X-rays are just like any other kind of electromagnetic radiation. They are produced in parcels of energy called photos just like light. There are two different atomic processes that can produce x-ray photons. One is called Bremsstrahlung, which is a fancy German name meaning “braking radiation.” The other is called K-shell emission. They can both occur in heavy atoms such as tungsten.
X-ray tubes produce photos by accelerating an electron beam to energies of several hundred kilovolts and colliding it into a heavy target material. The abrupt deflection of the charged particles (electrons) by other charged particles (protons in an atom) produces bremsstrahlung photons. X-ray radiation with a continuous spectrum of energies is produced ranging from a few keV to the maximum energy of the electron beam.
Atoms have their electrons arranged in set shells of different energies. The k-shell is the lowest energy state of an electron. An incoming electron can give a k-shell electron enough energy to knock it out of its energy state. About 0.1% of the electrons produce k-shell vacancies; most produce heat. Then, an electron of higher energy (from an outer shell) can fall into the k-shell. The energy lost by the falling electron produces an emitted x-ray photo. Meanwhile, higher energy electrons fall into the vacated energy state in the outer shell, producing more photons. K-shell emission produces higher-intensity x-rays than Bremsstrahlung, and the x-ray photon ejects at a single wavelength, giving a much sharper x-ray spectrum.
The attenuation of absorption of an x-ray beam is the fraction of the x-ray radiation that will be absorbed by the material. Usually attenuation is defined for a material by the linear absorption coefficient, µ (the absorption for a given thickness of material) for a narrow, well-collimated, monochromatic x-ray beam. The linear absorption coefficient is the sum of contributions of Photoelectric (PE), Compton Scattering also known as incoherent scattering, Pair Production (PP), Thomson Scattering (R), and Photodisintegration (PD).
Photoelectric (PE) absorption of x-rays
Photoelectric (PE) absorption of x-rays occurs when the x-ray photon is absorbed resulting in the ejection of electrons from the outer shell of the atom, resulting in the ionization of the atom. Photoelectron absorption is the dominant process for x-ray absorption up to energies of about 500 KeV. Photoelectron absorption is also dominant for atoms of high atomic number.
Compton Scattering (C), also known as incoherent scattering, occurs when the incident x-ray photon ejects an electron from an atom and an x-ray photo of lower energy is scattered from the atom. The scattering x-ray photo has less energy and therefore greater wavelength than the incident photon. Compton Scattering is important for low atomic number specimens. At energies of 100 KeV – 10 MeV the absorption of radiation is mainly due to the Compton effect.
Pair Production (PP)
Pair Production (PP) can occur when then x-ray photon energy is greater than 1.02 MeV. Pair Production is of particular importance when high-energy photons pass through materials of a high atomic number.
Thomson Scattering (R)
Thomson Scattering (R), also known as Rayleigh, coherent, or classical scattering, occurs when the x-ray photon interacts with the whole atm so that the photon is scattered with no internal energy to the scattering atom nor to the x-ray photon. Thomson scattering is never more than a minor contributor to the absorption coefficient.
The most important interaction processes are absorption or scattering. Information about the patient is conveyed by the primary photons; scattered photons, arising from interactions in the patient, reduce the image information content. Photon absorption and scattering in the patient result in the absorption of energy. The energy absorbed is called the absorbed dose and is measured J/kg or Gray, Gy.
At the energy used in medicine, two processes, PE and C, dominate. During these processes, absorption occurs and the new (characteristic) x-ray is emitted at the same time. The photons emitting from x-ray target lose their energy of the photon is transferred to the electron of the inner shell, K-shell. The electron is ejected from the atom and the electron of the outer shell will move into the inner shell in order to stabilize the nucleus of the atom. With this move, a new x-ray photon will be created.
We say this process is absorption and the loss of x-rays is attenuation. For low energy x-rays, PE and C occur easily while the x-ray is traveling through matter. The image we see is mostly from PE. Generally, Compton scattering (C) makes the image worse. It is seen as general grayness (fog) that degrades the quality of the image.