OCT: Past, Present and Future
OCT has become an essential tool in the diagnosis and management of ophthalmic disease
Since its development in the early 1990s by David Huang MD, PhD, James Fujimoto PhD, Carmen Puliafito MD and colleagues, optical coherence tomography (OCT) has evolved rapidly, with ongoing improvements in scanning speed, sensitivity, resolution and depth.
OCT relies on low-coherence interferometry and tomography to generate high-resolution images of ocular structures. It was Dr Huang who coined the term OCT and first reported the method for converting the signal intensities recorded at adjacent A-scan positions into grey or false colour values, combining the signals to form tomographic images of
The intervening years have seen systematic evolution from time domain, spectral domain and swept source domain OCT, with each system bringing greater speed, depth and resolution.
Oliver Findl MD, Chief, Department of Ophthalmology, Hanusch Hospital, Vienna, Austria, was among the very first to be involved with clinical investigations of ophthalmic applications of OCT. He has been involved in virtually every aspect of OCT research and development during the past 25 years.
“I got involved with OCT early on, in my first year in residence in Vienna. It was still called partial coherence interferometry. At the Institute of Medical Physics, the inventor of optical biometry produced a prototype and I was involved in some of the first clinical trials. It was huge apparatus, an optical bench one metre by one metre. We did our first measurements in eyes and published this in the 1990s. This eventually led to phase III clinical trials, and six years later the IOLMaster (Zeiss) arrived on the market,” said Dr Findl in an interview with EuroTimes.
This early work led to what is now called optical biometry. The OCT prototype proved to be a big step forward in measuring axial length, proving more accurate and reproducible than ultrasound biometry, the standard at the time. OCT proved easier to perform and much more comfortable for patients than the ultrasound technique.
“The IOLMaster really transformed biometry into what it is today. I bought the first IOLMaster in London, if not the whole of the UK. It was so accurate and had the convenience of non-contact. The whole thing transformed measurement of axial length, as the ultrasonic probe tended to indent the cornea, and you got artefact changes with them. The accuracy of optical biometry has continued to improve and is now extremely good,” said David J Spalton FRCS, FRCP, FRCOphth, President of the ESCRS.
Optical biometry also allowed accurate measurement of anterior chamber depth, corneal pachymetry, as well as of the lens and retina. Ultimately, when combined with better intraocular lens (IOL) power formulas, this provided better IOL power prediction and refractive outcomes with cataract surgery.
“Early on, we were just measuring axial length, and then we started measuring anterior depth and IOL position. We did a lot of trials looking at IOL position with different lens models, how they change their axial position in the first days and weeks after surgery. This was of great interest to lens manufacturers, trying to find the ideal materials and haptic designs,” noted Dr Findl.
The early OCT research showed for the first time how the IOL looked in the eye after surgery. The technique provided clear images showing how the posterior and anterior capsule enclosed the IOL within the first one to two weeks after surgery. It also showed differences in this process with different lens designs, haptic designs and materials.
Early research with intraoperative OCT revealed essential information about what happens in the eye after phacoemulsification and cataract removal, but before IOL implantation. “Once you’ve removed the cataract, done the phaco, and done the irrigation and aspiration, you have the empty bag. We learned that measuring the empty bag position actually gives you better prediction of the IOL position after surgery, and therefore better refractive outcomes,” Dr Findl explained.
OCT coupled with IOL power calculation formulas improved, and continue to improve the prediction of refractive outcomes after cataract surgery. Newer systems have added significant improvements in corneal measurement, which, combined with already accurate axial length measurements, have further improved the ability to predict outcomes.
“As far as swept source biometry is concerned, in terms of axial length it’s not a lot different from doing A-scans with partial coherence, but the advantage of swept source is that it penetrates denser cataracts and improves corneal measurements. If you make a 1.0mm error on the A-scan it works out to about 2.5D of IOL power error. If you make a dioptre of error in the K value it is about 0.9D in IOL power. New systems such as the Zeiss IOLMaster 700 and the Lenstar (Haag-Streit) provide more accurate biometry and better prediction of K values,” explained Prof Spalton.
IMPROVING ELP PREDICTION
However, the main problem cataract surgery has yet to solve is accurate prediction of the effective lens position (ELP). In spite of improvements in optical biometry, real-world figures from ‘big data’ sources such as the European Registry of Quality Outcomes for Cataract and Refractive Surgery (EUREQUO) suggest that getting to within +/-0.5 dioptres of predicted refraction is problematic, with seven or eight per cent of patients still outside of +/-1.0 dioptres.
“This is real-world data. Some surgeons may have better numbers. It may be that they don’t have patients with very long or short eyes. But at the end of the day, if we look at big data, that is the kind of number we see. We are not where we want to be, especially if we are talking about premium lenses. You want to be close to emmetropia, that is when these lenses start working properly,” Dr Findl told EuroTimes.
The prediction of ELP is also limited by the biological wound healing process that occurs once the lens is removed. This process varies with the individual patient, with the amount of fibrosis having an effect on the healing process as the capsule shrinks around the implant.
“The problem with swept source biometry, even though it is so good, is that you don’t visualise the equatorial part of the lens because you can’t see through the iris. The only way you do that is with ultrasound B-scan, or MRI, neither of which is practical. We don’t have a technology to show us the real position of the lens, and even if we did, you are still dealing with the biological healing process,” notes Prof Spalton.
However, new IOL calculation formula may help cataract surgeons reach the goal of getting patients to within at least 0.5 dioptre of emmetropia. The radial basic function formula, developed by US ophthalmologist Warren E Hill MD, does not depend on vergence formulas or ELP, but is based on artificial intelligence and pattern recognition.
