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Part of the problem, unfortunately, is that the "gold standard" method for
measuring % diam. stenosis is flawed.  Angiography cannot tell where the true
vessle diameter is, so they must measure downstream in a PRESUMED normal
segment.  Of course the vessel lumen becomes increasing smaller with distance
and the distal segment may or may not have atherosclerotic narrowing.
The angio findings inherently under exaggerate the degree of stenosis.
For dimensional or area measurement of stenosis ultrasound is not truly
accurate unless the region is perpendicular to the transducer and superficial
allowing adequate visualization of the lateral and medial segments.  Then of
course you may be deemed inaccurate by the "gold standard" for reasons
mentioned above.
We rely on doppler criteria : 0-15%, 16-49%, 50-79%, 80-99%, 100% because it
most closely follows angio grading categories.  This a common criteria and is
available in most references.
Doppler still tends to over estimate stenosis relative to angiography.
Personally I think one reason may the accepted procedure of correcting all
angles to 60 degrees.  We set our H-P to correct to 50 degrees - less
mathmatical "correction", less exaggeration.
Good Luck,
T Cunningham, RVT, RDCS