Vascular ultrasound contrast agents are gas microbubbles with a diameter less than 10 μm (2 to 5 μm on average for most of the newer agents) to pass through the lung capillaries and enter into the systemic circulation. Air bubbles in that size persist in solution for only a short time; too short for systemic vascular use in medical ultrasound imaging. So the gas bubbles have to be stabilized to persist long enough and survive pressure changes in the heart.
Most vascular contrast media are stabilized against dissolution and coalescence by the presence of additional materials at the gas-liquid interface. In some cases, this material is an elastic solid shell that enhances stability by supporting a strain to counter the effect of surface tension. In other cases, the material is a surfactant, or a combination of two or more surfactants.
Typically the effective duration of vascular enhancement is a few minutes, after which the microbubbles dissipate. This rather short duration of vascular enhancement makes it easy to perform repeated dynamic studies. Intravenous vascular contrast agents will be used in imaging malignant tumors in the liver, kidney, ovary, pancreas, prostate, and breast. Tumor neovascularization can be a marker for angiogenesis, and Doppler signals from small tumor vessels may be detectable after contrast injection. Contrast agents are useful for evaluating vessels in a variety of organs, including those involved in renal, hepatic, and pancreatic transplants. If an area of ischemia or a stenosis is detected after contrast administration, the use of other more expensive imaging modalities, including CT and MRI, can often be avoided.

See also Acoustically Active Lipospheres.

Peripheral veins are easily tested using a 5 to 10 MHz transducer. The venous walls are smooth, thin, and compressible. Venous ultrasound imaging requires the compression of the veins in the transverse view. If compression is performed in the longitudinal view, the vein may roll away from the transducer possibly creating a false-negative examination.
The lumen of the normal vein is echo free. Increasing the gain will display low level echoes representing venous blood moving towards the heart. When performing Doppler spectral analysis or color Doppler the gate should be placed in the center of the vessel. In case of a non-obstructing or recanalized thrombosis, the Doppler gate should be placed within the remaining vessel lumen for flow detection.

See also Maximum Venous Outflow and Zero Offset.

The wavelength is a unit of relative distance equal to the length of a wave. This could be a light wave, a radio wave, or even a sound wave. For sound waves the formula is:
l=c/f (wavelength = propagation speed/frequency)
In ultrasound imaging is the wavelength the distance between the onset of peak compression or cycle to the next. The wave propagates as bands of compression and rarefaction. One wavelength is the distance between two bands of compression, or rarefaction. Maximum compression corresponds to maximum pressure. The wavelength (see also Angstrom) is important in image resolution.

See also Spectral Reflector.