University of Houston handheld scanner offers improved views of tissues

A project at the University of Houston has developed a compact hand-held fiber-optic platform for mapping the molecular structure and chemical composition of tissues and other samples.

Described in Optica, the device could lead to point-of-care diagnostics and in situ chemical analysis, assisting clinical workflows and surgical assessments.

The platform also represents a further advance in the miniaturization of mid-infrared optical platforms, a long-standing hurdle for biophotonics developers, since conventional IR sources and detectors have proven hard to shrink while maintaining the sensitivity needed at those wavelengths.

Recent approaches to the mid-IR challenge have included the use of colloidal quantum dots as emitters, and upconverting mid-infrared to near-infrared to circumnavigate the problem.

But photothermal imaging, in which a visible laser beam is used to detect heat-induced changes caused by mid-infrared light absorption, remains challenging.

"Mid-infrared spectroscopic imaging (MIRSI) has emerged as a particularly compelling route toward optical biopsy because it is inherently label-free and provides rich biochemical contrast based on vibrational absorption signatures of endogenous molecules," wrote the Houston team in its paper.

"Despite these advantages, photothermal MIRSI has remained confined to bulky benchtop instruments, restricting its use to controlled laboratory settings and limiting its translational potential. Miniaturization, which is now routine in visible and near-infrared optics, remains exceptionally difficult in the mid-infrared."

The Houston solution was to design the first compact fiber-optic platform for handheld photothermal MIRSI, combining specialty mid-infrared fiber delivery, custom beam-combining optics, reflective optical designs and compact custom-fabricated components.

Assessing tissue during cancer surgery

Photothermal MIRSI requires the visible and mid-infrared light to both be focused onto the same point, although optical materials suitable for visible light often absorb mid-infrared, while those that work for mid-infrared light can introduce dispersion and other wavelength-dependent distortions.

The project used optical fibers made from chalcogenide materials to deliver mid-infrared light directly from the laser, eliminating bulky free-space optics; and replaced traditional lenses with mirrors able to reflect both visible and mid-IR, allowing both beams to share the same optical path without lenses.

A final off-axis parabolic mirror was used to focus both beams onto the sample, and raster scanning was also introduced into the optical system.

"Careful optical design and alignment helped to minimize the image distortions that mirrors can cause," commented Rohith Reddy from the University of Houston. "We also designed the handheld scanner to connect to the light source through a fiber-optic cable, allowing it to be maneuvered easily around a sample."

In trials, the new mid-IR device was applied to human cervical and ovarian cancer tissues, human bone marrow biopsy tissue and mouse kidney tissue. Spectra and images exhibited comparable chemical contrast and imaging performance to a benchtop chemical imaging system, while also resolving features as small as 2 microns - five times finer than direct infrared detection allows, according to the project.

The next steps will include broadening the system's mid-infrared spectral bandwidth, currently 1150 to 1400 cm⁻¹, to provide more complete molecular signatures and improve the system’s ability to distinguish different biochemical constituents. The ultimate goal for the Houston team is to apply the platform to the assessment of tissues during cancer surgery.

"After removing a suspected tumor, a surgeon could scan the freshly excised tissue to help determine whether it is malignant or whether cancer cells remain at the surgical margin," said Rohith Reddy. "This complementary information would be available while the patient is still in the operating room, instead of having to wait for results from laboratory testing."