Structural MRI is used to measure the anatomy of the human brain. The signal used to produce structural MRI images is sensitive to the water concentration in different types of tissue. There are different kinds of structural MRI images.
T1-weighted images (top) have high signal in areas with low water concentrations (such as the skull and white matter) and low signal in areas with high water concentrations (such as the cerebrospinal fluid).
T2-weighted images (bottom) have high signal in areas with high water concentrations (such as the cerebrospinal fluid) and low signal in areas with low water concentrations (such as the skull and white matter).