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.

The main lobe is the main acoustic transducer beam. There are other, smaller lobes called side lobes that are located around the main lobe.

See also Side Lobe, Grating-lobe Artifact, Beam Steering, Beamforming.

Standard scanners allow visualizing microbubbles on conventional gray scale imaging in large vascular spaces. In the periphery, more sensitive techniques such as Doppler or non-linear gray scale modes must be used because of the dilution of the microbubbles in the blood pool. Harmonic power Doppler (HPD) is one of the most sensitive techniques for detecting ultrasound contrast agents.
Commonly microbubbles are encapsulated or otherwise stabilized to prolong their lifetime after injection. These bubbles can be altered by exposure to ultrasound pulses. Depending on the contrast agent and the insonating pulse, the changes include deformation or breakage of the encapsulating or stabilizing material, generation of free gas bubbles, reshaping or resizing of gas volumes.
High acoustic pressure amplitudes and long pulses increase the changes. However, safety considerations limit the pressure amplitude and long pulses decrease spatial resolution. In addition, lowering the pulse frequency increases destruction of contrast bubbles. However, at low insonation power levels, contrast agent particles resist insonation without detectable changes. Newer agents are more reflective and will usually allow gray scale imaging to be used with the advantages of better spatial resolution, fewer artifacts and faster frame rates.

Feasible imaging methods with advantages in specific acoustic microbubble properties:
Resonating microbubbles emit harmonic signals at double their resonance frequency. If a scanner is modified to select only these harmonic signals, this non-linear mode produces a clear image or trace. The effect depends on the fact that it is easier to expand a bubble than to compress it so that it responds asymmetrically to a symmetrical ultrasound wave. A special array design allows to perform third or fourth harmonic imaging. This probe type is called a dual frequency phased array transducer.

See also Bubble Specific Imaging.

The multiple frame trigger adjusts the acquisition of a series of consecutive frames, usually with ECG triggering. Multiple frame triggers are used in harmonic power Doppler modes to identify motion artifacts in contrast enhanced Doppler imaging.