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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