(H) The Hounsfield scale displays radiodensity in a linear scale of gray shades expressed in Hounsfield units (HU). The Hounsfield scale is a quantitative transformation of the attenuation coefficient.
The Hounsfield value -1000 is defined as the radiodensity of air, 0 H that of distilled water at standard pressure and temperature, and denser tissues like for example cranial bone can reach 2000 H. The radiation attenuation of dental fillings or artificial implants depends on atomic number of the elements used. Titanium usually has an amount of +1000 HU. Iron steel can have a density greater than the highest range (traditional 3095 H) covered by the standard Hounsfield scale of a CT scanner. Areas with attenuation coefficients that exceed the scale's maximum are white areas in which no detail is visible.
Some CT machines are relatively tolerant, precise representing regions with very high densities. Sometimes, an option is available to select an extended CT number scale.

An ion is an atomic particle that is electrically charged, either negatively or positively by loss or addition of one or more electrons. The simplest ions are for example hydrogen ions (a proton, H+), or an alpha particle (helium ion, He2+).
Positively-charged ions have fewer electrons than protons. They are cations due to the attraction to cathodes.
Negatively charged ions have more electrons in the electron shells than they have protons in the core. Due to their attraction to anodes they are named anions.

Iodinated contrast agents (typically iodine-substituted benzene derivatives) are bound either in nonionic or ionic compounds. Ionic contrast agents consist of the negatively charged anion and the positively charged cation. Used components of the anion are for example diatrizoate, iodamide, iothalamate or metrizoate and of the cation the sodium or meglumine ion. The osmotic pressure depends on the number of particles in solution. Ionic contrast agents have a greater osmolarity; double that of nonionic contrast agents due to delivering more iodine atoms per molecule.
Ionic contrast agents were developed first and are still in use depending on the examination. Iodine based contrast media are water soluble and as harmless as possible to the body. However, ionic agents have more side effects compared to nonionic contrast agents due to their high osmolarity.

See also Ionic Dimer.

Ionic dimers are used as x-ray contrast agents. They are formed by joining 2 ionic monomers and eliminating 1 carboxyl group and contain 6 iodine atoms for every 2 particles in solution (CM ratio=3).
Ionic dimers have an intermediate osmolarity, intermediate viscosity and an intermediate hydrophilicity between that of ionic and nonionic monomers. The only commercially available ionic dimer is ioxaglate (Hexabrix). Ioxaglate is used primarily for peripheral arteriography.

Ionization is the process of converting an atom or molecule with no electrical charge into an ion by adding or removing charged particles such as electrons or other ions, through high temperatures, electrical discharges, radiation etc.

See also Ion.