Acoustic impedance (Z) is the product of a tissue’s density and the speed of sound through it (Z = ρ × c). When an ultrasound beam crosses a boundary between two tissues with different acoustic impedances, some energy is reflected back toward the transducer and some is transmitted forward. The greater the impedance mismatch, the stronger the reflection. This principle is what makes ultrasound imaging possible — organ boundaries and tissue interfaces reflect sound back to form the image.