Sound and ultrasound waves consist of a mechanical disturbance of a medium such as air. The disturbance passes through the medium at a fixed speed causing vibration. The rate at which the particles vibrate is the frequency, measured in cycles per second or Hertz (Hz).
The pressure of sound is reported on a logarithmic scale called sound-pressure level, expressed in decibel (dB) referenced to the weakest audible 1 000 Hz sound pressure of 2*10^-5 Pascal (20 mP). Sound level meters contain filters that simulate the ear's frequency response. The most commonly used filter provides what is called 'A' weighting, with the letter 'A' appended to the dB units, i.e. dBA.
Sound becomes inaudible to the human ear above about 20 kHz and is then known as ultrasound. Diagnostic imaging uses much higher frequencies, in the order of MHz.
See also Spatial Peak Intensity.

Sound frequencies:

The international system for units.
Le Systeme international d'Unites officially came into being in October 1960 and has been adopted by nearly all countries, though the amount of actual usage varies considerably.
It is based upon 7 principal units:
Length - meter (m)
Mass - kilogram(kg)
Time - second (s)
Electric current - ampere (A)
Temperature - Kelvin (K)
Amount of substance - mole (mol)
Luminous intensity - candela (cd)
From these basic units many other units are derived and named.

See also Ultrasound Physics.

(TRUS) Transrectal sonography (also called transrectal ultrasonography, transrectal echography (TRE), endorectal ultrasound (ERUS or EUS)) is an ultrasound procedure used to examine the prostate gland, the rectum or bladder.
A small, lubricated transducer placed into the rectum releases sound waves, which create echoes as they enter the region of interest. A computer creates a picture called a sonogram.
TRUS is commonly used for guidance during a prostate needle biopsy and may be used to deliver brachytherapy and monitor cancer treatment. Transrectal ultrasonography detects enlargement, tumors and other abnormalities of the prostate, rectal polyps, rectal cancer, perianal infection, and sphincter muscle injuries. TRUS is also performed on male patients with infertility to view the prostate and surrounding structures and on patients with suspected bladder conditions or disease to view the bladder.

See also Transurethral Sonography, Endoscopic Ultrasound, Pelvic Ultrasound, Rectal Probe, Biplane Probe, Endocavitary Echography and High Intensity Focused Ultrasound.

Transurethral echography or sonography is used to detect small tumors of the urinary bladder or to visualize the urethra and surrounding muscles with special transducers. The bladder neck can be visualized using a transrectal probe.
In addition, high intensity focused ultrasound provides treatment of benign prostatic hyperplasia and adenocarcinoma of the prostate. Small catheter-based sectored tubular or planar transducers with highly directional energy deposition and rotational control are used for precise treatment. Regions of the prostate can be selective coagulatet while monitoring and controlling the treatment with MRI.

See also Urologic Ultrasound, Lithotripsy, Reflux Sonography, Ultrasound Therapy, Interventional Ultrasound and Thermotherapy.

(UCA / USCA) Ultrasonography is the most commonly performed diagnostic imaging procedure. The introduction of sonographic contrast media into routine practice modifies the use of ultrasound in a variety of clinical applications. USCAs consist of microbubbles filled with air or gases and can be classified according to their pharmacokinetics. Among the blood pool agents, transpulmonary ultrasound contrast agents offer higher diagnostic potential compared to agents that cannot pass the pulmonary capillary bed after a peripheral intravenous injection. In addition to their vascular phase, some USCAs can exhibit a tissue- or organ-specific phase.
The sonogram image quality is improved either by decreasing the reflectivity of the undesired interfaces or by increasing the backscattered echoes from the desired regions.

Different types of ultrasound contrast agents:
Ultrasound contrast agents act as echo-enhancers, because of the high different acoustic impedance at the interface between gas and blood. The enhanced echo intensity is proportional to the change in acoustical impedance as the sound beam crosses from the blood to the gas in the bubbles.

The ideal qualities of an ultrasound contrast agent:
A typical ultrasound contrast agent consists of a thin flexible or rigid shell composed of albumin, lipid, or polymer confining a gas such as nitrogen, or a perfluorocarbon. The choice of the microbubble shell and gas has an important influence on the properties of the agent.
Current generations of microbubbles have a diameter from 1 μm to 5 μm. The success of these agents is mostly dependent on the small size and on the stability of their shell, which allows passage of the microbubbles through the pulmonary circulation. Microbubbles must be made smaller than the diameter of capillaries or they would embolize and be ineffective and perhaps even dangerous.
The reflectivity of these microbubbles is proportional to the fourth power of a particle diameter but also directly proportional to the concentration of the contrast agent particles themselves.
Ultrasound contrast agents produce unique acoustic signatures that allow to separate their signal from tissue echoes and to depict whether they are moving or stationary. This enables the detection of capillary flow and of targeted microbubbles that are retained in tissues such as normal liver.
The new generation of contrast media is characterized by prolonged persistence in the vascular bed which provides consistent enhancement of the arterial Doppler signal. Contrast agents make it also possible to perform dynamic and perfusion studies. Targeted contrast imaging agents are for example taken up by the phagocytic cell systems and thus have liver/spleen specific effects.

See also Ultrasound Contrast Agent Safety, Adverse Reaction, Tissue-Specific Ultrasound Contrast Agent, and Bubble Specific Imaging.