X-ray and CT images can be considered to be a map of density of tissues in the body; white areas on X-ray and CT images represent high density structures.
MRI images are different.
In simple terms, MRI images can be considered as a map of proton energy within tissues of the body. A variety of MRI images can be produced which emphasise different tissue types, in particular those that contain a large amount of fat or water.
Bright areas on an MRI image represent high ‘signal’ given off by protons in the body during the scanning process.
White areas on an X-ray or CT image = high density
White areas on an MRI image = high signal