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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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