Diagnostic ultrasound ( PDFDrive )

CHAPTER 1 Physics of Ultrasound 25
A
FIG. 1.37 Continuous Wave and Pulsed Wave Doppler. (A) Continuous wave (CW) Doppler uses separate transmit and receive crystals
that continuously transmit and receive ultrasound. Although able to detect the presence and direction of low, CW devices are unable to distinguish
signals arising from vessels at different depths (green-shaded area). (B) Using the principle of ultrasound ranging (see Fig. 1.4), pulsed wave
Doppler permits the sampling of low data from selected depths by processing only the signals that return to the transducer after precisely timed
intervals. The operator is able to control the position of the sample volume and, in duplex systems, to view the location from which the Doppler
data are obtained.
B
Limitations of Color Doppler Flow Imaging
Angle dependence
Aliasing
Inability to display entire Doppler spectrum in the image
Artifacts caused by noise
Advantages of Power Doppler
No aliasing
Much less angle dependence
Noise: a homogeneous background color
Increased sensitivity for low detection
display of low throughout the image ield allows the position
and orientation of the vessel of interest to be observed at all
times. he display of spatial information with respect to velocity
is ideal for display of small, localized areas of turbulence within
a vessel, which provide clues to stenosis or irregularity of the
vessel wall caused by atheroma, trauma, or other disease. Flow
within the vessel is observed at all points, and stenotic jets and
focal areas of turbulence are displayed that might be overlooked
with duplex instrumentation. he contrast of low within the
vessel lumen permits visualization of small vessels that are not
visible when using conventional imagers and enhances the visibility
of wall irregularity. CDFI aids in determination of the
direction of low and measurement of the Doppler angle.
Power Doppler
An alternative to the display of frequency information with color
Doppler imaging is to use a color map that displays the integrated
power of the Doppler signal instead of its mean frequency shit 18
(see Fig. 1.38B). Because frequency shit data are not displayed,
there is no aliasing. he image does not provide information
related to low direction or velocity, and power Doppler imaging
is much less angle dependent than frequency-based color Doppler
display. In contrast to color Doppler, where noise may appear in
the image as any color, power Doppler permits noise to be assigned
to a homogeneous background color that does not greatly interfere
with the image. his results in a signiicant increase in the usable
dynamic range of the scanner, permitting higher efective gain
settings and increased sensitivity for low detection (Fig. 1.39).
Interpretation of the Doppler Spectrum
Doppler data components that must be evaluated both in spectral
display and in color Doppler imaging include the Doppler shit
frequency and amplitude, the Doppler angle, the spatial distribution
of frequencies across the vessel, and the temporal variation
of the signal. Because the Doppler signal itself has no anatomic
signiicance, the examiner must interpret the Doppler signal
and then determine its relevance in the context of the image.
he detection of a Doppler frequency shit indicates movement
of the target, which in most applications is related to the presence
of low. he sign of the frequency shit (positive or negative)
indicates the direction of low relative to the transducer. Vessel
stenosis is typically associated with large Doppler frequency
shits in both systole and diastole at the site of greatest narrowing,
with turbulent low in poststenotic regions. In peripheral vessels,
analysis of the Doppler changes allows accurate prediction of the
degree of vessel narrowing. Information related to the resistance
to low in the distal vascular tree can be obtained by analysis of
changes of blood velocity with time, as shown in the Doppler
spectral display. Doppler imaging can provide information
about blood low in both large and small vessels. Small vessel
impedance is relected in the Doppler spectral waveform of
aferent vessels.