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PRESENTATION BY:   DR. CHARUSMITA
                       CHAUDHARY
Dual characteristics of X-rays
 X-rays belong to a group of radiation called
  electromagnetic radiation .

 Electromagnetic radiation has dual characteristic,
  comprises of both
   Wave
   Particle

   Wave concept : Propagated through space in the form of waves.
 Waves of all types have associated wavelength and frequency
      Relationship : c=λν.
      c=velocity of light
      λ=wavelength
      ν=frequency
   The wavelength of diagnostic X-rays is very short around 0.1 to
 1A. Wave concept explains why it can be reflected.
   Particle concept of EM radiation: Short EM waves such as X-
    rays predominantly react with matter as if they were particles rather
    than waves. At high frequencies electrons interact with EM radiation
    as if the EM radiation were an energy bundle. These particles are
    actually discrete bundles of energy and each of these bundles is
    called a quantum or photon.
   Particle concept used to describe interaction between radiation &
    matter
    .The amount of energy carried by each quantum is given by
       E=h ν      E=photon’s energy
                 h=planck’s constant
                 ν =frequency
   c= νλ or ν=c/λ so substituting c/ λ for ν we get
   E=hc/ λ        h=4.13x 10-18 keV/sec
                  c=3 x 108 m/sec
   E=12.4
E=Energy in keV. λ=wavelength in A0
METHODS OF INTERACTIONS

   Photons : absorbed / scattered.
   Attenuation : Reduction of intensity. Difference in attenuation gives
    the radiographic image.
   Absorbed : completely removed from the x-ray beam & cease to exist.
   Scattered : Random course. No useful information. No image only
    darkness. Adds noise to the system. Film quality affected : “film fog”.

   About 1% of the x rays that strike
     a patient's body emerge from the
    body to produce the final image.
    The radiographic image is formed
    on a radiographic plate that is
    similar to the film of a camera.
   Remaining 99% of the x-rays ---
    Scattered / Absorbed.
ATOMIC STRUCTURE
X-ray photons may interact either with orbital electrons or
 with the nucleus. In the diagnostic energy range, the
 interactions are always with orbital electrons.

The molecular bonding energies ,however are too small to
 influence the type and number of interactions .

The most important factor is the atomic make up of a
 tissue and not its molecular structure.
Atomic structure
                              Basic structure of an ATOM :
                                                                           PROTON ( +ve charge )
                                 An atom is made up of       NUCLEUS
                                                                           NEUTRON ( neutral )


                                                             ORBITAL ELECTRONS ( -ve charge )


                                                             ORBITS / SHELLS ( K, L, M, N etc. )




 K shell : 2 electrons
 L shell : 8 electrons
 Each shell has a specific binding energy & closer the shell is to the nucleus,
  the tighter it is bound to the nucleus. The electrons in the outermost shell
  are loosely bound to the nucleus & are hence called “free electrons”.
physical interaction of x ray with matter
 Energy value of electronic shells is also determined by the
  atomic number of the atom.

 K-shell electron are more tightly bound in elements of
  high atomic number. Pb : 88keV while Ca : 4keV.

 Electrons in the K -shell are at a lower energy level than
  electrons in the L-shell. If we consider the outermost
  electrons as free ,than inner shell electrons are in energy
  debt. The energy debt is greatest when they are close to
  nucleus in an element with a high atomic number.
BASIC INTERACTIONS BETWEEN X-
RAYS AND MATTER
  There are 12 mechanism, out of which five basic ways in which an x-ray
    photon may interact with matter.
  These are :Broadly classified on the basis of-

     A: PHOTON                               B: PHOTON
                                                 DISAPPEARANCE
         SCATTERING:
                                               - PHOTOELECTRIC
        - COHERENT                               EFFECT
         SCATTERING                            - PAIR
        - COMPTON                                PRODUCTION
         SCATTERING                            -
                                                 PHOTODISINTEGR
                                                 ATION
1. COHERENT SCATTERING


 Radiation undergoes
          Only Change in direction. No change in wavelength
thats why sometime called “ unmodified scattering”
Coherent scattering of X-rays is an interaction of the wave
type in which the X-ray is deflected.
Coherent Scattering occurs mainly at low energies.
It is of
two types :Both type described in terms of “ wave Particle Interaction”
          ( also called “ Classical scattering”)
      Thomson scattering : Single electron involved in the interaction.

