A megahertz-speed optical coherence tomography (MHz-OCT) instrument, integrated with a neurosurgical microscope, has demonstrated the ability to acquire high-quality, volumetric, cross-sectional brain scans in seconds. The images obtained by the system are immediately available for post-processing. The microscope-integrated MHz-OCT system could be used by medical personnel in operating rooms to precisely identify tumor margins during brain surgery, so that damage to healthy tissue is minimized. It could also be used to improve surgical results in procedures that require comprehensive information about subsurface brain anatomy. The development of the MHz-OCT system is the result of efforts by a Universität zu Lübeck team to speed up OCT technology. The MHz-OCT system produces over one million depth scans per second and provides real-time signal processing and visualization capabilities. It incorporates swept laser source technology to improve imaging performance parameters like coherence length and relative intensity noise, and enable the use of imaging techniques like speckle variance, online spectral zooming, and long-range OCT. “The MHz-OCT system we developed is very fast, about 20 times faster than most other OCT systems,” researcher Wolfgang Draxinger said. “This allows it to create 3D images that reach below the brain’s surface. These could be processed, for example with AI, to find and show parts that are not healthy and further need treatment, yet would remain hidden with other imaging methods.” Researchers have developed an MHz-OCT system and integrated it into a commercially available neurosurgical microscope. (Upper left) The typical surgical microscope view of a tumor below the brain’s surface illuminated with white light. The only visible hint of the tumor is a peculiar growth of new blood vessels. (Lower left) With a fluorescent agent and viewed under blue-violet light, parts of the tumor become slightly visible. (Upper right) The new microscope-integrated MHz-OCT system can look below the surface to reveal the structure in depth, which shows the slice indicated by the blue line. (Lower right) When projected into a top view, the tumor becomes clearly visible. Courtesy of Universität zu Lübeck, Medizinisches Laserzentrum Lübeck GmbH & Universitätsklinikum Schleswig-Holstein. The researchers recognized that, although the system might be very useful to surgical staff, it would need to be integrated into the microscopes already in use as surgical tools to prevent any disruptions in the surgical workflow. The team integrated the MHz-OCT system into a commercially available surgical microscope to make it practical for surgical use. Although the optical properties of the microscope system were not ideally suited to support the wavelength of the OCT imaging system, early identification of the primary obstacles enabled the researchers to work out ways to overcome them. Using adaptive optics techniques, they designed a scanner optics and beam delivery system to work within the constraints of the mechanical and optical parameters of the microscope. The researchers tested the integrated system with calibration targets and tissue analog phantoms. Once they were satisfied with the results, they performed patient safety testing and began a clinical study to investigate the use of the technology in brain tumor resection neurosurgery in 30 patients. The microscope-integrated OCT system was used to capture high-density, volumetric OCT images at specified regions of interest. The image quality of the OCT C-scan volumes acquired in-vivo exceeded the team’s expectations and showed that MHz-OCT imaging could be used in a clinical setting. “We found that our system integrates well with the regular workflow in the operating room, with no major technological issues,” Draxinger said. “The quality of the images acquired surpassed our expectations, which were set low due to the system being a retrofit.” During the clinical study, the researchers acquired about 10 total backscatter (TB) of OCT imaging data along with matched pathological histology information. The team is currently developing AI methods to classify the tissue, using the OCT image data acquired in the study. Should these classification systems demonstrate the ability to reliably discern malign from healthy tissue, a new contrast channel, enhancing surgeons’ view of the target area, could be possible. The researchers are also preparing a study in which the microscope-integrated MHz-OCT system will be used to demonstrate the exact location of brain activity during neurosurgery in response to, for example, an external stimulus. The results could lead to more precise implantation of neuroprosthetic electrodes, which would allow more accurate control of prosthetic devices. “We see our microscope-integrated MHz-OCT system being used not just in brain tumor surgeries, but as a tool in every neurosurgery setting, since it can acquire high contrast pictures of anatomy such as blood vessels through the thick membrane that surrounds the brain,” Draxinger said. “This could significantly improve outcomes for procedures requiring detailed information about anatomical structures beneath the brain’s surface, such as deep brain stimulation for Parkinson’s disease.” The research was published in Biomedical Optics Express (www.doi.org/10.1364/BOE.530976).