Chapter 10 X-ray Production & Chapter 11 X-ray Emissions by f8vPH0

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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

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