OBJECTIVES

• From studying this chapter, the reader should be able to:

a. Explain the properties of ultrasound waves.

b. Describe the decibel notation for ultrasound intensity and
pressure.
c. Delineate ‫ تحديد‬the ultrasound properties of velocity,
attenuation, and absorption.
d. Depict ‫ تصور‬the consequences of an impedance mismatch at
the boundary between two regions of tissue.
e. Explain ultrasound reflection, refraction and scattering.
 Introduction
• Ultrasound is a mechanical disturbance that moves as a
pressure wave through a medium.
• When the medium is a patient, the wavelike disturbance is
the basis for use of ultrasound as a diagnostic tool.
• Appreciation of the characteristics of ultrasound waves
and their behavior in various media is essential to
understanding the use of diagnostic ultrasound in clinical
medicine.
• WAVE MOTION
• During the propagation of an ultrasound wave, the
molecules of the medium vibrate over very short
distances in a direction parallel to the longitudinal
wave.
Frequency (Hz) Classification

20-20,000 Audible sound Ultrasound

20,000-l,000,000 l,000,000-30,000,000 Diagnostic medical
ultrasound
• Ultrasound Intensity
• As an ultrasound wave passes through a medium, it transports
energy through the medium.
• The rate of energy transport is known as "power.“
• Medical ultrasound is produced in beams that are usually focused
into a small area, and the beam is described in terms of the power
per unit area, defined as the beam's "intensity".

• Ultrasound frequencies of 1 MHz and greater correspond to ultrasound

wavelengths less than 1 mm in human soft tissue.
• Intensity is usually described relative to some reference
intensity
• For example, the intensity of ultrasound waves sent into the body may be
compared with that of the ultrasound reflected back to the surface by
structures in the body
• For many clinical situations the reflected waves at the surface may be as
much as a hundredth or so of the intensity of the transmitted waves.

• where Io is the reference intensity Table 19-2 shows examples of

decibel values for certain intensity ratios.
• Table 19-2 shows examples of decibel values for certain
intensity ratios.
Several rules can be extracted from this table:
•
• Several rules can be extracted from this table:
• Positive decibel values result when a wave has a higher intensity than the
reference wave; negative values denote a wave with lower intensity
• Increasing a wave's intensity by a factor of 10 adds 10 dB to the intensity,
and reducing the intensity by a factor of 10 subtracts 10 dB.
• Doubling the intensity adds 3 dB, and halving subtracts 3 dB.
• Because intensity is power per unit area and
• (Table 19-1), Eq. (19-1) may be used to compare the power or the energy
contained within two ultrasound waves. Thus we could also write

• Ultrasound wave intensity is related to maximum pressure (Pm) in the

medium by the following expression

• where ρ is the density of the medium in grams per cubic centimeter and
c is the speed of sound in the medium. Substituting Eq. (19-2) for I and Io
in Eq. (19-1) yields
• The acoustic impedance Z of a medium is the product of the density ρ
of the medium and the velocity of ultrasound in the medium:
• where ρ is the density of the medium in grams per cubic centimeter and
c is the speed of sound in the medium.
• ULTRASOUND VELOCITY
• The velocity of an ultrasound wave through a medium varies with the physical
properties of the medium.
• In low-density media such as air and other gases, molecules may move over
relatively large distances before they influence neighboring molecules. In these
media, the velocity of an ultrasound wave is relatively low.
• In solids, molecules are constrained in their motion, and the velocity of ultrasound is
relatively high.
• Liquids exhibit ultrasound velocities intermediate between those in gases and solids.
•In ultrasound, the term propagation speed is preferred over the term velocity.
•The velocity of ultrasound in a medium is virtually independent of the
ultrasound frequency.
• The velocity of ultrasound is determined principally by the compressibility
of the medium. A medium with high compressibility yields a slow ultrasound
velocity, and vice versa. Hence, the velocity is relatively low in gases,
intermediate in soft tissues, and greatest in solids such as bone.

• Properties of ultrasound such as reflection, transmission, and

refraction are characteristic of the wave velocity
 • Example 19-1
•Find the percent reduction in intensity for a 1-MHz ultrasound beam traversing 10 cm of material having an attenuation of 1 dB/cm. The
reduction in intensity (dB) = (1 dB/cm) (10 cm)= -10 dB (the minus sign corresponds to a decrease in intensity compared with the
reference intensity, which in this case is the intensity of sound before the attenuating material is encountered).
•

There has been a 90% reduction in intensity.

Determine the intensity reduction if the ultrasound frequency were increased to 2MHz.

Because the attenuation increases approximately linearly with frequency, the at tenuation coefficient at 2 MHz would be 2 dB/cm, resulting
in a -20 dB (99%) intensity reduction.

