For perpendicular beam of
incidence, if f
is 90, then cos f
is 0, therefore there is no Doppler shift. If the angle is 0, then cos
f
is 1, and there is maximum Doppler shift. In practice however, this is
mostly not possible. Therefore to get an optimum Doppler shift, the
transducer beam is usually oriented to make a 30 to 60 degree angle
with the arterial lumen. This gives an optimal color fill in. The
angle is also important for measurements of velocities; it is known as
angle corrected velocities. After obtaining the spectral tracing from
a vessel the angle is corrected so that it is parallel to the vessel
lumen. This gives us the true velocity. But in practice, when we try
to attempt to do this (especially so in carotid tracing, which is
parallel to the skin surface), the angle goes beyond 60. This will
give a spurious increase in velocities.
For
example: If angle is changed from 50 to 55, then the velocity
difference moves from 45 cm/s to 50 cm/s. (5 cm/s)
If the
angle is changed from 70 degrees to 75 degrees, the velocity
difference moves from 84 cm/s to 112 cm/s (a difference of 28 cm/s).
(See Fig 1)
Since the grading of stenosis in carotid arteries is
done according to velocities, taking the angle corrected velocities
beyond 60 will give a false positive high grade stenosis, since the
velocities measured will be much higher than actual. Thus the angle
should be kept between 3060.
2. What happens if I cannot align
the angle to absolutely parallel to the vessel without crossing 60
degrees – should I take a reading as it is?
As
described earlier, it is vital to keep the angle between 3060
degrees, max at 60 and not beyond. Another option is to use a
wedgeshaped water bag kit, which can be attached to the transducer;
this has an advantage as it gives same angle to the scanner image, so
even if the vessel is parallel to the skin surface, it appears at the
same angle (usually 30 degrees). See Fig 2, below.
