Third-Harmonic Generation Microscopy Provides In Situ Brain Tumor Imaging

A technique involving third-harmonic generation microscopy could allow neurosurgeons to image and assess brain tumor boundaries during surgery, providing optical biopsies in near-real time and increasing the accuracy of tissue removal.

Pathologists typically use staining methods, in which chemicals like hematoxylin and eosin turn different tissue components blue and red, revealing its structure and whether there are any tumor cells. A definitive diagnosis can take up to 24 hours, meaning surgeons may not realize some cancerous tissue has escaped from their attention until after surgery -- requiring a second operation and more risk.

Tissue from a patient diagnosed with low-grade glioma. The green image is taken with the new method, while the pink uses conventional hematoxylin and eosin staining. From the upper left to the lower right, both images show increasing cell density due to more tumor tissue. The insets reveal the high density of tumor cells. Courtesy of N.V. Kuzmin et al./VU University Amsterdam.

Brain tumors — specifically glial brain tumors — are often spread out and mixed in with the healthy tissue, presenting a particular challenge. Surgery, irradiation and chemotherapy often cause substantial collateral damage to the surrounding brain tissue.

Now researchers from VU University Amsterdam, led by professor Marloes Groot, have demonstrated a label-free optical method for imaging cancerous brain tissue. They were able to produce most images in under a minute; smaller ones took <1 s, while larger images of a few square millimeters took 5 min.

The study involved firing short, 200-fs, 1200-nm laser pulses into the tissue. When three photons converged at the same time and place, the photons interacted with the nonlinear optical properties of the tissue. Through the phenomena of third harmonic generation, the interactions produced a single 400- or 600-nm photon (in the case of third or second harmonic generation, respectively).

The shorter-wavelength photon scatters in the tissue, and when it reaches a detector — in this case a high-sensitivity GaAsP photomultiplier tube — it reveals what the tissue looks like inside. The resulting images enabled clear recognition of cellularity, nuclear pleomorphism and rarefaction of neuropil in the tissue.

While this technique has been used in other applications — to image insects and fish embryos, for example — the researchers said this is the first time it’s been used to analyze glial brain tumors.

Groot and her team are now developing a handheld device for tumor border detection during surgery. The incoming laser pulses can only reach a depth of about 100 μm into the tissue currently; to reach further, Groot envisions attaching a needle that can pierce the tissue and deliver photons deeper.

The research was published in Biomedical Optics Express, a publication of The Optical Society (OSA) (doi: 10.1364/boe.7.001889).