How can sound make a picture inside the body?

An ultrasound image can reveal a boundary inside the body without light traveling through the skin. Its information comes from echoes. A probe sends out very short sound pulses above the range of human hearing and listens for some of that sound to return.

Many probes use piezoelectric material: an electrical input makes it change shape and launch a sound pulse, while a returning sound wave makes it produce an electrical response. Where the pulse meets tissues with different acoustic properties, some energy is reflected and some continues onward. The gel between probe and skin replaces air gaps that would reflect much of the sound before it entered the body.

The scanner records how long each echo takes to return. In the simple example, sound travels at an assumed 1540 meters per second and returns after 80 microseconds. Multiplying speed by time gives 0.1232 meters of travel. That includes the outward journey and the return, so the reflecting boundary is half as far away: 61.6 millimeters.

Echo strength supplies information about the reflected signal, while delay supplies a location along the beam. Repeating this measurement in many directions lets the scanner place echoes across an image. The picture is assembled from measured travel times and signal strengths rather than captured as an ordinary photograph.

Real tissues do not all have exactly the same sound speed, and a beam has width rather than following one perfect line. Reflections and processing also affect what appears. The single-boundary calculation explains the principle of locating an echo; it does not by itself identify the tissue or interpret a clinical scan.