ULTRASONIC ELASTOGRAPHY

Abstract

Elastography is a new ultrasound-based imaging technique that provides images (called elastograms) of internal strain in soft tissues under a static compression. The strain is related to the stiffness of the tissues, which is in turn related to the pathological state of tissues. For example, it has been known for long that breast and prostate cancer are stiffer than normal tissues, and palpation is a standard medical practice. Elastography is able to provide new quantitative information about the stiffness of the tissues that would not be obtainable with existing imaging modalities. Elastograms are created by digitizing ultrasonic signals before and after applying a small static compression (usually about 1%) to the tissues being scanned. For small compressions, the displacement and the compression of the tissues induce a displacement and a temporal compression of the ultrasonic signal. These displacements are tracked by locating the position of the maximum of the cross-correlation function between the pre- and post-compression signals. Strain is obtained by taking the gradient of the displacements, and a strain image is formed. Applications include breast, prostate, kidney and intravascular elastography, as well as thermal therapy follow-up. Although elastography is still only used in research today, it is expected that this imaging modality may become available for standard practice within a few years.