Novel Technique Uses Lasers to Image Living Tissues

A new imaging technique uses tiny embedded particles to illuminate cellular structures in deep tissue and other dense and opaque materials.

Developed by researchers at the Massachusetts Institute of Technology (MIT, Cambridge, MA, USA), Massachusetts General Hospital (MGH; Boston, USA), and the Jožef Stefan Institute (Ljubljana, Slovenia), the particles are made from lead iodide perovskite, a material that absorbs and traps light efficiently. When a laser beam is projected at the particles, they emit a normal, diffuse fluorescent light. But if the incoming laser's power is tuned to certain threshold, the particles will instantly generate laser light.

The tiny, 6-micron-long particles have a rod-shaped geometry – much like chopsticks – and can allow a specific wavelength of light to bounce back and forth along the particles' length, resulting in images that can be captured at resolutions six times higher than current fluorescence-based microscopes. According to the researchers, the optical technique, called LAser particle Stimulated Emission (LASE) microscopy, could be used to image a specific focal plane, or a particular layer of biological tissue. The study was published on November 4, 2016, in Physical Review Letters.

“Theoretically, scientists can shine a laser beam into a three-dimensional sample of tissue embedded throughout with laser particles, and use a lens to focus the beam at a specific depth,” said lead author MIT graduate student Frederick Sangyeon Cho, MSc. “Only those particles in the beam's focus will absorb enough light or energy to turn on as lasers themselves. All other particles upstream of the path's beam should absorb less energy and only emit fluorescent light.”

“Our idea is, why not use the cell as an internal light source? We can collect all this stimulated emission and can distinguish laser from fluorescent light very easily using spectrometers,” concluded Mr. Cho. “We expect this will be very powerful when applied to biological tissue, where light normally scatters all around, and resolution is devastated. But if we use laser particles, they will be the narrow points that will emit laser light. So we can distinguish from the background and can achieve good resolution.”

Perovskite, named after Russian mineralogist Lev Perovski (1792–1856), is composed of calcium titanate, and was discovered in the Ural Mountains by Gustav Rose in 1839. Perovskites have excellent light-emitting properties, and have been used to make light-emitting diodes and optically pumped lasers. They may also be used to design solution-processed lasers that can be tuned across the entire visible and near infrared (NIR) spectrum by changing the chemical composition of the perovskite film.

Related Links:
Massachusetts Institute of Technology
Massachusetts General Hospital
Jožef Stefan Institute