FUNDAMENTAL MECHANICAL LIMITATIONS ON THE VISUALIZATION OF ELASTICITYCONTRAST IN ELASTOGRAPHY

Elastography is a new ultrasonic imaging technique that produces image s (elastograms) of the elastic properties of compliant tissue. To dete rmine the Young's modulus it is necessary to measure or estimate any f ive of seven relevant variables. In elastography, the measured quantit y is the normal strain component in the direction of the applied load, and the three normal components of stress may be estimated using the modified Love's analytical models while assuming a value close to 0.5 (incompressible) for Poisson's ratio. The distribution of Young's modu li can thus be computed and displayed in the form of two-dimensional i mages called elastograms. The analytical models used for the estimatio n of the three normal components of stress assume that the target is s emi-infinite and homogeneous in composition. The objective of this art icle is to determine some of the errors associated with the assumption of homogeneity of the target. Experiments using computer simulations were performed to study the efficiency with which elastograms display the contrast in the Young's modulus of a lesion or target, with respec t to its background under certain conditions. It was observed (using t he definition of contrast-transfer efficiency of elastography as the r atio of the elasticity contrast as measured from an elastogram, to the true contrast) that elastograms were consistently efficient in quanti tatively depicting the elasticity contrast of hard lesions; however, t hey showed suboptimal contrast-transfer efficiency in cases of soft le sions in a hard background. In general, elastograms are efficient in d isplaying the elasticity contrast of hard or soft lesions which have a low contrast level with respect to the surroundings, irrespective of their size and location.