Friday, January 20, 2017

On the Front Line of Movie Making

With a new camera system, researchers at Washington University in St. Louis can capture 100 billion frames per second in a single shot. This record-breaking design won’t improve the quality of your YouTube uploads (even very high speed video cameras only record a few thousand frames per second)—but it could improve your health.

An artist’s impression of the new camera system capturing the light cone produced
when a particle moves faster than light.
Image Credit: Jinyang Liang and Lihong V. Wang.

Biomedical imaging is a cornerstone of modern healthcare. From x-ray to MRI to ultrasound and PET, doctors regularly use imaging techniques to identify problems and guide treatments. As scientists work on developing next generation imaging systems that can provide doctors with even more detailed information, one of their aims is to better capture how light is scattered as it travels through materials.

The way light is scattered in a material depends on the properties of the material. Some of these properties can change as a result of diseases, injuries, or other processes. Therefore, measuring the way light scatters as it passes through biological tissue can reveal information about the structure and health of the tissue. This knowledge can also help develop higher-resolution imaging techniques.

There are models that simulate this scattering well, but it’s much harder to see experimentally. The biggest problem is that light travels fast. Really fast. Mind-bogglingly fast. In the words of Douglas Adams, “Nothing travels faster than the speed of light with the possible exception of bad news, which obeys its own special laws.” It’s so fast that CCD technology (which underlies modern digital cameras) can’t store and readout the information fast enough to capture the 100 billion frames per second necessary to break down the process.

There are some ultrafast imaging methods that can reach this frame rate, but they don’t work very well in real time. Many require capturing an event with multiple shots that are taken at different times and stitched together. This limits the use of these methods to recording events that happen exactly the same way every time, which isn’t often the case when light scatters within biological tissue.

In research out today in the open-access journal Science Advances, published by the nonprofit science society AAAS, the scientists describe their camera and demonstrate its potential. The camera is a collection of mirrors, lenses, and other optical components that fit on a tabletop, connected to a computer program that analyzes the data and constructs the image. The researchers call it a lossless-encoding compressed ultrafast photography system, and it builds on a previous design by the same team. The project was carried out in a lab directed by Lihong Wang at Washington University in St. Louis, with the individual project led by Jinyang Liang.

In a cool demonstration of its potential, the team captured the first single-shot image of a light scattering event called a photonic (for photon) Mach cone, which is kind of like the pressure cone that causes a sonic boom. When an airplane travels faster than the speed of sound, it leaves a cone-shaped pressure wave in its wake. When this pressure wave reaches the ground, a person standing there will hear a loud “boom” – a sonic boom. Similarly, when a small particle in a medium (like water) travels faster than light can travel in that medium, it leaves a light cone in its wake.

Shock waves festoon a small-scale model of the X-15 traveling to the left
in Langley Research Center’s Supersonic Pressure Tunnel. March 1962.
Image Credit: NASA.

In this case, the team induced such a cone using light in place of the particle. They sent a super short laser pulse into a tunnel filled with air and dry ice fog. The tunnel was surrounded by a different material through which light travels more slowly. When the light in the tunnel scattered off of fog particles, it created ripples of light that, because the light in the tunnel traveled faster than the light scattered into the surrounding material, formed a photonic Mach cone.

Image of a laser beam traveling to the right and traveling faster in
the source tunnel than scattered light does in the surrounding material.
Credit: Liang et al. Sci. Adv.2017;3:e1601814.

How do you capture this kind of an event with one click of the shutter? The camera system is designed to take in three views, one that records a direct image like you would get with a traditional camera, and two that record different sets of time-related data. The specialized software then integrates the information from these three views to recreate the event. The results match theoretical predictions well, demonstrating that the technique has great promise for imaging scattering events and taking medical imaging to the next level.

As illustrated by the capture of the photonic Mach cone, this technique is fast enough that with the right setup, you could actually watch neurons fire! I’ll leave it at that.

Kendra Redmond

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