Scatterings image

Collimated polarized light illuminates nerves (black arrows) in human cadaver hand tissue. [Image: Biomed. Opt. Express, doi: 10.1364/BOE.8.004122]

A team of scientists from the Netherlands reports validating a method for using collimated polarized light imaging (CLPi) to identify nerve tissue during surgery (Biomed. Opt. Express, doi: 10.1364/BOE.8.004122). A working CLPi prototype created by cousins Kenneth Chin, Academic Medical Center (AMC), and Patrick Chin, Utrecht University, was further developed and tested at AMC by surgeon Thomas van Gulik’s group. During a demonstration with the new CLPi system, a CLPi operator could identify nerves in a human cadaver hand more accurately than a surgeon using standard identification techniques (100 percent versus 77 percent).

The researchers believe that non-invasive CLPi imaging could someday be used in the operating room to help surgeons identify nerves in real time, thereby reducing the chances of accidental nerve damage and improving patient outcomes.

Hands-off approach

The AMC and Utrecht University paper cites studies showing that accidental nerve injuries account for 25 percent of all injuries caused by non-surgical diagnoses or treatments, and 15 percent of all injuries incurred during orthopedic surgery. These numbers, van Gulik and his colleagues believe, highlight the gains in patient outcomes that could come from improving intraoperative nerve visualization.

Current methods of identifying nerve tissue during surgery rely on the surgeon’s naked eye and dissection skills, as well as ultrasound, fluorescent dyes and optical coherence tomography. These methods have limitations, including potentially harmful physical contact with delicate nerve tissue, application of toxic dyes and extra time for image processing. The researchers say their new, non-invasive CLPi system—which uses only collimated polarized light to illuminate and image nerves in real time—could thus constitute an improvement on the current state-of-the art.

The microtubule difference

Polarized light illuminates a nerve cell differently than other cells because it has a patterned internal structure. Microtubules in a nerve cell are arranged lengthwise in the direction of the axon (the long conductive “arm” of a nerve cell). The orderly orientation of the microtubules creates a strong anisotropic optical reflection under polarized light. This reflection is different from the light scattering that happens in cells in the surrounding tissue. Rotating the polarized light 45 degrees switches this reflection on and off—making the nerves, in a sense, blink at the observer. Van Gulik and his team found that by collimating the light (making all the light waves parallel) they could maximize the amount of light reflected by nerve cells. 

In previous studies, the researchers tested a proof-of-concept on animal models to visualize nerves during surgery. The aim of this latest study was to “adapt and validate” CPLi as a clinical tool by testing it with tissue from a human wrist and hand. Tissues in this area have dense, hard-to-see networks of sensory and motor nerves, and are particularly susceptible to accidental damage during surgery.

Testing CLPi with human tissue

The CLPi system consists of a modified stereo microscope equipped with rotatable polarizers and two light sources with adjustable collimation lenses. Images were recorded with a scientific camera through the microscope’s ocular lens.

For the study, two participants conducted the subcutaneous nerve identification: a surgeon using conventional illumination techniques and an operator using the new CLPi system. The two participants viewed seven ex situ and six in situ tissue samples from a defrosted, fresh frozen human cadaver hand and wrist. The CLPi operator correctly identified nerves in all 13 tissue samples, while the surgeon correctly identified nerves in 10 of the 13 samples. (The results were validated using standard histopathological methods.) The team also conducted a preliminary test of the CLPi system during a live surgical procedure to relieve nerve pain in the wrist. The in vivo imaging results were also promising.

In addition to expanding the CLPi system’s field-of-view, the researchers hope to do additional tests during live surgery to see how nerve-specific anisotropic optical reflection could vary from patient to patient and under different treatment conditions.