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.

Power modulation is a non-linear method, based on a multi-pulse technique where the acoustic amplitude (and hence power) of the transmitted pulses is changed. Full and half amplitudes pulses are used to induce changes in the response of the contrast agent. The received echoes from the emitted half amplitude pulse are adjusted with the full amplitude pulse and this pairs of pulses are subtracted. Power modulation is used to separate contrast agent echoes at low mechanical index, allowing real-time perfusion imaging.
Power modulation can be used with a low frequency wide band transducer to increase the depth and transmit the sound beam homogenous allowing ultraharmonic imaging.

Ringing means the rise and decay time before and after the transducer reaches the maximum amplitude. Expressed as the mechanical Q of the transducer, which is the number of cycles it takes to get up to 90% of maximum amplitude or down to 10% above zero amplitude.

A-mode (Amplitude-mode) ultrasound is a technique used to assess organ dimensions and determine the depth of an organ. While A-mode technology was previously employed in midline echoencephalography for rapid screening of intracranial mass lesions and ophthalmologic scanning, it is now considered obsolete in medical imaging. Nonetheless, the A-mode scan has found applications in early pregnancy assessment (specifically the detection of fetal heartbeats), cephalometry, and placental localization.
When the ultrasound beam encounters an anatomic boundary, the received sound impulse is processed to appear as a vertical reflection of a point. On the display, it looks like spikes of different heights (the amplitude). The intensity of the returning impulse determined the height of the vertical reflection and the time it took for the impulse to make the round trip would determine the space between verticals. The distance between these spikes can be measured accurately by dividing the speed of sound in tissue (1540 m/sec) by half the sound travel time.
During an echoencephalography scan, the first A-mode scan is acquired from the right side of the head and captured on film. Subsequently, the probe is positioned at the corresponding point on the left side, and a second exposure is captured on the same film, displaying inverted spikes. The A-mode ultrasound could be used to identify structures normally located in the midline of the brain such as the third ventricle and falx cerebri. The midline structures would be aligned in normal patients but show displacement in patients with mass lesion such as a subdural, epidural, or intracranial hemorrhage.

See also 2D Ultrasound, 3D Ultrasound, 4D Ultrasound, Ultrasound Biomicroscopy, A-scan, B-mode and the Infosheet about ultrasound modes.

In a color map, colors are allocated to Doppler shift frequencies corresponding to flow. Color (colour, Brit.) maps may also display Doppler amplitude, signal power, variance, or the gray levels of the B-mode image.

See also Color Amplitude Imaging, Color Priority, and Color Saturation.