Blood moving toward or away from the transducer causes a Doppler shift. From the Doppler shift, it is possible to calculate the velocity of the moving blood. This principle is analogous to the change in pitch that occurs when a train with its horn blowing sounds different when it approaches you compared to what it sounds like after it passes you. Frequencies used in diagnostic echocardiography can resolve objects 1 mm apart or less. As frequency increases, penetration decreases.
Frequencies used diagnostically are a compromise which provide adequate penetration and resolution.
This is the basis for two-dimensional echocardiography. The M-mode tracing displays distance from the transducer on the vertical axis, and time on the horizontal axis. This is the view that nearly all veterinarians can easily relate to because it displays the beating heart and its various components. Measurements of structures can be done from this view but this is typically done from the M-mode. Two-dimensional echocardiograms are routinely performed from both sides of an animal's thorax.