Small Animal Diagnostic Ultrasound E-Book

John S. Mattoon, Rance K. Sellon, Clifford Rudd Berry

• Publisher: Saunders
• Year: 2020
• eBook ISBN: 9780323533331
• Edition: 4
• Topic: Medicine
• Subtopic: Veterinary Medicine

- NEW! Updated content on diagnostic ultrasound ensures that you are informed about the latest developments and prepared to meet the challenges of the clinical environment.- NEW! Coverage of internal medicine includes basic knowledge about a disease process, the value of various blood tests in evaluating the disease, as well as treatment strategies.- NEW editors Rance K. Sellon and Clifford R. Berry bring a fresh focus and perspective to this classic text.- NEW! Expert Consult website includes a fully searchable eBook version of the text along with video clips demonstrating normal and abnormal conditions as they appear in ultrasound scans.- NEW! New and updated figures throughout the book demonstrate current, high-quality images from state-of-the-art equipment.- NEW contributing authors add new chapters, ensuring that this book contains current, authoritative information on the latest ultrasound techniques.

1: Fundamentals of diagnostic ultrasound

John S. Mattoon, Clifford R. Berry

Diagnostic ultrasound uses high-frequency sound waves that are pulsed into the body, and the returning echoes are then analyzed by computer to yield high-resolution cross-sectional images of organs, tissues, and blood flow. The displayed information is a result of ultrasound interaction with tissues, which is based on the tissue’s acoustic impedance, and does not necessarily represent specific microscopic or macroscopic anatomy. Indeed, organs may appear perfectly normal on an ultrasound image in the presence of dysfunction or failure. Conversely, organs may appear abnormal on the ultrasound examination but be functioning properly. This basic tenet must be understood and respected for diagnostic ultrasound to be used properly.

High-quality ultrasound studies require a firm understanding of the important physical principles of diagnostic ultrasound. In this introductory chapter, we strive to present the necessary fundamental physical principles of ultrasound without excessive detail. In-depth sources on the subject are recommended to interested readers.^1-5 These textbooks uniformly stress that image quality depends on knowledge of the interaction of sound with tissue and the skillful use of the scanner’s controls. Ultrasound examinations are highly interactive; a great deal of flexibility is often required for good images to be obtained. Accurate interpretation depends directly on the differentiation of normal and abnormal anatomy. Unlike with other imaging modalities, interpretation is best made at the time of the study. It is very difficult to render a meaningful interpretation from another sonographer’s static images or video clips.

Basic acoustic principles

Wavelength and frequency

Sound results from mechanical energy propagating through matter as a pressure wave, producing alternating compression and rarefaction bands of molecules within the conducting medium (Fig. 1.1). The distance between each band of compression or rarefaction is the sound’s wavelength (λ), the distance traveled during one cycle. Frequency is the number of times a wavelength is repeated (cycles) per second and is expressed in hertz (Hz). One cycle per second is 1 Hz; 1000 and 1 million cycles per second are 1 kilohertz (kHz) and 1 megahertz (MHz), respectively. The range of human hearing is approximately 20 to 20,000 Hz. Diagnostic ultrasound is characterized by sound waves with a frequency up to 1000 times higher than this range. Sound frequencies in the range of 2 to 15 MHz and higher are commonly used in diagnostic ultrasound examinations. Even higher frequencies (20 to 100 MHz) are used in special ocular, dermatologic, and microimaging applications.

Fig. 1.1 Ultrasound waves and echoes. A, Ultrasound emitted from the transducer is produced in longitudinal waves consisting of areas of compression (C) and rarefaction (R). B, The wavelength, depicted overlying the longitudinal wave, is the distance traveled during one cycle. Frequency is the number of times a wave is repeated (cycles) per second. The wavelength decreases as frequency increases. Switching from a lower frequency to a higher frequency transducer (e.g., from 3.0 to 7.5 MHz) shortens the wavelength and provides better resolution. C, In pulsed ultrasound systems, sound is emitted in pulses of two or three wavelengths rather than continuously as seen in A and B. A portion of the sound is reflected, whereas the remainder is transmitted as it passes through interfaces in tissues.

