No, demyelinating plaques aren't the same thing, but they would also show up as white bits, but just in different places - like within the spinal cord, or nerves or brain, I think (the myelin sheath is the covering around nerves).
Lots of different things will show up as white areas on different MRI sequences (T1, T2, fat suppressed, STIR, etc). Thats why they do the different sequences or use contrast dye for looking for different things
Inflammation in bone shows up as white bits on STIR sequences, and with contrast.
A full bladder will show up as a big white area too.
