CHARACTERISTIC X RAYS CHARACTERISTIC X RAYS con • The high energy electron can also cause an electron close to the nucleus in a metal atom to be knocked out from its place. • This vacancy is filled by an electron further out from the nucleus. • The well defined difference in binding energy,characteristic of the material, is emitted as a monoenergetic photon. • When detected this X-ray photon gives rise to a characteristic X-ray line in the energy spectrum. Characteristic X-rays con… Characteristic radiation are emitted at discrete energies hv = Ek-El Ek and El are the electron binding energies of the K shell and the L shell Factors affecting x-ray beam Quality and Quantity • The energy of the x-rays is determined by the voltage applied. • The amount of x-rays is determined by the current. Factors affecting x-ray beam Quality and Quantity Anode material Voltage applied (kVp) Tube Current (mA) Filters used Anode material • Different anode materials will produce different characteristic x-ray spectra and different amounts of bremsstrahlung radiation. Voltage (kVp) • Note that increasing the applied voltage or kVp will increase the maximal energy, the average energy and the intensity of the x-rays. • Characteristic x rays do not change with a change in kVp Tube current (mA) • Increasing the current (ie mA) will not change energy of the beam only the intensity (i.e. the amount) of x-rays. • The quantity of x-rays is directly proportional to the tube current Power Ratings • The energy per unit time that can be supplied by the x-ray generator or received by the x-ray tube during operation. • Power delivered by an electric circuit is equal to the product of the voltage and the current X-ray Exposure Rating charts • Operational limits of the x-ray tube for single and multiple exposures and the permissible heat load of the anode and the tube housing. • The single-exposure chart contains the information to determine whether a proposed exposure is possible without causing tube damage. • Rating chart is specific to a particular x-ray tube and must not be used for other tubes. • Charts show the limitations and allowable imaging techniques for safe operation X-Rays Interactions • When X-ray photon enters a layer of matter such as human body, it is possible that it will penetrate through without any interaction, or it may interact and transfer energy to the matter in one or two of the interactions. • There are four major types of interactions of x-ray photons with matter, the first three of which play a role in diagnostic radiology . Types of interactions of x-ray photons with matter. 1- Rayleigh (Coherent) scattering 2- Photoelectric effect 3- Compton effect 4- Pair production (Annihilation radiation) Rayleigh (Coherent) Scattering (elastic scattering) During the Rayleigh scattering event, the electric field of the incident photon's electromagnetic wave expends energy, causing all of the electrons in the scattering atom to oscillate in at the same frequency. The x-ray photon is scattered through small angle without change of x-ray energy or loss of energy to the medium. (i.e. the scattered photon has the same energy as the incident photon): Rayleigh (Coherent) Scattering (elastic scattering) This interaction occurs mainly with very low energy x-rays, (15 to 30 keY). In this interaction, electrons are not ejected, and thus ionization does not occur. Rayleigh (Coherent) Scattering (elastic scattering) Compton Scattering (inelastic scattering) Compton Scattering (inelastic scattering) Compton scattering is the predominant interaction of x-ray with soft tissue in the energy range approximately from 30 KeVto 24 MeV. This interaction is most likely to occur between photons and outer ("valence") shell electrons. The electron is ejected from the atom, and the photon is scattered with some reduction in its energy. Compton Scattering (inelastic scattering) • The energy of the incident photon (Eo) is equal to the sum of the energy of the scattered photon (Ese) and the kinetic energy of the ejected electron (Ee-) • The binding energy of the electron that was ejected is very small and can be ignored. Photoelectric Effect Photoelectric Effect In the photoelectric effect, all of the incident photon energy is transferred to an electron which is ejected from the atom. After ejection of the electron, the neutral atom becomes a positively charged ion with a vacancy in an inner shell that must be filled with a nearby less tightly bound electron. Photoelectric Effect The kinetic energy of the ejected photoelectron (Ee) is equal to the incident photon energy (Eo) minus the binding energy of the orbital electron (Eb). Ee= Eo-Eb In order for photoelectric absorption to occur, the incident photon energy must be greater than or equal to the binding energy of the electron that is ejected. Photoelectric Effect con…. • The probability of photoelectric absorption per unit mass is approximately proportional to Z*3/E*3, • where Z is the atomic number and E is the energy of the incident photon. • The photoelectric effect predominates when lower energy photons interact with high Z materials • The benefit of photoelectric absorption in x-ray transmission imaging is that there are no additional secondary photons to degrade the image. Pair production (Annihilation radiation) Pair production can only occur when the energies of x-rays exceed 1.02 MeV. (i.e. if the energy of the X-ray photon is less than 1.02 MeV, this interaction cannot happen). In pair production, an x-ray photon interacts with the electric field of the nucleus of an atom. The photon's energy is completely converted into an electronpositron pair. Pair production (Annihilation radiation) The rest mass energy equivalent of each electron is 0.511 MeV, and this is why the energy threshold for this reaction is 1.02 MeV. Photon energy in excess of 1.02 MeV appears as kinetic energy, which may be distributed in any proportion between the electron and the positron. Pair production (Annihilation radiation) The electron and positron lose their kinetic energy via excitation and ionization. When the positron comes to rest, it interacts with an electron. Then both particles undergo mutual annihilation, with the appearance of two annihilation photons each with an energy of 0.511 MeV traveling in opposite directions. Pair production (Annihilation radiation) Pair production becomes more likely with increasing atomic number and increasing photon energy. Pair production has NO importance in diagnostic x-ray imaging because of the extremely high energies required for this interaction to occur. X-Rays Interactions • Photon interactions probabilities Photoelectric effect • Directly proportional to Z*3 • Inversely proportional to E*3 Compton scattering • Directly proportional to the electron density • Independent of Z Pair production • Directly proportional to Z*2 • Directly proportional to E Relative importance of photon interactions For soft tissues (Z = 7) Photoelectric effect is the predominant interaction for beams below about 30 keV. Compton is predominant interaction for beams above 30 keVand below 24 MeV Pair production becomes the predominant interaction Above 24 MeV References The Essential Physics of Medical Imaging. JT Bushberg, JA Seibert, EM Leidholdt, JM Khan’s Lectures: Handbook of the physics of radiation therapy