Interference artifacts occur if decreasing of the echo amplitude is not exponential with penetration depth caused by inhomogeneous tissue layers and fluid or air-filled regions. If ultrasound waves have opposite phases, i.e. if the phase difference is 180°, their amplitudes will always be in opposite directions and their sum is a weaker wave. This is destructive interference and artifacts occur.

Huygens principle states that an expanding sphere of waves behaves as if each point on the wave front were a new source of radiation of the same frequency and phase. The principle explains how a flat ultrasound transducer can transmit a narrow ultrasound beam, which in the near field is confined to the dimensions of the transducer surface.
Spherical wavelets are emitted from numerous point sources on the transducer surface. They interfere to form a narrow, slightly converging beam of ultrasound in the near field. The wavefronts in the beam are nearly parallel. A precondition for this interference is that the transducer surface is much larger than the ultrasound wavelength.

See also Interference Artifact.

Interference is the interplay of two or more waveforms. When two or more waves with equal frequency and wavelength interfere, a new wave is created whose amplitude at any point in time and space is the sum of the amplitudes of the original waves at the same point in time and space. Constructive interference occurs when two waves of equal frequency are in phase. The amplitudes will always be in the same direction, and the waves will combine to produce a stronger one. Two equally strong waves with the same amplitude that are 180° out of phase will cancel each other out.

See also Interference Artifact.

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.