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Chapter 10 X-ray Production &
Chapter 11 X-ray Emissions
• Electron- Anode Interaction
– Imagine the energy needed to propel electron
from 0 to half the speed of light in one to three
centimeters.
– The electrons that travel from the cathode to
the anode are called projectile electrons.
X-ray Production
• Electron- Anode Interaction
– When they strike the heavy metal atoms of
the anode they interact with the atoms and
transfer their kinetic energy to the target.
– These interactions happen at a very small
depth of penetration into the target.
Electron Interaction with Target
• The electrons interact
with either the orbital
electrons or nucleus
of the target atoms.
• Interaction with the
outer shell electrons
produce heat
Electron Interaction with Target
• There is no ionization
but there is excitation.
• More than 99% of
the kinetic energy of
the projectile
electron is
converted to
thermal energy.
Electron Interaction with Target
• The production of
heat increases
directly with tube
current.
• Through the
diagnostic range, heat
production increases
directly with the
increase of kVp.
X-ray Efficiency
• The efficiency of x-ray production is
independent of the tube current.
• Regardless of what mA setting is used, the
x-ray production remains constant.
• The efficiency increases with the
increasing projectile electron energy. At 60
keV only 0.5% of the energy is converted
to x-rays, at 20 MeV, it is 70%.
Characteristic Radiation
• When the projectile
electron interacts with
an inner shell electron
of the target atom
rather than with the
outer shell electron,
Characteristic X-
radiation can be
produced.
Characteristic Radiation
• The interaction is
sufficiently violent to
Ionize the target atom
by removing a K shell
electron.
• A outer shell electron
falls down to replace
the lost electron.
Characteristic Radiation
• The translation from
outer shell electron to
fill the hole in the K
shell is accompanied
by the emission of an
x-ray photon.
• The K shell has an
average energy of 69
keV.
Characteristic Radiation
• Only the K-
characteristic x-rays
are useful and
contribute greatly to
diagnostic
radiographs.
Characteristic Radiation
• Characteristic x-
rays are produced by
transitions of orbital
electrons from the
outer shell to the
inner shell and is
characteristic of the
target element.
Bremsstrahlung Radiation
• Heat and
Characteristic x-rays
are the product of
interaction with the
electrons of the target
atom.
• There is a third type
of interaction.
Bremsstrahlung Radiation
• The projectile electron
can also interact with
the nucleus of the
target atom.
• The nucleus has a
strong positive
charge.
• The projectile electron
misses all if the orbital
electrons.
Bremsstrahlung Radiation
• And comes close to
the nucleus.
• The strong positive
charge of the nucleus
causes it to slow, lose
kinetic energy and
change direction.
Bremsstrahlung Radiation
• The lose of kinetic
results in a low
energy x-ray photon.
• This type of x-rays
are called
Bremsstrahlung X-
rays.
Bremsstrahlung Radiation
• Bremsstrahlung is a
German word for
braking.
• This energy of x-ray
is dependent upon
the amount of kinetic
energy in the
interaction.
Bremsstrahlung Radiation
• A 70 keV electron
can lose all, none or
any intermediate level
of kinetic energy.
• The x-ray can have
an energy range of 0
to 70 keV.
Bremsstrahlung Radiation
• This is different from
Characteristic X-ray
that have a specified
energy.
• Low energy
Bremsstrahlung x-ray
result from slight
interaction with the
nucleus.
Bremsstrahlung Radiation
• Maximum strength
Bremsstrahlung X-ray
happen when the
projectile electron
looses all of it’s
kinetic energy.
Characteristic vs.
Bremsstrahlung X-rays.
• Characteristic X-ray require 70 kVp or
higher. Based upon the energy of the k-
shell electron.
• Bremsstrahlung X-rays can be produced
at any projectile electron energy. In
diagnostic radiography most of the x-rays
are bremsstrahlung x-rays.
X-ray Emission Spectrum
• If a relative number of x-ray photons were
plotted as a function of their energies we
can analyze the x-ray emission spectrum.
