Alpha particles consist of two neutrons and two protons (nucleus of He), have a positive charge of 2 and a velocity in air of approximately one-twentieth the speed of light.
Discovered by Ernest Rutherford in 1899 (Rutherford-Bohr planetary atom model) alpha particles became emitted by very large atoms in an unstable energy state (high atomic number mostly over 82, with a too low neutrons//protons ratio (e.g. <= 1.5)).
Through their relative slow travel speed, they get stopped by e.g. a thin sheet of paper or the outer layers of human skin or travel only inches; once stopped they pick up free electrons and become helium.
These alpha particles are only dangerous to humans when the alpha-emitting material is inhaled or ingested (it causes damage that may lead to cancer) or comes into contact with the lens of the eye, caused by their less penetrating properties.

Barium sulfate (BaSO4) is an inert and insoluble white powder with high density. Barium belongs chemically to the group of heavy metals. Mixed with water and additional ingredients (e.g., sweetening agents), barium sulfate is the preferred positive contrast agent for abdominal x-ray and computed tomography examinations. The extremely low solubility of barium sulfate protects patients from absorbing harmful amounts of the metal (water soluble metal compounds are often highly toxic). The high density in x-ray examinations is related to the high atomic number, since large nuclei absorb x-rays much better than smaller nuclei.
Barium sulfate agents for opacification of the gastrointestinal tract are not absorbed or metabolized and are resistant to dilution. These contrast agents are opaque white suspensions and usually swallowed or administered as an enema. They provide better delineation of mucosal details and are less expensive than water-soluble iodinated contrast media. The elimination rate is a function of gastrointestinal transit time. After GI application, it leaves the body with the feces.

Contraindications of barium sulfate products in case of known or suspected:

A film consists of a thin, transparent sheet of polyester or similar material. Films are coated on one or both sides with an emulsion sensitive to radiation, light or heat. Films are relatively radiolucent due to their relatively low atomic number (that of silver halide).

See also X-Ray Film, Direct Exposure Film and Intensifying Screen.

Gallium is a metal with the chemical symbol Ga and atomic number 31. Gallium salts such as gallium citrate and gallium nitrate (with the radioactive isotope 67Ga) are used as radiopharmaceuticals in diagnostic nuclear medicine procedures, for example abscess or inflammation scintigraphy.

(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.