X-RAY PRODUCTION &
X-RAY EMISSIONS
RADIATION PHYSICS
TUBE INTERACTIONS
• 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.
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 of
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 loses 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.
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.
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.
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%.