Had to look it up...

[Increased disc signal on T2-weighted images is associated with dehydration of the lumbar discs. Change in the disc signal, or darkening of the signal, is associated with dehydration or loss of hydrogen ions within the disc. This is often associated with lumbar disc degeneration. Decreased hydration leads to a loss of signal intensity on the T2 images which leads to darkening of the disc on the image ]
http://www.hindawi.com/journals/aorth/2012/413913/