• Understanding the x-ray emission spectra
is key to understanding how changes in
kVp, mA, time and filtration affects the
optical density and contrast of the
radiograph.
Discrete X-ray Spectrum
• Characteristic x-rays have a precisely
fixed or discrete energies.
• These energies are characteristic of the
differences between electron binding
energies of a particular element.
• For tungsten you can have one of 15
energies .
Discrete X-ray Spectrum
• There are 15
energies
• There are 5 vertical
line representing K x-
rays.
• 4 representing L x-
rays.
• Remaining represent
lower energy outer
shell electrons.
Discrete X-ray Spectrum
• K x-rays are the only
characteristic x-rays
of tungsten that have
sufficient energy to be
of value in
radiography.
Continuous X-ray Spectrum
• The Bremsstrahlung
x-ray energies range
from zero to a peak
and back to zero.
• This is referred to as
the Continuous X-ray
Spectrum.
Continuous X-ray Spectrum
• The majority of the
useful x-rays are in
the continuous
spectrum.
• The maximum energy
will be equal to the
kVp of operation.
• This is why it is called
kVp (peak).
Four Factors Influencing the
X-ray Emission Spectrum
• 1. The electrons accelerated from the
cathode do not all have the peak kinetic
energy. Depending upon the type of
rectification and high voltage circuits,
many electrons will have very low energy
that produces low energy x-rays.
Four Factors Influencing the
X-ray Emission Spectrum
• 2. The target is relatively thick. Many of
the bremsstrahlung x-ray emitted result
from multiple interactions of the projectile
electrons.
• Each successive interaction results in less
energy.
Four Factors Influencing the
X-ray Emission Spectrum
• 3. Low energy x-rays are more likely
absorbed by the target.
• 4. External filtration is always added to
the tube assembly. This added filtration
serves to selectively remove the lower
energy photon.
Minimum Wavelength
• As a photon wavelength increases, the
photon energy decreases. Therefore the
maximum x-ray energy is associated with
the minimum x-ray wavelength.
• Since the minimum wavelength of x-ray
emissions corresponds to the maximum
photon energy, the maximum photon
energy is equal to the kVp.
Integration
• The total number of x-rays emitted from an
x-ray tube could be determined by adding
the number of x-rays emitted at each
energy level over the entire spectrum. This
is referred to as integration.
Factors affecting the size and
relative position of the x-ray
emission spectra.
• Tube Current (mA) effects the amplitude
• Tube Voltage effects the amplitude and
position.
• Added Filtration effects Amplitude most
effective at low energies.
Factors affecting the size and
relative position of the x-ray
emission spectra.
• Target material effects spectrum and
position of the line spectrum.
• Voltage waveform effects the amplitude,
most effective at high energies
Influence of Tube Current
• A change in mA or
mAs results in a
proportional change
in the amplitude of the
x-ray emission
spectrum at all
energies and the
intensity of the output.
Influence of Tube Potential
• Unlike tube current, a
change in kVp affects
both the amplitude
and the position of the
x-ray emission
spectrum.
Influence of Tube Potential
• When kVp increases
the relative
distribution of emitted
photons shifts to the
right or to higher
energies.
• 15% increase in kVp
is equivalent to
doubling the mAs.
Influence of Added Filtration
• Adding filtration to an
x-ray machine has an
effect on the relative
shape of the
spectrum similar to
that of increasing the
kVp.
Influence of Added Filtration
• Added filtration
effectively absorbs
more low energy x-
ray than high energy
x-rays, therefore the
spectrum is reduced
more to the left.
X-ray Filtration
• Filtration of the x-ray
beam has two
components:
– Inherent Filtration
– Added Filtration
• Filtration is required
by law.
• Aluminum is most
common material.
Filtration Affects the Beam
Spectrum
• Filtration removes the
lower energy photons
that do not contribute
to image production.
• Added filtration
results in an
increased half value
layer or higher quality
beam.
