http://www.gouttreatmenttip.com A cyst is a fluid-filled sac and typically, the fluid is a clear liquid that is easy to detect on film. A brain cyst, then, is no different. It is a fluid-filled sac and sits on the brain. They can be quite small, or quite large.
My particular cyst is quite large, as I've mentioned before, taking up over a 1/3 of my brain. It covers the whole back portion of my brain and wraps around to the right of my brain. If you have never seen a brain cyst, here are a few pictures of a cyst.
This picture is similar to the pictures of mine. This is an MRI, showing someone's cyst at the back of the brain. The white, clear area is the cyst. Typically you would see the brain extending to that area and not a light, clear looking area with no brain visible. This is only because the brain is hidden behind the cyst; however.
My particular cyst takes up the entire back portion of the brain, even extending up the back of the brain, and wrapping around to the right of my brain. My cyst is so large, it doesn't leave much of my brain truly visible. When I first looked at my films, I thought my brain was gone. But I was assured that my brain is still there; it's just hiding beneath the cyst.
Today, my brain sags, which means that on top of the brain, my dura matter, is visible when it shouldn't be. What else is visible is that my brain stem has herniated into my spine.
I will download some of my own, personal MRIs soon and post them. But this gives you an idea of what a large, Posterior Fossa Arachnoid Cyst looks like.