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