      Rayleigh scattering : Co-operative interaction of all the electrons.
1. COHERENT SCATTERING


What happens in coherent scattering ?

        Low energy radiation encounters electrons


          Electrons are set into vibration



          Vibrating electron, emits radiation.


         Atom returns to its undisturbed state




                                                    Fig : Rayleigh scattering
1. COHERENT SCATTERING



 No ionization --- why??? because, no energy transfer. Only
  change of direction.

 Only effect is to change direction of incident photon.

 Less than 5%. Not important in diagnostic radiology.
  Produces scattered radiation but of negligible quantity.
2. PHOTOELECTRIC EFFECT


  What happens in Photoelectric effect ?
                An incident PHOTON encounters a K shell electron and ejects it
    from the orbit


       The photon disappears, giving up ( nearly) all its energy to the
 electron

  The electron ( now free of its energy debt) flies off into space as a
 photoelectron carrying the excess energy as kinetic energy.

      The K shell electron void filled immediately by another electron and
 hence the excess energy is released as CHARACTERISTIC RADIATION.

                          The atom is ionised.
PHOTOELECTRIC EFFECT
Percentage of photoelectric reactions

Radiation   Water        Compact      Sodium
energy(keV)              bone         iodide
20          65           89           94


60          7            31           95


100         2            9            88
CHARACTERISTIC RADIATION
Characteristic radiation generated by the photoelectric effect is exactly the same
The only difference in the modality used to eject the inner shell electron.

   In x ray tube a high speed electron ejects the bound electron,
  while
  In photoelectric effect an X ray photon does the trick.
  In both cases
the atom is left with an excess of energy = the binding energy of an ejected electron



  Usually referred to as Secondary Radiation to differentiate
  It from scatter radiation……
  End result is same for both,
  “A Photon that is deflected from its original path”
Characteristic radiation
   How does this happen ?
      After the electron has been ejected, the atom is left with a
  void in the K shell & an excess of energy equivalent to the
  binding energy.

 This state of the atom is highly unstable & to achieve a low
  energy stable state ( as all physical systems seek the lowest
  possible energy state ) an electron immediately drops in to fill
  the void.

 As the electron drops into the K shell, it gives up its excess
  energy in the form of an x-ray photon. The amount of energy
  released is characteristic of each element & hence the
  radiation produced is called Characteristic radiation.
physical interaction of x ray with matter
2. PHOTOELECTRIC EFFECT



 Thus the Photoelectric effect yields three end products
 :

       Characteristic radiation
       A -ve ion ( photoelectron )
       A+ve ion (atom deficient in one electron )
2. PHOTOELECTRIC EFFECT


Probability of occurrence :
 The incident photon energy > binding energy of the
 electron.

 Photon energy similar to electron binding energy
             Photoelectric effect     1
                                  (energy)³



 The probability of a reaction increases sharply as the
 atomic no. increases
                      Photoelectric effect  (atomic no.)³
 Low atomic number : interaction mostly at the K shell.
 High atomic number : interaction mostly at L and M shell.


 In summary, Photoelectric reactions are most likely to
  occur with low energy photons and elements with high
  atomic numbers provided the photons have sufficient
  energy to overcome the forces binding the electrons in
  their cells.
For eg : I2
K shell :33.2keV
L-shell : 4.9keV
M shell 0.6 Kev.

From L-shell to K-
shell a 28.3 kev(33.2-
4.9=28.3) keV photon
is released.

