Chapter 8: Artifacts
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amplitude of the true color signals above the color threshold. Of
course,if the color gain is set too low,color dropout will occur,giving
the artificial appearance of no or limited flow.
Fig. 51
Color bleeding because of color tail
6.7 Wall Filter Saturation
In Chapter 7, we reviewed the function of wall filters. We also
discussed the fact that when the “clutter” signals are not adequately
attenuated, the signal dynamic range becomes too large causing
circuit saturation. The following example demonstrates saturation
of the wall filters when using CW Doppler.
Fig. 52
Saturation of Doppler wall filters
In this case,the saturation occurs from the strong specular reflection
from a mechanical valve. Notice that as the valve opens and closes,
the Doppler insonification angle changes. When the incident angle
is close to 0 degrees, (the beam is close to perpendicular to the valve
leaflets),the specular reflection returns to the transducer and drives
the Doppler circuit to saturation. During this period of time, the
electronics are ringing between the voltage supply rails resulting in
the bright white spikes from top to bottom of the spectrum.
6.8 Color and Power Doppler Flash Artifact
Flash artifact in color Doppler and color power Doppler appears as
a short term flash of color within the color box related to relative
motion between the patient and the transducer, and not actual flow.
Flash is a relatively obvious artifact when viewing real time or cine
loop (video) imaging. With still images, if the capture happened
contemporaneously with the flash artifact occurring, the image can
be somewhat deceiving,as seen in the color power Doppler image of
a hematoma
(Figure 53)
captured during flash artifact. Ultrasound
systems have implemented flash reduction algorithms which make
flash artifact less common than in years past. However,flash artifact
still occurs, especially when color wall filters and color scales are set
low, and when power Doppler is being used.
Fig. 53
Flash artifact with color power Doppler
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6.9 Doppler Spectral Dropout
Spectral dropout can occur for many reasons including:
• poor Doppler angle (close to 90 degrees)
• high wall filter setting relative to low flow velocities
• inadequate sensitivity
• transmit frequency too high (usually when depth set to
deeper than 3 cm)
• transmit power too low
• weak signal from insignificant number of red blood cells
(low flow)
• inappropriate settings such as low receiver gain or improper
gray scale setting
• refraction artifact
• shadowing artifact (sample volume in acoustic shadow from
superior attenuator)
With the exception of refraction artifact, each of these causes for
Doppler spectral dropout has been discussed in prior sections.
Refraction artifact as relates to imaging was discussed extensively
in Chapter 2 and again within this chapter. With respect to spectral
Doppler,refraction artifact can significantly weakenDoppler signals,
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