Piezo means pressure, so piezoelectric means that pressure is generated when electrical energy is applied to a quartz crystal. When electrical energy is applied to the face of the crystal, the shape of the crystal changes as a function of the polarity of the applied electrical energy. As the crystal expands and contracts it produces compressions and rarefactions, and creates sound waves. When this material is struck by sound waves it creates electrical currents.
Thus, a piezoelectric crystal can produce a pulse of mechanical energy (pressure pulse) by electrically exciting the crystal (transmitter), and they can produce a pulse of electrical energy by mechanically exciting the crystal (receiver). This ultrasound physics principle is called the piezoelectric effect (pressure electricity), which was discovered by Pierre and Jacques Curie in 1880, and is used to generate ultrasound waves. Instead of quartz crystals, piezoelectric ceramics such as barium titanate or lead zirconate titanate are also used, which are crystalline materials with similar piezoelectric properties.

See also Temporal Peak Intensity.

In the field of medical ultrasound imaging, the term 'probe' specifically refers to the ultrasound transducer and represent the handheld device that emits and receives ultrasound waves during an examination.
The probe encompasses various components such as the elements, backing material, electrodes, matching layer, and protective face that are responsible for both emitting and receiving the sound waves. Aperture, known also as the footprint, is the part of the probe that is in contact with the body. When the emitted sound waves encounter body tissues, they generate reflections that are received by the probe, which then generates a corresponding signal. In most cases, the probe emits ultrasound waves for only about 10% of the time and receives them for the remaining 90%.
Probes are available in different shapes and sizes to accommodate various scanning situations. The footprint is linked to the arrangement of the piezoelectric crystals and comes in different shapes and sizes e.g. linear array transducer//convex transducer. The transducer plays a huge role in image quality and is one of the most expensive parts of the ultrasound machine. Mechanical probes steer the ultrasound beam driven by a motor and are capable of producing high-quality images, but they are prone to wear and tear. Mechanical probes have been mostly replaced by electronic multi-element transducers, but mechanical 3D probes still remain for abdominal and Ob-Gyn applications.
In summary, the terms 'ultrasound transducer,' 'probe,' and 'scanhead' are often used interchangeably to refer to the same component of the ultrasound machine. Probes consist of multiple components and are available in different shapes and sizes depending on the sonographer's needs.

See also Handheld Ultrasound, Ultrasound System Performance, Omnidirectional, Probe Cleaning, and Multi-frequency Probe,

Pulsed ultrasounds are cycles of ultrasound separated in time with gaps of no signal. Pulsed sound waves are generated by short, strong pulses of sound from a phased array of piezoelectric crystals. The transducer, though emitting ultrasound in rapid pulses, acts as a receiver most of the time. In sonography, pulsed ultrasound is used to perform diagnostic or therapeutic procedures.

See also Pulse Average Intensity, Release Burst Imaging.

Transducers used for the real-time mode are different than for the A-mode, B-, or M-modes. A linear array transducer with multiple piezoelectric crystal elements that are different arranged and fired, transmits the needed larger sound beam.
A subgroup of x adjacent elements (8-16; or more in wide-aperture designs) is pulsed simultaneously; the inner elements pulse delayed with respect to the outer elements. The interference of the x small divergent wavelets generates a focused beam. The delay time determining the focus depth of a real-time transducer can be changed during imaging.
Similar delay factors applied during the receiving phase, result in a dynamic focusing effect on the return. This forms a single scan line in the real-time image. To produce the following scan line, another group of x elements is selected by shifting one element position along the transducer array from the previous group. This pattern is then repeated for the groups along the array, in a sequential and repetitive way.

A transducer assembly is the configuration of an ultrasound transducer. Electronic array transducers are composed of multiple crystal elements e.g., linear array and phased array transducer assemblies. A handheld mechanical probe may contain a mechanically driven single element transducer, or a rotating wheel transducer. Static B-scanner (obsolete) had a cylindrical probe with a single disk-shaped piezoelectric crystal.
Some probes are designed to be inserted in body cavities (vagina, rectum, esophagus) so that they can get closer to the organ being examined (uterus, prostate gland, heart); getting closer to the region of interest allow for a more detailed view.

See also Endocavitary Echography, Endoscopic Ultrasound, Vaginal Transducer and Rectal Probe.