As it turns out, in the studies that we've done of the caroitd arteries,
the Doppler frequency changes little with examination angle, much less
than expected from the Doppler equation. Of course, a similar test of a
string phantom, the frequency changes with angle as expected from the
Doppler equation. So, you are right, since in the carotid arteries, the
frequency doesn't change much with examination angle, then the differences
are nearly a factor of the inverse of the cosine of the examination angle
when the Doppler equation is applied automatically by the instrument.
We first began to recognize this problem when David Philips, Gene
Strandness, Jean Primozich and I were asked by Ray and Wendy Powis to
verify the velocity measurements on the Quantum Quad 1 color Doppler
scanner vs the ATL Mark 5 scanner. The Quad 1, with proper Doppler
equation angle "correction", gave consistently lower "velocities" than the
Mark 5. This was a surprise to us. We found that because of the
design of the Quad 1 non-steerable linear array scanhead with the 20
degree wedge to provide a non-perpendicular Doppler angle combined with
the tilt of the carotid artery caused the Doppler examination angle to
range from 45 degrees to 55 degrees. The mechanical scanhead of the
Mark 5 allowed the consistent use of a 60 degree angle. We'd already
verified that two examiners could reproduce the Doppler angle within a
couple of degrees. Contrary to the Doppler equation that we'd learned in
physics, the Doppler frequency shift (after properly accounting for
ultraosund frequency, 5 MHz for the Mark 5 and 4.5 MHz for the Quad 1) was
the same for the Mark 5 and the Quad 1. Thus, when divided by the cosine
of the examination angle, the Quad 1 "velocity" measurements are lower
than the Mark 5 measurements. The results have been the same with all
subsequent scanhead designs by all manufacturers.
The source of the problem is the complexity of laminar blood flow.
I think that Ray Powis advocated aligning the Doppler cursor with the
color streak on the color flow image. To test this, simply look at the
color streak with the color lines tilted cephalad, and look again at the
same time in the cardiac cycle with the color lines tilted caudad. See if
the color streak is in the same orientation. To be sure that you are at
teh same time in the cardiac cycle, include an ECG tracing in the image.
I'd love to be able to do what cardiologists do: use Doppler velocity
measurements to compute volume flow rate and Bernoulli pressure drop.
They are successful when they use a Doppler examination angle of ZERO
degrees. In vascular examinations, we can't. The "velocities" that we
measure at 45 or 60 degrees are not "correct" velocities, but they are
"precise", that is the measurements are reporducible, if the same
examination methods are used consistently. An these measurements are
useful when compared against empiricaly created diagnostic criteria, when
gathered by the same methods.
On Mon, 27 Mar 2006, Giselle Barnes BS,RVT wrote:
> But I think what Kirk is saying is that velocities taken at less than 60
> degrees will be less than velocities taken at 60 degrees, therefore it is like
> comparing apples and oranges. You can't use velocity criteria written for a 60
> degree angle for velocities obtained at a 30 degree angle. Also, ya can't get an
> accurate ratio if you keep changing angles..... Kirk, was the difference in
> your velocities taken at the different angles a factor of the cosine of the
> Giselle Barnes BS RVT
> In a message dated 3/27/2006 9:15:16 AM Pacific Standard Time,
> [log in to unmask] writes:
>> Personal preferences or procedures aside, I believe there was an article
>> published several years ago which studied this and they found that as long as
>> the angle was 60 degrees or less, there was no clinically significant
>> difference in velocity between using the angle of the flow jet or parallel to the
>> vessel wall. That the velocities may vary somewhat, but that it did not change
>> the category of stenosis in any of the cases they studied.
>> I am sorry that I can’t remember the reference, but am sure you could find
>> it if you did a reference search or googled it.
> To unsubscribe or search other topics on UVM Flownet link to:
To unsubscribe or search other topics on UVM Flownet link to: