The finding of air within the disc space or a ‘vacuum phenomenon’ simply means that your disc is worn out and that there is excessive movement around that disc space. Normally, the disc maintains its shape and height, and the vertebral spine bones above and below the disc have very little movement, typically only 5 degrees of movement.
However, when the inner layer of the disc (ie. Nucleus pulposus) either degenerates or herniates, it leaves behind a void or space in the center. Then, due to excessive movement of the bones above and below, and especially when they are standing upright so that there is gravity pressure, that new void or space becomes compressed in between the two bones.
As a result, when a person lies down, that space opens up again and the only thing that can quickly fill up that space is air that is sucked in from the surrounding tissue. Think of the disc like an empty ketchup bottle where the inner nucleus pulposus or ketchup is completely finished.
When you squeeze the ketchup bottle together like when you are standing, it flattens out. But when you lie down it expands to its normal shape and sucks in air. Overall, this finding on a CT scan typically suggests that there is excessive movement between the bones and implies that you may require a fusion.