Diagnostic ultrasound ( PDFDrive )

CHAPTER 1 Physics of Ultrasound 23
F T
F R
FIG. 1.32 Backscattered Information. The backscattered ultrasound
signal contains amplitude, phase, and frequency information. Signals B
and C differ in amplitude but have the same frequency. Amplitude
differences are used to generate B-mode images. Signals A and B differ
in frequency but have similar amplitudes. Such frequency differences
are the basis of Doppler ultrasound.
∆F = (F R − F T ) = 2 • F T • v
c
v
A
Stationary target: (F R − F T ) = 0
A
B
Target motion toward transducer: (F R − F T ) > 0
F T F R
c
C
Target motion away from transducer: (F R − F T ) < 0
θ
FIG. 1.33 Doppler Effect. (A) Stationary target. If the relecting
interface is stationary, the backscattered ultrasound has the same
frequency or wavelength as the transmitted sound, and there is no
difference in the transmitted frequency (F T ) and the relected frequency
(F R ). (B) and (C) Moving targets. If the relecting interface is moving
with respect to the sound beam emitted from the transducer, there is
a change in the frequency of the sound scattered by the moving object.
When the interface moves toward the transducer (B), the difference
in relected and transmitted frequencies is greater than zero. When the
target is moving away from the transducer (C), this difference is less
than zero. The Doppler equation is used to relate this change in frequency
to the velocity of the moving object.
Doppler Signal Processing and Display
Several options exist for the processing of ΔF, the Doppler frequency
shit, to provide useful information regarding the direction
and velocity of blood. Doppler frequency shits encountered
clinically are in the audible range. his audible signal may be
analyzed by ear and, with training, the operator can identify
many low characteristics. More oten, the Doppler shit data are
displayed in graphic form as a time-varying plot of the frequency
B
∆F = (F R − F T ) = 2 • F T • v • cos θ
FIG. 1.34 Doppler Equations. The Doppler equation describes the
relationship of the Doppler frequency shift to target velocity. (A) In its
simplest form, it is assumed that the direction of the ultrasound beam
is parallel to the direction of movement of the target. This situation is
unusual in clinical practice. More often, the ultrasound impinges on the
vessel at angle θ. (B) In this case the Doppler frequency shift detected
is reduced in proportion to the cosine of θ. ΔF, Frequency shift; F R ,
relected frequency; F T , transmitted frequency; v, velocity.
spectrum of the returning signal. A fast Fourier transformation
is used to perform the frequency analysis. he resulting Doppler
frequency spectrum displays the following (Fig. 1.36):
• Variation with time of the Doppler frequencies present in the
volume sampled
v