Frame averaging is a form of lowpass filtering. With frame averaging temporal variation are smoothed by addition of consecutive frames in a real-time mode.

A filter in electronics is a circuit that only passes certain signals. In ultrasound is a filter a device to suppress acoustic or electromagnetic waves of certain frequencies, letting other frequencies pass, e.g. high pass filter, low pass filter. The high pass filter is the wall filter (also called thump filter) used in Doppler devices to eliminate low frequency Doppler shifts caused by clutter. For blood flow measurement, a low pass filter is often used to strip out high frequency noise, leaving only the biological components of interest. Frame averaging is a form of a low pass filter.

Real-time mode has been developed to present motion like a movie of the body's inner workings, showing this information at a high rate. The special real-time transducer uses a larger sound beam than for A, B or M-modes. A linear array transducer with multiple crystal elements displays real-time compound B-mode images with up to 100 images per second.
At each scan line, one sound pulse is transmitted and all echoes from the surface to the deepest range are received. Then the ultrasound beam moves on to the next scan line position where pulse transmission and echo recording are repeated.

See also Compound B-Mode, Pulse Inversion Doppler, and Frame Averaging.