Imaging method to observe fast movements in 3D

In the past, many discoveries have been made because more better measurement methods have become available, making it possible to obtain data from previously unexplored phenomena. Researchers at the University of Bonn and the center of advanced european studies and research (caesar) have now developed a method that uses multifocal images to reconstruct the movement of fast biological processes in 3D.

Many biological processes happen on a nano- to millimetre scale and within milliseconds. Established methods such as confocal microscopy are suitable for precise 3D recordings, but lack the temporal or spatial resolution to resolve fast 3D processes and require labelled samples. For many investigations in biology, image acquisition at high frame rates is essential to record and understand the principles that govern cellular functions or fast animal behaviours, but the challenge facing scientists can be compared to following a thrilling tennis match: sometimes it is not possible to follow the fast-moving ball with precision, or the ball is not discovered before it is already out of bounds.

With previous methods, researchers were unable to track the shot because the image was blurred or the object of interest was simply no longer in the field of view after the picture was taken. Standard multifocal imaging methods allow high-speed 3D imaging but are limited by the compromise between high resolution and large field of view, and they often require bright fluorescent labels. The Bonn scientists have now enabled multifocal imaging with both a large field of view and a high spatio-temporal resolution, which they used to track the movement of non-labelled spherical and filamentous structures quickly and accurately.

As described in the journal Nature Communications, the new method has provided novel insight into the dynamics of flagellar beating and its connection to the swimming behaviour of sperm. This connection was made possible because the researchers were able to precisely record the flagellar beat of free-swimming sperm in 3D over a long period of time and simultaneously follow sperm trajectories of individual sperm. In addition, the scientists determined the 3D fluid flow around the beating sperm. Such findings not only open the door to understand causes of infertility, but could also be used in bionics — the transfer of principles found in nature to technical applications.

In addition to observing sperm, the method could also be used to determine the 3D flow maps that result from the beating of motile cilia. Motile cilia beat in a similar way to the sperm tail and transport fluid. Cilia-driven flow plays an important role in the ventricle of the brain or in the airways where it serves to transport mucus out of the lungs and into the throat — this is also how pathogens are transported out and warded off.

The multifocal imaging concept reported in this study is said to be cost-effective and easily implemented, and does not rely on object labelling. The researchers assert that their new method can find its way into other fields as well, and they see many other potential applications.

Image caption: The new method provides novel insight into the dynamics of flagellar beating and its connection to the swimming behaviour of sperm. Image ©René Pascal

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