The void in the L-
shell is then filled
with a photon from
the M shell with the
production of a ( 4.9-
0.6 KeV)4.3 keV
photon.
K-shell electron binding energies of elements important in
diagnostic radiology



Atom            Atomic number       K-shell binding energy(keV)

Calcium         20                  4.04
Iodine          53                  33.2
Barium          56                  37.4
Tungsten        74                  69.5
Lead            82                  88.O
2. PHOTOELECTRIC EFFECT :
Applications in diagnostic radiology :

                                      Disadvantage:
   Advantages :
                                       Maximum radiation exposure.
   Excellent radiographic images :
    No scatter radiation.             All the energy is absorbed by the
                                         patient whereas in other reactions
                                         only part of the incident photon’s
    Enhances natural tissue             energy is absorbed.
     contrast. Depends on 3rd power
     of the atomic no., so it
     magnifies the difference in
     tissues composed of different
     elements, such as bone & soft
     tissue

    Lower energy photons : total
     absorption. Dominant upto 500
     keV.
3. COMPTON EFFECT


          The Compton effect occurs when the incident x-ray
     photon with relatively high energy ejects an electron
     from an atom and a x-ray photon of lower energy is
     scattered from the atom.




The reaction produces an ion pair
    A +ve atom
    A –ve electron ( recoil
    electron )
COMPTON SCATTERING

Almost all the scatter radiation that we encountered
In diagnostic radiology comes from Compton Scattering
3. COMPTON EFFECT


                                    Kinetic energy of recoil electron
 Energy of photon distributed

                                    Retained by the deflected photon.

 Two factors determine the amount of energy the photon transmits :
      The initial energy of the photon.
      Its angle of deflection.
       1.Initial energy :- Higher the energy more difficult to deflect.
               High energy : Travel straight retaining most of the
      energy.
               Low energy : Most scatter back at angle of 180º
       2. Angle of deflection :- Greater the angle, lesser the energy
      trasmitted. With a direct hit, maximum energy is transferred to
      the recoil electron. The photon retains some energy & deflects
      back along its original path at an angle of 180º.
ENERGY OF COMPTON SCATTERED PHOTONS




The change in wavelength of a scattered photon is calculated as :
                         Δλ = 0.024 ( 1 – cos θ ) ,
                 where Δλ = change in wavelength
                          θ = angle of photon deflection
3. COMPTON EFFECT

Probability of occurence :
    It depends on :-
    Total number of electrons : It further depends on density and number
   of electrons per gram of the absorber. All elements contain approx. the
   same no. of electrons per gram, regardless of their atomic no. Therefore the
   no. of Compton reactions is independent of the atomic no. of the absorber.

    Energy of the radiation : The no. of reactions gradually diminishes as
   photon energy increases, so that a high energy photon is more likely to pass
   through the body than a low energy photon.

Two subsequent points should also be noted:
 Firstly, the photoelectron can cause ionizations along its track.

 Secondly, X-ray emission can occur when the vacancy left by the
  photoelectron is filled by an electron from an outer shell of the atom.
3. COMPTON EFFECT
Disadvantages of Compton reaction :
  Scatter radiation : Almost all the scatter radiation that we encounter in diagnostic
  Radiology comes from Compton scattering. In the diagnostic energy range, the
  photon retains most of its original energy. This creates a serious problem, because
  photons that are scattered at narrow angles have an excellent chance of reaching an x-
  ray film & producing fog.

  Exceedingly difficult to remove –
          ► cannot be removed by filters because they are too energetic.
          ► cannot be removed by grids because of narrow angles of deflection.


     It is also a major safety hazard. Even after 90˚ deflection most of its original
    energy is retained.
    Scatter radiation as energetic as the primary radiation.
    Safety hazard for the radiologist, personnel and the patient.
4. PAIR PRODUCTION

No importance in diagnostic radiology.

What happens in Pair production ?
A high energy photon interacts with the nucleus of an atom.
The photon disappears & its energy is converted into matter in the form
of two particles
   An electron
   A positron (particle with same mass as electron, but with +ve
   charge.)

