LECTURE 4 PRINCIPLE OF IMAGE FORMATION KAMARUL AMIN BIN ABDULLAH
Lesson Objectives At the end of the lesson, student should able to: Define attenuation Explain interactions between x-rays and matter in the following : a) coherent (elastic) scatter b) photoelectric absorption c) Compton (inelastic) scatter d) pair production
Introduction
Interaction of x-ray photons with matter When a beam of x-rays passes through matter, its intensity is reduced: attenuated x-rays are attenuated by either absorption or scatter Some x-ray photons are transmitted
Attenuation of x-ray photons X-ray Tube scattered photons absorbed photons transmitted photons
Interaction of x-ray photons with matter Attenuation increases with: A. physical density B. atomic number C. electron density Attenuation decreases with beam energy
Inverse Square Law Reduction in intensity of a diverging x-ray beam is also due to beam geometry the inverse square law: The intensity of a beam of x-rays, from a point source, is inversely proportional to the square of the distance from the source, in a vacuum
Inverse Square Law
example If the output of an x-ray tube is measured as 0.2 mgy at 100 cm, what will be the approximate output at 3 m?
Absorption Some x-ray photons are absorbed in matter: their energy is transferred and the photon disappears The energy deposited per unit mass of matter is called absorbed dose 1 joule per kg = 1 Gray (Gy) The energy deposited as absorbed dose causes chemical changes due to ionisation which may result in biological effects
Scattering Some photons are scattered: deflected from their original path At high photon energies, scatter is in the forward direction and contributes to absorbed dose At low photon energies, scatter occurs in all directions and may contribute to loss of image quality by increasing the overall density of the film Scattered x-ray photons contribute to exposure of staff, patient and public
Scattering
Exponential Attenuation When a beam of x-rays passes through matter, its intensity is reduced i.e. attenuated, due to absorption and scatter The quantity of x-rays removed depends on the x- ray photon energy and the density of the matter
Interaction Processes coherent (elastic) scatter photoelectric absorption Compton (inelastic) scatter pair production
Coherent (elastic) scatter This type of attenuation process occurs when the energy of photons in the beam is small compared with electron binding energies in the atoms of the attenuating medium. This type of attenuation process therefore occurs with low energy radiation and is sometimes referred to as "elastic" scattering This process is not important at the X-ray energies normally used in radiography.
Photoelectric absorption This process occurs when the energy of the incoming photon in the X-ray beam is equal or not much greater than the electron binding energy The photon transfers all it s energy to an electron The electron is ejected from the atom (photoelectron) True absorption has taken place This process leaves an electron vacancy, which is filled by electron transition from a higher energy level There is also the emission of characteristic radiation The photon energy of characteristic radiation is generally low, because the elements interacted with in the body have low proton numbers and low binding energies.
Compton scatter If an X-ray photon has an energy very much greater than the binding energy of the electron with which it interacts, Compton scattering can take place. As the photon energy is so great compared with the electrons binding energy, the electron recoils from the collision and is ejected at speed from the atom. The amount of energy transferred to the electron increases with the angle of scatter
Pair production This type of absorption process can only take place when the incoming photon energy exceeds 1.02 MeV As the photon passes the nucleus of an atom in the attenuating material, it experiences the strong electric forces around the nucleus caused by the positive charges on the protons there. As a result, the photon undergoes a dramatic change of state - its energy is transformed into matter in the form of two minute particles, hence the given term pair production.
Summary: Interaction of x-ray photons with matter When a beam of x-rays passes through matter, its intensity is reduced: attenuated x-rays are attenuated by either absorption or scatter attenuation increases with density of matter, atomic number and electron density interaction process depends on x-ray photon energy
Next lesson Radiographic Processing