Principles of Imaging Science I (RAD119)
X-ray Production & Emission
X-ray Production
• X-rays are produced inside the x-ray tube
when high energy projectile electrons from
the filament interact with the atoms of the
anode
• Conditions necessary:
– Source of electrons
– Target (anode)
– High potential difference
– Sudden deceleration of projectile electrons
Target Interactions
• All occur within 0.25 to 0.5 mm of target
surface
– Heat production
– Bremsstrahlung interactions
• Braking or slowed-down
– Characteristic interaction
• Target material
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Heat Production
• 99.8 percent of incident electrons’ kinetic
energy converted to heat
• Incident electrons transfer kinetic energy to
outer shell electrons of target atoms
– Causes them to emit infrared radiation
• Heat (heat units = joules)
X-ray Production
Most of the kinetic energy of projectile electrons is
converted to heat by interactions with outer-shell electrons
of target atoms. These interactions are primarily excitations
rather than ionizations
Bremsstrahlung Radiation
• Projectile electron enters an atom in the metal
of the anode and does not strike any of the
electrons
• It may continue toward the center of the atom
and come near the nucleus due to the
electrostatic attraction. This attraction slows
the electron down as it passes the nucleus
and alters the direction of the projectile
electron.
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Bremsstrahlung
• As incident electrons get closer to nucleus, the
following occurs:
– Photon energy increases
• Photon energy dependent on how close
electron comes to nucleus
• Due to larger deflection of incident electron
Bremsstrahlung
• Direct interaction between nucleus and
incident electron
– Possible, but not probable
– Maximum energy photon
Bremsstrahlung Radiation
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Video
Characteristic Radiation
• Characteristic of the target element
• Projectile electron strikes an atom and knocks
a K shell electron out of its orbit. This leaves
the atom in an unstable state
– Tube potential must be at least 70 kVp to eject a
tungsten K shell electron
• K shell binding energy is 69 keV
– Only electron that drops into K-shell will
contribute to beam
Characteristic Radiation
• An electron from a higher orbit moves down
to the “hole”
– Called a K characteristic x-ray
• X-ray photons are produced when the
electron changes orbital shells.
– Cascade effect
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Characteristic Radiation
• Characteristic x-rays are
produced after the
ionization of a K-shell
electron. When an outershell electron fills the
vacancy in the K shell, an xray is emitted.
 The energy is calculated by
the difference between the
electron orbits
Video
Characteristic Radiation
– Calculate the the
energy of a K
characteristic photon
with the transition
from the L shell. M
shell?
– Calculate the energy
of an L characteristic
photon with the
transition from the
M shell. N shell?
Graphed as discrete
spectrum (measurable)
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X-ray Production (Mostly Brems)
• First, only a high-energy projectile
electron has enough energy to
knock a K-shell electron out of its
orbit to produce a characteristic
x-ray.
– At kVp settings lower than 70, all brems. .
• Second, the projectile electron is
more likely to miss the K-shell
electron of the target atom than
it is to hit it due to atom’s open
space
Off-Focus Radiation Effect on Image
Emission Spectrum
• General form of an x-ray
emission spectrum.
– Characteristic radiation
– Bremsstrahlung radiation
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Emission Spectrum
• Graphical representation of characteristic (discrete) and bremsstrahlung
radiation (continuous)
– Y axis = x-ray quantity
• Height of the curve or bar graph
• Change in amplitude = change in quantity
– X axis = x-ray quality (keV)
• Shown on horizontal axis
• Change in position horizontal axis = change in quality
Emission Spectrum: Brems Radiation
– Tube potential based on manufacturer specs
• kVp range <70 - 120+
– Graphed as continuous spectrum (wide range of
energies)
• Selected kVp will determine maximum keV possible
for any photon
• Minimum kev could be just above zero
Factors Affecting the Emission Spectrum
• Milliamperage (mA)
– Quantity, number of
photons
– Amplitude of continuous
and discrete spectra are
affected
– No change in position
• Milliamperage-seconds
(mAs)
– mA X time
Change in mA results in
a proportionate change
in the amplitude of the xray emission spectrum at
all energies.