“Dr Hill reckons that 90 per cent of patients will be within 0.5D of emmetropia, which is pretty outstanding. This is going to take out some of that variability that we see due to ELP,” Prof Spalton predicted.
A macula OCT is an essential part of the preoperative assessment of any patient who wants a multifocal IOL, says Prof Spalton. Missing subtle macula pathology such as small holes or an epiretinal membranes can lead to a poor visual outcome and a dissatisfied patient. To be forewarned is to be forearmed.
Anterior segment OCT has also become an indispensable tool for penetrating and lamellar keratoplasty. It provides useful information during the surgery planning stage and is used postoperatively to evaluate graft and host interaction, and for detecting corneal problems. It has become key in lamellar procedures, helping in a variety of ways such as demonstrating Descemet’s membrane folds and detachment after deep anterior lamellar keratoplasty (DALK), and graft position and thickness after Descemet’s stripping automated endothelial keratoplasty (DSAEK). Anterior segment OCT has also proven essential for Descemet’s membrane endothelial keratoplasty (DMEK) surgery.
“I believe that OCT is absolutely essential in DMEK. Intraoperatively we can judge the proper positioning of the graft injected into the anterior chamber by observing the orientation of the scrolled edges of the membrane. This may be very challenging, especially for the beginner surgeon or sometimes even for the experienced one, when the corneal stroma is cloudy.
“Intraoperatively, OCT also allows us to check whether or not there are any irregularities of the posterior corneal contour that may lead to Descemet’s membrane detachment postoperatively. I can also say that, without having OCT in your diagnostic armamentarium, one would not be able to perform and manage DMEK patients pre- and postoperatively according to the current standards of care,” Boris Malyugin MD, Chief of the Department of Cataract and Implant Surgery, Fyodorov Eye Microsurgery Complex, Moscow, Russia, told EuroTimes.
In the last couple of years a number of microscope-mounted intraoperative systems have become available. Two large prospective clinical studies, the PIONEER and DISCOVER studies, provided considerable support for the use of these systems in a variety of ophthalmic surgical settings. However, the cost of these systems continues to be an issue.
“I could imagine in a few years’ time most microscopes will have integrated OCT. It’s a nice thing, you don’t even have to look up. You have an overlay that you see in the eyepiece. It doesn’t slow you down during surgery, and helps you stay very concentrated and focused. These are the tools that will definitely move us forward into the future,” Dr Findl commented.
While initially used primarily for retinal surgery, intraoperative systems are now finding many clinical applications in cataract, refractive and corneal surgery. For example, OCT has also proven to be an indispensable element in femtosecond laser-assisted cataract surgery (FLACS). While FLACS surgical systems show a lot of variety in terms of hardware and software, most include some form of high-resolution OCT image-guidance system. The integrated systems provide detailed visualisation of the cornea, iris, iridocorneal angle, and lens, including both the anterior and posterior capsules.
Intraoperative systems are also potentially very useful research tools. For example, one recent study (Amir-Asgari et al, JCRS, Volume 42, Issue 7, 1029-1036) used intraoperative OCT to look at the eye during the phacoemulsification stage, to study how many lens fragments actually hit the endothelium. This helped to explain why the endothelium gets damaged during surgery. The study showed a correlation between the number of fragments hitting the endothelium and endothelial cell loss, one day after surgery. The researchers developed a fragment score that predicts the operative endothelial cell loss.
Continuing developments in the technology can also open new avenues of clinical benefit. For example, the new IOLMaster 700 uses swept source OCT biometry, providing excellent images of the cornea and lens. But it also provides a small image of the central fovea. This is already showing promise as a way to screen cataract patients for unsuspected macular disease.
Microscope-integrated OCT also shows promise as a surgical training aid, another recent study suggests (B Todorich et al, Investigative Ophthalmology & Visual Science July 2016, Vol.57, OCT146-153). In the randomised study, 14 ophthalmology residents performed anterior segment surgical manoeuvres on porcine eyes with or without the aid of swept source intraoperative OCT.
Residents using the integrated OCT system performed better when doing corneal suture passes at 50 per cent and 90 per cent depth and corneal laceration repair, when compared with those not using the system. Moreover, the residents who had used the system showed superior skills subsequently, even when not using the system.
“The current study demonstrates that microscope-integrated feedback can not only guide a surgeon’s intraoperative decision-making, but also has the potential to improve surgical performance, enhance anatomic outcomes, and be used as a training tool that is directly translatable to the operating room for use in live human surgery. As intraoperative OCT technology continues to evolve, so will our understanding of its potential, which will allow for its increasing creative applications in many aspects of ophthalmic surgery,” the researchers concluded.
OCT AND OCULAR SURFACE DISEASE
New highly sensitive OCT platforms have also extended research to the very front of the eye, providing hitherto unavailable objective data on dry eye disease. Leopold Schmetterer MD and colleagues at the Medical University of Vienna, Austria, report developing a method to image the human cornea with a resolution of approximately 1.3 microns (Schmetterer et al, IOVS, Vol 56, 4482.). They have used the technique to visualise and quantify the pre-corneal tear film, and to observe the effect of various eye drop lubricants on tear film thickness.
“It is amazing. They are looking at the tear film thickness with OCT resolution of 1-2 microns. That will be very interesting in the management of dry eye problems, where obtaining objective measurements has always been challenging. We’re going to get a lot of good objective information from the very front of the eye,” Prof Spalton observed.
David J Spalton: email@example.com
Oliver Findl: firstname.lastname@example.org
Boris Malyugin: email@example.com