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
X-ray Production
• Anode interactions with
projectile electrons result
in:
• .
– Heat (heat units = joules)
• Outer shel l electron
excitation
– Characteristic radiation
– Bremsstrahlung radiation
(braking or slowed-down)
1
X-ray Production (Mostly Brems)
• Only a high-energy projectile
electron has enough energy to
knock a K-shell electron out of its
orbit to produce this characteristic
x-ray.
• The projectile electron is more likely
to miss the K-shell electron of the
target atom than it is to hit it—
because there is much open space
inside the atom through which the
projectile electron travels.
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
Characteristic Radiation
• An electron from a higher orbit moves down
to the “hole”
• X-ray photons are produced when the
electron changes orbital shells.
2
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
x-ray is emitted.
Characteristic Radiation
– The energy is calculated by
the difference between the
electron orbits
– Examples:
Graphed as discrete
spectrum (measurable)
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.
3
Bremsstrahlung Radiation
• Bremsstrahlung x-rays
result from an interaction
between a projectile
electron and a target
nucleus. The electron is
slowed and its direction is
changed.
Bremsstrahlung Radiation
• The slowing of the electron means that it loses
kinetic energy and is transformed into the
release of x-ray photon energy.
– Graphed as continuous spectrum
• Maximum kev is kvp value set
• Minimum kev could be zero
Emission Spectrum
• Graphical representation of
characteristic (discrete) and
bremsstrahlung (continuous)
radiations.
• 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
4
Emission Spectrum
• General form of an x-ray
emission spectrum.
– Characteristic radiation
– Bremsstrahlung radiation
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 high-energy side.
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 res ults in
a proportionate change
in the am plitude of the xray emis sion spectrum at
all energies.
5
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 w ith an increase in the
atomic number of the target
material. The continuous spectrum
increases slightly in amplitude,
particularly to the high-energy side,
w ith an increase in target atomic
number.
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. Show n are
representative spectra for 92-kVp
operations.
Factors Affecting the Emission Spectrum
• Filtration
– Inherent
• Window of x-ray tube
• O.5 mm Al equival ent
– Added
• Al uminum added
between tube housing
and col l imator
• 1.0 mm Al equival ent
– Total Filtration = Inherent +
Added
• 2.5 mm Al equival ent
6
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 w ith
different filtration.
Filtration Types
• Inherent
– 0.5 mm Al equivalent
– Glass or metal envelope
• Added
– 1.0 mm Al equivalent
– Thin layers of aluminum permanently added
between the collimator and protective housing
– Collimator mirror
Total Filtration =
Inherent + Added Filtration
7
Half-Value Layer (HVL)
• Numerical value applicable to x-ray quality
• Thickness of absorbing material (Al) that will
reduce the x-ray beam to half of its original
intensity
– Al is inexpensive, easily molded, absorbs low
energy x-ray photons (Z# 13)
– X-ray beam: HVL 3-5mm Al, or 3-6 cm soft tissue
• Filtration increases x-ray quality, decreases xray quantity
Attenuation
• Reducing the intensity of x-ray photons by
absorption or scatter
• High energy x-ray photons penetrate through
the body as compared to low energy x-ray
photons
– 100 keV x-rays are attenuated at a rate @3%/cm
soft tissue
– 10 keV x-rays are attenuated at a rate @ 15%/cm
soft tissue
HVL Calculation
• Experimentation using the equipment shown
• Data obtained is plotted on graph
• HVL extrapolated from graph
8
HVL Relationship to kVp
Kilovoltage (kVp)
HVL (mm Al)
50
1.9
75
2.8
100
3.7
125
4.6
150
5.4
HVL Calculations
9