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Ultrasonography forms medical images using ultrasound. In pulse-echo imaging, a piezoelectric transducer emits a short acoustic pulse and receives echoes from changes in acoustic impedance. If an echo returns after time $\Delta t$ from a reflector at distance $d$ in a uniform medium with sound speed $c$, the round trip gives $d=c\Delta t/2$. Scanning the beam assembles spatial information from many such measurements.^[ref]

A water-tank experiment with two reflectors at known separations tests the depth calculation without biological interpretation. An incorrect assumed sound speed changes the inferred depths; overlapping pulses limit the separation of nearby reflectors. In tissue, acoustic attenuation, scattering and the geometry of the beam further affect the image. These mechanisms explain why the displayed anatomy depends on both physical propagation and the acquisition model.

Pulse-echo imaging sends a short acoustic pulse and records echoes returning from interfaces or scatterers. If sound speed is approximately constant and the path is straight, an echo delay $t$ corresponds to range $r=ct/2$, because sound travels to the scatterer and back.

Scanning beam direction and measuring echo strength produces an image. Pulse duration and bandwidth affect axial resolution, while beam width affects transverse resolution. Incorrect sound-speed assumptions distort inferred positions, and multiple reflections can create echoes that do not correspond to a single direct round trip.