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Extra cranial veins are just like traffic routes If you block 1 road it's no big deal to get out of a city, but block 2 or more and the problem is compounded. The flow is slowed. Cars run out of gas. The more blocks the worse the problem. Even a temporary block such as head position is not a problem unless there are no other or few alternate routes.

Of course this is very simplified but it's true.

2 things...

when upright the IJ's collapse but still have flow.

rotation has a significant impact on the appearance of the mid and distal IJ. in most instances, turning the head away from the vein in question, that vein gets bigger.

the importance of this is that many mid and distal IJ "stenosis" are not real and there is NO need to balloon or treat these.

IVUS makes it very easy to look and evaluate the IJ in different head positions.