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.

The operator (sonographer) sets and changes the frequency and duration of the ultrasound pulses, as well as the scan mode of the machine with the transducer pulse control. The commands from the operator are translated into changing electric currents that are applied to the piezoelectric crystals in the transducer probe.

See also Blanking Distance.

Transducers can be divided in:
1.) Transducers where the sound wave is transmitted and received by different elements.
2.) Transducers where multiple elements part of the time transmit and part of the time receive sound energy.
The first type of ultrasound transducer is used in detection of blood flow (also called nonimaging transducers). For example, the continuous wave transducer (Pedoff transducer) has two separate elements, where one element is always transmitting while the other element is always receiving.
Probes of the second type are used to image cardiac structures and have the capability to use various Doppler techniques to detect blood flow (also called imaging transducers). For example, continuous wave, pulsed wave, high pulse repetition frequency, color flow, M-mode, and 2D-mode are the various modes that this type of transducer can perform.

Transducers can also be divided in mechanical and electronic or phased scan types.
Mechanical transducers use a combination of single element oscillation, multiple element rotation, or a single element and set of acoustic mirrors to generate the sweeping beam for 2D mode. Caused by the vibration (created as the mirrors rotate or oscillate inside the cover) is this type sometimes called the 'wobbler'. Mechanical transducers are cheaper than electronic transducers.
Different types of electronic or phased array probes can create a linear or rectangular shaped scan plane as well as a sector or pie shaped scan plane. Sector scanners are most useful for cardiac ultrasound examinations where the beam is directed between the ribs to image the heart. A linear array transducer is more useful in abdominal, OB/GYN, and small parts examinations. Electronic transducers are more expensive but they provide dynamic focusing and smaller probe.

See also Rectangular Array Transducer.

(TEE) Transesophageal echocardiography provides a superior view of cardiac anatomy compared with transthoracic echocardiography. TEE is performed by the introduction of a probe attached to a fiberoptic endoscope into the esophagus. Caused by the position close to the heart e.g., clot finding and the view of the mitral valve are improved.

Indications:
The piezoelectric crystal creating the acoustic power is mounted on the gastroscope that must be swallowed by the patient. This endoscopic transducer is miniaturized to approximately the size of a fingernail. Usually the probe is in place for an average of 15 minutes, to numb the surface a topical anesthetic is sprayed into the throat, in addition a conscious sedation is recommended.

See also Myocardial Contrast Echocardiography, Stress Echocardiogram, M-Mode Echocardiography, Contrast Enhanced Ultrasound and Vascular Ultrasound Contrast Agents.

The transforaminal or sub-occipital acoustic window is found in the space between the atlas and the base of the skull (through the foramen magnum insonated from the top of the neck below the occiput). Practiced in the prone position (or sitting).
This acoustic window allows the insonation of the vertebral arteries, basilar artery and some of the other branches of the posterior circulation (e.g. posterior inferior cerebellar artery).

See also Transcranial Doppler.