(CHI) Contrast harmonic imaging is an ultrasound technique to improve the measurement of blood perfusion or capillary blood flow. Based on the nonlinear properties of contrast agents, CHI transmits at the fundamental frequency but receives at the second harmonic. Contrast enhanced echo signals contain significant energy components at higher harmonics (bubbles acts as harmonic oscillators), while tissue echoes do not. Caused by that contrast signal can be separated from tissue echoes by the characteristic signal.
In combination with the pulse inversion technique, CHI promises very high contrast agent sensitivity with high spatial resolution.

See also Ultrasound Contrast Agent Safety and Hemoglobin.

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.

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Harmonic is an oscillation of a system at a frequency that is a simple multiple of its fundamental frequency. The fundamental frequency of a sinusoidal oscillation is called the first harmonic. The second harmonic has a frequency doubled that of the fundamental.

See also Fundamental Imaging, Harmonic Imaging, Subharmonic Imaging and Superharmonic Imaging.

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