When surgically removing a tumor, it is important for surgeon and patient to know as soon as possible whether all tumor tissue has been successfully removed. For this purpose, a biopsy is now often taken. Its analysis takes time. Thanks to a newly developed imaging technology, PUV-PAM, by American bioengineers, it may soon be possible to perform this analysis under a microscope as early as during surgery.
Currently, tissue samples taken during surgery via biopsy are frozen, colored (to improve visibility) and cut into thin slices. An optical histology microscope is then used for detailed examination of the tissues. This takes time, and should the examination reveal that tumor cells are still present on the surface of the tissue, the patient must be operated on again.
Analysis in OR
Thanks to the PUV-PAM technique developed by Caltech scientists, the “time-consuming” process can be shortened to such an extent that it becomes possible to analyze tissue samples in the OR, during surgery, meaning additional surgery can be avoided. This is because the new method no longer requires freezing, cutting or the coloring of tissue samples. Even relatively thick samples with irregular surfaces that are normally too thick to image with microscopy can be imaged directly with the new method. PUV-PAM stands for parallel ultraviolet photoacoustic microscopy.
“We hope that this new imaging system can provide more opportunities for intraoperative pathological examination of specimens without slides in oncology surgeries. We believe it has the potential to revolutionize intraoperative histology,” said Rui Cao, lead author of the study recently published in Science Advances. The research was conducted in the lab of Lihong Wang, Caltechs Bren Professor of Medical Engineering and Electrical Engineering. Cao is now an assistant professor of biomedical engineering at Case Western Reserve University.
Innovations in microscopy are being worked on in more places. For example, Cornell researchers in November presented a new advanced imaging technology that enables unprecedented deep and wide visualization of brain activity at single-cell resolution. The innovative microscope, called DEEPscope, combines two- and three-photon microscopy techniques to capture large-scale neuronal activity and structural details previously inaccessible.
Photoacoustic microscopy
The new approach is based on a technique developed by Wang's laboratory called photoacoustic microscopy (PAM). In PAM, a tissue sample is struck with a low-energy laser, causing the tissue to vibrate. The system measures the ultrasonic waves emitted by the vibrating tissue. Because the nuclei of cells absorb more light than the surrounding material, PAM reveals the size and distribution of those nuclei, along with the packing density of cells. Cancer tissue usually has larger nuclei and more densely packed cells.
Several PAM systems have been developed in Lihong Wang's laboratory for imaging bone and breast tissue samples. But to make the systems useful for use in the operating room, the imaging speed of the technique had to be significantly increased. Until now, it has been limited by the speed of the ultraviolet lasers used to excite the tissues.
Laser speed problem solved
To get around this laser speed problem, the researchers divided a single laser beam into eight smaller laser “spots” that all operate in parallel. This makes imaging much faster because the spots can cover the sample area much faster. In addition, PUV-PAM uses a combination of two scanning techniques to achieve high frame rates for tissues without slides. Together, these improvements make the new technique about 40 times faster than state-of-the-art methods previously developed in Wang's laboratory.
“With the current system, we can image a 1 square cm sample with a resolution of 1.3 microns within about five minutes, and the new technique is effective in various tissue types,” Cao said. “We believe that the use of advanced ultraviolet lasers with higher pulse rates and the integration of more parallel channels could significantly improve the speed of imaging with this technique,” adds Professor Wang.
