(HIFU / FUS) High intensity focused ultrasound is used in thermotherapy or thermoablation e.g., for the treatment of benign prostate hyperplasia or under study for the treatment of cancer.
An applied ultrasound probe (see transrectal sonography) focuses sound waves at one spot, elevating the tissue temperature to a point that the tissue destroys. Generally, lower frequencies (from 250 kHz to 2000 kHz) are used than for medical diagnostic ultrasound, but significantly higher time-averaged intensities.

See also Magnetic Resonance Guided Focused Ultrasound, Low Intensity Pulsed Ultrasound, and Lithotripsy.

The Nyquist frequency describes the highest frequency that can be sampled in a pulsed ultrasound system. To record higher frequencies the zero baseline can be shifted in the desired direction.

See also Nyquist Limit, and Sampling Rate.

Pressure is the force per unit area applied on a surface in a direction perpendicular to that surface.
Pressure can also be described as a form of potential energy in a fluid. The maximum pressure of the fluid medium obtained during propagation of an ultrasonic pulse.
The negative peak pressure is the peak rarefaction pressure attained during the negative portion of a propagating ultrasound pulse in a medium such as tissue.
Sound pressure can be measured using a microphone in air and a hydrophone in water. The SI unit for sound pressure is the Pascal.
Blood pressure is the pressure exerted by the blood on the walls of the blood vessels.

See also Rarefactional Pressure, Low Intensity Pulsed Ultrasound, and Projector.

(PWD) Pulsed wave (PW) Doppler is a Doppler ultrasound mode that evaluates blood flow velocities in a range specific area along the length of the sound beam. Measured are changes in received frequency due to relative motion (flow) between a sound source (transducer) and sound receiver (transducer).
PW Doppler produces an audible signal as well as a graphical representation of flow. The Doppler shift produced by moving blood flow is calculated by the ultrasound system.

See also Amplitude Indicator, Pulsed Ultrasound.

Ultrasound therapy uses high energy sound waves to treat different diseases. Historically, the use of ultrasonic waves in therapy began before the wide use as a diagnostic medical imaging tool. Dependend on the intensity, ultrasound therapy reach from the thermal effect used in physical therapy to the destruction of tissue with lithotripsy.

Types of ultrasound treatment:
See also Thermal Index, History of Ultrasound, Interventional Ultrasound, and B-Mode Acquisition and Targeting.