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Ultrasound (US) uses the transmission and reflection of mechanical waves to generate an image. US frequency (f) is the number of wavelengths per second and is measured in hertz (Hz) (Fig. 2-1). As humans can hear sound in the 20–20,000 Hz range, US is defined as sound with a frequency >20,000 Hz, or 20 kilohertz (kHz). Diagnostic sonography for most medical applications uses frequencies of 2–20 megahertz (MHz).
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US frequency is independent of the medium through which the sound is traveling and is a property of the crystals in the US transducer. Modern transducers now often include a range of frequencies ("broadband") and/or allow for frequency adjustment. Propagation speed (c) describes how fast the US travels through a given medium. Unlike frequency, the propagation speed depends on the medium through which the sound travels. The wave velocity in fluid or tissue is approximately 1540 m/s (although it does vary slightly depending on the type of tissue) as compared to the propagation speed through air at a velocity of approximately 330 m/s. The relationship of frequency, wavelength, and propagation speed is described by the equation c = f λ. Because frequency is constant, wavelength varies directly with propagation speed (ie, f = c/λ). Thus, when propagation speed increases, wavelength increases, and vice versa. The change in the speed of sound at tissue interfaces results in a change of wavelength, which is responsible for determining image contrast, and resolution.
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The power of an US wave refers to the amount of energy passing through the tissue in a unit of time and is expressed in watts. In the majority of compact US units, the power is fixed, though it may be adjusted on more sophisticated machines. If using one of these units, one should always use the lowest power that will produce the desired imaging as higher powers (>1 W) can result in cellular and tissue damage. This principle is commonly referred to as ALARA (As Low As Reasonably Achievable).
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Transducers convert one form of energy into another. Piezoelectric transducers convert electrical energy into mechanical energy by inducing vibration of the ferroelectric materials in the transducer head. These vibrations are transmitted through the tissue, echo back at boundaries of tissue that have different acoustic impedance, and are then converted back to an electrical signal. The transducer thus acts as an US transmitter and receiver. When the boundary between two tissues has high acoustic impedance, most of the US is reflected back to the transducer. If two materials have the same acoustic impedance, their boundary will not produce an echo. Typically, only a small fraction of the US pulse is reflected back, with the majority of the pulse continuing along the beam line, but can also be scattered or ...