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UVMFLOWNET  January 2009

UVMFLOWNET January 2009

Subject:

Re: angles

From:

LORI LEVY <[log in to unmask]>

Reply-To:

UVM Flownet <[log in to unmask]>

Date:

Tue, 27 Jan 2009 11:13:21 -0500

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (153 lines)

I have a student who was taught to angle correct to the posterior wall,
if the walls are not parallel, as in a proximal ICA.  I always place the
angle beween the walls, pretty much with the flow, not either wall but
with both.  This only makes a difference occasionally but we got caught
on QA for it.  Comments???

-----Original Message-----
From: UVM Flownet [mailto:[log in to unmask]] On Behalf Of Kristy
Peeler
Sent: Tuesday, January 27, 2009 10:54 AM
To: [log in to unmask]
Subject: Re: angles

Debbie,
Thank you for your response and the well written explanation of the
history of angle correction.  I am going to present this information to
the university vascular instructor along with the ICAVL standards
recommending 60 degrees or less. When I explain this to my students who
are the better physics minded, they get it.
And thanks to everyone who posted this blog.
 
Kristy Peeler LPN, RVT
The Heart Group, PC
Evansville, IN

________________________________

From: UVM Flownet on behalf of Debbie Anderson
Sent: Tue 1/27/2009 9:42 AM
To: [log in to unmask]
Subject: Re: angles



Kristy, here is a note from Frank Miele...    The 60 degree angle is a
ubiquitous dilemma.  A few pieces of information might help clarify. 

It is important to realize that 60 degrees is a very suboptimal angle
for Doppler in terms of errors and artifacts.  According to Doppler
theory, it should not matter what angle is used since angle correction
can be employed.  This theoretical assumption is false since it ignores
the fact that there is always error in specifying the Doppler angle (no
one can place the flow indicator exactly in the true direction of flow
all of the time) and that there are many artifacts that are angle
dependent.  Since the Doppler Effect is affected by the cosine of the
Doppler angle, and since the cosine is very non-linear with angle, the
error associated with the Doppler angle is also very non-linear.  The
worst Doppler angle is clearly 90 degrees at which there is infinite
error and no amount of angle correction can "fix" the problem.  The best
angle is 0 or 180 degrees (the cosine changes very slowly around 0 and
180 degrees - if you know calculus it is determined by the first
derivative (the slope)).  One thing is for sure, angles greater than 60
degrees yield horrendous error and should be avoided as much as
possible.  However, 60 degrees is not very good either.  50 degrees is
certainly better than 60 degrees, and of course 40 degrees is better
still.  Additionally, artifacts such as spectral spread are exacerbated
with larger angles, again leading to the conclusion that angles smaller
than 60 degrees are better.  (Note that for carotid studies, at 60
degrees, spectral broadening can sometimes result in an overestimation
of the peak velocity by as much as 50%.)

So how then did 60 degrees become a standard?   The answer is related to
what was practical.  In the early days before electronic steering,
"steering" was manual.  Since vessels tend to be parallel to the skin
surface, the worst possible situation for Doppler existed unless the
transducer was manually angled (without manually angling the probe, an
angle of 90 degrees would result, yielding no Doppler information).  As
it turns out, the transducer can be manually angled about 30 degrees
while maintaining good skin contact and still be before the point of
actually having to push so hard as to affect flow in the vessel being
assessed.  If the steering angle is 30 degrees, the resulting Doppler
insonification angle (Doppler angle) is 60 degrees.  Furthermore, in the
early days, spectral analysis was not performed (this was well before
the days of applying Fast Fourier Transforms with slick DSP chips) and
velocity information was not presented.  Instead, the detected frequency
shift was presented.  As it turns out, the cosine of 60 degrees is 0.5,
which when used in the denominator of the calculation to correct for the
angular effect, is simply a factor of 2, a factor that anyone can
mathematically apply in their head.  

The standard then became 5 MHz transducers at 60 degree Doppler angles.
The correlations were performed using this standard.  Now move ahead to
the point at which electronic steering was created. The question was
what angle should now be used.  Initially, many systems had very limited
steering, so maintaining a 60 degree angle within a patient, let alone
from patient to patient, was challenging, but usually "doable."  Since
all the correlations were at 60 degrees, the 60 degree criterion was
preserved.  

Now move ahead to modern times.  Most systems now can achieve angles
better than 60 degrees pretty routinely.  So why continue to use 60
degrees?  Quite often the answer is historic.  Since that was the
standard, many people now mistakenly believe that 60 degrees gives the
best Doppler.  In contrast, many labs now recommend a range of 45 to 60
degrees (a very acceptable practice).  There is a reason that many give
for specifying a range and not just specifying 60 or less.  That
argument is associated with repeatability. The goal is to maintain
repeatability in how studies are performed both within a lab and from
lab to lab.  From a mathematical standpoint, this argument is not
completely true since the error bands associated with 0 to 60 degrees
are encompassed within the error bands associated with 45 to 60 degrees.
Furthermore, with vessel tortuosity, often times maintaining a 60 degree
angle is almost impossible and often leads to the use of suboptimal
views and steering angles.  In these cases, the correct answer is to use
the lower angle and make a note.  The interpreter should already be
acutely aware of potential velocity variations (acceleration effects)
associated with the tortuosity.  The best position nowadays is probably
to try to maintain angles between 45 and 50 degrees, realizing that
there will be times when larger or smaller angles will just make more
sense.  Of course, you should always perform correlation studies to show
that your Doppler measurements are consistent with the current gold
standard. 

Debbie K. Anderson
Pegasus Lectures, Inc. 
972.668.1842 (direct line)
[log in to unmask] 

________________________________

From: UVM Flownet [mailto:[log in to unmask]] On Behalf Of Kristy
Peeler
Sent: Monday, January 26, 2009 9:59 AM
To: [log in to unmask]
Subject: angles

 

I work in a small city with a new ultrasound program at the university.
There is also a new vascular ultrasound program with students rotating
through my lab.  I am finding these students are being taught that the
angle of insonation should be 60 degrees and nothing else no matter how
the vessel is coursing. My students are telling me that he other
surgeons' lab and the general US depts. in the 2 hospitals also
incorporate this same school of thought.  For me, it has always been 60
degrees or less. Is this common practice in other communities? I would
appreciate your feedback.

 

Kristy

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