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PERIPHERAL VASCULAR ULTRASOUND
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Angle of insonation
Figure 6.7 Graph showing the relationship between
maximum velocity, although this can be misleading as the color image displays mean frequency, which is related to the blood motion in the direction of the beam, rather than the actual blood velocity. What appears to be the maximum velocity on the image may instead be the site at which the angle between the Doppler beam and the direction of the blood flow is smallest. It is important to consider this when estimating the site of maximum velocity and the direction of flow from the color image. The blood velocity may need to be measured at a few points through and beyond a stenosis to ensure the highest velocity has been obtained.
Errors relating to the out-of-imaging plane angle of insonation
It is important to remember that the interception of the ultrasound beam with the blood flow occurs in a three-dimensional space and not just in the two-dimensional plane shown on the image. An underestimate of the true velocity will be obtained if the out-of-imaging plane angle of insonation is not close to 0°. Therefore, the transducer should be aligned with a reasonable length of the vessel, as seen on the image, to ensure a minimal error.
Creation of a range of insonation angles by the Doppler ultrasound beam aperture
The large aperture used by linear array transducers not only results in ISB but also leads to another problem. For velocity to be calculated from the Doppler shift frequency, the cos term is required, but clearly only a single value for the angle can be used. Substituting the two extreme angles shown in Figure 6.4 (1 and 3) into the Doppler equa- tion would obviously give different values for the velocity. A decision has to be made as to which angle is most suitable for use in converting the detected Doppler frequency into velocity. Typically, ultrasound scanners use the angle between the cen- ter of the active elements and the direction of flow (i.e., angle 2). This would be an appropriate angle to select for estimation of the mean velocity, but it leads to an overestimation of the calculated maxi- mum velocity. In fact, in order to obtain a correct value for the peak velocity from the frequency spectrum, the smallest angle of insonation present
the percentage error in the velocity measurements as the angle of insonation increases, for a 5° error in the placement of the angle correction cursor. (After
Evans D H & McDicken W N 2000 Doppler ultrasound: physics, instrumentation, and signal processing. © John Wiley & Sons Limited, with permission.)
to an error in the velocity measurement. The velocity calculation depends on the cos term, so the error created will be greater for larger angles of insonation. Figure 6.7 shows the relationship between the percentage error in the velocity mea- surement as the angle of insonation increases where there is a 5° error in the placement of the angle correction cursor. For example, Figure 6.7 shows that this 5° error in cursor placement causes an error in velocity measurement of 23% when the angle of insonation is 65°. In order to minimize this error, angles of insonation of greater than 60% should not be used. However, estimating the angle of insonation is not always straightforward, espe- cially in the presence of disease. Some of the limi- tations are listed below.
Errors relating to the direction of flow
relative to the vessel walls
The direction of the blood flow may not be paral- lel to the vessel wall, especially in the presence of a stenosis, vortices or helical flow. Therefore, in these cases lining the angle correction cursor paral- lel to the walls may lead to large errors. If there is a clear image of the flow channel through a nar- rowing it may be possible to line up the angle cur- sor with the flow channel. However, the maximum velocity may be just beyond the stenosis, and the direction of flow may be less obvious at that point. The color image may be used to identify the site of
Percentage error in velocity measurement