Microbubbles filled with air or inert gases are used as contrast agents in ultrasound imaging. Compression and rarefaction created by an ultrasound wave insonating a gas-filled microbubble along with the mechanical index of the ultrasonic beam lead to volume pulsations of the bubbles, and it is this change that results in the signal enhancement.
Microbubbles have diameters from 1 μm to 10 μm and a thin flexible or rigid shell composed of albumin, lipid, or polymer confining a gas such as nitrogen, or a perfluorocarbon. These microbubbles can cross the pulmonary capillaries and have a serum half-life of a few minutes. Microbubbles in the 1-10 μm range have their resonance at the frequencies used in diagnostic ultrasound (1−15MHz). Smaller bubbles resonate at higher frequencies. Caused by this coincidence, they are such effective reflectors.
The intrinsic compressibility of microbubbles is approximately 17,000 times more than water, and they are very strong scatterers of ultrasound. Under acoustic pressure the vibrating bubble radius may have a conventional linear response or a harmonic non-linear response. Microbubbles usually increase the Doppler signal amplitude by up to 30 dB.

Molecular Biosystems, Inc. (MBI), founded in 1980, is a biomedical company developing a range of contrast agents for use with diagnostic ultrasound, magnetic resonance imaging (MRI), and computed tomography (CT).
Alliance Pharmaceutical Corp. acquires Molecular Biosystems in Nov. 2000.

A projector (transmitter) converts the energy from the power amplifier (generator) into an acoustic pressure output. Projectors are usually driven near their resonance frequencies where they provide the highest acoustic output.

Quadrature detection is used in Doppler ultrasound as well as in magnetic resonance imaging and is also called quadrature demodulation or phase quadrature technique. Quadrature detection is the acquisition of Mx and My simultaneously as a function of time by using two separate detector channels. This signal processing method is used for directional Doppler in which the signal reference frequency for the two channels has a phase shift of 1/4 period. The output Doppler signal phase for both channels also depends on the Doppler shift whether positive or negative.
The fast Fourier transform analyzer performs spectral Doppler analysis in ultrasound machines and displays different quadrature Doppler frequencies, when a sample volume cursor is used along time.

The range of diagnostics and imaging systems of Siemens Medical Systems covers ultrasound, nuclear medicine, angiography, magnetic resonance , computer tomography and patient monitoring.
In September 2000 Siemens Medical Engineering Group, bought Acuson Corporation (Mountain View, California) for approximately $700 million.

Ultrasound Systems:

Ultrasound Systems (older):