Nanjing University miniaturizes OCT for imaging within the brain

The potential value of OCT imaging during live surgical procedures has made miniaturization of the OCT probe itself a highly desirable research goal.

Past approaches have included the work at University of Colorado Boulder incorporating an electrowetting beam-steering prism into the OCT architecture to carry out the platform's lateral scanning operation, a non-mechanical approach that reduces the power requirements and complexity.

A project at the Medical School of Nanjing University and Nanjing University of Aeronautics and Astronautics has now developed a new miniaturized OCT probe intended specifically for imaging inside blood vessels in the brain, and published its findings in Biomedical Optics Express.

"Doctors currently have limited tools for looking at microscopic details directly inside small and tortuous brain arteries," said research team leader Rui Liu. "Measuring just 0.55 millimeters in diameter and 4 millimeters long, our new probe is designed to fit inside these arteries and acquire full 360-degree images by using a tiny piezoelectric actuator to rotate an optical lens."

Carrying out OCT within brain blood vessels poses some site-specific problems. Most existing intravascular OCT systems are based on designs suitable for coronary arteries, vessels generally larger than the small arteries in the brain and with fewer bends and twists, commented the Nanjing project. Consequently those coronary OCT systems can successfully use a motor outside the device body to rotate the optical probe.

In addition, long imaging catheters passing through the smaller, more curved vessels of the brain can experience friction and twisting, causing the probe to rotate unevenly and producing the imaging flaws known as nonuniform rotational distortion.

Putting a very small motor directly at the tip of the catheter has been investigated as a possible solution. However, conventional electromagnetic micromotors become difficult to miniaturize below about one millimeter, and their electrical wires can block part of the optical field of view, according to the Nanjing team.

A fundamentally different miniature drive mechanism

The project's answer is a probe designed to work as a microscopic rotating camera positioned inside a blood vessel. At the center of the probe is a tiny optical fiber carrying near-infrared light to a miniature angled lens. Instead of rotating the entire catheter from outside the body, only this tiny lens on the tip of the catheter is rotated.

The rotating motion is driven by a piezoelectric material changing shape when an electrical voltage is applied. As the lens rotates, it directs the OCT light around the entire inner wall of the vessel, making it possible to collect a complete 360-degree cross-sectional image. By gradually moving the probe along the vessel, many cross-sectional images can also be combined into a 3D view.

“We wanted to develop a fundamentally different miniature drive mechanism that could be placed at the distal tip of the catheter, remain extremely small, and still provide an unobstructed 360-degree optical scan,” said Liu. “By rotating the lens directly at the catheter tip, we can also avoid the rotational distortion that can occur when a long catheter is rotated from outside the body.”

In trials the new probe produced 2D and 3D OCT images of several increasingly complex samples, including the fine vein network of a magnolia leaf, a vascular stent positioned in a middle cerebral artery model and an ex vivo porcine blood vessel. It then acquired OCT images of excised human atherosclerotic plaque tissue, showing features associated with lipid-rich regions and fibrotic tissue, observations that were consistent with the histological findings.

The team's next steps will include further reductions in the probe's dimensions and increasing the stability and speed of the rotating lens, to confirm the design's performance and reliability under conditions that more closely match actual neurointerventional procedures.

"Although the probe would need more development and testing to be used clinically, intravascular OCT could one day give physicians a much closer look at what is happening inside the small arteries of the brain," said Dawei Wu from the Nanjing University of Aeronautics and Astronautics.