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Factors Affecting the Emission Spectrum
• Kilovoltage (kVp)
– Quality, penetrability
– Amplitude and position
of continuous spectrum
are affected
– Amplitude of discrete
spectrum is affected
Change in kVp results in an increase in the
amplitude of the emission spectrum at all
energies, but a greater increase at high
energies than at low energies. Therefore,
the spectrum is shifted to the right or highenergy side.
Emission Spectrum
Factors Affecting the Emission Spectrum
• Anode atomic number
– Slight change in
Amplitude of continuous
spectrum
– Amplitude and position
of discrete spectrum is
affected
Discrete emission spectrum shifts
to the right with an increase in the
atomic number of the target
material. The continuous spectrum
increases slightly in amplitude,
particularly to the high-energy side,
with an increase in target atomic
number.
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Factors Affecting the Emission Spectrum
• Voltage Waveform
– Amplitude and position
of continuous spectrum
is affected
– Amplitude of discrete
spectrum is affected
Three-phase and high-frequency
operation are considerably more efficient
than single-phase operation. Both the xray intensity (area under the curve) and
the effective energy (relative shift to the
right) are increased. Shown are
representative spectra for 92-kVp
operations.
Filtration
• Process of eliminating undesirable low-energy
x-ray photons by insertion of absorbing
materials into primary beam
• Allows radiographer to shape emission
spectrum
Factors Affecting the Emission Spectrum
• Filtration
– Inherent
• Window of x-ray tube
• O.5 mm Al equivalent
– Added
• Aluminum added
between tube housing
and collimator
• 1.0 mm Al equivalent
– Total Filtration = Inherent +
Added
• 2.5 mm Al equivalent
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Filtration
Filtration
• “Hardening” of beam
– Removes low energy “soft” photons
– Increases average beam energy
• Soft tissue penetration requires approximately
30-40 kiloelectronvolt (keV) photons
Filtration
• Low energy photons cannot penetrate the
part
– Only contribute to patient dose
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Factors Affecting the Emission Spectrum
• Purpose of added
filtration is to remove low
energy, long wavelength
photons
– Amplitude and position of
continuous spectrum is
affected
– Amplitude of discrete
spectrum is affected
Adding filtration to an x-ray
tube results in reduced x-ray
intensity but increased
effective energy. The emission
spectra represented here
resulted from operation at the
same mA and kVp but with
different filtration.
Measurement
• Aluminum
– Standard filtering material
– Filtration expressed as Al/Eq
• Half-value layer (HVL)
– Filtration needed to reduce beam to one half of its
original intensity
Types of Filtration
•
•
•
•
•
Inherent filtration
Added filtration
Compound filtration
Compensating filtration
Total filtration
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• Inherent
Filtration Types
– 0.5 mm Al equivalent
• X-ray tube design
• Added
– 1.0 mm Al equivalent
– Any filtration outside x-ray tube and housing
• Silver on collimator mirror
• Thin layers of aluminum or copper permanently added
between the collimator and protective housing
• Filters may be changed
Inherent Filtration
• Glass or metal envelope
• Dielectric oil bath
• Glass window of housing
Inherent Filtration
• Tube aging increases inherent filtration
– Vaporized tungsten coats tube window
– HVL testing important
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Total Filtration =
Inherent + Added Filtration
Does not take into account any compound or compensating filtration
Effect on Tube Output
• Ideally, filtration would only remove lowenergy photons
• Some high energy photons are removed
• Results in decrease in radiographic density
that must be compensated for with increase in
technique
Compound Filtration
• K-edge filters
– Two or more materials
– Each layer absorbs characteristic photons created
in previous layer
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Compensation Filtration
• Evens radiographic density with parts that
have uneven tissue thickness or densities
– E.g., wedge for foot or T-spine, trough for CXR
Compensating Filters
Compensation Filtration Applications
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