Optical coherence tomography (OCT), long considered the gold standard for imaging and diagnosing diseases of the eye, could be used to identify and evaluate conditions deep beneath the skin. A team led by Duke University’s Adam Wax has developed a method to increase the depth at which light can penetrate skin. The team adapted dual-axis OCT (DA-OCT) for this purpose and increased the imaging depth of conventional OCT by almost 50%, providing depth penetration in skin imaging at 1.3 µm. Though OCT can easily access the retina through the eye’s cornea and lens, it is limited in its ability to penetrate most other biological tissues because of their highly scattering nature. The more deeply light goes into the tissue, the more likely it is to scatter. The more the light scatters, the more difficult it is to detect. To realize this increase and create a dual axis, the researchers directed the light they shone on an object at a slight angle. They set up the detector at an equal, opposite angle. The sensitivity of the detector benefited from the slight scattering angle introduced by the object. “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,” researcher Evan Jelly said. “And OCT is so sensitive that just a little bit more of that scattered light is all you need.” To mitigate an issue that the researchers observed — that the greater the angle used to identify a deep signal, the smaller the field of view — they used a tunable lens and coordinated focal plane selection with image acquisition to enhance the depth of field. They coherently combined the images from different scans using a computer algorithm. By using dynamic focusing to enhance the depth of field, the researchers avoided the limitations in the system’s scanning geometries and preserved the contrast from deep signals. A newly developed 'dual-axis' approach to OCT allows researchers to look deeper beneath biological tissue. Here, a needle point more than 1 mm beneath a mouse’s skin can be seen toward the bottom of the dual-axis image (left) but not in the standard image (right). The signal of the needle in both has been adjusted for better visualization of the needle. Courtesy of Evan Jelly. The researchers imaged a complex tissue model, various optical scattering phantoms, and mouse skin samples to analyze the penetration capability of their DA-OCT system. The experiments showed that the system could offer improved depth penetration in skin imaging compared to conventional OCT. In the live mice, the DA-OCT system imaged the tip of a needle at 2 mm beneath the skin’s surface. The landmark depth for such imaging is 1.2 mm. “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,” Jelly said. The DA-OCT system for deep tissue scanning encompasses a custom spectrometer design and a micro-electromechanical systems mirror for fast beam scanning. With GPU-assisted processing, the system achieved a rate of about 20 frames per second and performed volumetric imaging in seconds. By reducing scattering at the 1.3-µm wavelength band and exploiting the depth enhancement of a dual-axis geometry, the new DA-OCT system improves signal contrast and increases penetration depth significantly, which could open new possibilities for OCT. In a spin on the eye-imaging technology OCT, beams of light and their reflections come in at an angle to capture details deeper than traditional techniques that focus straight in and straight out. Courtesy of Matthews et al., Optica 1, 105-111 (2014). “Being able to use OCT even 2 or 3 mm 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,” Wax said. The researchers plan to work on providing their system with a more significant advantage over conventional OCT when the system is used in tissues with substantial forward scattering, such as brain and breast tissues. The DA-OCT system could be used for identifying skin cancers and assessing burn damage and healing progress, as well as in surgical procedures. “The technology is still in its infancy, but it is primed to be highly successful for biosensing or guiding surgical procedures,” Jelly said. The research was published in Biomedical Optics Express (www.doi.org/10.1364/BOE.438621).