MRIs are acquired as thin slices in the axial, coronal, or sagittal planes. PD (proton density) weighting is a technique used to acquire the images that results in a standard, specific appearance of normal tissues. On PD images of the brain, fluid appears light gray, and can be hard to differentiate from gray matter. Gray matter is also light gray, and white matter is darker gray. On PD images, the contrast between white and gray matter is excellent. Most vessels are black. The subcutaneous fat is white.