ABSTRACT
This chapter introduces the reader to absorptiometric methods of the measurement of bone. The intention of the introduction section is to make the reader aware of the basic physics of bone densitometry. Measurement of bone is usually carried out with either γ-rays or X-rays. Both are penetrating electromagnetic radiation. Radiation is the propagation of energy. If an object or instrument emits energy in any form then it is said to be emitting radiation. γ-rays are the result of changes of energy of the nuclei of radioactive atoms. Gamma radiation is always emitted as characteristic radiation for a particular isotope. The nature of photon interactions with matter explains the dependence of attenuation on atomic number and photon energy. When a beam of collimated photons interacts with matter, some of the photons may be scattered and some absorbed. When a photon interacts with the atom it is totally absorbed and an electron is dislodged from its orbit around a nucleus.
