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.

(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.

Ultrasound physics is based on the fact that periodic motion emitted of a vibrating object causes pressure waves. Ultrasonic waves are made of high pressure and low pressure (rarefactional pressure) pulses traveling through a medium.

Properties of sound waves:

The speed of ultrasound depends on the mass and spacing of the tissue molecules and the attracting force between the particles of the medium. Ultrasonic waves travels faster in dense materials and slower in compressible materials. Ultrasound is reflected at interfaces between tissues of different acoustic impedance e.g., soft tissue - air, bone - air, or soft tissue - bone.
The sound waves are produced and received by the piezoelectric crystal of the transducer. The fast Fourier transformation converts the signal into a gray scale ultrasound picture.

The ultrasonic transmission and absorption is dependend on:
See also Sonographic Features, Doppler Effect and Thermal Effect.

A piezoelectric ceramic is made of crystalline substance which creates charges of electricity by the application of pressure and vice versa. This material is used in ultrasound transducers to create the sound waves.

See also Composite Array.

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.