Researchers have been perfecting the art of cellular imaging for centuries. Many methods exploit differences in optical contrast caused by variations in cellular structure or molecular composition. But there are other ways to interrogate a cell.
Sound energy, for instance, can reveal details of mechanical stiffness and density – think about how the ripples would differ if a stone were dropped in a pond of water as opposed to a pond filled with maple syrup.
“To understand a cell, a good place to start is its structure and mechanics,” explained Oliver Wright, Professor of Engineering at Hokkaido University, Japan, who led one of the teams.  Using the method, called picosecond ultrasonics, “You will learn about what holds the cell together.”
Here’s how it works. Fixed cells sitting atop a metal surface—a thin titanium film on a sapphire slide—are interrogated from below by a titanium-sapphire laser firing 0.2-picosecond pulses more than 75 million times per second to a tiny, micron-sized spot. These light pulses cause the titanium to expand and generate gigahertz-frequency acoustic waves that propagate through the cell. Those waves, which form a “pancake-shaped packet,” are then tracked as they pass through the cell using a second, focused laser beam, building up a picture of a cell’s elastic stiffness and density distribution.
“So as this pancake of sound is coming up through the cell, we check point-by-point, time window-by-time window, what is happening to that pancake,” Wright said.
Wright’s team used their setup to image both adipose and endothelial cells in three dimensions. The team collected 800 sections over 7 hours, each 1.5-nm apart, for a total of ~1 m of cellular depth. The resulting images demonstrate that their technique can distinguish cells from the surrounding media and different cell types from each other. Fat cells are more like water, he said, whereas endothelial cells are “more like treacle,” transmitting the acoustic pulses less effectively. But the team was unable to distinguish subcellular features, including the nucleus.
“This method is not at all optimized but shows promise,” said Wright. He noted, for instance, that in the future he could enhance lateral resolution several-fold by using shorter-wavelength lasers.
In a second study, Bertrand Audoin at the University of Bordeaux in France described a similar setup, which they called an “inverted pulsed opto-acoustic microscope.”  Audoin’s rig also allowed the team to collect both optical and acoustic data, although not in three dimensions. They did, however, distinguish the nucleus and lamellipodia of human mesenchymal stem cells.
According to Wright, the method, if optimized, could provide researchers a way to probe cellular infrastructure in three dimensions. “Using our method, you won’t learn anything about chemical reactions, but you will learn about what holds the cell together.”
 Danworaphong, S, et al., “Three-dimensional imaging of biological cells with picosecond ultrasonics,” Appl Phys Lett, 106:163701, 2015.
 Dehoux, T, et al., “All-optical broadband ultrasonography of single cells,” Sci Rep, 5:8650, 2015.