```Terry and Kirk, linear array Dopplers are a mess!
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

To unsubscribe or search other topics on UVM Flownet link to:
http://list.uvm.edu/archives/uvmflownet.html

```