Few technologies have had such a swift and profound impact on eyecare as much as optical coherence tomography. Insight takes a deep dive into the evolution of OCT and the bearing this may have on future models.
Since its development in 1991, OCT has become a cornerstone in the diagnosis and assessment of most vision-threatening diseases and has helped transform vision research.
As to be expected, there were many early adopters in ophthalmology – and to a lesser extent optometry – from the beginning. But as the utility of OCT became undeniable, and often despite the lack of a financial incentive to incorporate OCT into a practice, uptake of the technology began to trend upward.
Its ubiquity in Australia’s ophthalmic sector is perhaps best demonstrated by its increasing prevalence in both independent and corporate optometry practices and its inclusion in MBS items for ophthalmology (but not optometry) since November 2016.
OCT’s developmental path started with time-domain (TD)-OCT, followed by spectral-domain (SD)-OCT, and ultimately swept source (SS)-OCT models. The technology is advancing through its next major evolutionary step in the form of OCT-angiography (OCTA), introduced in 2014, with other variations also inching closer to commercialization.
And eyecare isn’t the only sector benefitting, with the technology now extending into oncology, cardiology and dermatology.
Engineer and Massachusetts Institute of Technology alumni Mr Eric Swanson is a co-inventor of OCT technology and a co-founder of the first ophthalmic OCT company, later acquired by Zeiss. He is also editor of OCT News, a not-for-profit website providing the latest news in the field.
In a presentation he gave at a conference in Singapore in 2017, Swanson says OCT continues to benefit from “tectonic advancements” in other fields, such as augmented reality, and says high-speed lasers, integrated photonics, and computer-aided detection and diagnosis are among the most active areas of OCT research and design.
He also notes that the commercialization and growth of OCT during the past 25 years has had enormous scientific, clinical, and economic impact. To illustrate the latter, he cites statistics that between 2008-2015, OCT-guided anti-VEGF therapy in age-related macular degeneration saved more than US$9 billion (AU$12.6 b) by avoiding unnecessary injections.
In a paper he co-authored in Biomedical Optics Express, Swanson estimated there was more than US$500 million (AU$693 m) of venture capital and corporate research and development investment developing OCT related products, and more than US$500m (AU$693 m) of government funding towards OCT research.
But despite the tremendous accomplishments in advancing OCT technology and clinical applications, Swanson believes the best of OCT technology is still to come.
“We’re still at the beginning of this technology – we’re not even at an inflection point.”
Building on breakthroughs
With the relatively recent introduction of OCTA, eyecare professionals now have access to an additional imaging modality for the retinal and choroidal microvasculature, helping inform decisions in glaucoma and retinal diseases.
Compared with the gold standard fluorescein angiography, it has unique benefits as well as certain disadvantages. However, software and hardware improvements are evolving to mitigate these limitations.
Two recent projects, reported in Biomedical Optics Express in July this year, have published findings that could assist greater translation of OCTA into ophthalmology clinics.
In one project, a team at Boston University School of Medicine has developed what is said to be the first visible light OCTA (vis-OCTA) for human retinal imaging.
The other project, involving a team of researchers at Oregon Health and Science University (OHSU) in Portland, including Swanson’s OCT co-inventor Dr David Huang, devised a prototype to improve OCTA image quality without excessive sacrifice in field of view and device complexity, which they believe may have potential for clinical translation.
“Over the past decade,” the authors note, “we have witnessed the rise of optical coherence tomographic angiography (OCTA) in retinal imaging. Unlike fluorescein angiography, OCTA does not require intravenous dye injections and uses intrinsic motion contrast provided by the flowing blood cells. OCTA is acquired within a few seconds; making it ideal for translation into routine ophthalmic clinical practice.”
While OCTA can image the retinal blood flow, they argue that visualization of the capillary caliber is limited by the low lateral resolution.
The researchers developed a sensorless AO-OCTA prototype with an intermediate numerical aperture (NA) to produce depth-resolved angiograms with high resolution and signal-to-noise ratio over a 2×2 mm field of view (FOV), with a focal spot diameter of six microns, which is about three times finer than typical commercial OCT systems.
“Despite OCTA’s advantages over fluorescein angiography, a historical limitation of OCTA has been the smaller field of view. Because of the constraint imposed by the shorter depth of focus, adaptive optics instruments restrain OCTA’s field of view even further,” they note.
“Designs like ours based on intermediate-NA imaging beams alleviate this problem and present a potential to either achieve a clinically useful field of view in a single scan or to reduce the number of acquisitions needed for larger field of views by montaging partially overlapping scans.”
Translating research into real-world
Speaking from her clinical experience, optometrist Ms Janelle Tong at the Centre for Eye Health (CFEH), says while some retinal vascular abnormalities can be difficult to visualize on fundoscopy, photographs and routine OCT, OCTA can be invaluable in identifying these subtle changes.
“In particular, the ability to detect both retinal and choroidal neovascularisation and areas of ischaemia, associated with diabetic retinopathy for example, means that it can definitely improve the way in which retinal diseases are diagnosed and managed. Research into the applicability of OCTA in optic nerve disease is also showing great potential,” Tong says.
Prior to joining the CFEH, Tong worked in a full-scope private practice in Sydney, where she developed her interest in managing posterior ocular disease. She is involved in both clinical and research aspects of CFEH, with her current research looking at modelling normal ageing changes to the eye using advanced imaging.
She says that while OCTA is not often seen in clinical practice settings, current models are rapidly evolving through faster scan speeds and subsequent wider fields of view.
“Combined with ongoing research in this field, it is likely that the uptake of this technology will expand rapidly in the future.”
She says Huang’s OHSU prototype is interesting because it presents an option which combines higher resolution imaging of the retinal capillaries with a wider field of view than most adaptive optics-based OCT scans.
“This presents a key concern of commercially available OCTA, which uses a slightly different principle to adaptive optics, where despite the larger field of view the resolution is often too coarse to visualize the retinal capillary networks in detail,” Tong says.
“However, it is important to recognize that acquiring imaging using adaptive optics techniques and scan montaging are skill-intensive and time-consuming procedures, so without further developments to improve ease of use it is difficult to see it translating to routine clinical practice at this stage.”
She adds: “Nevertheless, I am excited to see what this research holds for the future, and there is lots of potential to facilitate better detection of vascular anomalies in the eye.”
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