Attenuation As x-rays pays through matter, the exit beam will contain less photons than the entrance beam. This reduction in the quantity of photons is termed ATTENUATION Dependent upon: Energy of the radiation Atomic number of the absorber Thickness of the absorber Density of the absorber

X-ray Interactions with matter II

Compton Effect Also known as: Modified scattering or Incoherent scattering

Compton Effect Occurs within the patient Predominates at higher x-ray energies Ionizing interaction between an x-ray photon and a loosely bound outer shell electron Incident photon is diverted from its original path after ejecting an outer shell electron of the target atom.

Compton Effect The ejected electron may be refered to as: Compton electron Secondary electron Recoil electron The incident x-ray photon continues in a different direction as a scattered photon with less energy yet, the scattered photon will usually retain most of its energy. Most of the energy is diverted between the scattered photon and the ejected Compton electron. The scattered photon and Compton electron have enough energy to continue ionizing interactions within the patient

Compton Effect Importance to Radiography Is In A Negative Sense Scattered x-rays produce no useful information in radiography The scattered photon has enough energy to reach the image receptor and reduce radiographic contrast by fogging the image. Creates a serious exposure hazard especially in fluoroscopy Source of most of the occupational radiation exposure that radiologic technologists receive. !

Compton Effect Importance to Radiography Is In A Negative Sense The incident photon may be deflected from its original path anywhere from 0 to 180 degrees At a 0 degree deflection angle, no energy is transferred because the incident photon is still traveling in its original direction. At a 180 degree deflection angle, more energy is transferred to the recoil electron and less remains with the scattered photon. !

Compton Effect Importance to Radiography Is In A Negative Sense Backscatter Radiation: This occurs when the incident photon is deflected back towards its source, it is traveling in a direction opposite to the original incident photon. This happens when the deflection angle approaches 180 degrees. Note: Most photons will scatter in a more forward, especially when incident photon energy increases. (Higher kVp) !

Compton Scattering Probability The probability of the occurrence of Compton scattering relative to the photoelectric effect increases as the energy of the x-ray photon increases. This means that: Relative to the photoelectric effect, there will be more Compton scattering at 90 kVp than at 60 kVp.

Compton Effect Key Points to Remember Most Likely To Occur With loosely bound outer-shell electrons As x-ray energy increases Increased Compton relative to the photoelectric effect scattering As atomic number of absorber increases No effect on Compton scattering As mass density of absorber increases Proportional increase in Compton scattering SummarySummary

Compton Effect Mathematical Application E i = E s + E b + E KE E i = Energy of the incident photon E s = Energy of the Compton scattered photon E b = Electron binding energy of the Compton electron E KE = Kinetic energy of the ejected Compton electron

Compton Effect Mathematical Application E i = Energy of the incident photon E s = Energy of the Compton scattered photon E b = Electron binding energy of the Compton electron E KE = Kinetic energy of the ejected Compton electron

Compton Effect Mathematical Application E i = E s + E b + E KE E i = Energy of the incident photon E s = Energy of the Compton scattered photon E b = Electron binding energy of the Compton electron E KE = Kinetic energy of the ejected Compton electron

- - K L - - Incident X-ray Photon with 30 KeV Barium Atom Shell Binding Energy In keV K37 L6 M1.3 N0.3 O0.04 P - N O M Compton Scattered Photon Compton Electron

- - K L - - Incident X-ray Photon with 60 KeV Barium Atom Shell Binding Energy In keV K37 L6 M1.3 N0.3 O0.04 P Secondary X-ray Photon created with 36.7 KeV E x = K b - N b E x = 37 keV keV E x = 36.7 keV E x = energy of secondary photon K b = K shell binding energy N b = N shell binding energy Photoelectron with a kinetic energy of 23 KeV E KE = K i - K b E KE = 60 keV - 37 keV E KE = 23 keV E KE = kinetic energy of photoelectron K i = kinetic energy of incident photon K b = binding energy of k shell -

Photoelectric Effect Important to diagnostic radiography including mammography (23 to 150 kVp) Most important mode of interaction between x-ray photons and the atoms of the patient In the human body, this energy transfer results in increased patient dose and contributes to biologic damage of tissues Responsible for contrast on a radiographic image (No photoelectric effect = no radiographic contrast)

Photoelectric Effect Occurs within the patient Predominates at lower x-ray energies Ionizing interaction with an inner shell electron Low energy incident x-ray photon is completely absorbed by an inner shell electron The electron shell absorbing the incident x-ray photon’s energy ejects its electron (photoelectron) leaving a “hole” where the former electron was.

