the time taken for the source sound to return was taken to indicate the depth of the object below the ship. A long pause or no return signal would indicate deep water while a rapid return would indicate an object close by or immediately beneath the ship. This is now used in all manner of applications, eg deep-sea fishing.
The advent of ultrasound body examination is a direct spin-off from this invention. As with any waveform based method of investigation the resolution (ability to discriminate two close objects as being two not one) is dependent on wavelength. Figure 1 shows the variation in resolution and object size in various methods used for non-invasive examination of the body.
There are two aspects of the principle, first the reflection of the analysing waveform from the various layers of the structure and second the 'time of flight' of the signal back to the sensor, which is used to locate the specific tissue.
Light obviously has a much shorter wavelength than sound and the utilisation of a method analogous to ultrasound with light as the medium clearly offers a dramatic increase in resolution. However, there is an inherent problem in this approach, namely the speed of sound is approximately 1,500ms-2 while the speed of light is 225,000,000ms-2. The first component of the principle will stand (reflection) but the time of flight is simply too quick to allow assessment in the same way as ultrasound.
This problem is managed by using another property of a wave based radiation, namely that of interference. This is familiar in optometry as it is the science behind anti-reflection coatings. When a waveform reflects off a surface it may reflect back in phase (additive), increasing the amplitude of the waveform or reflect back out of phase (subtractive) reducing or eliminating the wave amplitude entirely. This is shown in Figure 2.
A-scan or B-scan
In ultrasound or OCT the A-scan is the axial path (or depth) of reflection across a narrow width. In the Zeiss Stratus OCT this is 1,024 pixels deep shown in Figure 3. A B-scan is simply a summary of many axial scans to form a longitudinal scan. This is typically achieved using a rotating mirror to 'scan' several A-scans one after the other. Figures 4 and 5 show the principle applied to anterior segment scans.
Figure 6 shows that the size of the red spot determines the resolution and this is further defined as lateral resolution ie between each a-scan (limited by the human eye to approximately 20 microns) and axial resolution between each tissue layer (typically 6-10 microns). It is not possible to scan the entire eye and so sections of interest are typically selected and scanned.
OCT Imaging
First on the scene, Zeiss led the way in selecting the colours to use for the false colour image, necessary because the OCT image itself is monochrome. Figure 7 shows an OCT B-scan in false colour where some layers of the retina show up as a band of colour or reflectance and some do not. The inner limiting layer is the first interface reached, followed by the nerve fibre layer. The in-between layers like the outer plexiform layer and the outer nuclear layer do not reflect and show up as dark areas. The RPE is shown as a bright red line with the photoreceptors lying above in blue. Most experienced users seem to prefer to use the grey scale image, as it appears to show more detail.
Time Domain vs Fourier Domain (Spectral)
The first commercially available OCT was the Zeiss Stratus. This provided four B-scans in the cardinal compass points (north-south, east-west, north east-south west etc). However, the gaps in between were filled according to a mathematical algorithm. For the first time there was a non-invasive technique that could be used to isolate individual retinal layers and also see through and identify opacities anterior to the retina (such as vitreous floaters etc).
Essentially the difference between time domain and spectral/Fourier domain instruments is that the former uses time delay to identify the layer interface reflections from a source beam whereas the spectral domain uses the phenomenon of Fourier analysis to assemble a waveform from a collection of other waveforms. Previously, Fourier analysis, if used at all in optometry, lay behind the understanding of the contrast sensitivity function derived from sine wave gratings.
International optical science, ophthalmology and optometry meetings are now dominated by the presence of spectral domain OCT and no discussion on clinical examination or differential diagnosis is complete without this facility. There are at least five different spectral domain OCT instruments on the market, only two are marketed directly by suppliers who undertake R&D, software development, design, manufacture, marketing, support and ongoing clinical research. These are Topcon and Zeiss. Of these only Topcon has the unique feature of a high-resolution fundus image in perfect registration with the OCT image. This is done by incorporating a fundus camera within the system.
Topcon 3D-OCT is a four-in-one instrument
A few years ago it seemed that to adequately analyse and visualise the retinal structures needed to identify and manage the optic disc, nerve fibre layer and the macular regions required various different instruments, all competing for the scarce hospital or practice clinical equipment budget. Instruments such as the Heidelberg HRT3, the GDx nerve fibre analyser and time domain OCT all seemed to have their own specific role, the clear implication that you need one of each.
Unlike the US where any new instrument competes aggressively for an insurer rebate code, UK practitioners are not used to separately billing items of service for different tests and so new equipment frequently waits its turn finding gradual acceptance.
In the Topcon 3D-OCT the practitioner has the choice of a four-in-one instrument:
- First, it is a good fundus camera (perhaps not quite as good as a stand alone digital SLR type which have chased resolution above all else) but certainly good enough to exceed the NSF requirements for diabetic screening (and thus eligible for Scottish GOS grants)
- Second, it is a spectral domain OCT enabling conventional B-scans of retinal structures
- Third, it has a three-dimensional capacity to integrate all B-scans into a single composite mobile image that graphically represents the real structure of the retina. This enables the user to, non-invasively, separate retinal layers, peel them back, rotate and even upend the tissue so that the retina can be viewed from behind
- Fourth, the instrument can separately measure and graphically represent the nerve fibre layer thickness in microns. Typically done in a circle surrounding the disc, a clear idea of glaucoma risk factors for nerve fibre defects can be assembled in an instant and used alongside other measures such as IOP, pachymetry and visual fields. Figure 8 shows the retinal nerve fibre layer reports which include normative data arranged in traffic light: red for danger, yellow is borderline and green a safe graphical form.
No other instrument offers this four-way capability.
OCT can keep you out of court
A female patient attended for a routine eye examination and was found to have a fairly ordinary hypermetropic Rx that required updating. At R&L: +2.50 with a +2.25 add she was dispensed with a rimless Lindberg glazed with Rodenstock Impressions lenses. At collection three weeks later she expressed some query over the adequacy of her vision. Contrary to custom and practice she was allowed to take the spectacles and was advised to return shortly for review and, of course, to settle the account. She did not. Some three months later and after exhausting our usual approach of phone calls and letters we elected to take the matter to the Small Claims Court. Needless to say she attended immediately. Although the left VA was excellent at 6/5, the right was a poorer 6/6-2, accompanied by a small scotoma exactly covering one letter along the Snellen chart 6/6 line. She was RE dominant. We were unable to visualise any fundal lesion and, although the scotoma was present on the Snellen chart, she did not fail Amsler and colour vision was normal.
We undertook 3D-OCT and were amazed to see what appeared to be a piece of retina directly above the fovea attached to the posterior vitreous face (Figure 9). Clearly she had been experiencing vitreous traction - in some cases a precursor to a macular hole - but with the vitreous face fully detached there was little risk of further damage. On viewing the left eye we observed essentially the same appearance. As it was early days with an OCT we elected to refer - she was then referred to a vitreo-retinal surgeon who, impressed with our OCT plots, advised her that she was at no further risk of mechanical retinal damage and should just attend for regular eye examinations.
Happily the patient paid for her spectacles and has been back for Rx sunglasses since. The OCT is first class at explaining the unexplainable. Never underestimate that even if there is no treatment or cure for a visual or ocular problem, being able to visualise and understand is of huge value for our clients.?
? Nicholas Rumney is a paid consultant to Topcon GB. and is managing director of BBR Optometry which specialises in low vision, contact lenses and children's vision. Richard Petrie is not a paid consultant to Topcon GB. He was, until recently, a sole practitioner in Derby. He is a strong advocate of offering the best possible eye examination in the firm belief that this sets out the ethos of the practice