Clutters are unwanted components of the received signal. In contrast enhanced ultrasound, clutter often describes the part of the received echo that is not from microbubble contrast agents.

See also Release Burst Imaging.

In contrast enhanced ultrasound, the focus is the point at which maximum bubble destruction occur.
The transmit focus is the region on the axis of an ultrasound beam where the width of the beam has a minimum value. All waves crossing the focus are in phase in relation to the transducer surface or to the electronic summing point of an electronically focused array.

(MI) The mechanical index is an estimate of the maximum amplitude of the pressure pulse in tissue. It is an indicator of the likelihood of mechanical bioeffects (streaming and cavitation). The mechanical index of the ultrasound beam is the amount of negative acoustic pressure within a ultrasonic field and is used to modulate the output signature of US contrast agents and to incite different microbubble responses.
The mechanical index is defined as the peak rarefactional pressure (negative pressure) divided by the square root of the ultrasound frequency.
The FDA ultrasound regulations allow a mechanical index of up to 1.9 to be used for all applications except ophthalmic (maximum 0.23). The used range varies from 0.05 to 1.9.
At low acoustic power, the acoustic response is considered as linear. At a low MI (less than 0.2), the microbubbles undergo oscillation with compression and rarefaction that are equal in amplitude and no special contrast enhanced signal is created. Microbubbles act as strong scattering objects due to the difference in impedance between air and liquid, and the acoustic response is optimized at the resonant frequency of a microbubble.
At higher acoustic power (MI between 0.2-0.5), nonlinear oscillation occurs preferentially with the bubbles undergoing rarefaction that is greater than compression. Ultrasound waves are created at harmonics of the delivered frequency. The harmonic response frequencies are different from that of the incident wave (fundamental frequency) with subharmonics (half of the fundamental frequency), harmonics (including the second harmonic response at twice the fundamental frequency), and ultra-harmonics obtained at 1.5 or 2.5 times the fundamental frequency. These contrast enhanced ultrasound signals are microbubble-specific.
At high acoustic power (MI greater than 0.5), microbubble destruction begins with emission of high intensity transient signals very rich in nonlinear components. Intermittent imaging becomes needed to allow the capillaries to be refilled with fresh microbubbles. Microbubble destruction occurs to some degree at all mechanical indices. A mechanical index from 0.8 to 1.9 creates high microbubble destruction. The output signal is unique to the contrast agent.

(MCE) Myocardial contrast echocardiography is a contrast enhanced ultrasound method that utilizes intravenous injected microbubbles as red blood cell tracers. MCE can assess myocardial perfusion both at rest and stress to evaluate viable myocardium after acute infarction.
MCE perfusion imaging improves the blood echoes during the microbubble passage and the imaging system suppresses the clutter represented by non-contrast-bearing tissue.

See also Injection Rate, Stress Echocardiogram, and Myomap.

Radio frequency (RF) thermal ablation is a technique that uses the thermal effect created by radio frequencies to destroy tumors or metastases in the liver.
This treatment for liver cancer can accurately be evaluated by contrast enhanced ultrasound. RF thermal ablation monitored by sonography can lead to immediate re-treatment, preventing a second anesthesia and shortening the hospitalization time.