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Dual-axis OCT imaging probes deeper under the skin

16 Dec 2021

Duke University device tackles applications where scattering limits depth penetration.

OCT has achieved widespread use in several clinical applications thanks to its imaging resolution and easy fit with clinical workflows, and a recent report concluded that the technique is set to support a $1.5 billion market by 2023.

One hurdle has always been the technique's limited penetration depth, and approaches to improving it have included the development of novel VCSEL sources and the use of tunable lenses.

A project from the Duke University lab of biomedical optics specialist Adam Wax has now demonstrated a further route to enhanced imaging depth, combining a tunable lens with dual-axis optics.

As described in Biomedical Optics Express, the new dual-axis optical coherence tomography (DA-OCT) platform could allow improved imaging in cases where a high degree of scattering limits the depth penetration of existing OCT devices.

Dual-axis OCT architectures have been available for some time, designed to counter attenuated signals from ballistic photons by collecting multiply forward-scattered photons as well, and image deeper subsurface morphology. However, compromises in the depth of field can hinder this approach.

The Duke University platform combines the inherently reduced scattering at the 1.3-micron wavelength band with the depth enhancement of a dual-axis geometry, to improve signal contrast with increasing penetration depth significantly over previous iterations, according to the project's paper.

"By tilting the light source and detector, you increase your chances of collecting more of the light that’s scattering off at odd angles from a tissue's depths," said Evan Jelly from Duke University. "And OCT is so sensitive that just a little bit more of that scattered light is all you need."

Crossing the beams

The platform incorporates a MEMS mirror for fast beam scanning, a dual-window speckle reduction approach previously developed in Wax's lab for OCT imaging, and a suitable GPU processor to assist the image processing. The result was a frame rate of around 20 frames per second, and volumetric imaging performed in seconds.

Trials on both tissue phantoms and mouse skin confirmed that DA-OCT at 1.3 microns provided improved penetration capability compared to conventional on-axis OCT. Imaging depths of around 2 millimeters were demonstrated in skin tissue, with sensitivity to highly reflective objects at even greater depths according to the Duke paper.

"It’s actually a fairly simple technique that sounds like something out of Ghostbusters — you get more power when you cross the beams," said Wax. "Being able to use OCT even 2 or 3 millimeters into the skin is extremely useful, because there are a lot of biological processes happening at that depth that can be indicative of diseases like skin cancer."

The next steps for the project will include ways to increase the advantages of this architecture when used on tissues with more substantial forward scattering, such as brain and breast tissues.

"The dual-axis OCT gave us images and information from the layers of skin where blood and molecular exchanges are occurring, which is extremely valuable for detecting signs of diseases,” said Jelly. “The technology is still in its infancy, but it is primed to be highly successful for biosensing or guiding surgical procedures."

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