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Similarly, the width of the ultrasound beam from a transducer depends in part on the wavelength. The size of an object is most meaningfully expressed if given relative to the ultrasonic wavelength for the frequency of the sound beam. Wavelength has relevance when describing dimensions of objects, such as reflectors and scatterers in the body. * Assuming a speed of sound of 1540 m/sec. TABLE 2-2 Wavelengths for Various Ultrasound Frequencies Higher frequencies have shorter wavelengths and vice versa. For example, if the frequency is 5 MHz, the wavelength in soft tissue is approximately 0.3 mm. A good rule of thumb for tissues is the wavelength λ t = 1.5 mm/F, where F is the frequency expressed in MHz. Table 2-2 presents values for the wavelength in soft tissue, where the speed of sound is taken to be 1540 m/sec, for several frequencies. Where c is the speed of sound and f is the frequency. The diagram schematically illustrates compressions and rarefactions at an instant of time.
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The pressure amplitude is the maximum pressure swing, positive or negative. The fluctuations propagate through the medium in waves. Vibrations of the transducer are coupled into the medium, producing local fluctuations in pressure. It is defined by the equationįIGURE 2-1 Sound waves produced by an ultrasound transducer. The wavelength λ is the distance over which a property of a wave repeats itself. It illustrates accompanying compressions and rarefactions in the medium that result from the particle oscillations. Higher frequencies are associated with improved spatial detail, or better resolution.įigure 2-1 shows what might be called a snapshot of a sound wave, captured at an instant of time. Manufacturers of ultrasound equipment and clinical users strive to use as high a frequency as practical that still allows adequate visualization depth into tissue (see section on attenuation). Diagnostic ultrasound applications use frequencies in the 1-MHz to 30-MHz (1 million to 30 million Hz) frequency range. Ultrasound refers to any sound whose frequency is above the audible range (i.e., above 20 kHz). Audible sounds are in the range of 30 Hz to 20 kHz. Frequency is expressed in cycles per second, or hertz (Hz). The number of oscillations per second of the piezoelectric element in the transducer establishes the frequency of the ultrasound wave.
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In Fullerton G, Zagzebski J, editors: Medical physics of CT and ultrasound, New York, 1980, American Institute of Physics, p 381. TABLE 2-1 Speed of Sound for Biologic Tissueįrom Wells PNT: Propagation of ultrasonic waves through tissues. Adipose tissues have sound speeds that are lower than the average, whereas muscle tissue exhibits a speed of sound that is slightly greater than 1540 m/sec. Slight variations exist in the speed of sound from one tissue to another, but as Table 2-1 indicates, speeds of sound in specific soft tissues deviate only slightly from the assumed average. 1 Most diagnostic ultrasound instruments are calibrated with the assumption that the sound beam propagates at this average speed. For soft tissues, the average speed of sound has been found to be 1540 m/sec. Speeds of sound in common media and tissues are listed in Table 2-1. As a general rule, gases, including air, exhibit the lowest sound speed liquids have an intermediate speed and firm solids such as glass have very high speeds of sound. Sound propagation speeds depend on the properties of the transmitting medium and not significantly on frequency or wave amplitude. The speed of sound in tissue must be known to apply pulse-echo methods. The time between transmitting a pulse and receiving an echo is used to determine the depth of the interface. Most ultrasound applications involve transmitting short bursts, or pulses, of sound into the body and receiving echoes from tissue interfaces.