A bolus is a rapid infusion of high dose contrast agent. Dynamic and accumulation phase imaging can be performed after bolus injection. Since the transit time of the bolus is only a short time, images with high frame rate show the wash in and wash out of the contrast material. The injection rate and the total injected volume modifies the bolus peak profile. Substantial changes in the concentrations during signal acquisition induce artifacts. Furthermore, the hemodynamic parameters (cardiac output, blood pressure) influence the bolus profile. However, the characteristics of ultrasound contrast agents are favorable with a continuous perfusion.

See also Negative Bolus.

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.

Ultrasound contrast agents (USCAs) improve the sensitivity of various ultrasound applications. They usually stay within the vascular space and can be injected several times. Nevertheless the contrast enhancement is limited caused by physiologic clearance and bubble destruction.

Different injection techniques to improve the imaging:
Continuous infusion yield a steady-state concentration of the USCAs, greater examination time with optimal enhancement, avoid bloom and possibly other artifacts. Continuous infusion also allows the sonographer to optimize the effective dose individually during the examination.

See also Power Modulation.

A liver sonography is a diagnostic tool to image the liver and adjoining upper abdominal organs such as the gallbladder, spleen, and pancreas. Deeper structures such as liver and pancreas are imaged at a lower frequency 1-6 MHz with lower axial and lateral resolution but greater penetration. The diagnostic capabilities in this area can be limited by gas in the bowel scattering the sound waves.
The application of microbubbles may be useful for detection of liver lesions and for lesion characterization. Some microbubbles have a liver-specific post vascular phase where they appear to be taken up by the reticuloendothelial system (RES). Dynamic contrast enhanced scans in a similar way as with CT or MRI can be used to studying the arterial, venous and tissue phase.
After a bolus injection, early vascular enhancement is seen at around 30sec in arterialized lesions (e.g., hepatocellular carcinomas (HCC), focal nodular hyperplasia (FNH)). Later enhancement is typical of hemangiomas with gradually filling towards the center. In the late phase at around 90sec, HCCs appear as defects against the liver background. Most metastases are relatively hypovascular and so do not show much enhancement and are seen as signal voids in the different phases.
Either with an intermittent imaging technique or by continuous scanning in a nondestructive, low power mode, characteristic time patterns can be used to differentiate lesions.

See also Medical Imaging, B-Mode, High Intensity Focused Ultrasound, Ultrasound Safety and Contrast Medium.