Shear wave vs. strain

Philips wasn't the only company with an elastography unit on the floor. French-based SuperSonic Imagine, S.A. showed their Aixplorer (the company is in Aix-en-Provence). This ultrasound cart offers something a bit different: shear-wave elastography.


"Everybody on the market today does strain elastography," says Michele Debain, an executive at SuperSonic. "Strain elastography measures the displacement of the tissue. If the transducer is on top of the tissue, I have to push down to measure the lesion underneath. When I displace that with strain elastography, the measurement is of the displacement of that tissue."

Debain says SuperSonic offers a newer, more promising technique, called shear wave elastography. Shear waves are low-frequency waves that push tissue away from the wave source through what's known as acoustic radiation force.

"The shear wave happens naturally in your body," says Debain. "Every time your heart beats, it's a shear wave. When you tap your skin, you create a shear wave." As these waves ripple along tissue, they're picked up again by a transducer on the Aixplorer's wand, and algorithms calculate tissue firmness based on the speed of the wave.

"We compute shear wave velocity," Debain says. If you know the speed of a shear wave, you know the tissue elasticity according to a formula, Young's modulus, that helps determine the elasticity of a substance based on deformation from external stress if the tissue density and wave speed are known. (The computer can add this together, as human tissue density is well known and the wave speed is recorded by the transducer.)

Unlike strain elastography, shear wave doesn't require clinicians to compress the tissue with the ultrasound wand. "In shear-wave elastography, we do not push on the tissue. We generate a shear wave to move through the tissue. Then we capture the shear wave as it moves, and we compute it," Debain says. By not requiring tissue compression, she believes it offers an easier learning curve and more real-world accuracy.

"[One of the] greatest benefits of shear-wave elastography is the fact that it's user-skill independent," says Debain. "So you could have a very seasoned radiographer or radiologist use this system and then you can also have a novice use this system and they would have the same results."

Shear-wave elastography also promises precise quantitative measurements, but as with Philips' software, SuperSonic's quantitative measuring tool is not available in the U.S., as it's still undergoing clinical review.

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