Diagnostic ultrasound ( PDFDrive )

CHAPTER 1 Physics of Ultrasound 5
A
B
FIG. 1.5 Specular and Diffuse Relectors. (A) Specular relector. The diaphragm is a large and relatively smooth surface that relects sound
like a mirror relects light. Thus sound striking the diaphragm at nearly a 90-degree angle is relected directly back to the transducer, resulting in
a strong echo. Sound striking the diaphragm obliquely is relected away from the transducer, and an echo is not displayed (yellow arrow).
(B) Diffuse relector. In contrast to the diaphragm, the liver parenchyma consists of acoustic interfaces that are small compared to the wavelength
of sound used for imaging. These interfaces scatter sound in all directions, and only a portion of the energy returns to the transducer to produce
the image.
propagation velocities of sound in the media forming the interface
(Fig. 1.7). Refraction is important because it is one cause of
misregistration of a structure in an ultrasound image (Fig. 1.8).
When an ultrasound scanner detects an echo, it assumes that
the source of the echo is along a ixed line of sight from the
transducer. If the sound has been refracted, the echo detected
may be coming from a diferent depth or location than the image
shown in the display. If this is suspected, increasing the scan
angle so that it is perpendicular to the interface minimizes the
artifact.
FIG. 1.6 Ultrasound Speckle. Close inspection of an ultrasound
image of the breast containing a small cyst reveals it to be composed
of numerous areas of varying intensity (speckle). Speckle results from
the constructive (red) and destructive (green) interaction of the acoustic
ields (yellow rings) generated by the scattering of ultrasound from small
tissue relectors. This interference pattern gives ultrasound images their
characteristic grainy appearance and may reduce contrast. Ultrasound
speckle is the basis of the texture displayed in ultrasound images of
solid tissues.
Refraction
When sound passes from a tissue with one acoustic propagation
velocity to a tissue with a higher or lower sound velocity, there
is a change in the direction of the sound wave. his change in
direction of propagation is called refraction and is governed by
Snell law:
sinθ
sinθ
= c c
1 2 1 2
where θ 1 is the angle of incidence of the sound approaching
the interface, θ 2 is the angle of refraction, and c 1 and c 2 are the
Attenuation
As the acoustic energy moves through a uniform medium, work
is performed and energy is ultimately transferred to the transmitting
medium as heat. he capacity to perform work is determined
by the quantity of acoustic energy produced. Acoustic power,
expressed in watts (W) or milliwatts (mW), describes the amount
of acoustic energy produced in a unit of time. Although measurement
of power provides an indication of the energy as it relates
to time, it does not take into account the spatial distribution of
the energy. Intensity (I) is used to describe the spatial distribution
of power and is calculated by dividing the power by the area
over which the power is distributed, as follows:
I( W/cm
2 ) = Power( W) Area( cm
2 )
he attenuation of sound energy as it passes through tissue
is of great clinical importance because it inluences the depth in
tissue from which useful information can be obtained. his in
turn afects transducer selection and a number of operatorcontrolled
instrument settings, including time (or depth) gain
compensation, power output attenuation, and system gain levels.
Attenuation is measured in relative rather than absolute units.
he decibel (dB) notation is generally used to compare diferent
levels of ultrasound power or intensity. his value is 10 times
the log 10 of the ratio of the power or intensity values being