Head CTs are performed by acquiring discrete, axial slices through the brain, ranging in thickness from 2-5mm/slice. In some cases, the CT data is acquired as a helix of density data which is then reformatted into the axial, coronal, or sagittal planes.
Head CTs can be performed without iv contrast (unenhanced), or following iv contrast injection (enhanced), depending on what the clinical situation is. On enhanced scans, the arteries, veins, dura and meninges all become brighter since they take up the contrast. The blood brain barrier prevents the contrast from being taken up by the brain parenchyma, so it does not become brighter.
This is an enhanced CT of the brain after intravenous contrast was administered.