A handle or probe contains the transducer elements and is attached to the system via a relatively long coaxial cable. Handles are selected via high voltage (HV) relays. In most ultrasound machines, several different handles are available to be connected to the system, allowing the operator to select the appropriate transducer for optimal imaging.

See also Probe Cleaning, and Transducer Assembly.

(Pa) The SI unit of pressure.
Definition: 1 Pascal is equal to 1 N/m² = 1 J/m³ = 1 kg*m^-1*s^-2
1 kPa = 0.145 lbf/in².
Air pressure is measured in hectoPascal (hPa), with 1 hPa = 1 millibar.
The unit is named for Blaise Pascal (1623-1662), French philosopher and mathematician.

See also Open Circuit Voltage, and Source Level.

A phased array probe applies voltage pulses to all elements as a group but with small time differences (phasing). The time difference is changed each time so that the sound pulses will be sent out in different directions.

A piezoelectric crystal changes the physical dimensions when subjected to an electric field. When deformed by external pressure, an electric field is created across the crystal. Piezoelectric ceramic and crystals are used in ultrasound transducers to transmit and receive ultrasound waves.
The piezoelectric crystal in ultrasound transducers has electrodes attached to its front and back for the application and detection of electrical charges. The crystal consists of numerous dipoles, and in the normal state, the individual dipoles have an oblique orientation with no net surface charge.
In ultrasound physics, an electric field applied across the crystal will realign the dipoles and results in compression or expansion of the crystal, depending on the direction of the electric field. For the transmission of a short ultrasound pulse, a voltage spike of very short duration is applied, causing the crystal to initially contract and then vibrate for a short time with its resonant frequency.

See also Composite Array, Transducer Pulse Control, and Temporal Peak Intensity.

The resonant frequency is the frequency at which a piezoelectric ceramic (crystal) will vibrate most efficiently. At the resonant frequency the crystal will produce the highest output with the least amount of voltage applied.