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Boston University spots stalled blood flow in brain capillaries

19 Sep 2023

Two-photon Bessel beam approach allows fast volumetric imaging to spot circulation problems.

A new method for imaging blood circulation and the flow within blood vessels could lead to faster and more precise assessments of capillary dynamics.

In particular it should help the understanding of "stalling," brief interruptions in capillary flow thought to be both a potential sign of acute neurological events and connected to longer chronic conditions like Alzheimer's.

OCT imaging has previously been applied to the task due to its ability to monitor capillaries within a small volume, although conventional OCT has proven to lack the temporal resolution to catch short stalling events of significance. The modified technique termed OCT angiography can improve matters, but a more suitable technique is desirable.

A project at Boston University (BU) Neurophotonics Center has now demonstrated an alternative method, combining two-photon imaging with a Bessel beam optics architecture, and reported the findings in Neurophotonics.

Bessel beams are theoretical forms of illumination in which a beam of light does not diffract or spread out as it travels. Real-world approximations of Bessel beams have proven useful in bioimaging, as in the Thorlabs platform designed to study neuron activity using light sheets that are as thin as possible.

For imaging of capillary stalling, a two-photon excitation approach combined with Bessel beam illumination can exploit the way that intensity along a stalled capillary in a two-photon image would remain relatively unchanged. However, the quantities of data involved compared to OCT meant that BU had to develop new computational analysis methods to get the task done.

The BU team implemented an algorithm to calculate the intensity correlation between frames for individual capillaries, with a high correlation implying that the capillary has stalled. By visualizing the calculated correlation instead of the raw intensity image, the researchers were able to identify stalling events more easily and faster.

Intricate workings of the circulatory system

For in vivo trials on blood vessels of mice the technique was used to study the changes involved in stalling, before and after a stroke. An instrument able to monitor a volume 713 x 713 x 120 microns at 0.57 Hz was built and used in the trials, and proved to be 3 to 4 times faster than an OCT angiography method used in previous studies to monitor a similar volume, according to the team.

Although the new approach could not achieve full automation, and a relatively high false positive rate was observed according to the project's published paper, the platform still identified the vast majority of stalling events, including many that blind observation missed, and cut the time required in half.

Enhancements to the deep-learning tools used should help to accelerate the process and perhaps allow full automation, said the BU project.

Use of a Bessel beam two-photon microscopy approach also allowed the diameter of blood vessels to be estimated from the fluorescence intensity. This could help to reveal the relationship between stalling events and arterial dilations, since enlarged vessels are able to transiently reduce stalls.

"The findings of this study demonstrate the power of Bessel beam two-photon microscopy to explore the intricate workings of the brain's circulatory system and its implications for neurological health," commented Ji Yi from Neurophotonics journal.

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