"Real-time, hand-held elastography is now a commercial reality. With several companies offering it, the clinical applications are expected to accelerate," he told a symposium on ultrasound imaging at the 50th annual meeting of the American Association of Physicists in Medicine (AAPM) in Houston, TX, this week.
So why study tissue elasticity? "Many pathological changes are associated with changes in tissue stiffness," Ophir explained. And with some isoechoic lesions (certain prostate cancers, for example) stiffer than surrounding tissue, "you can see things that you don't see otherwise."
It's not just a case of finding lesions that can't be detected with standard ultrasound, however. 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 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. This could be used as a sign for cancer."
Going forward, it's possible that 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 highlighted 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 tissue, and its evolution over time, as another possible 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 tissue (an elastic matrix containing fluid) and to provide information on fluid flow in tissues affected by diseases such as lymphoedema (swelling caused by an abnormal collection of lymph in the body tissues).