What are A, B, and M modes?

Different methods of displaying an ultrasound signal:
A mode (amplitude mode): Shows amplitude of the reflected signal versus depth along a single direction; rarely used except in ophthalmology.

B mode (brightness mode): Displays an image obtained by sweeping the sound beam through a plane. The amplitude of the reflected wave determines the brightness of each pixel in the display; the common, familiar display mode.

M mode (motion mode): Shows the amplitude of the reflected signal as dots, which can be observed over time; used primarily in echocardiography.