X-ray Production
and Interactions
This unit will explain the
process of x-ray
production and how xrays interact with matter
Two Sets of Interactions
• Interactions in the
x-ray tube target
between filament
electrons and anode
target atoms
– Bremsstrahlung
– Characteristic
• Interactions in the
body between x-ray
photons and tissue
atoms.
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–
–
–
–
Photoelectric
Coherent
Compton
Pair Production
Photodisintegration
Converting Electrons into X-ray Photons
• Production of diagnostic x-rays is
extremely inefficient
– Therapeutic x-ray production, where mega
electron volts (MeV) are used, has a higher
conversion of electrons into photons
– In the diagnostic range (KeV), there is more
conversion of the electrons to heat
• Total number of electrons converted to heat is
99%
• Only 1% of the electrons
are converted to photons
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Bremsstrahlung Target Interaction
• Created when incident (filament) electron
interacts with the nucleus of an anode target
atom
• Occurs at all kV settings
– Electron approaches nucleus
– Nuclear force field is too strong for electron to
penetrate
•
•
•
•
Electron slows down (“brakes”)
Braking causes a loss of energy
Energy loss is released as a Brems x-ray photon
Energy of the photon is exactly the difference between the
entering & exiting filament electron energy
• Electron changes course & keeps going in new direction
Brems Target Interaction
• The closer the electron gets to the nucleus,
the more it brakes; resulting in a higher
energy Brems photon
• Electron can collide
with nucleus losing
all of its energy, pass
close to the nucleus
and lose most of its
energy, or pass at a
distance and lose
little of its energy.
• Average energy of
Brems is 1/3 of the
maximum kV used
Characteristic Target Interactions
• Occur when incident (filament) electron interacts
with an orbital electron of the anode target atom.
– Incident (filament) electron has energy ≥ binding
energy of orbiting electron
– Filament electron knocks inner-shell electron from orbit
creating “hole”. The atom is now unstable
– Outer-shell electron drops into hole, must give up
energy to do this
– Energy is released as characteristic x-ray photon
• Characteristic photon energy is equal to the difference between
binding energy of the electron shells involved. (ex. If L shell
electron fills K shell vacancy K-L= characteristic photon
energy)
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Characteristic Target Interactions
• When outer shell electrons fill inner shell
vacancies, a characteristic cascade occurs. This
produces several x-ray photons at different
energies from each atom
• Photon is named for the
“hole” filled (k-characteristic,
L-characteristic, etc.)
Characteristic Target Interactions
– Any outer shell electron can fill an inner
shell vacancy, the most likely is the
adjacent shell
– k-shell emissions are the highest in energy
and are the only emissions useful to us
– To get K-characteristic we must use at
least 70 kV. (k-shell binding energy of
tungsten is 69.5 keV). Below 70 kV the
beam is basically Brems.
X-ray Photon Emission Spectrum
• The emission spectrum for tungsten (most
common target material)
– Overall smooth shape
– X-ray production starts ≃ 15 keV
– Increases rapidly to
30-40% of max. energy
(peak of the curve)
– After peak, there is a
gradual down-slope
to x-axis (maximum
energy)
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X-ray Photon Emission Spectrum
• At the 70 keV point, a slight “spike” occurs
which is representative of the characteristic
interactions taking
place in the target
material. In
tungsten targets,
this will always
occur at 70 keV
X-ray Photon Emission Spectrum
• The position of the
spectrum on the x-axis
represents quality, the
further to the right it is
the higher the quality.
• The area under the
spectrum represents
the quantity of x-rays,
the greater the area
the greater the
quantity.
quality
quantity
Factors Affecting the Emission Spectrum
• Tube current (mA): increasing the mA (mAs)
increases the amplitude of the spectrum
(area under the spectrum)
• Tube voltage (kV): increasing the kV
increases the amplitude of the spectrum and
shifts it to the right.
• Added filtration: adding a compensating filter
will decrease the amplitude, more so on the
left side than the right.
• Target material: changing to a better target
material will increase amplitude AND shift
discrete spike to the right.
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Attenuation
• Attenuation - reduction in the number of
photons as they pass through matter
• Attenuation occurs in several different
ways:
– Some photons are absorbed by matter they
pass through
– Other’s change course in
matter, called “scatter”
Attenuation
• In short, this is how we get an image on
our x-ray film
– High density bone attenuates the photons that
try to pass through its structure
• This creates light areas on our x-ray film
– In other places there is little attenuation, such
as an air-filled cavity, the majority of the
photons will reach the x-ray
film
• This creates dark areas on
our x-ray film
Photoelectric Absorption
• X-ray photon ejects a k-shell electron
• KE = Ex - Ek
• “True absorption” - Photon is completely
absorbed in process
• Also called “photoelectric effect” & is
what gives the clear
areas of our films
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Coherent Scattering
(classical scattering)
• Change in direction of incident x-ray without
change in kinetic energy
– As photon approaches atom:
• Photon is absorbed by the atom causing excitation
• Atom immediately releases this energy as a scattered
photon with energy equal to incident photon but in a
new direction
• Only occurs with incident
photons of about 10 keV
energy
• At 70 kVp, only 3% of
photons are result of this
• Adds to patient dose,
minor effect on film (fog)
Compton Scattering
• Occurs between moderate-energy x-ray photons
& outer-shell electrons
• Results of interaction are:
– Ionization of target atom
– Change in direction of photon
• Can deflect in any direction
• Backscatter - x-rays that scatter back to origin (180° angle)
– Reduction in energy of photon
– Both scatter photon (former incident photon) and
orbital electron possess enough energy to undergo
many more ionization events.
• Can occur with all x-rays
Compton Scattering
• Creates an exposure hazard in radiography
– Primary contributor to film fog
– Can leave patients & cause interactions in the
radiographer, resulting in radiation exposure
– A serious problem
in fluoroscopy and
is the major source
of occupational
exposure
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Pair Production
• Occurs with photons having energies > 1.02
MeV.
– Used only in radiation therapy applications
– Does not occur in the diagnostic range
• Photon reacts with the nuclear force field &
creates 2 particles
with opposing electrostatic
charges:
– Negatron (electron)
– Positron
– Each has .51 MeV of
energy
Photodisintegration
• Occurs when a photon is absorbed by the
nucleus of atom & nucleus must release a
nuclear fragment (nucleon)
• Occurs only with photons with energy
level greater than
10 MeV
– only in radiation
therapy procedures
– Outside diagnostic
range
Selecting Proper Technical Factors
• Considerations
– kV determines type of interaction in the body
• As kV increases photoelectric decreases (everything
is penetrated)
• As kV decreases photoelectric increases (more
absorption by thicker or denser tissues)
• As kV increases Compton increases but Compton
occurs throughout the diagnostic range.
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Selecting Proper Technical Factors
• Select kV based on:
– Type of exam (ex. Chest vs. ribs)
– Desired level of penetration
– Desired film contrast
– Minimizing patient dose
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