Signal in MR images is high or low (bright or dark), depending on the pulse sequence used, and the type of tissue in the image region of interest. The following is a general guide to how tissue appears on T1- or T2- weighted images.
Dark on T1-weighted image:
increased water, as in edema, tumor, infarction, inflammation, infection, hemorrhage (hyperacute or chronic)
low proton density, calcification
flow void
Bright on T1-weighted image:
fat
subacute hemorrhage
melanin
protein-rich fluid
slowly flowing blood
paramagnetic substances: gadolinium, manganese, copper
calcification (rarely)
laminar necrosis of cerebral infarction
Bright on T2-weighted image:
increased water, as in edema, tumor, infarction, inflammation, infection, subdural collection
methemoglobin (extracellular) in subacute hemorrhage
Dark on T2-weighted image:
low proton density, calcification, fibrous tissue
paramagnetic substances: deoxyhemoglobin, methemoglobin (intracellular), iron, ferritin, hemosiderin, melanin
protein-rich fluid
flow void
source : http://www.med.harvard.edu/AANLIB/basicsMR.html
