Although the first investigations on it began in the 1980s, elastography appears to have finally come into its own as a viable method, at least that's the impression given at the Radiological Society of North America's annual meeting last week.

One of the complaints against conventional ultrasound is that, while non-invasive, radiation-free and reasonably sensitive, it is often not very specific, and can require a highly trained expert to get the most out of it.

Enter elastography, an ultrasound technique related to one of the oldest modalities: a hands-on clinical examination. Since ancient Egypt, supposedly, physicians have palpated body parts to determine tissue stiffness. What they didn't know, but what we suspect now, is that malignant lesions are stiffer, or less elastic, than benign ones. Elastography allows doctors to use ultrasound waves to determine the stiffness of a lesion compared with surrounding tissue, which could be useful in telling apart benign fibrous growths from deadly malignancies in the breast or other areas.

Best for breast?

At RSNA, Dr. Stamatia Destounis, a radiologist from Rochester, NY, presented evidence from an ongoing study that elastography was about 98 percent sensitive for malignant lesions in the breast and 78 percent specific -- far higher than conventional ultrasound. (See DM 10903).

Dr. Destounis calls elastography an "old-new" technique, because, though research began more than 20 years ago, it's only now becoming truly commercially available. She says it appears to offer promise in breast cancer detection. And, it seems, vendors agree.

On the floor, Philips was touting its iU22 Ultrasound system, the latest version of their flagship ultrasound cart, which now features elastography -- in Europe at least, as it's pending FDA clearance, which is expected sometime early next year.

Philips had on hand a plastic model breast for testing, and this reporter, a non-medically-trained correspondent, was able to move the wand around to detect a suspicious "lump," whose hardness rating came up on screen. In Europe, the iU22 can ship with statistical software that actually assigns a true numerical rating to the tissue's toughness. But because the software isn't FDA-cleared, and won't be, presumably, until clinical trials are wrapped up, U.S. doctors may have to settle for a scale that color-codes lesions along a qualitative spectrum from "hard" to "soft." Philips says the underlying algorithms are just about the same. Note that software clearance may come well after the machine itself is cleared for U.S. sale.

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.

According to Debain, in preliminary clinical studies involving 254 patients, doctors' palpations appeared to line up with elastography scorings. "Shear-wave elastography is directly related to clinical palpation," she says.

While it's too early to give results, Debain says the study, ultimately expected to involve 2,300 patients at 17 sites around the world, should be written up by next fall. For now, she can say that from what they've seen, the ROC curve, a tool which helps judge the signal-to-noise ratio of a test, gives their system between .88 and .93 (out of one) for accuracy, depending on how many shear wave elements they add.

"If we stick with the ROC curve for accuracy, we will go up every time we add a shear wave feature," Debain says, but the jury is out until the results are seen next year.

Elastograms for livers, too

At the show, Philips also announced its own works-in-progress shear-wave ultrasound device, which currently is being used primarily for liver research.

Because it's not practical to perform biopsies to screen people at risk for cirrhosis or liver diseases, due to risks associated with liver biopsies, doctors are on the lookout for imaging tools that will help them identify people with mild fibrosis, or tissue-hardening, early, before symptoms show up. It might be possible to treat them before lasting damage sets in. At least, that's the hope, and a shear-wave elastography device could be useful in detecting that faint fibrotic tissue in early onset liver disease.

Not much is known about the results with the Philips device so far, but in a statement released at the show, they said that early studies with an experimental elastography device were able to accurately measure elasticity of tissue. "Based on the results of these studies," reads the statement, "the experimental system has been modified in preparation for further clinical research into shear-wave elastography.

Whatever the results of this or SuperSonic's clinical studies, elastography's future appears to be stretching comfortably forward.