From the earliest days of medicine, the palpable “feel” of tissues and organs has been used in diagnostic evaluations. Even today, screening exams include palpation of the liver, breast, prostate, thyroids, skin lesions, and other tissues to characterize their condition. However, manual palpation is limited to accessible surfaces and is not quantitative. By the 1980s, there existed a wide gap between the impressive imaging abilities within radiology and the important but limited information that was obtained from palpation. How could these two disparate domains be bridged to add hidden biomechanical properties into modern radiological imaging systems? Some early work took advantage of the availability of Doppler ultrasound devices to study the tissue motion and abnormalities in the 1970s and 1980s. Out of this background came a remarkable series of imaging approaches to map out the elastic properties of tissues. Initially, images were produced by vibration sonoelastography, where vibrational shear waves (typically between 50 and 1000 Hz continuous wave) are excited within tissue and the resulting vibrations are imaged and displayed using the Doppler-displacement or phase estimators. Other innovative approaches applied step compressions, transient forces, and a variety of additional imaging modalities, such as magnetic

resonance elastography (MRE) and optical imaging, to uncover the biomechanical properties of tissue that were previously unobservable with conventional radiology.