4: Ultrasonography


CHAPTER 4
Ultrasonography


Elizabeth Huyhn1, Elodie E. Huguet2,, and Clifford R. Berry3


1 VCA West Coast Specialty and Emergency Animal Hospital, Fountain Valley, CA, USA


2 Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL, USA


3 Department of Molecular Biomedical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC, USA


Overview: Uses and Advantages


Ultrasound is a valuable and noninvasive modality used for the identification and diagnosis of small animal diseases. With advanced training and a good understanding of cross‐sectional anatomy, ultrasound can also be used to thoroughly evaluate anatomic structures and abnormalities based on their acoustic impedance. In an emergency room setting, ultrasound is routinely used for the Thoracic or Abdominal Focused Assessment with Sonography in Triage (TFAST or AFAST), for the identification and tracking of abnormal fluid collections. The portability of today’s ultrasound equipment allows for cage‐side evaluation of veterinary patients. However, this is not the ideal environment for complete abdominal ultrasound evaluations to be done. Using a darkened, quiet room where dogs and cats can be laid on their backs or in lateral recumbency for the scan is important.


Ultrasound can be used to assess all abdominal organs and the heart. The use of echocardiography to assess the cardiovascular structures will be discussed in the cardiovascular chapter.


Ultrasound can provide information related to size, shape, position, margin or contour, echogenicity, and echotexture of the organ being evaluated. Other uses of ultrasound include Doppler ultrasound, elastography, and use of ultrasound‐specific contrast agents.


Basic Physics and Principles of Ultrasound in Diagnostic Imaging


Ultrasound consists of high‐frequency sound waves (MHz or 1 000 000 Hz), with the normal human hearing range being between 2000 and 20 000 Hz. Ultrasound waves are thus not audible to the human ear. The ultrasound waves are generated by nonionizing, mechanical compression and relaxation of a special piezoelectric crystal inside the transducer that creates a mechanical wave which then travels through the tissues. The sound wave can be generated and recorded at a specific frame rate, depending on the features that are engaged (abdomen typically has a frame rate of 40–80 frames per second compared with echocardiography which will have frame rates of greater than 100 frames per second), allowing the evaluation of static and dynamic structures.


The ultrasound wave will travel through and interact with tissues in a number of different ways. Echoes (reflected ultrasound waves) are created based on the specific intrinsic property of tissue through which the sound wave is passing. This property is called acoustic impedance (Z = physical density of the tissue × the speed of sound in the tissue; defined in units of Rayl [gm/m2 s]). These mechanical sound waves return to the ultrasound probe, where they are detected and converted from mechanical into electrical energy and then changed into an anatomic image. This pulse–echo technique results in the transducer “listening” for returning pulses 99% of the time and generating outgoing (sending) pulses 1% of the time.


The ultrasound beam is created by a series of piezoelectric crystals arranged in a curved, linear, or annular format. The first two arrangements are found in transducers used for the abdomen and small body parts. The last is used specifically for echocardiography where crystals do not act in unison but can act independently. This results in the ability to do spectral continuous wave Doppler ultrasound where independent crystals send US waves 100% of the time and different crystals receive and process incoming echoes 100% of the time (see Doppler section of this chapter for more details).


Interaction of Sound Waves in the Tissues


The ultrasound transducer creates pressure variations in the form of ultrasound waves which travel through the tissues, with resultant interactions being based on variations in physical properties within the tissue and between tissue boundaries. These sound waves have a characteristic speed, frequency, and wavelength with a relationship represented by the following equation:


Wavelength λ (m) = speed of sound in tissues [c (m/s)]/frequency [f (MHz)]. The speed of sound propagating in soft tissues is an average speed of sound and ultrasound machines will use 1540 m/s as the average speed of sound in tissues. The propagation speeds of sound vary for different tissue types as listed in Table 4.1. The frequency corresponds to the number of cycles, or complete waveform of the US wave, per second. Ultrasound imaging transducers used in veterinary medicine have a frequency ranging between 1 and 20 MHz (1 megahertz [MHz] defined as 1 × 106 cycles per second or Hz).


