ULTRASOUND PRINCIPLES AND ECHOCARDIOGRAPHIC MODALITIES

Ultrasound Principles

Sound waves are mechanical vibrations that create refraction and compression of the medium through which they travel. Humans can hear sound waves with frequencies ranging from 20 cycles/sec (Hz) to 20 kHz. Beyond this range is ultrasound. Echocardiography uses ultrasound waves with frequencies of 2.5 to 7.5 million cycles/sec (MHz). Frequencies greater than 7.5 MHz are not routinely used in echocardiography because they produce wavelengths too short for adequate penetration into tissues (penetration is limited to about 200 to 400 times the wavelength). Frequencies less than 2.5 MHz are not routinely used because they produce wavelengths too long for resolution of small objects (resolution is limited to about twice the wavelength). The interrelationship of wavelength and frequency is described by the following equation:

C = λ · F

where C = velocity of sound in tissue (1540 m/sec), F = frequency (Hz), and λ = wavelength (m). Because the velocity of sound is assumed to be constant, the wavelength of any frequency can be calculated as

λ (m) = 1540 (m/sec)/F (Hz)

, or

λ (mm) = 1,540,000 (mm/sec)/F (Hz)

or

λ (mm) = 1.54/F (MHz)

Thus, ultrasound waves in the range of 2.5 to 7.5 MHz produce wavelengths of 0.2 to 0.6 mm, resolve objects 0.4 to 1.2 mm in size, and penetrate tissues up to 24 cm.

Ultrasound waves are generated by piezoelectric crystals that emit ultrasound when electricity is applied to them and emit electricity when struck by ultrasound. Thus, the same crystals can serve as emitters and receivers of ultrasound. Typically in echocardiography, the crystals emit very short pulses of ultrasound (approximately 1 µsec) and receive or "listen" for the reflected ultrasound for 250 to 500 µsec. When the emitted waves strike an interface of tissues of differing density, such as the pericardium and the heart, a portion of the ultrasound is reflected. The portion reflected depends on the difference in tissue density; that is, the greater the difference, the greater the portion reflected. For example, air in the left ventricle reflects a much greater portion of the transmitted ultrasound than blood does and is translated as a brighter signal on the display screen. Tissue is localized according to the time that it takes a sound wave to bounce back to the transducer; that is, the longer a sound wave takes to bounce back, the greater its distance from the transducer. For example, if it takes 26 µsec for the wave to return to the transducer, the ultrasonograph (often called an "echo machine") displays a structure that is 1 cm from the transducer. No ionizing radiation of any type is used in echocardiography, and no adverse effects of ultrasound have been demonstrated in humans.

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