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Ultrasound: Physical Principles of Ultrasound Imaging
1
Robert Cole and Adrien-Maxence Hespel 2
1 Department of Clinical Sciences, College of Veterinary Medicine, Auburn, AL, USA
2 College of Veterinary Medicine, University of Tennessee, Knoxville, TN, USA

3.1 Introduction Propagation speed is also determined by the stiffness and
density of the medium in which it travels (Table 3.1). The
Diagnostic ultrasound is a commonly used and invaluable stiffer the medium, the higher the sounds speed. Propagation
imaging modality in veterinary medicine. To make the speeds are thus slowest in gases and highest in solids. The
most of ultrasound, a basic understanding of the physics average speed of sound in soft tissue is 1540 m/s [1–3, 5].
behind this modality is needed. This chapter will focus on For most diagnostic sonography, pulsed ultrasound is used
the principles of the ultrasound wave, sound–tissue inter- rather than continuous wave. A pulse of ultrasound is a few
actions, transducers, Doppler basics, and artifacts. cycles of ultrasound separated by gaps of no ultrasound.
The pulse repetition frequency (PRF) is the number of
pulses occurring in one second and is typically expressed in
3.2 Ultrasound Waves kilohertz (kHz) [5]. This is usually controlled automatically
in the machine. However, with Doppler techniques, the
Diagnostic ultrasound uses high‐frequency sound waves to operator has control of the PRF. The PRF will control how
produce images of the anatomy and flow. Sound, as a wave, fast images are generated and thus affect the frame rate. The
is a traveling variation of the acoustic variables including spatial pulse length or length of the ultrasound pulse is an
pressure, density, and particle motion. Sound is described important quantity as it relates to image resolution. Shorter
by terms such as frequency, wavelength, and propagation spatial pulse length improves image resolution [1, 5].
speed [1–4]. Frequency is a count of how many wave cycles
occur in one second (Figure 3.1). The unit of frequency is a
hertz (Hz). A person is able to hear sound in a frequency 3.3 Sound–Tissue Interactions
range of 20–20 000 Hz or cycles per second Hz [2, 3]. Sound
with a frequency of greater than 20 000 Hz is called ultra- Attenuation (weakening of the sound wave) occurs as the result
sound. Frequencies commonly utilized in veterinary medi- of absorption, reflection, and scattering of the sound wave as it
cine range between 3 and 15 MHz (1 MHz = 1 000 000 Hz). travels and interacts with tissue [1, 4, 5]. Attenuation of the
The wavelength is the length of space that one wave cycle sound wave increases with increasing frequency and decreases
takes up (Figure 3.2). Wavelength will be important when the depth at which you can image (Table 3.2). Absorption, the
resolution is discussed. Propagation speed is the rate at conversion of the mechanical energy of the sound wave to heat,
which sound travels through a medium. The relationship is the dominant factor in attenuation of the sound wave [5].
between wavelength, frequency, and propagation speed is Reflection of the sound wave occurs as the sound wave
demonstrated by the formula: encounters tissues with different acoustic impedance values.
/
cf Acoustic impedance is the product of the tissue density and
sound velocity within the tissue [3–5]. If the difference
where λ represents the wavelength (m), c represents the between impedance increases, there will be an increase in
speed of sound in the media (m/s), and f the frequency the reflection of the sound wave. If the impedance values are
(Hz) [1–3, 5]. equal, there is no echo generated [2,5]. The largest acoustic

Feline Diagnostic Imaging, First Edition. Edited by Merrilee Holland and Judith Hudson.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.