A narrow or tuned Bandwidth describes a small frequency spectrum of pulses. With the Fourier transformation method a pulse or amplifier can be subscribed with its bandwidth. It is usually expressed with a 6dB drop of maximum amplitude, subscribing the bandwidth between the edges of the curve.
The selection of bandwidth is essential for achieving certain test results; narrow bandwidth for highly sensitive scans or broad banded for high resolution scans. The fast Fourier transformation uses beside the echo amplitude evaluation method the capability of the echo frequency / bandwidth information.

The near field (also called Fresnel zone) is the proximal part of an ultrasound beam. The Fresnel zone is adjacent to the transducer surface and has a converging sound beam profile. A narrow beam shape is maintained in the near field owing to constructive and destructive interference patterns of sound wavelets emitted from the transducer crystal.
The length of the near field is equal to
r²/l = d²/4l
where r is the radius, l is the ultrasound wavelength in the medium of propagation and d the diameter of the piezoelectric crystal.

See also Beam Pattern, and Sonographic Features.

A negative bolus is created after microbubble destruction at a specific location by the absence of bubbles.

(N) The SI unit of force.
Definition: 1 Newton will accelerate a mass of 1 kilogram at the rate of 1 meter per second per second.
The relationship between force (F), mass (m), and acceleration (a) is expressed by the formula F = ma.
The Newton is named for Isaac Newton (1642-1727), the British mathematician, physicist, and natural philosopher.

An undesirable background interference or disturbance that affects image quality.
The noise is commonly characterized by the standard deviation of signal intensity in the image of a uniform object (phantom) in the absence of artifacts. The measured noise may depend on the particular phantom used due to variable effects. Noisy images appear when the signal to noise ratio is too low. There are various noise sources in any electronic system, including Johnson noise, shot noise, thermal noise.

See also Interference Artifact.