Mass of one electron is 0.51 MeV.
2 electron masses are produced.
So the interaction cannot take place with photon energy less than 1.02
   MeV.
4. PAIR PRODUCTION



                     Positron annihilation.
                     What happens to the
                     Positron ?

                     Slowly moving Positron
                     combines with a free electron
                     to produce two photons of
                     radiation.
                     2 mass units are converted,
                     giving a total energy of 1.022
                     MeV.
                     To conserve momentum, two
                     photons each with 0.511 MeV
                     energy are ejected in opposite
                     direction.
5. PHOTODISINTEGRATION

  A photon with extremely high energy ( 7-15 MeV), interacts directly
  with the nucleus of an atom.
  May eject a neutron, proton or on rare occasions even an alpha
  particle.

  No diagnostic importance.
  We rarely use radiation >150 KeV in diagnostic radiology.

What happens in Photodisintegration ?
   A high energy photon encounters the nucleus of an atom.


Part of the nucleus which may be a neutron, a proton, an alpha particle or
                      a cluster of particles, is ejected.
RELATIVE FREQUENCY OF BASIC INTERACTIONS

     Coherent scattering : About 5% .
      Minor role throughout the diagnostic energy range.

     Compton scattering : Dominant interaction in water.
      Water is used to represent tissues with low atomic nos.
      such as air, fat and muscle.

     Photoelectric reaction : usually seen in the contrast
      agents because of their high atomic numbers.

      Bone is intermediate between water & the contrast agents.
      At low energies, Photoelectric reactions are more common,
      while at high energies, Compton scattering is dominant.
RELATIVE FREQUENCY OF BASIC INTERACTIONS
X-ray photon    Photoelectric    Compton scatter   Pair production%
energy
                absorption   %   %
 10 keV         95               5                 0
 25 keV      50                  50                0
(Mammography
  60 keV        7                93                0
 (Diagnostic)
150 keV         0                100               0
                0                94                6
4 MeV

10 MeV          0                77                23
(Therapy)
24 MeV          0                50                50
Scatter Radiation
Scatter Radiation
 Definition

   A type of secondary radiation composed of
   photons of lower energy than the photons that
   produced them and which travel in a different
   direction.

   The term scatter radiation is synonymous with
   secondary radiation in the context of x-rays
Scatter radiation & Contrast - overview

  Radiographic images are maps of radiation attenuation. Bones
   attenuate the most, air in lungs the least.
  Good radiograph : maximum contrast difference between
   different tissues.

                      X-Ray beam enters body.

   Large number of interactions producing scatter radiation.

        Image contrast reduced depending on scatter radiation
        content reaching film.
CONTRAST REDUCTION
Assumed that the object shown here is not penetrated and
  would produce 100% contrast if no scatter radiation.
Sources of scattered radiation
  Transmitted scatter
    constitutes greater portion of scattered radiation and
    originates from the patient under examination.
  Scatter from cassette
  Side scatter
    Side scatter originates from walls, or objects on the source
    side of the film
  Reflection scatter or Back scatter
    It is often called backscatter when it comes from objects
    behind the film.
  Undercut
    Undercut occurs due to scattering within the film
Factors affecting scatter radiation


  Field size


  Kilo voltage (kVp)


  Anatomical volume (Part thickness)
Factors affecting scatter radiation
 Scatter radiation is maximum with high kvp
  technique, large field , and thick parts----
 Unfortunately, this is what we usually deal with in
  diagnostic radiology.

 The only variable we can control is kvp , but we have
 less control .
Factors affecting scatter radiation
 Field Size
   Most important factor in the production of
    scatter radiation.
   A small x ray field usually called Narrow beam
    irradiates less tissue and generates fewer
    scattered photons.




Contrast Improvement by
Reducing X-Ray Beam Size
Factors affecting scatter radiation
 Kvp

           The effect of kvp on the production of
 scatter radiation is probably not as important as part
 thickness , and as field size.
Factors affecting scatter radiation
KvP


                   Photoelectric      Less
         Low Kvp
                      effect       scattering




       Higher      Compton           More
        Kvp         effect         scattering
Factors affecting scatter radiation


  Part thickness

    Scatter radiation is directly proportional to the
     part thickness.

