From the 1991 and 1998 NASCET studies, surgery was shown to be of benefit in pts with 70% or greater stenosis. How is ICA stenosis measured according to the NACSET technique?
Carotid Stenosis Reporting
All studies of the carotid (CTA, MRA, US) must describe the degree of stenosis using the distal carotid as a reference (as per the NASCET trial)
e.g. if there is 76% stenosis of the right ICA:
Severe stenosis of the right ICA.
Note:
Internal carotid artery stenosis reported are based on the distal internal carotid artery as a reference as used in the 1998 NASCET study.
Mild stenosis <50 %
Moderate stenosis 50-69 %
Severe stenosis 70-99 %
Q: How can a 75% stenosis by US (not based on NASCET) and 50% stenosis by CTA (using NASCET) describe the same anatomy?
A: If the patent lumen measures 2mm, the plaque thickness 6mm at the ICA at the level of the stenosis, and the distal ICA measures 4mm, the result will be as above. In other words, an US measuring a stenosis can overestimate the stenosis as compared to using the NASCET technique. This could lead to unindicated surgery! This is why CMS is forcing docs to use the NASCET criteria.
One can think of the NASCET technique as representing a ratio, as opposed to one’s instinct to measure at the level of the lesion. One reason for this is that the NASCET trial used catheter angiography to assess the vessels. Since the wall of the vessel could not be visualized, they used the distal vessel lumen for their references/comparison.
Q: What is the difference in management of a 50 vs. 75 percent stenosis?
A: 75% is in the severe category and meets the criteria for surgery, while 50% just makes the moderate category, likely leading to medical management.
References:
The NASCET technique as described by Allan Fox (NYU Fellow Classmate of Richard Pinto and Thomas Naidich)
http://radiology.rsna.org/content/186/2/316.full.pdf+html
http://stroke.ahajournals.org/content/25/12/2445.full.pdf+html
Carotid Stenosis Reporting/PQRS (Physician Quality Reporting System measure 11)
From the ACR
http://members.asnr.org/cpc/asnrpqriwebsite/ACR_PQRI_Guide.pdf
From the ASNR
http://members.asnr.org/misc/ASNR_PQRI.pdf
http://www.ajnr.org/content/early/2011/11/17/ajnr.A2912.full.pdf
Q: What is the disadvantage of assessing for signal void at the distal ICA?
A: A stenotic ICA can have normal flow in the distal ICA from the posterior communicating artery.
Q: What is a better place to assess for signal void?
A: Look at the cavernous ICA.
Q: What is the next test that should be done for following up this finding?
A: CTA
Q: Why is there often signal dropout at the carotid bifurcation on MRA (posterior wall of the proximal ICA)?
A: There is "disordered flow"(flow separation) at the bulb. (the MR PhDs do not use the term “turbulence” (it kind of gets beyond the scope of Radiology http://en.wikipedia.org/wiki/Turbulence)