Frequencies in the millions of cycles per second have short wavelengths (submillimeter) that are essential for high-resolution imaging. The shorter the wavelength (or higher the frequency), the better the resolution. Frequency and wavelength are inversely related if the sound velocity within the medium remains constant. Because sound velocity is independent of frequency and nearly constant (1540 m/sec) in the body’s soft tissues^1,5 (Table 1.1), selecting a higher frequency transducer will result in decreased wavelength of the emitted sound, providing better axial resolution (see Fig. 1.1, A, and Fig. 1.2). The relationship between velocity, frequency, and wavelength can be summarized in the following equation:

$\begin{array}{c}\mathrm{Velocity}\left(\text{m}/\text{sec}\right)\text{=}\mathrm{frequency}\left(\text{cycles}/\text{sec}\right)\text{×}\\ \mathrm{wavelength}\left(\text{m}/\text{cycle}\right)\end{array}$

Fig. 1.2 Axial resolution. Higher frequency transducers produce shorter pulses than lower frequency transducers because the wavelength of the emitted sound is shorter. Pulse length actually determines axial resolution and is directly dependent on the wavelength from the primary ultrasound beam. In this example, the near and far walls of a cyst can be resolved if the echoes returning to the transducer from each wall remain distinct. The echo from the near wall must clear the wall before the echo from the far wall returns to merge with it. The ability to resolve the cyst is dependent on the pulse length and distance between walls. Axial resolution cannot be better than half the pulse length. In this example, a 0.5-mm cyst could theoretically be resolved with a 7.5-MHz but not a 3.0-MHz transducer because of the superior axial resolution of the higher frequency.

TABLE 1.1

Velocity of Sound in Body Tissues

Tissue or Substance	Velocity (m/sec)
Air	331
Fat	1450
Water (50°C)	1540
Average soft tissue	1540
Brain	1541
Liver	1549
Kidney	1561
Blood	1570
Muscle	1585
Lens of eye	1620
Bone	4080

Data from Curry TS III, Dowdey JE, Murry RC Jr. Christensen’s Physics of Diagnostic Radiology. 4th ed. Philadelphia: Lea & Febiger; 1990.

The wavelengths for commonly used ultrasound frequencies can be determined by rearranging this equation (Table 1.2).

TABLE 1.2

Commonly Used Ultrasound Frequencies *

Frequency (MHz)	Wavelength (mm)
2.0	0.77
3.0	0.51
5.0	0.31
7.5	0.21
10.0	0.15

^*Assume velocity = 1.54 mm/µsec (1540 m/sec).

Propagation of sound

Diagnostic ultrasound uses a “pulse echo” principle in which short pulses of sound are transmitted into the body (see Fig. 1.1, C). Propagation of sound occurs in longitudinal pressure waves along the direction of particle movement as shown in Fig. 1.1. The speed of sound (propagation velocity) is affected by the physical properties of tissue, primarily the tissue’s resistance to compression, which depends on tissue density and elasticity (stiffness). Propagation velocity is increased in stiff tissues and decreased in tissues of high density. Fortunately, the propagation velocities in the soft tissues of the body are very similar, and it is therefore assumed that the average velocity of diagnostic ultrasound is 1540 m/sec.

The ultrasound transducer both sends pulses into the tissue (1% of the time) and receives the returning echoes (99% of the time). The assumption of a constant propagation velocity (1540 m/sec) is fundamental to how the ultrasound machine calculates the distance (or depth) of a reflecting surface. Suppose it takes 0.126 msec from the time of pulse until the return of the echo. The depth of the reflective surface would be calculated as follows:

$1540\frac{\text{m}}{\sec }\text{×}0.126\text{msec}\text{×}\frac{1\text{sec}}{\text{1000 msec}}=\text{0.194 or 19.4 cm}$

This value must be divided by 2 to account for the round trip to and from the reflector, so the depth of the reflective surface equals 9.70 cm.

It should be intuitive then that if the sound travels through fatty tissue at 1450 m/sec, the reflector depth will be erroneously calculated as greater (or deeper) than it actually is. This is termed the speed propagation error and is discussed and illustrated further in later sections that focus on artifacts.

Further, when the ultrasound beam encounters gas (331 m/sec) or bone (4080 m/sec), marked velocity differences in these media result in high reflection and improper echo interpretation with characteristic reverberation and shadowing artifacts (see later sections on artifacts) (Fig....

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Contributors
• Preface
• Acknowledgments
• Video contents
•     List of Tables
•     List of Illustrations
• 1.  Fundamentals of diagnostic ultrasound
• 2.  Ultrasound-guided aspiration and biopsy procedures
• 3.  Point-of-care ultrasound
• 4.  Abdominal ultrasound scanning techniques
• 5.  Eye
• 6.  Neck
• 7.  Thorax
• 8.  Echocardiography
• 9.  Liver
• 10.  Spleen
• 11.  Pancreas
• 12.  Gastrointestinal tract
• 13.  Peritoneal fluid, lymph nodes, masses, peritoneal cavity, and great vessel thrombosis
• 14.  Musculoskeletal system
• 15.  Adrenal glands
• 16.  Urinary tract
• 17.  Prostate and testes
• 18.  Ovaries and uterus
• Index