(Vmax) The maximum velocity is the highest velocity found with significant amplitude within the sampled region normally for use in Bernoulli type gradient estimations.

The Bernoulli effect describes the reduction in pressure that occurs with an increase in velocity of fluid flow.

See also Bernoulli Equation, and Maximum Velocity.

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.

A transducer is a device, usually electrical or electronic, that converts one type of energy to another. Most transducers are either sensors or actuators. A transducer (also called probe) is a main part of the ultrasound machine. The transducer sends ultrasound waves into the body and receives the echoes produced by the waves when it is placed on or over the body part being imaged.
Ultrasound transducers are made from crystals with piezoelectric properties. This material vibrates at a resonant frequency, when an alternating electric current is applied. The vibration is transmitted into the tissue in short bursts. The speed of transmission within most soft tissues is 1540 m/s, producing a transit time of 6.5 ms/cm. Because the velocity of ultrasound waves is constant, the time taken for the wave to return to the transducer can be used to determine the depth of the object causing the reflection.
The waves will be reflected when they encounter a boundary between two tissues of different density (e.g. soft tissue and bone) and return to the transducer. Conversely, the crystals emit electrical currents when sound or pressure waves hit them (piezoelectric effect). The same crystals can be used to send and receive sound waves; the probe then acts as a receiver, converting mechanical energy back into an electric signal which is used to display an image. A sound absorbing substance eliminates back reflections from the probe itself, and an acoustic lens focuses the emitted sound waves. Then, the received signal gets processed by software to an image which is displayed at a monitor.
Transducer heads may contain one or more crystal elements. In multi-element probes, each crystal has its own circuit. The advantage is that the ultrasound beam can be controlled by changing the timing in which each element gets pulsed. Especially for cardiac ultrasound it is important to steer the beam.
Usually, several different transducer types are available to select the appropriate one for optimal imaging. Probes are formed in many shapes and sizes. The shape of the probe determines its field of view.
Transducers are described in megahertz (MHz) indicating their sound wave frequency. The frequency of emitted sound waves determines how deep the sound beam penetrates and the resolution of the image. Most transducers are only able to emit one frequency because the piezoelectric ceramic or crystals within it have a certain inherent frequency, but multi-frequency probes are also available.
See also Blanking Distance, Damping, Maximum Response Axis, Omnidirectional, and Huygens Principle.

According to Shannon's sampling theorem, the sampling frequency should be twice the frequency being sampled. The nyquist frequency is the maximum frequency that can be sampled without aliasing. In ultrasound imaging, it is defined as half of the pulse repetition frequency.
NF = PRF/2 (nyquist frequency = pulse repetition frequency/2)
This is the so-called Nyquist limit. If the velocity of flow exceeds the Nyquist limit, the direction and velocity are inaccurately displayed and appear to change direction. Color flow Doppler capitalizes on this effect. This allows detecting flow disturbances from laminar to turbulent flow.

See also Aliasing Artifact, Repetition Rate, and Sampling Rate.