Tissues can be characterised on CT by their Hounsfield Units or H.U. Each tissue has it's own range of densities measured in Hounsfield units, and the scan can be manipulated by altering the window level and / or width of densities viewed to give more information (the CT machine can capture more dynamic range than can be displayed in gray scale and perceived by the human eye). Water is defined as zero, air as -1000 and substances greater than 0 are scored according to the attenuation coefficient (μ) of the material relative to water:
The large density contrast between bone (containing calcium) or metallic implants and other soft tissues can cause artifact and resolution issues, which limits the usefulness of CT in central nervous system imaging. This is interesting mainly as an historical footnote, as the first CT scanners revolutionised scanning of the brain, but were not able to accommodate the adult body.