TABLE 4.1 The propagation speeds of sound waves in different tissues.




























Tissue Propagation speed of sound (m/s)
Gas 331
Fat 1450
Liver 1549
Kidney 1561
Brain 1541
Blood 1570
Bone 4080

Multiple acoustic variables affect the way sound waves travel in tissues, including pressure, physical density of the tissue, and relative speed within the tissue as well as elastic motion of the tissues themselves. As previously stated, reflection of ultrasound waves within and between tissues is based on differences in acoustic impedance. The acoustic impedance increases if the physical density of the tissue and/or the propagation speed of the US sound wave increases. This increase in different acoustic impedances will then result in more ultrasound waves being reflected toward the transducer.


When the ultrasound waves travel in tissues, there are five potential interactions: reflection, refraction, scattered, absorption or no interaction and therefore the wave is transmitted further into the tissues.



  • Reflection: occurs when there are differences in acoustic impedance and the ultrasound wave is reflected back toward the transducer (Figure 4.1). If there are no differences in acoustic impedance, then the ultrasound waves are propagated further into the tissues (called transmission). Reflectors perpendicular to the incident beam of the ultrasound waves are the best whereas incident ultrasound waves interacting with acoustic boundaries that are parallel to ultrasound beam are poor reflectors. The larger the differences in acoustic impedance, the greater the number of ultrasound waves reflected. For example, there are large differences in acoustic impedance between soft tissues (1.65 × 106 Rayls) and bone or mineral (7.8 × 106 Rayls) that will result in reflection of all sound waves without transmission of waves to a depth below the soft tissue–mineral interface.
  • Refraction: refraction differs from reflection in that sound waves being transmitted into bordering tissues with a different acoustic impedance will undergo a change in direction (Figure 4.2). These sound waves may eventually return to the transducer and provide misinformation regarding the position of a tissue in relation to another. This results in refraction artifacts on the image. The degree of displacement or refraction of the sound wave is directly proportional to the propagation speed of the second tissue through which the sound wave travels.
  • Scattering: when the sound wave encounters irregular surfaces, heterogeneous tissues, or objects equal to or smaller than the size of its wavelength, it can be redirected in many directions (Figure 4.3). Some of these sound waves may return to the transducer and result in loss of resolution. In comparison, specular reflections occur when sound waves encounter a smooth and flat interface and are returned to the transducer without a change in direction and therefore recorded accurately as to depth.
  • Absorption: sound waves attenuated in tissues may be converted into heat, and therefore may not contribute to the final image. Most of the sound waves attenuated in tissues are absorbed. The heat generated consequently contributes to some of the risks associated with ultrasonography and will be discussed later in this chapter.
Schematic illustration of the interaction of ultrasound waves at different acoustic interfaces (tissue) whereby some of the incident US waves are reflected toward the transducer and will be used to create an image at depth.

FIGURE 4.1 Schematic diagram of the interaction of ultrasound waves at different acoustic interfaces (tissue) whereby some of the incident US waves are reflected toward the transducer and will be used to create an image at depth.

Schematic illustration of the interaction of ultrasound waves at different acoustic interfaces (tissue) whereby some of the incident US waves are refracted and continue into the tissue.

FIGURE 4.2 Schematic diagram of the interaction of ultrasound waves at different acoustic interfaces (tissue) whereby some of the incident US waves are refracted and continue into the tissue. These US waves may never contribute to the image.

Schematic illustration of the interaction of ultrasound waves at different acoustic interfaces (tissue) whereby some of the incident US waves are scattered in different directions other than being reflected toward the transducer and will not aid in image creation.

FIGURE 4.3

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Apr 16, 2023 | Posted by in ANIMAL RADIOLOGY | Comments Off on 4: Ultrasonography

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