(CCI) A major limitation of the use of ultrasound contrast agents is the problem that signals from the microbubbles are mixed with those from tissue, so that the distribution of the microbubbles is not optimally displayed either in Doppler or gray scale.
Coherent contrast imaging is a high frame rate implementation of inverting the phase of alternate sound pulses and summing the resulting echoes. The symmetrical signals from linear reflectors are cancelled leaving those from non-linear scatterers, with the advantage that the cancellation is performed without the need to transmit two pulses per image line so that bubble destruction is minimized. Coherent contrast imaging yields best results in the vascular phase of phospholipid microbubbles (such as Definity and SonoVue).

See also Coherence.

(CPS) Contrast pulse sequencing is a technique to exploit contrast agent properties with series of three pulses that differ in phase and amplitude. CPS allows bubble specific imaging with non-linear fundamental and higher order harmonics, low MI, and extremely high microbubble-to-tissue background ratio.

See also Ultrasound Contrast Agent Safety.

Contrast agents improve the sensitivity of vascular Doppler ultrasound, for example in cerebrovascular sonography or examinations of deep abdominal vessels. They also enlarge the role of transcranial Doppler. Microbubbles can be used with various modes e.g., color and power Doppler imaging, as well as pulsed-wave Doppler to increase the signal intensity. However, the ultrasound system must be suitable for contrast enhanced technology.
Microbubbles usually stay within the vascular space; nevertheless, the contrast enhancement is limited to 2−6 minutes caused by physiologic clearance and bubble destruction.
Depended on the application, contrast agents can be administered with a different injection rate e.g., bolus injection, slow injection, or continuous infusion. Stable, homogeneous, and prolonged enhancement can be obtained with perfusion, lasting until the infusion is stopped.

See also Cerebrovascular Ultrasonography, Multiple Frame Trigger.

Contrast microbubbles can be destroyed by intense ultrasound and the scattered signal level can increase abruptly for a short time during microbubble destruction, resulting in an acoustical flash (sudden increase in echogenicity).
Intermittent imaging with high acoustic output utilizes the properties of contrast microbubbles to improve blood-to-tissue image contrast by imaging intermittently at very low frame rates.
The frame rate is usually reduced to about one frame per second, or it is synchronized with cardiac cycles so that enough contrast microbubbles can flow into the imaging site where most microbubbles have been destroyed by the previous acoustic pulse. Because bubbles are destroyed by ultrasound, controlling the delay time between frames produces images whose contrast emphasizes regions with rapid blood flow rate or regions with high or low blood volume.

Non-linear imaging is used to detect primary non-linear components of the received echo. Non-linear methods like harmonic imaging and pulse inversion imaging are designed to detect ultrasound contrast agents.

See also Contrast Pulse Sequencing, and Power Modulation.

(PII) Pulse inversion imaging (also called phase inversion imaging) is a non-linear imaging method specifically made for enhanced detection of microbubble ultrasound contrast agents. In PII, two pulses are sent in rapid succession into the tissue; the second pulse is a mirror image of the first. The resulting echoes are added at reception. Linear scattering of the two pulses will give two echoes which are inverted copies of each other, and these echoes will therefore cancel out when added.
Linear scattering dominates in tissues. Echoes from linear scatterers such as tissue cancel, whereas those from gas microbubbles do not. Non-linear scattering of the two pulses will give two echoes which do not cancel out completely due to different bubble response to positive and negative pressures of equal magnitude. The harmonic components add, and the signal intensity difference between non-linear and linear scatterers is therefore increased. The resulting images show high sensitivity to bubbles at the resolution of a conventional image.
In harmonic imaging, the frequency range of the transmitted pulse and the received signal should not overlap, but this restriction is less in pulse inversion imaging since the transmit frequencies are not filtered out, but rather subtracted. Broader transmit and receive bandwidths are therefore allowed, giving shorter pulses and improved axial resolution, hence the alternative term wideband harmonic imaging. Many ultrasound machines offer some form of pulse inversion imaging.

See also Pulse Inversion Doppler, Narrow Bandwidth, Dead Zone, Ultrasound Phantom.

Release burst imaging is a method based on the combination of multiple high frequency, broadband-detection pulses with a separate release burst. Release burst imaging is optimally suited to the characteristic of ultrasound contrast agents.
This method allows to improve both contrast sensitivity and imaging resolution. Imaging and transient enhanced scattering can be optimized separately. The release burst mode can be combined with a Doppler method to remove residual clutter signals from moving tissue.

See also Repetition Rate.