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Seeing things you wouldn’t otherwise

The idea of measuring tissue stiffness using ultrasound is nothing new - articles on this technique appeared in the literature back in the early 1980s. But according to Jonathan Ophir, from the University of Texas Medical School (Houston, TX), the area has progressed at pace of late.

"Real-time, hand-held elastography is now a commercial reality, with several companies offering it," Ophir told AAPM delegates at a symposium on ultrasound imaging. "The clinical applications are expected to accelerate."

So why study tissue elasticity? "Many pathological changes are associated with changes in tissue stiffness," Ophir explained. And with some isoechoic lesions (HIFU-ablated tissue, for example, or some prostate cancers) stiffer than surrounding tissue, "you can see things that you don’t see otherwise".

But it’s not just a case of finding lesions that can’t be detected with standard ultrasound. Ophir presented in-vivo sonograms and elastograms, both showing the same breast-cancer lesion. While the images looked similar, the lesion appeared obviously larger in the elastogram. He explained that this discrepancy appears to be a trend, with three or four other research groups also noting the same effect.

"Nobody has proved this yet, but we think it’s due to scar formation around cancers that’s not typical for benign disease," Ophir suggested. "This could be used as a sign for cancer."

Looking forward, elastography could provide several additional ways to glean more information relating to the mechanics of tissues. One possibility is offered by imaging shear strains at tissue boundaries.

Ophir showed an example comparing the shear strains of breast carcinoma and fibroadenoma. In the benign case, the areas showing the greatest shear strain were tight up against the lesion/tissue boundary, as viewed in a standard ultrasound image.

In the carcinoma, however, these areas appeared much further from the boundary, possibly due to the aforementioned ‘scarring’ effect. The high-shear-strain areas were also much larger in this case, assigned to the fact that cancers tend to be more strongly fixed between the surrounding tissue layers. "Shear strain may be able to tell us if a lesion is benign or malignant and where the lesion boundaries are," Ophir explained.

He also cited imaging of the Poisson’s ratio of a tissue, and its evolution in time, as another future development. Poisson’s ratio can be calculated by measuring both the lateral and axial strains, and can be used to study the behaviour of poroelastic (an elastic matrix containing fluid) tissue and provide information on fluid flow in tissues affected by diseases such as lymphoedema.

 

 

Posted by Tami Freeman on July 28, 2008 11:12 PM | Permalink