Influence of Added Filtration
• The overall result is
an increase in the
effective energy of the
beam
• The discrete and
maximum energy of
the x-ray spectrum is
not effected.
Influence of Target Material
• As the atomic number of the target
material increases, the efficiency of the
continuous spectrum x-rays increase.
• The discrete spectrum also shifts to the
right representing higher energy
characteristic radiation.
• Tungsten is used for general radiography.
Influence of Target Material
• Some specialty tube use gold.
• Molybdenum is used for mammography. It
has a lower atomic number so the discrete
spectrum is of a lower energy. This is ideal
for soft tissue studies such as
mammography.
Influence of Voltage Waveform
• As the voltage across
the x-ray tube
increases for zero to
its peak, the intensity
and energy increase
slowly at first and
then rapidly as the
peak voltage is
obtained.
Influence of Voltage Waveform
• The x-ray intensity is
not proportional to the
voltage.
• The intensity is much
higher at peak voltage
than at lower
voltages.
Type of X-ray Voltage
• High frequency or
three phase voltage
waveforms will result
in considerably more
intense x-ray
emission.
Type of X-ray Voltage
• Operation on three
phase equipment is
equivalent to a 12%
increase over single
phase equipment.
• High Frequency is a
16% increase.
Single-Phase to High
Frequency
• With the spectrum shifted to the right or
higher intensity, the change in mAs for this
conversion is to reduce mAs by 50%.
• 30 mAs single phase = 15 mAs High
Frequency or Three Phase.
Chapter 11 X-ray Emission
• The output intensity is measured in
roentgens ( R) or milliroentgens (mR) and
is termed the X-ray Quantity.
• Radiation Exposure is often used instead
of x-ray intensity or X-ray Quality.
• The number of x-rays in the useful beam is
the Radiation Quantity.
Estimating X-ray Intensity
• Using a nomogram,
we can estimate the
exposure output over
a wide range of
technical factors.
• Important factors are:
• Filtration
• kVp
Estimating X-ray Intensity
• Exposure is
expressed as
mR/mAs.
• With 3mm of Al
filtration at 70 kVp the
output is about 5
mR/mAs
• At 100 mAs, the
exposure would be
500 mR.
Factors Affecting X-ray Quantity
• A number of factors affect X-ray Quantity.
Theses same factors also control
radiographic film density:
• Milliamperage- Seconds
• kVp
• Distance
• Filtration
mA x time (s)= mAs
• The X-ray quantity is directly proportional
to the mAs. If we double the mAs, the
number of electrons striking the target is
doubled.
• 300 mA @ 1/30 second = 10 mAs
• 200 mA @ 1/20 second = 10 mAs
• 100 mA @ 1/10 second = 10 mAs
Kilovoltage
• X-ray quantity varies rapidly with changes
in kVp.
• The change in quantity is proportional to
the square of the ratio of the change.
• If the kVp is doubled, the intensity would
increase by a factor of four.
Kilovoltage
• What really happens when the kVp is
increased?
• When kVp is increased, the penetrability of
the x-rays is increased and relatively fewer
x-rays are absorbed in the patient.
• More rays pass through the patients to
interact with the film.
Kilovoltage
• To maintain a constant exposure of the
film, an increase of 15% in kVp should
be accompanied by a reduction of one
half the mAs.
Distance
• Radiation intensity from an x-ray tube
varies inversely with the square of the
distance from the target. This is referred to
as the inverse square law.
• It is the same for any type of
electromagnetic energy.
• We will explores distances in the Lab.
Filtration
• X-ray machines have
metal filters inserted
into the useful beam.
• The primary purpose
is the remove the low
energy beam that
reach the patient and
are absorbed
superficially.
Filtration
• These low energy
photons contribute
nothing to the
formation of the
radiographic image.
• Filters therefore
reduce patient
exposure.
Filtration
• Calculation of the
amount of exposure
reduction requires a
knowledge of the
Half-Value Layer.
