Microbubbles filled with air or inert gases are used as contrast agents in ultrasound imaging. Compression and rarefaction created by an ultrasound wave insonating a gas-filled microbubble along with the mechanical index of the ultrasonic beam lead to volume pulsations of the bubbles, and it is this change that results in the signal enhancement.
Microbubbles have diameters from 1 μm to 10 μm and a thin flexible or rigid shell composed of albumin, lipid, or polymer confining a gas such as nitrogen, or a perfluorocarbon. These microbubbles can cross the pulmonary capillaries and have a serum half-life of a few minutes. Microbubbles in the 1-10 μm range have their resonance at the frequencies used in diagnostic ultrasound (1−15MHz). Smaller bubbles resonate at higher frequencies. Caused by this coincidence, they are such effective reflectors.
The intrinsic compressibility of microbubbles is approximately 17,000 times more than water, and they are very strong scatterers of ultrasound. Under acoustic pressure the vibrating bubble radius may have a conventional linear response or a harmonic non-linear response. Microbubbles usually increase the Doppler signal amplitude by up to 30 dB.

A mode is an operational state that a system has been switched to. A normal mode occurs when all parts of a system oscillate with the same frequency.
For example, a standing wave is a continuous form of normal mode. In a standing wave, all the parts are oscillating in the same frequency and phase but each has a different amplitude.

Periorbital Doppler is a continuous wave Doppler examination, determining the amplitude, flow direction, and compression effect of the frontal or supraorbital arteries in the periorbital region.

See also Acoustic Window, and Cerebrovascular Ultrasonography.

Phase in ultrasound describes where the sound wave is in its cycle of amplitude change. Different waves oscillate at different frequencies, so time is often not a suitable measure of phase.
The phase shift is a difference in the phase or the temporal offset of the peaks of a waveform along one scan line.

See also Coherence, and Histogram.

A point scatterer is a reflector with a diameter much smaller than the ultrasound wavelength. The reflection from blood is a typical example of point scattering. Red blood cells are with 7μm versus 0.44 mm wavelength at 3.5 MHz, smaller than any US wavelength. The individual cells are not only the point scatterers, ultrasound is scattered whenever there is a change in acoustic impedance, and in blood such changes are caused by variable cell concentration. These local fluctuations in cell concentration have a spatial extent that is also much smaller than the ultrasound wavelength, and they therefore act as point scatterers.
A point scatterer gives rise to spherical wavelets spreading out in all directions with the scatterer itself at the center of the sphere. The spherical wavelets from one single point scatterer are much too weak to be detected by the transducer, but constructive interference between numerous wavelets will produce backscattering of higher amplitude echoes with parallel wavefronts, also in the direction of the ultrasound transducer.

See also Rayleigh Scattering.