AsianScientist (Apr. 29, 2015) – Researchers led by Professor Park YongKeun from the Korea Advanced Institute of Science and Technology (KAIST) have developed a method to simultaneously visualize and track the movements of microscopic particles in three dimensions (3D). Their study has been published in the journal Optica.
Called optical diffraction tomography (ODT), the method uses a principle similar to X-ray CT imaging commonly used in hospitals for visualizing the internal organs of patients. Like X-ray CT imaging, which takes several images from various illumination angles, ODT measures 3D images of optically-trapped particles by illuminating them with a laser beam in various incidence angles. This technique of locating the optically-trapped particles differs from the traditional method of using optical microscopes.
Optical microscopes measure light signals scattered by the optically-trapped microscopic particles and the positions of the particles in two dimensions. However, it was difficult to quantify the particles’ precise positions from a single image along the direction of the beam, analogous to the difficulty of determining the front and rear positions of objects when closing an eye due to a lack of depth perception.
Furthermore, it became more difficult to measure precisely 3D positions of particles when scattered light signals were distorted by optically-trapped particles having complicated shapes or other particles occlude the target object along the optic axis.
Instead, the KAIST team used optical tweezers to trap a glass bead with a diameter of two micrometers and moved the bead toward a white blood cell. Optical tweezers employ a tightly-focused laser whose beam diameter is smaller than one micrometer (1/100 of hair thickness), which generates attractive force on neighboring microscopic particles moving toward the beam focus. Controlling the positions of the beam focus enabled researchers to hold the particles and move them freely to other locations.
The team measured the 3D dynamics of the white blood cell as it responded to an approaching glass bead via ODT with a high acquisition rate of 60 images per second. They were able to localize the 3D positions of the bead precisely as well as measure the composition of the internal materials of the bead and the white blood cell simultaneously.
“Our technique has the advantage of measuring the 3D positions and internal structures of optically-trapped particles in high speed without labeling exogenous fluorescent agents and can be applied in various fields including physics, optics, nanotechnology and medical science,” said Park.
The researchers also note that their research could be used for further applications, such as cellular surgeries:
“This ODT technique can also apply to cellular-level surgeries where optical tweezers are used to manipulate intracellular organelles and to display in real time and in 3D the images of the reaction of the cell membrane and nucleus during the operation or monitoring the recovery process of the cells from the surgery,” added Dr. Kim Kyoohyun, the lead author of this paper.
The article can be found at: Kim et al. (2015) Simultaneous 3D Visualization And Position Tracking Of Optically Trapped Particles Using Optical Diffraction Tomography.
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Source: KAIST.
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