We present a review of elastography from a systems standard medical practice of soft-tissue palpation is based on point of view. We show that elastography can be viewed as a cascade qualitative assessment of the low-frequency stiffness of tissue.
of two distinct processes. The first process involves the mapping of Pathologic changes are generally correlated with changes in the distribution of local elastic moduli in the target into a distribution tissue stiffness as well. Many cancers, such as scirrhous carciof local longitudinal strains. This process is governed by the theory noma of the breast, appear as extremely hard nodules [2]. In of elasticity as applied to a particular experimental setup under some many cases, despite the difference in stiffness, the small size of specific boundary conditions and some assumptions. Since this proa pathologic lesion and/or its location deep in the body preclude cess involves errors due to the simplified mechanical model used, its detection and evaluation by palpation. In general, the lesion artifacts such as target hardening, stress concentrations, and limited may or may not possess echogenic properties which would make contrast-transfer efficiency are usually encountered. These errors may be recursively minimized by solving the inverse problem, thus increas-it ultrasonically detectable. For example, tumors of the prostate ing the contrast-transfer efficiency such that a more accurate modulus or the breast could be invisible or barely visible in standard image may be obtained. The second process involves the production ultrasound examinations, yet be much harder than the embedding of the strain image (elastogram) from ultrasonically estimated values tissue. Diffuse diseases such as cirrhosis of the liver are known of local strains. Here, the limitations of the ultrasound system [such to increase the stiffness of the liver tissue significantly as a whole as time-bandwidth product, center frequency, and sonographic sig-
[2], yet they may appear normal in conventional ultrasound examnal-to-noise ratio (SNR)] as well as the signal-processing algorithms ination. Since the echogenicity and the stiffness of tissue are used to process the signals cause additional corruption of the data generally uncorrelated, it is expected that imaging tissue stiffness through the introduction of constraints in the attainable elastographic will provide new information that is related to tissue structure SNR, resolution, sensitivity, and strain dynamic range. This process is described in terms of a stochastic strain filter. These two system and/or pathology.
components are discussed in detail, and it is concluded that both Biologic tissues can be considered as approximating homogemust be optimized in a specific order to result in quality elastograms.
neous gels [3]. Different modes of propagation of elastic waves