New Optical Imaging Technique Probes Deeper into Tissue
The technique, called TRAFIX, uses patterns of light focused in time to gather information from greater depths without the need for prior knowledge about tissue properties.
Optical imaging for medical applications is often limited by the fact that light can reach only a certain tissue depth before becoming rapidly diffused from scattering off cell structures like nuclei and mitochondria.
Some optical imaging techniques try to overcome this challenge by measuring the propagation of light in the sample with an embedded “guide star” -- for instance, a single fluorescent bead with known size and depth. The imaging system's response to the guide star can then help reconstruct images of unknown objects within similar samples. However, inserting a bead into living tissue can be laborious and impractical for clinical applications.
A new optical imaging method proposed by an interdisciplinary research team from the University of St. Andrews in Scotland has the ability to image deeper inside tissue without the need for a guide star. The technique, called TRAFIX, uses patterns of light focused in time to gather all the necessary information from the sample and doesn't require prior knowledge about the properties of the medium. The study was published by Science Advances in October.
“We believe this is an exciting new way for neuroscientists to image at depth, as well as researchers in pathology looking for early signs of disease,” said senior author Kishan Dholakia, a professor of physics at the University of St. Andrews. “Quite simply, we have shown that we can do depth imaging where other standard approaches will have difficulty to succeed.”
TRAFIX -- also known as temporal focusing microscopy with single-pixel detection -- starts with a femtosecond laser beam that is expanded onto a spatial light modulator (SLM), a device used to spatially modulate the amplitude and phase of an optical wavefront in two dimensions. The SLM forms the beam into unique checkerboard-type patterns. These patterns of light are diffracted through a grating, which splits up the beam into multiple beams of different wavelengths.
After going through a microscope objective, the beams recombine at the focal point and reach the sample. To create an image, the authors collect a fraction of the backscattered light from the sample on a single-pixel detector. The resulting signal depends on how well each checkerboard pattern overlaps the object hidden within the tissue.
“By weighting the recorded signal from each pattern and summing up appropriately, we can thus recover the whole wide-field image through the scattering medium without ever knowing anything about the scattering properties of the medium,” said Dholakia.
Dholakia and his colleagues tested their TRAFIX system by imaging through tissue phantoms, rat brain and human colon tissue. For instance, they were able to successfully discern fluorescent structures through a maximum of 400 microns of rat brain tissue. Conventional temporal focusing microscopy, another optical imaging technique, could not discern the structures, and the images appeared as noise.
Dan Oron, principal investigator of the Nanophotonics Group at the Weizmann Institute of Science in Israel, who was not involved in the study, remarked that this new optical method has exciting potential to go deeper into scattering media. He feels that the work is a good proof-of-principle demonstration of a new idea, although he wonders whether performance could be further improved by a different choice of patterns.
“This new optical method overcomes in a clever manner a significant difficulty in imaging deep inside scattering media,” said Oron. “This is a technique which could potentially be used for fast volumetric imaging of biological samples, either by fluorescence or by nonlinear scattering.”