Due to the absorption of ultrasound, heating of tissue (including bone) can occur. For this reason, the sonographer should follow the ALARA principle to minimize the potential for ultrasonic heating of tissue during for example M-mode ultrasound. The thermal effect of Doppler ultrasound flow examinations is significantly greater.

See also Thermal Index and Ultrasonic Power.

Ultrasound imaging procedures are widely used in medicine. It is possible to perform diagnostic or therapeutic procedures with the guidance of ultrasonography (interventional ultrasound biopsies or drainage of fluid collections). Sonography or ultrasound scanning involves the application of an ultrasound transducer used to transmit high frequency sound waves, which bounce off internal structures to produce an image that can be displayed and recorded.
Ultrasound imaging procedures include for example:

Urologic ultrasound includes the examination of the kidneys, renal vessels, urinary tract, bladder, prostate, and scrotum.
Usual gray scale ultrasound equipment and standard probes are sufficient to examine the kidney parenchyma and renal pelvis, the urinary tract and bladder. Doppler ultrasound is a useful adjunct to kidney ultrasound. High ultrasound system performance is desirable to show the arterial system, because advanced power Doppler is significantly more sensitive to blood flow than standard color Doppler.
Transurethral sonography may be used to examine the bladder and urethra. Transrectal sonography is used to scan and treat the prostate e.g., with brachytherapy or high intensity focused ultrasound. Very small probes are used for these applications. Reflux sonography is especially used in pediatric ultrasound.

See also Ultrasound Imaging Procedures, Ultrasound Picture, Ultrasound Imaging Modes, Lithotripsy, Thermotherapy, Brachytherapy and Ultrasound Therapy.

Sound waves must have a medium to pass through. The velocity or propagation speed is the speed at which sound waves travel through a particular medium measured in meters per second (m/s) or millimeters per microsecond (mm/μs). Because the velocity of ultrasound waves is constant, the time taken for the wave to return to the probe can be used to determine the depth of the object causing the reflection.
The velocity is equal to the frequency x wavelength.
V = f x l
The velocity of ultrasound will differ with different media. In general, the propagation speed of sound through gases is low, liquids higher and solids highest. The speed of sound depends strongly on temperature as well as the medium through which sound waves are propagating. At 0 °C (32 °F) the speed of sound in air is about 331 m/s (1,086 ft/s; 1,192 km/h; 740 mph; 643 kn), at 20 °C (68 °F) about 343 metres per second (1,125 ft/s; 1,235 km/h; 767 mph; 667 kn)

Velocity (m/s)

Doppler ultrasound visualizes blood flow-velocity information. The peak systolic velocity and the end diastolic velocity are major Doppler parameters, which are determined from the spectrum obtained at the point of maximal vessel narrowing. Peak systolic velocity ratios are calculated by dividing the peak-systolic velocity measured at the site of flow disturbance by that measured proximal of the narrowing (stenosis, graft, etc.).

See Acceleration Index, Acceleration Time, Modal Velocity, Run-time Artifact and Maximum Velocity.