•
•
 • Example 19-2
•Suppose that a block of tissue consists of 2 cm fat, 3 cm muscle (ultrasound propagated parallel to the fibers),
and 4 cm liver. The total energy loss is
• Total energy loss = (Energy loss in fat) + (Energy loss in muscle)
• + (Energy loss in liver)
• = (0.6 dB/cm) (2 cm)+ (1.2 dB/cm) (3 cm)
• + (09 dB/cm) (4 cm)
• = 1.2 dB + 3.6 dB + 3.6 dB
• = 8.4 dB
•For an ultrasound beam that traverses the block of tissue and, after reflection, returns through the tissue
block, the total attenuation is twice 8.4 dB or 16.8 dB.
•
• REFLECTION
•In most diagnostic applications of ultrasound, use is made of ultrasound waves reflected from interfaces
between different tissues in the patient. The fraction of the impinging energy reflected from an interface depends
on the difference in acoustic impedance of the media on opposite sides of the interface.
• The acoustic impedance Z of a medium is the product of the density ρ of the medium and the velocity of
ultrasound in the medium:

• where Z1 and Z2 are the acoustic impedances of the two media. The fraction of the incident energy that is
transmitted across an interface is described by the transmission coefficient ar, where
• Acoustic impedance may be expressed in units of
rayls where a

rayl = 1 kg.m−2.sec−1
•
• Ultrasound Reflection

• Ultrasound reflection at an interface, where the angle of incidence θi equals the angle of reflection θR .
• Angle of incidence = Angle of reflection
•
• REFRACTION
• As an ultrasound beam crosses an interface obliquely between two media, its direction is changed (i.e., the beam is bent).
If the velocity of ultrasound is higher in the second medium, then the beam enters this medium at a more oblique (less
steep) angle. This behavior of ultrasound transmitted obliquely across an interface is termed refraction. The relationship
between incident and refraction angles is described by Snell's law:

• For example, an ultrasound beam incident obliquely upon an interface between muscle (velocity 1580 rn/sec) and fat
(velocity 1475 rn/sec) will enter the fat at a steeper angle.
 but
• therefore

•For an incidence angle θc equal to the critical angle,

refraction causes the sound to be transmitted along the surface
of the material.
•For incidence angles greater than θc, sound transmission
across the interface is prevented by refraction.
• Ultrasound wave

• Lateral displacement of an ultrasound beam as it

traverses a slab interposed in an otherwise homogeneous medium.
• TABLE 19-5 Variation of Ultrasound Attenuation Coefficient a with Frequency in Megahertz,
Where α1 Is the Attenuation Coefficient at 1 MHz and υ is the frequency of ultrasound wave
• If gas bubbles are present in a material through which a
sound wave is passing, the compressions and rarefactions
cause the bubble to shrink and expand in resonance with
the sound wave.
• The oscillation of such bubbles is referred to as stable
cavitation. Stable cavitation is not a major mechanism for
absorption at ultrasound intensities used diagnostically,
but it can be a significant source of scatter.
• SUMMARY
• Properties of ultrasound waves include:
• Compression and rarefaction

• Requires a transmissive medium

• Constructive and destructive interference

• The relative intensity and pressure of ultrasound waves are described in units of decibels.
• Ultrasound may be reflected or refracted at a boundary between two media. These
properties are determined by the angle of incidence of the ultrasound and the
impedance mismatch at the boundary.
• Energy may be removed from an ultrasound beam by various processes, including
relaxation energy loss.
• The presence of gas bubbles in a medium may give rise to stable and dynamic
cavitation.
‫نهاية المحاضرة•‬
• ATTENUATION OF ULTRASOUND
• As an ultrasound beam penetrates a medium, energy is removed from the beam by
absorption, scattering, and reflection. These processes are summarized in Figure 19-2. As
with x rays, the term attenuation refers to any mechanism that removes energy from the
ultrasound beam. Ultrasound is "absorbed" by the medium if part of the beam’s

• FIGURE 19-2
•Constructive and destructive interference effects
characterize the echoes from nonspecular reflections.
Because the sound is reflected in all directions, there are
many opportunities for waves to travel different pathways.
The wave fronts that return to the transducer may
constructively or destructively interfere at random.
The random interference pattern is known as "speckle."
 •Contributions to attenuation of an ultrasound beam may include:
• Absorption
• Reflection
• Scattering
• Refraction
• Diffraction
• lnterferrence
• Divergence

• FIGURE 19-3
• Summary of interactions of ultrasound at boundaries of materials.
• The behavior of a sound beam when it encounters an obstacle
depends upon the size of the obstacle compared with the
wavelength of the sound. If the obstacle's size is large compared
with the wavelength of sound (and if the obstacle is relatively
smooth), then the beam retains its integrity as it changes
direction. Part of the sound beam may be reflected and the
remainder transmitted through the obstacle as a beam of lower
intensity.
• If the size of the obstacle is comparable to or smaller than the
wavelength of the ultrasound, the obstacle will scatter energy in
various directions. Some of the ultrasound energy may return to
its original source after "nonspecular" scatter, but probably not
until many scatter events have occurred.
• ABSORPTION
• ultrasound is propagated by displacement of molecules of a
medium into regions of compression and rarefaction.
• This displacement requires energy that is provided to the medium
by the source of ultrasound.
• Therefore, the energy of the ultrasound beam is gradually reduced
as it passes through the medium.
• This reduction is termed relaxation energy loss. The rate at
which the beam energy decreases is a reflection of the
attenuation properties of the medium.
• The effect of frequency on the attenuation of ultrasound in
different media is described in Table 19-5.
 •In the audible range, sound power and intensity are referred to as "loudness."

•Pulsed ultrasound is used for most medical diagnostic applications.

•Ultrasound pulses vary in intensity and time and are characterized by four variables:
1- spatial peak (SP),
2- spatial average (SA),
3- temporal peak (TP), and
4- temporal average (TA).
•
•Temporal average ultrasound intensities used in medical diagnosis are in the
mW/cm2 range.
•