"Carotid narrowing should be reported as a percent stenosis as calculated using the NASCET criteria. I also include a statement documenting the residual lumen diameter (optional). The stenosis should be measured at the point of maximal narrowing relative to the vessel DISTAL to the stenosis. The report must state that the NASCET methodology was utilized." -- Pierre Sasson MD

Carotid artery stenosis (Radiopaedia)

NASCET trial demonstrated benefit of carotid endarterectomy for stenosis 70-99%.

% ICA stenosis = [1 - (narrowest ICA diameter / distal normal ICA diameter) ] * 100%

NASCET Trial: Beneficial effect of carotid endarterectomy in symptomatic patients with high-grade carotid stenosis. N Engl J Med 1991;325:445-453