In response to your question about proportions that are impossible to annotate, here is my quick breakdown of 100 random real movies.
45% OK to annotate. Reasonable outline of target vessel and clear enough to make a reasonable determination.
29% Too many different vessels that could be considered to be the target. The movies are too long or penetrate through too many layers of vessels. Multiple vessels could be considered as the targets throughout the length of the movie. As Mike indicated, many times some are stalled and others are flowing which creates a real dilemma. I've been marking the stalled vessels to bring these to the experts' attention, but this has increased my overall percentage of stalls.
21% Outline does not sufficiently target a single vessel. Different vessels may cross at oblique angles to the outline, but none could be considered as a reasonable target for annotating based on the outline. I suspect that the lack of clarity in the movies and the depth of field is throwing off the outlining algorithm. Again, I am defaulting to marking any stalls seen hear if I can't locate an obvious target vessel.
5% Too blurry to make a legitimate call on flowing versus stalled. This is even using a laptop. I would suspect that users with tablets or other handheld devices would rate this percentage much higher.
Hope this helps quantify our challenge,