The new approach, demonstrated by a team at North Carolina State University, works by using special algorithms to carefully analyse the echoes of multiple ultrasound waves and thereby build up a picture of the fluid levels in the lungs.
"Historically, it has been difficult to use ultrasound to collect quantitative information on the lung, because ultrasound waves don't travel through air- and the lung is full of air," explained Marie Muller, an assistant professor of mechanical engineering at North Carolina State University and co-author of a paper on the work. "However, we've been able to use the reflective nature of air pockets in the lung to calculate the amount of fluid in the lung."
When ultrasound waves travel through the body, most of each wave's energy passes through the tissue. But some of that energy is reflected as an echo. By monitoring these echoes, an ultrasound scanner is able to create an image of the tissue that the waves passed through. All of this happens in microseconds.
But when ultrasound waves hit air, all of the energy is reflected - which is why ultrasound images of the lung tend to look like a big, grey blob, with little useful information for health-care providers.
However, no two ultrasound waves take the same path - they may bounce in different directions as they travel through the lung meaning that their echoes take different amounts of time to return to the scanner.
By looking at all of the echoes, and how those echoes change over time, Muller’s team was able to calculate the extent to which the space between the air pockets was filled with fluid.
To test the approach, the researchers conducted two sets of experiments using rats and rat lung tissue.
In the first set of experiments, researchers used rat lung tissue that had been injected with saline solution to mimic fluid-filled lung tissue. The new approach allowed researchers to quantify the amount of fluid in the lung to within one millilitre.
In the second set of experiments, researchers found significant differences between fluid-filled and healthy lungs in rats. Specifically, the researchers were calculating the mean distance between two "scattering events" - or how far an ultrasound wave travelled between two air pockets. For fluid-filled lungs, the mean distance was 1,040μm, whereas the mean distance in healthy lungs was only 332μm.
"This is important, because one could potentially track this mean distance value as a way of determining how well pulmonary edema treatment is working," Muller said.
The technique makes use of conventional ultrasound scanning equipment, though the algorithm used by the researchers would need to be incorporated into the ultrasound software.
The researchers are now preparing to carry out human and animal trials of the technology.
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