So here’s a view of Morison’s pouch and when you’re looking at the space between the liver and the kidney - here you may be tempted to think that there is some hypoechoic space that might represent fluid,
but if you look closely at it it has some internal echoes to it, and there’s a line on each side and this is what we call the double line sign, it’s actually perirenal fatty tissue and should not be mistaken for free fluid.
Another possibility is a clip like this, where we are seeing the liver and there is a black area sort of near the right of the screen between the liver and the kidney, but if you look closely at this one it has a line around it, it’s encapsulated and this actually is
the gall bladder. In this case you’ll note that there are not these sharp crevasses which you typically see with fluid that is tracking into spaces between the liver and the kidney. So this is gallbladder and not free fluid.
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