Ultrasound

Physics of Ultra sound

Ultrasound is the term used to describe sound of frequencies above 20 000 Hertz (Hz),beyond the range of human hearing. Frequencies of 1–30 megahertz (MHz) are typical for diagnostic ultrasound. Therapeutic ultrasound has a frequency range of 0.7 and 5.0 MHz. Diagnostic ultrasound imaging depends on the computerized analysis of reflected ultrasound waves, which non-invasively build up fine images of internal

body structures. The resolution attainable is higher with shorter wavelengths, with the wavelength being inversely proportional to the frequency. However, the use of high frequencies is limited by their greater attenuation (loss of signal strength) in tissue and thus shorter depth of penetration. For this reason, different ranges of frequency

are used for examination of different parts of the body:

■ 3–5 MHz for abdominal areas
■ 5–10 MHz for small and superficial parts and
■ 10–30 MHz for the skin or the eyes.

Generation of ultrasound

Piezoelectric crystals or materials are able to convert mechanical pressure (which causes alterations in their thickness) into electrical voltage on their surface (the piezoelectric effect). Conversely, voltage applied to the opposite sides of a piezoelectric material causes an alteration in its thickness (the indirect or reciprocal piezoelectric effect). If the applied electric voltage is alternating, it induces oscillations which are

transmitted as ultrasound waves into the surrounding medium. The piezoelectric crystal, therefore, serves as a transducer, which converts electrical energy into mechanical energy and vice versa.

Ultrasound transducers are usually made of thin discs of an artificial ceramic material such as lead zirconate titanate(PZT). The thickness (usually 0.1–1 mm) determines the ultrasound frequency. The basic design of a plain transducer is shown in Fig. 1.1.

In most diagnostic applications, ultrasound is emitted in extremely short pulses as a narrow beam comparable to that of a flashlight. When not emitting a pulse (as much as 99% of the time), the same piezoelectric crystal can act as a receiver.The quality of the crystal is what makes your US expensive.

Crystal quality depends on the following:

•  Beam Nonuniformity ratio: ranges from 2 to 6 – the smaller the better.
• Effective Radiating Area: as close to sound head area as possible