In contrast enhanced ultrasound, the focus is the point at which maximum bubble destruction occur.
The transmit focus is the region on the axis of an ultrasound beam where the width of the beam has a minimum value. All waves crossing the focus are in phase in relation to the transducer surface or to the electronic summing point of an electronically focused array.

(FDA) An agency of the US federal government established by Congress in 1912 and presently part of the US Department of Health and Human Services. The FDA gives classifications of medical devices according to potential risks and controls the safety of marketed drugs.

See also Class I II III Devices, Phase 1 2 3 4 Drug Trials and US FDA 510k Document.

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.

The frame rate is the number of images displayed per second. One frame represents one complete sweep of the ultrasound beam and contains many pulse-listen cycles. The frame rates decrease with an increased dwell time.

(F) The number of cycles of a periodic process per unit time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength. The frequency of ultrasound is expressed in units of hertz (Hz), where 1 Hz = 1 cycle per second.
The effect of different frequencies on tissue penetration:
The higher the frequency the less the penetration, the lower the frequency the greater the penetration. As frequency increases, resolution improves but the imaging depth or penetration decreases. The lower the axial resolution, the more detail can be seen.
Usual frequencies for pediatric ultrasound: 5.0mHz to 7.5mHz and 10mHz.
Usual frequencies for adult ultrasound: 2.0mHz to 3.0mHz.

See also Doppler Interrogation Frequency, Multi-frequency Probe, and Huygens Principle.