When undergoing an ultrasound, the transducer probe of the ultrasound machine transmits sound waves. It also receives the sound waves that are reflected back after it reaches a boundary.
The reflected waves are received by the probe and relayed to the ultrasound machine. The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue and the time of the each echo's return. It then displays the distances and intensities of the echoes on the display screen, forming a two dimensional image.