In this study, extensive efforts have been made to quantify soft tissue viscoelasticity and image lesions in vivo. Two independent quantitative methods, CRE and MM, were developed and validated for soft tissue characterization. The frequency- dependent complex Young's modulus of normal and cancerous prostate tissues was systematically investigated by the established mechanical measurement technique. Quantification of mechanical properties of human prostate tissue is highly necessary for the improvement of elasticity imaging for prostate cancer detection. As an important step towards clinical implementation in humans, an animal model was established to investigate the feasibility of sonoelastography for in vivo lesion detection. The results indicated that the area and volume of thermal lesions measured by sonoelastography correlated closely to the pathology findings, demonstrating that sonoelastography could reliably and more accurately image hepatic thermal lesions in vivo in comparison to conventional US.