    The operator has no control over this parameter.
Effects of scatter radiation

 Reduction of contrast: Scattered photons
      Carry no useful information
      Contribute to film blackness(film fog)


 Increased patient dose


 Increased risk to personnel
Control of scatter radiation
Backscatter : „B‟ is industry
code.

Lead „B‟ behind cassette to
assess backscatter.

If the letter "B" shows as a
"ghost" image on the film, a
significant amount of
backscatter radiation is
reaching the film.

Control of backscatter radiation
by : Backing film in the
cassette with a sheet of lead
that is at least 0.010 inch thick.

Industry practice : 0.005" lead
screen in front and a 0.010"
screen behind the film.
Prevention of scatter
            radiation
Different techniques are used to keep the
scatter radiation from reaching the films.
   X ray filters
   X ray beam
    restrictors
   Grids (most important)
SUMMARY
Only two interactions are important in diagnostic radiology, the Photoelectric effect &
  Compton scattering.
       The Photoelectric effect
        is the predominant interaction with low energy radiation & high atomic
        no. absorbers.
        It generates no significant scatter radiation & produces high contrast in
        the x-ray image.
                   But, unfortunately it exposes the patient to a great deal of
                 radiation.
         Compton scattering
           is the most common interaction at higher diagnostic energies.
           responsible for almost all scatter radiation.
           radiographic image contrast is less compared to photoelectric
            effect.
       Coherent scattering is numerically unimportant.
       Pair production & Photodisintegration occur at energies above the
        useful energy range.
SUMMARY
 Scatter Radiation
    secondary radiation composed of photons of lower
     energy than the photons that produced them and
     which travel in a different direction.
    Factors affecting it :
         Field size
         Kilo voltage (kVp)
         Anatomical volume (Part thickness)
     No useful information, causes film fog and
      increases patient exposure.
Thank you   Have A nice day




    THANK         YOU

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physical interaction of x ray with matter