Photoelectric Effect An electron from an outer shell moves in to fill the vacancy As an electron cascades to fill the vacancy, a secondary photon is created (characteristic photon) Energy of the secondary/characteristic photon created is the difference between the binding energies of the orbital shells involved in the cascade effect More likely to occur in absorbers of high atomic number (e.g., bone, positive contrast media)

Photoelectric Effect In human tissue, the energy of the secondary/characteristic photon created is very low and will be absorbed locally within the irradiated object. More likely to occur in absorbers of high atomic number (e.g., bone, positive contrast media)

Photoelectric Effect Important Fact: A photoelectric interaction cannot occur unless the incident x-ray photon has an energy equal to or greater than the electron-binding energy of the electron which it interacts with. A barium k shell electron with a binding energy of 37 keV cannot be removed by a 36 keV incident x-ray photon.

Photoelectric Effect Mathematical Application E i = E b + E KE E i = Energy of the incident photon E b = Electron binding energy E KE = Kinetic energy of the ejected electron

- - K L - - Incident X-ray Photon with 60 KeV Barium Atom Shell Binding Energy In keV K37 L6 M1.3 N0.3 O0.04 P Secondary X-ray Photon created with 36.7 KeV E x = K b - N b E x = 37 keV keV E x = 36.7 keV E x = energy of secondary photon K b = K shell binding energy N b = N shell binding energy Photoelectron with a kinetic energy of 23 KeV E KE = K i - K b E KE = 60 keV - 37 keV E KE = 23 keV E KE = kinetic energy of photoelectron K i = kinetic energy of incident photon K b = binding energy of k shell

- - K L - - Incident X-ray Photon with 60 KeV Barium Atom Shell Binding Energy In keV K37 L6 M1.3 N0.3 O0.04 P Secondary X-ray Photon created with 36.7 KeV E x = K b - N b E x = 37 keV keV E x = 36.7 keV E x = energy of secondary photon K b = K shell binding energy N b = N shell binding energy Photoelectron with a kinetic energy of 23 KeV E KE = K i - K b E KE = 60 keV - 37 keV E KE = 23 keV E KE = kinetic energy of photoelectron K i = kinetic energy of incident photon K b = binding energy of k shell

Photoelectric Effect Probability The probability is a function of: The x-ray energy The atomic number of the atom the photon interacts with.

Photoelectric Effect Probability: The probability of the photoelectric effect is inversely proportional to the third power of the x-ray energy. This means that: As photon energy increases, the probability of a photoelectric interaction markedly decreases Or conversely stated: As photon energy decreases, the probability of photoelectric interaction markedly increases

Photoelectric Effect Probability: The probability of the photoelectric effect is directly proportional to the third power of the atomic number of the absorbing material. This means that: There will be more photoelectric interactions in lead than in aluminum. More in bone than in soft tissue.

- - K L - - Incident X-ray Photon with 60 KeV Barium Atom Shell Binding Energy In keV K37 L6 M1.3 N0.3 O0.04 P Secondary X-ray photon created with 36.7 KeV (characteristic) E x = K b - N b E x = 37 keV keV E x = 36.7 keV E x = energy of secondary photon K b = K shell binding energy N b = N shell binding energy Photoelectron with a kinetic energy of 23 KeV is produced E KE = K i - K b E KE = 60 keV - 37 keV E KE = 23 keV E KE =kinetic energy of photoelectron K i = kinetic energy of incident photon K b = binding energy of k shell M N

- - K L - - Incident X-ray Photon with 60 KeV Barium Atom Shell Binding Energy In keV K37 L6 M1.3 N0.3 O0.04 P Secondary X-ray Photon created with 36.7 KeV E x = K b - N b E x = 37 keV keV E x = 36.7 keV E x = energy of secondary photon K b = K shell binding energy N b = N shell binding energy Photoelectron with a kinetic energy of 23 KeV E KE = K i - K b E KE = 60 keV - 37 keV E KE = 23 keV E KE = kinetic energy of photoelectron K i = kinetic energy of incident photon K b = binding energy of k shell M N

Pair Production Occurs within the patient Requires minimum incident photon energy of 1.02 MeV Incident photon comes close to the strong nuclear field and loses all of its energy in the interaction Energy of incident photon is converted into a pair of electrons: - one negatron - one positron (antimatter)

Pair Production Negatron is quickly absorbed by another atom Positron (antimatter) is attracted to a negative electron and both undergo an annihilation reaction Annihilation of the positron and electron results in the production of two photons each possessing an energy of 0.51 MeV. This interaction does not occur in diagnostic radiography because of the high energy required to create the positron and negatron!

K L - - Incident X-ray Photon with 1.02 MeV - Photon with 0.51 MeV Annihilation Reaction Pair Production