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a Chapter 2 Physical Principles of Doppler Ultrasonography 13
shadowº affects optimal imaging of structures lying
posterior to the bone. At most tissue interfaces, an
incident beam is echoed in different directions, a process
known as diffuse reflection. In relation to Doppler
ultrasonic reflections, however, an entirely different
phenomenon occurs, known as scattering. This
phenomenon is discussed below.
When an ultrasound beam travels with an oblique
angle of incidence across an interface, the beam path
deviates and the angle of transmission differs from
the angle of incidence (Fig. 2.10). The phenomenon is
similar to refraction of light. Refractive deviation in
the beam path may compromise image quality. For
pulsed Doppler duplex applications, where two-dimensional
gray-scale imaging is used for placing the
Doppler sample volume in deep vascular locations,
refraction may lead to error. However, the propagation
speed of sound does not vary appreciably in
most soft tissues; therefore only minimal refraction
occurs at most tissue interfaces. For example, an ultrasound
beam passing through a muscle-blood interface
at an incident angle of 308 undergoes only a
0847' refractive deviation [2].
Progressive decline in the pressure amplitude and
intensity of a propagating ultrasonic wave is known
as attenuation. Attenuation is caused by many factors,
including absorption, scattering, reflection, and wavefront
divergence. Absorption is the phenomenon
whereby sound energy is converted to heat and is
therefore responsible for thermal bioeffects. Attenuation
is also affected by the transmitting frequency of
a transducer: the higher the frequency, the greater
the attenuation. It limits the use of high-frequency
transducers for Doppler interrogation of deep vascular
structures. These considerations influence the
choice of a transducer for a specific use. Thus for
Doppler examinations of fetal circulation, a 5-MHz
transducer may by less efficient for obtaining adequate
signals than a 2-MHz transducer.
Doppler Effect
The Doppler effect is the phenomenon of observed
changes in the frequency of energy wave transmission
when relative motion occurs between the source of
wave transmission and the observer. The change in
the frequency is known as the Doppler frequency
shift, or simply the Doppler shift:
f d ˆ f t
f r
where f d is the Doppler shift frequency, f t is the
transmitted frequency, and f r is the received frequency.
When the source and the observer move closer,
the wavelength decreases and the frequency increases.
Conversely, when the source and the observer
move apart, the wavelength increases and the frequency
decreases. The Doppler effect is observed irrespective
of whether the source or the observer
moves (Fig. 2.11). The principle is applicable to all
forms of wave propagation. The utility of the Doppler
effect originates from the fact that the shift in frequency
is proportional to the speed of movement between
the source and the receiver and therefore can
be used to assess this speed. The Doppler effect is observed
irrespective of whether the source or the observer
moves.
Incident Beam
Acoustic
Interface
i
r
Reflected Beam
Medium 1
Doppler Ultrasound
For sound transmission, the Doppler shift sound is of
a higher frequency when the source and the receiver
move closer and of a lower frequency when they re-
Meduim 2
t
Transmitted Beam
Fig. 2.10. Reflection, refraction, and transmission of sound.
The beam, in this example, is encountering the acoustic interface
obliquely. A portion of the incident sound is reflected
back, and the rest is transmitted. The angle of reflection
(U r ) equals the angle of incidence (U i ). The transmitted
sound is refracted. The angle of refraction (U t ) depends
on the angle of incidence and the propagation
speeds of sound in medium 1 and medium 2
Fig. 2.11. Graphic depiction of the Doppler shift when a
source of sound transmission moves away or toward a stationary
observer