Wavelength is the distance the wave travels in a

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Wavelength is the distance the wave travels in a single cycle. Because velocity must remain constant in a given medium, wavelength is inversely related to frequency (velocity ¼ frequency ± wavelength). Therefore, high frequency decreases wavelength (and thus penetration), and lower frequency increases wavelength (and thus penetration). Attenuation is the progressive weakening of a sound wave as it travels through a medium. The attenuation coefficients for different tissue densities in the body are shown in Table 1.1 . Figure 1.2 Most ultrasound machines have 256 shades of gray, corresponding to the returning amplitude of a given ultrasound wave. 2 Fundamentals Fundamentals
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(A) (B) Figure 1.3 ( A ) The near field of the screen shows objects closest to the probe. ( B ) The far field of the screen shows images further from the probe. Courtesy of Dr. Manuel Colon, University of Puerto Rico Medical Center, Carolina, PR. Figure 1.4 Low- and high- frequency sound waves. Fundamentals 3 Fundamentals
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Several factors contribute to attenuation: the density of the medium, the number of interfaces encountered, and the wavelength of the sound. Diagnostic ultrasound does not transmit well through air and bone because of scatter and reflection. However, ultrasound travels well through fluid- containing structures such as the bladder. Attenuation also occurs as sound encounters interfaces between different types of media. If a tissue is homo- geneous and dense, then the number of interfaces is reduced and less attenu- ation occurs. If a tissue is heterogeneous and less dense, then more attenuation occurs. Reflection is the redirection of part of the sound wave back to its source. Refraction is the redirection of part of the sound wave as it crosses a boundary between different media (or crosses tissues of different propagation speeds such as from muscle to bone). Scattering occurs when the sound beam encounters an interface that is relatively small or irregular in shape (e.g., what happens when sound waves travel through air or gas). Absorption occurs when the acoustic energy of the sound wave is contained within the medium. Resolution refers to an ultrasound machine’s ability to discriminate between two closely spaced objects. Figure 1.5 shows two points that are resolved as distinct by a machine with higher resolution (the paired dots) and the same structures visualized by a machine with lower resolution (the two dots are seen as a single indistinct blob). Axial resolution refers to the ultrasound machine’s ability to differentiate two closely spaced echoes that lie in a plane parallel to the direction of the traveling sound wave. Increasing the frequency of the sound wave will increase the axial resolution of the ultrasound image. Lateral resolution refers to the ultrasound machine’s ability to differentiate two closely spaced echoes that lie in a plane perpen- dicular to the direction of the traveling sound wave. In some portable ultra- sound machines, the focal zone (or narrowest part of the ultrasound beam) Table 1.1
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