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Good points Rob.
This is a HUGE topic, but I will throw my two cents in on this as I am
very familiar with this issue and have been preaching about this for
years.
Intrinsic spectral broadening became a big issue when the world shifted
from round mechanical crystals to linear arrays. However certain types
of mechanical crystals can exhibit this as well.
One must remember that there are multiple crystals in the active
aperture of a linear array, all receiving the reflected Doppler shifts.
Every one of these crystals will receive the shift at a slightly
different point in time and based on the angles of those interfaces,
produce different frequency shifts. This is one of a few reasons that
spectral broadening is so much worse on electronic probes than on
mechanicals. Now, there is no standard among manufacturers as to how
many elements can or should be active in the apertures at any given
time. This is because the aperture is probe/frequency dependant and
dependant on the manufacturers own specifications,  technologies and
performance levels so it is different for all.
Another issue when comparing Doppler results from one system to another
is that operators assume that just because the manufacturer lists the
Doppler frequency as a 4 or 5 or 6 MHz, or whatever, that they are
indeed that frequency. This is simply not true. They could be 4.5, or
5.2, or 6.6. So, not all manufacturers use exactly the same Doppler
frequencies on all probes at all times.
However if we use intelligent angle correction and convert to
velocities, the detected frequency shift changes would be minimal. Of
course when all of these "minimal" effects start adding up, the
differences can be startling as your research noted.
Now before everyone gets upset,,,,  all manufacturers Doppler
technologies must meet accuracy guidelines to receive FDA approval and
to my knowledge, all current systems meet these. It is not a perfect
world however and we must accept the fact that there are differences in
performance and technology levels, and we must  educate ourselves so we
know how to compensate. (Remember, angle correction is a compensation,
which we accept and use).
And as you mention, when you factor in all of the different probe types,
frequencies, angles and apertures being used, and where in the aperture
the Doppler cursor is being generated and the fact that the aperture
automatically increases/decreases with image and sample volume depth and
the severity of the lesion well, the proverbial worms have exited the
can !!
Then,,, factor in all of the different stages/types of vessel lesions
and residual lumens, and that no one really knows the exact flow
direction, (we are making an assumption of a 3 dimensional area based on
a 2D X&Y scan plane), and I think it is amazing that we are as accurate
as we are !
Finally, I worked with a company that worked with you on your project
and we did implement a S/W program that would compensate for the
elements in the active aperture and correct the velocities by using the
"end element for angle correction" which most closely approximated the
correct angle, thereby eliminating the interference in the equation from
the other elements that did not sit at the "displayed" angle. It worked
very well in fact too well.
Why did this not become standard ?
Because another manufacturer, threatened to sue, claiming restraint of
trade if the word got out to customers that such inaccuracies in Doppler
actually existed.
In other words, who would buy the system if the customer knew it could
be innacurate ? So, this "fix" died a quiet death. Could it be
ressurected ?
Absolutly.
Will it be?
Too many lawyers.
Oh, what a tangled web.
Rob;
I wish you would reference the article you wrote so FLOWNET subscribers
who care about this,   could look it up and educate themselves on this
issue.
As for what can be done, there are new probe technologies in development
that operate differently from the Piezoelectric crystals and arrays in
use today.
You can "google" them by looking for capacitive micro machined
ultrasound transducers (cmuts).
One of the many hopes of these transducers is more control over not only
the aperture itself, but the shape of the aperture, in other words, we
could have a multi-element probe that mimics a mechanical aperture for
certain applications. Will it reduce intrinsic spectral broadening ?
Don't know yet,, but I am hopeful.
Big topic.
I look forward to discussion on this.
Don
 
 
 
 

  _____  

From: UVM Flownet [mailto:[log in to unmask]] On Behalf Of
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Sent: Thursday, April 13, 2006 10:13 AM
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Subject: Re: HELP


Terry and Kirk, linear array Dopplers are a mess!
Let me start by saying the more I know about this issue, the more I
realized how much I don't know..
The idea that a Doppler constant angle to flow solves all our
inconsistency issues is misleading. The Doppler steering angle has
perhaps a greater impact on variability than a 10 degree difference in
Doppler angle, eg, 50-60 degrees  This is instrument dependent.  On
phantom tests a few years back, we found a 5% difference in averaged
velocities between 50 and 60 degrees obtained at the same Doppler
steering angle. This isn't much. On the other hand, and on a different
high-end instrument, we found a 17 % difference in averaged velocity
between a precisely aligned 60 degree Doppler angle with the Doppler
beam steered versus a 60% angle to flow with Doppler unsteered.  
We also observed and recorded considerable frequency variation on some
ultrasound systems with the position of the Doppler beam cursor line
along the face of the transducer,  at the same steering angle and the
same Doppler angle. (When you more the line closer to the side of the
transducer, you change effective Doppler aperture size, change sample
volume size and change the amount of intrinsic spectral broadening and
change the frequency display.)
 
A few years back I worked design engineers for a major ultrasound
company that had designed and demonstrated an algorithm that would
compensate for the effects of intrinsic spectral broadening caused by
the above Doppler steering and angle issues and reduce or eliminate all
these variables.  They never  put it on their ultrasound systems,
because to do so meant they'd have to: 1) admit that their Doppler
frequency/velocity was not accurate (most aren't), 2) explain why linear
array Doppler technology is so "messy", 3) market the advantages of a
"fix" when 99.9% of their customers don't realize there needs to be a
"fixing". So why shoot themselves in the foot?
 
My passion against the "fixed angle" concept is based on seeing too many
exams being rejected by the interpreting physician because velocities
were obtained at 57 degree angles rather than 60 degrees. Too many
requests for a normal carotid patient to return for a repeat exam
because some radiologist went to a lecture in which the speaker said "if
you're not at 60 degrees, the exam is worthless". This stuff happens out
in the clinical world, I see it today (last week in fact). It's also one
of the reasons why we see too many instances of  uninformed users (read
morons) setting the Doppler angle to 60 degrees and unrelated to flow
direction or wall alignment (yes, it is ugly!) 
 
Perhaps rather than advocating a well-worn, partial solution (fixed 60
degree angle) to only one of the many variables affecting frequency
display and subsequent velocity calculation, we should be pushing
manufacturers to introduce solutions so that the Doppler systems don't
introduce these variables. It is possible to get accurate velocities at
70 degree angle, it's just that our new tools won't allow it.
Get on it Kirk; talk to those boys and girls at that company in Bothell!
Rob Daigle
 
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