Multiple Sclerosis 101
Air and bone appear dark because they have a low water content (but there are certain types of MRI scans where they appear bright), while things with a high water and/or fat content such as spinal fluid, blood, brain tissue are brighter. Although bone is dark on MRI scans, the fatty bone marrow also appears white which is why you can see the outline of the skull. Demyelination increases the water content of surrounding tissue, which is why lesions show up as bright white spots. Additionally, contrast dye lets us know if demyelination is new or old.
- National MS Society: Managing Relapses http://www.nationalmssociety.org/Treating-MS/Managing-Relapses