Optical Coherence Tomography
Extensive development activity is fueling OCT market growth.
The first commercial optical coherence tomography (OCT) imagers were introduced to ophthalmology by Advanced Ophthalmic Devices (now owned by Zeiss Meditec) in 1992. Since then, the market for OCT systems has grown at an impressive compounded annual rate of about 45%.
This segment of the biophotonics marketplace has flourished because OCT enables sub-surface imaging of translucent or opaque materials, such as human tissue, in real time at micron resolution and without the use of ionizing radiation.
As a result, last year’s global sales of all OCT systems exceeded $400 million and the market now supports more than 36 OCT systems companies, according to OCT News.
Rapid market growth will likely continue as clinical acceptance of OCT for biomedical imaging becomes more widespread and non-medical OCT applications emerge in areas like non-destructive evaluation and security. In fact OCT has so far been successfully commercialized for only two medical specialties — ophthalmology and cardiology — so it has plenty of room to grow!
At the same time, extensive development activity is leading to technical advances that are increasing OCT’s utility and will fuel market growth. Recent OCT developments are wide-ranging and include novel light sources that can extend the imaging range, innovative probe-delivery methods, and the combination of OCT with other imaging techniques.
Even the futuristic Google Glass has emerged as a potential component of an OCT system.
Optical coherence tomography is a non-invasive imaging technique that is analogous to ultrasound. However, instead of using reflected sound to create an image, OCT uses reflected infrared (~800-1310 nm) light.
The output image is constructed by measuring with low-coherence interferometry the echo time delay and wavelength of light that is backscattered or back-reflected from the microstructural features within the material being probed. Imaging can be performed in real time and in situ with resolution that ranges from 1 to 15 microns, orders of magnitude finer resolution than that of current clinical ultrasound systems.
Early OCT systems were based on time-domain (TD) optical technology. They used a mobile reference-arm mirror to sequentially measure the light echo time delay with acquisition speeds of 400 axial scans (A-scans) per second and axial resolution of 8-10 µm. Inherently long acquisition times were due to the moving reference mirror and limited the utility of the technique, especially in ophthalmology where eye movement such as blinking prevented precise mapping of retinal tissue.
Spectral domain OCT (SD-OCT) succeeded TD-OCT and addressed some of its limitations. Use of a broadband source centered at ~800-850 nm combined with a stationary reference arm and a high-speed camera spectrometer enabled the entire signal at all wavelengths to be acquired in parallel. Fourier transformation (FT) of the signal provided the spatial information. With no moving parts and high-speed processing the scan speed of SD-OCT was 50-1000 times faster than the TD technique.
The improved acquisition speed of SD-OCT reduced motion artifacts, but other SD benefits included an increased area of retinal coverage, the ability to produce three-dimensional data sets to create topographic maps with precise registration, and better resolution. Today’s current generation of spectral/Fourier OCT systems can achieve 100,000 A-scans per second with axial resolution of 3-5 µm and up to 250,000 A-scans/second at 5-10 µm resolution.
More recently, another form of FT-OCT emerged. Instead of a broadband superluminescent diode laser, swept-source OCT (SS-OCT) uses a narrowband light source typically centered at ~1050 nm that can emit at multiple wavelengths and is scanned sequentially through individual wavelengths to build up the interferogram. Since the axial resolution in OCT is related to the bandwidth of the source, a wider span of wavelengths in the light improves the axial resolution.
Also, the increased operating wavelength compared to SD-OCT means the light penetrates tissue better and with less scatter, allowing imaging of deeper structures. Typically SS-OCT also provides still faster scan speeds, achieving >200,000 A-scans/second with ~5-7 µm axial resolution.
In addition to ophthalmology and cardiology, OCT is being evaluated for many other clinical applications and there are uses for OCT that span medical specialties. These include, for instance, optical biopsy as an alternative to traditional excisional biopsy and real-time guidance of surgical procedures.
OCT-guided procedures are “just starting to happen,” according to Eric Swanson, editor of OCT News and co-inventor of OCT technology. “It’s exciting to watch the market start to transition from purely diagnostics to include therapeutic opportunities as well,” he said.
A recently introduced swept-source OCT system (from Thorlabs) uses a MEMS-based tunable vertical cavity surface emitting laser (VCSEL) optimized for OCT. Swept OCT sources generally use a relatively long external laser cavity — sometimes up to a few meters — with an intracavity filter or wavelength-selective mirror for tuning, but the output is multimode.
The MEMS VCSEL is based on a Fabry-Perot cavity that’s only a few microns long. The device has a free spectral range (FSR) of more than 100 nm, enabling mode-hop-free single-mode tuning over the entire FSR.
The result is that the VCSEL has a coherence length much longer than other OCT swept sources and delivers an OCT imaging range five times that of other SS systems, according to the manufacturer. See a paper in the SPIE Digital Library (Proceedings of SPIE 8213) for a description of this light source.
The long-range imaging capability has particular benefits in ophthalmology where it now makes possible simultaneous imaging of the anterior eye and measurement of the full length of the eye from the cornea to the retina in a single OCT acquisition. At the same time, long-range imaging offers benefits in non-medical applications because it allows non-destructive evaluation of 3D structures using rapid, high-resolution volumetric imaging that could be a major boon in manufacturing quality assurance.
