Diagnostic ultrasound ( PDFDrive )

22 PART I Physics
–0 dB
Uncorrected
–0 dB
Gain compensated
–10 dB
–25 dB
+10 dB
+10–25 = –15 db
–20 dB
–30 dB
+20 dB
+10–35 = –15 db
A
–30 dB
–40 dB
B
+30 dB
+30–5 = –15 dB
–0 dB
Gain compensated
–10 + 10 dB +10–3 = +7 dB
C
–20 + 20 dB +20–13 = +7 dB
FIG. 1.31 Shadowing and Enhancement. (A) Uncorrected image of a
shadowing breast mass shows that the mass attenuates 25 dB, 15 dB more
than the surrounding normal tissue, which attenuates only 10 dB. (B) Application
of appropriate time gain compensation (TGC) results in proper display of the
normal breast tissue. However, because of the increased attenuation of the
mass, a shadow results. (C) Similarly, the cyst attenuates 7 dB less than
the normal tissue, and TGC correction for normal tissue results in overampliication
of the signals deep to the cyst, producing enhancement of these tissues.
target movement. As the Doppler angle (θ) approaches 90 degrees,
the cosine of θ approaches 0. At an angle of 90 degrees, there
is no relative movement of the target toward or away from the
transducer, and no Doppler frequency shit is detected (Fig.
1.35). Because the cosine of the Doppler angle changes rapidly for
angles more than 60 degrees, accurate angle correction requires
that Doppler measurements be made at angles of less than 60
degrees. Above 60 degrees, relatively small changes in the Doppler
angle are associated with large changes in cosθ, and therefore
a small error in estimation of the Doppler angle may result in
a large error in the estimation of velocity. hese considerations
are important in using both duplex and color Doppler instruments.
Optimal imaging of the vessel wall is obtained when
the axis of the transducer is perpendicular to the wall, whereas
maximal Doppler frequency diferences are obtained when
the transducer axis and the direction of low are at a relatively
small angle.
In peripheral vascular applications, it is highly desirable that
measured Doppler frequencies be corrected for the Doppler angle
to provide velocity measurement. his allows comparison of data
from systems using diferent Doppler frequencies and eliminates
error in interpretation of frequency data obtained at diferent
Doppler angles. For abdominal applications, angle-corrected
velocity measurements are encouraged, although qualitative
assessments of low are oten made using only the Doppler
frequency shit data.