Elastography is an ultrasound technique that assesses tissue mechanical properties — primarily stiffness — by measuring how tissue deforms in response to applied force or acoustic radiation force impulse (ARFI). Stiffer tissues (such as fibrotic liver, malignant masses) deform less than softer tissues. Two main methods are strain elastography (compression-based) and shear wave elastography (quantitative, measuring shear wave speed in kPa or m/s). Clinical applications include liver fibrosis staging, breast lesion characterization, thyroid nodule evaluation, and lymph node assessment.