The application of OCT for endomicroscopic imaging of the gastrointestinal (GI) tract is relatively early in the process of gaining clinical acceptance and regulatory approval. Among the techniques being explored is a novel method of delivering the imager that is based on a small tethered capsule instead of the more traditional endoscope.
Researchers at the Wellman Center for Photomedicine at Massachusetts General Hospital (USA) have developed the capsule, which is about the size of a large pill and contains the optics required to transmit and collect light. The tether contains a driveshaft and an optical fiber.
Once the capsule is swallowed, it provides 3D images as it is moved up and down the digestive tract by means of the tether. Besides being less invasive than standard endoscopy, use of the capsule does not require that the patient be sedated.
Furthermore, the device is reusable and relatively inexpensive, making tethered capsule OCT imaging a potentially very attractive clinical option.
The new technique will be the subject of several presentations at BiOS, part of SPIE Photonics West in February. (Read more on tethered capsule endomicroscopy.)
Increasing the clinical utility of imaging techniques is generally a function of the physical apparatus used to create the images, but it is also dependent upon making the results (images) as unambiguous as possible. The combination of OCT and other imaging modalities can reduce ambiguity.
In one example of dual-modality imaging, researchers at the University of Western Australia recently reported demonstration of a needle probe that combined OCT and fluorescence imaging. The probe uses double-clad fiber that guides the OCT signal and fluorescence excitation light in its core and collects and guides the returning fluorescence in the inner cladding. The fiber is interfaced to a modified swept-source OCT system.
The researchers were able to show that the combined technique provided improved tissue differentiation compared to OCT alone.
Several emerging OCT technologies are poised to further expand the scope of OCT imaging. Micro-OCT, for instance, is currently the highest-resolution OCT available. It provides one-micron resolution in all directions and is capable of cellular-level imaging that enables researchers to see interactions between individual cells.
“The ability to focus down to one or two microns over a large depth provides remarkable images,” says Guillermo Tearney, a faculty member at the Wellman Center. “Cellular level resolution will allow better cancer diagnosis than does conventional OCT, as well as the ability to diagnose a wider range of diseases,” he says.
Such advances will enhance significantly our understanding and management of disease as well as ensuring that sales of OCT systems continue to grow at a healthy rate.
As for Google Glass, researchers working in SPIE Fellow Stephen Boppart’s group at the University of Illinois at Urbana-Champaign (USA) are exploring the potential of pairing Glass with an OCT system to improve patient care.
Developments in light sources, delivery, and detection have all served to advance OCT, but the output images still have to be displayed on a monitor. Graduate student Guillermo Monroy, an SPIE student member, notes that the integration of a head-up display, like Glass, allows real-time display and analysis of image and patient data while enabling the physician to stay focused on the patient.
Such technologies can help the physician efficiently and accurately assess the patient’s condition and ultimately benefit the overall quality of care.
Monroy will provide an update of this work 3 February at a BiOS conference on diagnostic and surgical guidance systems (Paper 8935-39).
US-based healthcare manufacturer St. Jude Medical launched its optical coherence tomography (OCT)-assisted system for imaging coronary arteries in the Japanese and US markets in 2013.
The “ILUMIEN OPTIS PCI Optimization” system is a physician’s tool for assessing patients with coronary artery disease.
The new platform combines fractional flow reserve (FFR) technology, used to measure blood-flow blockage inside coronary arteries, with intravascular OCT imaging.
The OCT imaging part, based on technology originally developed by James Fujimoto and colleagues at MIT, generates a 3D reconstruction of a patient’s blood vessel in real time. Fujimoto is a co-founder of LightLab Imaging, a company acquired by St. Jude Medical in 2010.
Clinical trials have demonstrated that the imaging technique makes it easier for physicians to visualize and examine diseased arteries and aids in the placement of stents that can keep the blood vessels open.
Pioneers in optical coherence tomography (OCT) technology are frequent presenters at SPIE Photonics West and other SPIE conferences.
To view an SPIE Newsroom collection of videos and articles from OCT experts, go to spie.org/techOCT.
Among the experts featured are:
- James Fujimoto, SPIE Board member and Fellow who led the research team at Massachusetts Institute of Technology that invented OCT in 1991. Fujimoto is the longtime chair of the SPIE BiOS symposium and recipient of the 2013 SPIE Britton Chance Biomedical Optics Award.
- Eric Swanson, editor of OCT News. Swanson will give a “Hot Topics” talk at BiOS 1 February on clinical translation in OCT and its role in research, funding, and entrepreneurism.
- SPIE member Benjamin Potsaid, research scientist at Thorlabs and a presenter at the 2013 Hot Topics session.
- SPIE Fellow Stephen Boppart from University of Illinois at Urbana-Champaign and presenter at the 2012 Hot Topics session.
- SPIE member David Huang, professor at the Casey Eye Institute (USA), OCT co-inventor with Swanson and Fujimoto, and 2011 Hot Topics presenter.
Stephen G. Anderson is industry and market strategist for SPIE. He will moderate a panel discussion on optics during SPIE Photonics West 2014 in San Francisco. He is also scheduled to give a keynote talk on the economic impact of photonics in the US and worldwide at a reception for optics and photonics clusters at Photonics West.
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