Diagnostic ultrasound ( PDFDrive )

CHAPTER 1 Physics of Ultrasound 21
A
B
FIG. 1.30 Multipath Artifact. (A) Mirror image of the uterus is created by relection of sound from an interface produced by gas in the rectum.
(B) Echoes relected from the wall of an ovarian cyst create complex echo paths that delay return of echoes to the transducer. In both examples,
the longer path of the relected sound results in the display of echoes at a greater depth than they should normally appear. In (A) this results in
an artifactual image of the uterus appearing in the location of the rectum. In (B) the effect is more subtle and more likely to cause misdiagnosis
because the artifact suggests a mural nodule in what is actually a simple ovarian cyst.
the beam will cause loss of clinically important information from
deep, low-amplitude relectors and small targets. Ultrasound
artifacts may alter the size, shape, and position of structures.
For example, a multipath artifact is created when the path of
the returning echo is not the one expected, resulting in display
of the echo at an improper location in the image (Fig. 1.30).
Shadowing and Enhancement
Although most artifacts degrade the ultrasound image and impede
interpretation, two artifacts of clinical value are shadowing and
enhancement. Again, shadowing results when an object (e.g.,
calculus) attenuates sound more rapidly than surrounding tissues.
Enhancement occurs when an object (e.g., cyst) attenuates less
than surrounding tissues. Failure of TGC applied to normal tissue
to compensate properly for the attenuation of more highly
attenuating (shadowing) or poorly attenuating (enhancing)
structures produces the artifact (Fig. 1.31). Because attenuation
increases with frequency, the efects of shadowing and enhancement
are greater at higher than at lower frequencies. he conspicuity
of shadowing and enhancement is reduced by excessive beam
width, improper focal zone placement, and use of spatial
compounding.
DOPPLER SONOGRAPHY
Conventional B-mode ultrasound imaging uses pulse-echo
transmission, detection, and display techniques. Brief pulses of
ultrasound energy emitted by the transducer are relected from
acoustic interfaces within the body. Precise timing allows
determination of the depth from which the echo originates. When
pulsed wave ultrasound is relected from an interface, the
backscattered (relected) signal contains amplitude, phase, and
frequency information (Fig. 1.32). his information permits
inference of the position, nature, and motion of the interface
relecting the pulse. B-mode ultrasound imaging uses only the
amplitude information in the backscattered signal to generate
the image, with diferences in the strength of relectors displayed
in the image in varying shades of gray. Rapidly moving targets,
such as red cells in the bloodstream, produce echoes of low
amplitude that are not usually displayed, resulting in a relatively
anechoic pattern within the lumens of large vessels.
Although gray-scale display relies on the amplitude of the
backscattered ultrasound signal, additional information is present
in the returning echoes that can be used to evaluate the motion
of moving targets. 16 When high-frequency sound impinges on
a stationary interface, the relected ultrasound has essentially
the same frequency or wavelength as the transmitted sound. If
the relecting interface is moving with respect to the sound beam
emitted from the transducer, however, there is a change in the
frequency of the sound scattered by the moving object (Fig.
1.33). his change in frequency is directly proportional to the
velocity of the relecting interface relative to the transducer and
is a result of the Doppler efect. he relationship of the returning
ultrasound frequency to the velocity of the relector is described
by the Doppler equation, as follows:
∆F = ( F − F ) = 2⋅F ⋅ v c
R T T
he Doppler frequency shit is ΔF; F R is the frequency of sound
relected from the moving target; F T is the frequency of sound
emitted from the transducer; v is the velocity of the target toward
the transducer; and c is the velocity of sound in the medium.
he Doppler frequency shit (ΔF) applies only if the target is
moving directly toward or away from the transducer (Fig. 1.34A).
In most clinical settings the direction of the ultrasound beam
is seldom directly toward or away from the direction of low,
and the ultrasound beam usually approaches the moving target
at an angle designated as the Doppler angle (Fig. 1.34B). In this
case, ΔF is reduced in proportion to the cosine of this angle, as
follows:
∆F = ( F − F ) = 2⋅F ⋅ v ⋅cosθ
c
R T T
where θ is the angle between the axis of low and the incident
ultrasound beam. If the Doppler angle can be measured, estimation
of low velocity is possible. Accurate estimation of target
velocity requires precise measurement of both the Doppler
frequency shit and the angle of insonation to the direction of