Tomography is imaging by sections or sectioning. A evice used in tomography is called a tomograph, while the image produced is a tomogram. The method is used in medicine, archaeology, biology, geophysics, oceanography, materials science, astrophysics and other sciences. In most cases it is based on the mathematical procedure called tomographic reconstruction. The word was derived from the Greek word tomos which means "a section" or "a cutting". A tomography of several sections of the body is known as a polytomography.