X-ray Quality
• As the effective energy of the beam is
increases, the penetrability is also
increased.
• Penetrability refers to the range of beam in
matter; high energy beams are able to
penetrate matter farther than low energy
beams.
• Beams with high penetrability are referred
to as hard.
X-ray Quality
• Beams of low quality are called soft
beams.
• X-ray quality is identified numerically by
HVL.
• The HVL is affected by the kVp of
operation and the amount of filtration in
the useful beam.
• X-ray quality is influenced by the kVp
and filtration.
Half-Value Layer (HVL)
• Half-value layer is the thickness of
absorbing material needed to reduce the
intensity to one half of its original value.
• HVL is a characteristic of the x-ray beam.
• A Diagnostic x-ray beam usually has an
HVL of 3 to 5 mm Al.
Determining the HVL
• An exposure is made
without filtration and
the intensity is
measured.
• Different thickness of
filtration is added and
intensity is measured.
• Results are graphed.
Determining the HVL
• From the graph, the
HVL can be
determined.
• The established
standard for filtration
is 2.5 mm Al for tube
operated above 70
kVp.
Half-Value Layer
• HVL is the best method for specifying x-
ray quality.
• Variations of kVp and filtration are not
simple relationships. A tube with 2mm Al
operated at 90 kVp may have the same
HVL as when operated at 70 kVp with 5
mm AL.
Half-Value Layer
• The penetrability remains constant as
does the HVL.
Factors Affecting X-ray Quality
• Kilovoltage. As the kVp is increased, so
is beam quality and therefore HVL.
• An increase in kVp results in a shift of the
x-ray emission spectrum towards the
higher energy side.
• This increases the effective energy of the
beam, making it more penetrating.
Relationship between kVp and
HVL with 2.5 mm Al
• kVp • HVL ( mm AL)
• 50 • 1.9
• 75 • 2.8
• 100 • 3.7
• 125 • 4.6
Factors Affecting X-ray Quality
• Filtration. The
primary purpose of
adding filtration to the
x-ray beam is to
remove low energy x-
rays that have no
chance of getting to
the film.
Factors Affecting X-ray Quality
• As filtration is
increased, so is the
beam quality, but
quantity is
decreased.
Types of Filtration
• There are three types of filtration:
• Inherent Filtration: Glass envelope
window equals about 0.5mm Al
• Added Filtration: Added in collimator
• Compensating Filtration: Used to
improve image quality or radiation
reduction
Inherent Filtration
• The glass envelope of the tube filters the
emerging beam. In diagnostic x-ray tubes
the glass is equal to about 0.5 mm Al.
• As tube ages and more tungsten is
vaporized, tungsten will build up on the
inside of the tube that will add more
filtration.
Added Filtration
• One or two mm of aluminum is added
filtration placed in the collimator. This
filtration is generally placed on the mirror
of the collimator.
• This filtration attenuates x-rays of all
energies emitted from the tube. This shifts
the spectrum to the high side.
Added Filtration
• This shift in the emission spectrum results
in a beam with higher effective energy,
greater penetrability and higher quality.
• This results in an increased half value
layer.
• The minimum filtration for tube operated
above 70 kVp is 2.5 mm Al equivalence.
Compensating Filters
• Compensating filters
are added to the
beam by the operator
to compensate for
differences in subject
tissue density or type.
• We use the Nolan
Filter System.
Compensating filters
• In areas of the body where there are great
differences in tissue density,
compensating filters are used to reduce
exposure in the area of less density.
• This reduced patient exposure and
improves image quality. The thoracic spine
and full spine x-rays need filtration.
Compensating Filters
• This is the 40”
Cervicothoracic
Compensating Filter.
• It may be called the
thyroid filter as it
reduces the exposure
to the upper thorax.
Compensating Filters
• This heart shaped
filter is used to reduce
exposure to the
ovaries of females of
child bearing age.
• It reduces exposure
by about 85%.
End of Lecture
