Shear strain elastography

In the breast, prostate and other organs increased tissue stiffness is often associated with the presence of disease. While palpation remains widely used, it has limited sensitivity which is highly dependent upon the skill of the clinician. To overcome this and other limitations a variety of automated approaches have been developed in recent years. These techniques generate tissue deformation by applying acoustic radiation force internally, shear waves superficially, or axial compression. In this paper we propose an alternate approach for elasticity imaging. A shear deformation is applied to the tissue surface. Ultrasound images obtained before and after shear application are processed to yield images of shear strain. We tested this method experimentally on custom acrylamide gel phantoms using a Philips SONOS 5500 imaging system. The phantoms consisted of a 5% acrylamide background with 15% acrylamide inclusions. 50 μm sephadex spheres were used in the background and inclusion to introduce ultrasonic scattering. 4% shear rates were generated by translating the upper surface of the 5 cm thick phantoms by 2.0 mm. The shear strains observed within the inclusions of each phantom were roughly one third those seen in the background region. As anticipated, little axial deformation was observed. Experimentally obtained shear strain images clearly delineate the inclusions with few artifacts, although some noise is visible. This noise undoubtedly arises from jitter in the lateral displacement estimates. Theoretical analysis suggests that signal decorrelation under lateral shear will be very small. This, coupled with the minimal artifacts visible in the shear strain images may make shear elastography an attractive alternative to techniques.