New Bioimaging Technique Diffuses Light Inside Living Tissue

By Chuck Seegert, Ph.D.

Researchers at Washington University in St. Louis have found a way to focus light deep inside tissues, a technology that may enable real-time optogenetic studies. Using a time-reversed ultrasonic encoding method, the non-invasive technique is the first to diffuse light inside a scattering medium that contains living tissue.

Using optical methods to non-invasively diagnose disease could play a key role in biomedical imaging. Historically, the use of light to image deep inside the living body has been challenging, since light scatters when it encounters tissues. Borrowing from astronomy, advances were made using a guidestar, or a reference point, to help refocus the scattered light. Initially, guidestars were invasively implanted, which was a drawback. Over time, however, virtual guidestars were developed, which enabled non-invasive techniques.

Though virtual guidestars were a significant step forward, non-invasive optical imaging of deep tissues was still notoriously slow — essentially no faster than 1 Hz (one image per second), according to a recent press release. The living body is a dynamic system with many moving parts, which changes the light scattering, or speckle pattern. At these slow imaging rates, real-time imaging wasn’t feasible because the motion in the body changed the speckle patterns.

To overcome these complications, Washington University of St. Louis (WUSTL) researchers recently developed a way to produce images in about 5.6 milliseconds.

To accomplish this, the WUSTL team used time-reversed ultrasonic encoded (TRUE) optical focusing, according to a recent study published by the team in Nature Communications. To increase speed, the team used a photorefractive crystal capable of fast response at the 790 nanometer wavelength. This wavelength was chosen because of its ability to penetrate and focus deep inside living tissues. After recording the light that is emitted from the tissue, the technology recalibrates based on the guidestar, which then provides the focus needed to create images of the deep tissues.

In the past, time reversal methods were used by the WUSTL team to detect portions of tissue that experienced high degrees of motion, according to a recent article published on Med Device Online. For example, the motion of blood flow is greater in tumors and other lesions compared to surrounding tissues, which could enable more accurate identification of diseases.