  • 1. PRESENTATION BY: DR. CHARUSMITA CHAUDHARY
  • 2. Dual characteristics of X-rays  X-rays belong to a group of radiation called electromagnetic radiation .  Electromagnetic radiation has dual characteristic, comprises of both  Wave  Particle Wave concept : Propagated through space in the form of waves. Waves of all types have associated wavelength and frequency Relationship : c=λν. c=velocity of light λ=wavelength ν=frequency The wavelength of diagnostic X-rays is very short around 0.1 to 1A. Wave concept explains why it can be reflected.
  • 3. Particle concept of EM radiation: Short EM waves such as X- rays predominantly react with matter as if they were particles rather than waves. At high frequencies electrons interact with EM radiation as if the EM radiation were an energy bundle. These particles are actually discrete bundles of energy and each of these bundles is called a quantum or photon.  Particle concept used to describe interaction between radiation & matter .The amount of energy carried by each quantum is given by E=h ν E=photon’s energy h=planck’s constant ν =frequency c= νλ or ν=c/λ so substituting c/ λ for ν we get E=hc/ λ h=4.13x 10-18 keV/sec c=3 x 108 m/sec E=12.4 E=Energy in keV. λ=wavelength in A0
  • 4. METHODS OF INTERACTIONS  Photons : absorbed / scattered.  Attenuation : Reduction of intensity. Difference in attenuation gives the radiographic image.  Absorbed : completely removed from the x-ray beam & cease to exist.  Scattered : Random course. No useful information. No image only darkness. Adds noise to the system. Film quality affected : “film fog”.  About 1% of the x rays that strike a patient's body emerge from the body to produce the final image. The radiographic image is formed on a radiographic plate that is similar to the film of a camera.  Remaining 99% of the x-rays --- Scattered / Absorbed.
  • 5. ATOMIC STRUCTURE X-ray photons may interact either with orbital electrons or with the nucleus. In the diagnostic energy range, the interactions are always with orbital electrons. The molecular bonding energies ,however are too small to influence the type and number of interactions . The most important factor is the atomic make up of a tissue and not its molecular structure.
  • 6. Atomic structure Basic structure of an ATOM : PROTON ( +ve charge ) An atom is made up of NUCLEUS NEUTRON ( neutral ) ORBITAL ELECTRONS ( -ve charge ) ORBITS / SHELLS ( K, L, M, N etc. )  K shell : 2 electrons  L shell : 8 electrons  Each shell has a specific binding energy & closer the shell is to the nucleus, the tighter it is bound to the nucleus. The electrons in the outermost shell are loosely bound to the nucleus & are hence called “free electrons”.
  • 8.  Energy value of electronic shells is also determined by the atomic number of the atom.  K-shell electron are more tightly bound in elements of high atomic number. Pb : 88keV while Ca : 4keV.  Electrons in the K -shell are at a lower energy level than electrons in the L-shell. If we consider the outermost electrons as free ,than inner shell electrons are in energy debt. The energy debt is greatest when they are close to nucleus in an element with a high atomic number.
  • 9. BASIC INTERACTIONS BETWEEN X- RAYS AND MATTER There are 12 mechanism, out of which five basic ways in which an x-ray photon may interact with matter. These are :Broadly classified on the basis of- A: PHOTON B: PHOTON DISAPPEARANCE SCATTERING: - PHOTOELECTRIC - COHERENT EFFECT SCATTERING - PAIR - COMPTON PRODUCTION SCATTERING - PHOTODISINTEGR ATION
  • 10. 1. COHERENT SCATTERING Radiation undergoes Only Change in direction. No change in wavelength thats why sometime called “ unmodified scattering” Coherent scattering of X-rays is an interaction of the wave type in which the X-ray is deflected. Coherent Scattering occurs mainly at low energies. It is of two types :Both type described in terms of “ wave Particle Interaction” ( also called “ Classical scattering”)  Thomson scattering : Single electron involved in the interaction.  Rayleigh scattering : Co-operative interaction of all the electrons.
  • 11. 1. COHERENT SCATTERING What happens in coherent scattering ? Low energy radiation encounters electrons Electrons are set into vibration Vibrating electron, emits radiation. Atom returns to its undisturbed state Fig : Rayleigh scattering
  • 12. 1. COHERENT SCATTERING  No ionization --- why??? because, no energy transfer. Only change of direction.  Only effect is to change direction of incident photon.  Less than 5%. Not important in diagnostic radiology. Produces scattered radiation but of negligible quantity.
  • 13. 2. PHOTOELECTRIC EFFECT What happens in Photoelectric effect ? An incident PHOTON encounters a K shell electron and ejects it from the orbit The photon disappears, giving up ( nearly) all its energy to the electron The electron ( now free of its energy debt) flies off into space as a photoelectron carrying the excess energy as kinetic energy. The K shell electron void filled immediately by another electron and hence the excess energy is released as CHARACTERISTIC RADIATION. The atom is ionised.
