VIS-OCT needs stable, versatile light sources

BY YUANBO WANG, ROMAN KURANOV, HAO F. ZHANG, AND KIEREN PATEL, OPTICENT HEALTH

Optical coherence tomography (OCT) is the gold standard for noninvasive retinal imaging, enabling ophthalmologists to diagnose and treat multiple ocular diseases. Compared to traditional OCT, which uses near-infrared (NIR) wavelengths of light, visible-light OCT (VIS-OCT) offers superior spatial resolution and novel functional imaging capabilities. VIS-OCT provides the most accurate means for measuring retinal oxygen saturation, a key biomarker for understanding changes in tissue metabolism associated with the pathology of various retinal diseases.

Traditional OCT has dominated ophthalmic diagnostics for the past 30 years. Thanks to investment in and development of key OCT components, there is an opportunity to accelerate adoption of VIS-OCT in clinical practice. With a global market size of ~$1 billion expected in 2022, the OCT market is primed for innovation. As described by David Huang, one of the original inventors of traditional OCT, “VIS-OCT is one of the most exciting new developments in OCT research, with the potential to improve imaging diagnostics in the eye and potentially beyond.”

In an OCT system, the light source is critical. Traditional OCT systems take advantage of compact and inexpensive light sources based on semiconductor amplifiers. The parameters of the light source for OCT are a broadband spectrum (from tens of nanometers to 100 nm), high spectral brightness that is achieved only in laser-like sources featuring strong coherent light amplification, and a high degree of spatial coherence. But there are no commercially available semiconductor amplifiers in the visible-light range. Single-mode fiber coupling is required to preserve axial (in-depth) resolution that is destroyed by intermodal dispersion in multimode fibers.

We recently investigated the performance of two commercially available lasers in the same VIS-OCT system. Both lasers had a pulse repetition rate of 150 MHz and were able to provide ~14-µm lateral resolution and ~1.3-µm axial resolution in biological tissue. The probing spectra were acquired using a Blizzard SR spectrometer (from Opticent Health) running at 25,000 A-lines/s. Both lasers performed similarly — as benchmarked by signal-to-noise (SNR), contrast-to-noise (CNR), and quality index (QI) values — and both required a warmup period of 20 min to reach stable power output. Incident light on the sample was controlled at 1.6 mW for both lasers. While the lasers’ general performance is suitable for VIS-OCT research and development, both light sources are poorly suited for clinical instrument design.

Current VIS-OCT instruments rely on high-power bulk-optical setups, which comprise discrete optical elements and use nonlinear processes to generate ultrawide spectrums known as a supercontinuum generation. These offerings are nonoptimal for clinical systems because only a fraction of the emission band is useful for VIS-OCT, laser power is generally more unstable as compared to superluminescent diodes used in traditional OCT, and bulky laser sizes prevent construction of smaller-footprint benchtop systems. A small, stable, high-performance light source that could be integrated into VIS-OCT systems would accelerate development and subsequent clinical adoption.

Future VIS-OCT light sources may employ a variety of designs such as an inexpensive LED to pump fluorescence in the visible range with Ce:YAG crystals. We have demonstrated proof of concept of this strategy, though further work is needed to improve spectral brightness and low spatial coherence. Another design may use fiber optic broadband amplifiers at popular NIR wavelengths such as 1032 or 1064 nm and frequency doubling with inexpensive but effective periodically poled lithium niobate (PPLN). Additionally, discovery of new semiconductor material that can amplify in the visible region, analogous to NIR light sources, could produce a viable alternative.

We invite scientists and engineers in the photonics industry to design new light sources for VIS-OCT and broaden access to the next wave of innovation in this important clinical field.

Meet the authors

Yuanbo Wang is a senior product engineer at Opticent Health. He received his doctorate from the University of Missouri-Columbia in 2017 and has over eight years of experience in optical imaging systems and biomedical image processing. Wang is leading the software design of Opticent products for clinical and preclinical markets; email: ybwang@opticenthealth.com.

Roman Kuranov is jointly appointed with Opticent Health and Northwestern University. He has over 15 years of experience in developing new OCT technologies, including swept-source OCT and VIS-OCT. Kuranov leads Opticent’s overall VIS-OCT product design for both humans and small animals; email: rkuranov@opticenthealth.com.

Hao F. Zhang is a professor in the Department of Biomedical Engineering and (by courtesy) the Department of Ophthalmology at Northwestern University. His lab develops OCT and superresolution imaging technology for ophthalmology and molecular biology. Zhang is a co-founder of Opticent Health; email: hfzhang@northwestern.edu.

Kieren Patel is Opticent Health CEO. He received his MBA from Kellogg School of Management at Northwestern University, and a J.D. from Northwestern School of Law. He also holds a doctorate in molecular and cell biology from the University of California, Berkeley. Patel is an expert in life science patent law, life sciences technology entrepreneurship, and business strategy in the fields of genomics, diagnostics, medical devices, and therapeutics; email: kpatel@opticenthealth.com.

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