The thermal effect of ultrasound is caused by absorption of the ultrasound beam energy. As the ultrasound waves are absorbed, their energy is converted into heat. The higher the frequency, the greater the absorbed dose, converted to heat according the equation: f = 1/T where T is the period as in simple harmonic motion. Ultrasound is a mechanical energy in which a pressure wave travels through tissue. Heat is produced at the transducer surface and also tissue in the depth can be heated as ultrasound is absorbed.
The thermal effect is highest in tissue with a high absorption coefficient, particularly in bone, and is low where there is little absorption. The temperature rise is also dependent on the thermal characteristics of the tissue (conduction of heat and perfusion), the ultrasound intensity and the length of examination time. The intensity is also dependent on the power output and the position of the tissue in the beam profile. The intensity at a particular point can be changed by many of the operator controls, for example power output, mode (B-mode, color flow, spectral Doppler), scan depth, focus, zoom and area of color flow imaging. The transducer face and tissue in contact with the transducer can be heated.

See also Thermal Units Per Hour and Ultrasound Radiation Force.

(TI) The definition of the thermal index is the ratio of the total acoustic power to that required raising a maximum temperature increase of 1 °C under defined assumptions. A thermal index of 1 indicates the acoustic power achieving a temperature increase of 1 °C. A thermal index of 2 has the doubled power but would not necessarily indicate a peak temperature rise of 2 °C. The temperature rise is dependent on tissue type and is particularly dependent on the presence of bone.
Classifications of thermal indices:
For fetal ultrasound, the highest temperature increase would be expected occurring at bone. Therefore, TIB gives the worst-case conditions. If the ultrasound system can exceed an index of 1, the mechanical index and thermal index must be displayed. The displayed indices are based on the manufacturer's data.

See also Cranial Bone Thermal Index, Bone Thermal Index, Soft Tissue Thermal Index.

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.

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.

(TRUS) Transrectal sonography (also called transrectal ultrasonography, transrectal echography (TRE), endorectal ultrasound (ERUS or EUS)) is an ultrasound procedure used to examine the prostate gland, the rectum or bladder.
A small, lubricated transducer placed into the rectum releases sound waves, which create echoes as they enter the region of interest. A computer creates a picture called a sonogram.
TRUS is commonly used for guidance during a prostate needle biopsy and may be used to deliver brachytherapy and monitor cancer treatment. Transrectal ultrasonography detects enlargement, tumors and other abnormalities of the prostate, rectal polyps, rectal cancer, perianal infection, and sphincter muscle injuries. TRUS is also performed on male patients with infertility to view the prostate and surrounding structures and on patients with suspected bladder conditions or disease to view the bladder.

See also Transurethral Sonography, Endoscopic Ultrasound, Pelvic Ultrasound, Rectal Probe, Biplane Probe, Endocavitary Echography and High Intensity Focused Ultrasound.