Two-photon fluorescence provides insights into blood flow

The development – from the University of Hong Kong in collaboration with the University of California, Berkeley - is expected to provide neuroscientists with a tool to better understand the inner workings of the brain, particularly how blood flow changes at the level of individual blood vessels and across the larger vessel network within the brain. This can help to explain how energy is distributed and regulated in healthy and diseased brains.

The team’s findings have been published in Proceedings of the National Academy of Sciences.

The team’s microscope is said to have super high-speed two-photon fluorescence imaging that penetrates light more deeply into tissue than other gold-standard microscopy technologies to capture blood cells in fast motion. Most other technologies work with one-photon fluorescence and have a slow focus-scanning mechanism, which limits them to capturing low-flow speed regimens in anaesthetised animals.

The two-photon imaging technology developed by the team captures faster moving flows – in this case, red blood cells –when the animals are awake and able to move somewhat. The flow of red blood cells is an important cue of brain activity, which is fuelled by energy from blood supply.

The interdisciplinary research team, which includes engineers and neurobiologists, was led by Professor Ji Na of UC Berkeley’s Department of Molecular & Cell Biology and Professor Kevin Tsia of the University of Hong Kong’s Department of Electrical and Electronic Engineering.

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“One of the key limitations in other brain imaging technologies has been the resolution of images,” Professor Tsia said in a statement. “Our technology is able to deal with these with very fast-moving images, at least 100 times faster than current state-of-the-art options, and capture them down to the individual blood cells. You can count the cells and trace their trajectories,”
In low-flow speed technology, the blood flow resembles a blur and smear. Images from the two-photon fluorescence technology are much more detailed and show individual cells in fast motion.

The research team initially reported on their high-speed two-photon fluorescence technology in 2020 when they used it to record the millisecond electrical signals of the neurons in an awake mouse. The same technology has also been applied by Professor Tsia’s team in cancer screening through imaging cancer cells in blood.

“The work on imaging individual blood cells in-vivo is an extension of that work and shows that the technology can be extended to other kinds of neuroscience research, especially cerebral haemodynamics, which is the dynamics of blood flow to the brain,” said Professor Tsia.