  • 15. Percentage of photoelectric reactions Radiation Water Compact Sodium energy(keV) bone iodide 20 65 89 94 60 7 31 95 100 2 9 88
  • 16. CHARACTERISTIC RADIATION Characteristic radiation generated by the photoelectric effect is exactly the same The only difference in the modality used to eject the inner shell electron. In x ray tube a high speed electron ejects the bound electron, while In photoelectric effect an X ray photon does the trick. In both cases the atom is left with an excess of energy = the binding energy of an ejected electron Usually referred to as Secondary Radiation to differentiate It from scatter radiation…… End result is same for both, “A Photon that is deflected from its original path”
  • 17. Characteristic radiation How does this happen ?  After the electron has been ejected, the atom is left with a void in the K shell & an excess of energy equivalent to the binding energy.  This state of the atom is highly unstable & to achieve a low energy stable state ( as all physical systems seek the lowest possible energy state ) an electron immediately drops in to fill the void.  As the electron drops into the K shell, it gives up its excess energy in the form of an x-ray photon. The amount of energy released is characteristic of each element & hence the radiation produced is called Characteristic radiation.
  • 19. 2. PHOTOELECTRIC EFFECT Thus the Photoelectric effect yields three end products :  Characteristic radiation  A -ve ion ( photoelectron )  A+ve ion (atom deficient in one electron )
  • 20. 2. PHOTOELECTRIC EFFECT Probability of occurrence :  The incident photon energy > binding energy of the electron.  Photon energy similar to electron binding energy Photoelectric effect  1 (energy)³  The probability of a reaction increases sharply as the atomic no. increases Photoelectric effect  (atomic no.)³
  • 21.  Low atomic number : interaction mostly at the K shell.  High atomic number : interaction mostly at L and M shell.  In summary, Photoelectric reactions are most likely to occur with low energy photons and elements with high atomic numbers provided the photons have sufficient energy to overcome the forces binding the electrons in their cells.
  • 22. For eg : I2 K shell :33.2keV L-shell : 4.9keV M shell 0.6 Kev. From L-shell to K- shell a 28.3 kev(33.2- 4.9=28.3) keV photon is released. The void in the L- shell is then filled with a photon from the M shell with the production of a ( 4.9- 0.6 KeV)4.3 keV photon.
  • 23. K-shell electron binding energies of elements important in diagnostic radiology Atom Atomic number K-shell binding energy(keV) Calcium 20 4.04 Iodine 53 33.2 Barium 56 37.4 Tungsten 74 69.5 Lead 82 88.O
  • 24. 2. PHOTOELECTRIC EFFECT : Applications in diagnostic radiology : Disadvantage: Advantages :  Maximum radiation exposure. Excellent radiographic images :  No scatter radiation.  All the energy is absorbed by the patient whereas in other reactions only part of the incident photon’s  Enhances natural tissue energy is absorbed. contrast. Depends on 3rd power of the atomic no., so it magnifies the difference in tissues composed of different elements, such as bone & soft tissue  Lower energy photons : total absorption. Dominant upto 500 keV.
  • 25. 3. COMPTON EFFECT The Compton effect occurs when the incident x-ray photon with relatively high energy ejects an electron from an atom and a x-ray photon of lower energy is scattered from the atom. The reaction produces an ion pair A +ve atom A –ve electron ( recoil electron )
  • 26. COMPTON SCATTERING Almost all the scatter radiation that we encountered In diagnostic radiology comes from Compton Scattering
  • 27. 3. COMPTON EFFECT Kinetic energy of recoil electron  Energy of photon distributed Retained by the deflected photon.  Two factors determine the amount of energy the photon transmits :  The initial energy of the photon.  Its angle of deflection. 1.Initial energy :- Higher the energy more difficult to deflect. High energy : Travel straight retaining most of the energy. Low energy : Most scatter back at angle of 180º 2. Angle of deflection :- Greater the angle, lesser the energy trasmitted. With a direct hit, maximum energy is transferred to the recoil electron. The photon retains some energy & deflects back along its original path at an angle of 180º.
  • 28. ENERGY OF COMPTON SCATTERED PHOTONS The change in wavelength of a scattered photon is calculated as : Δλ = 0.024 ( 1 – cos θ ) , where Δλ = change in wavelength θ = angle of photon deflection
  • 29. 3. COMPTON EFFECT Probability of occurence : It depends on :-  Total number of electrons : It further depends on density and number of electrons per gram of the absorber. All elements contain approx. the same no. of electrons per gram, regardless of their atomic no. Therefore the no. of Compton reactions is independent of the atomic no. of the absorber.  Energy of the radiation : The no. of reactions gradually diminishes as photon energy increases, so that a high energy photon is more likely to pass through the body than a low energy photon. Two subsequent points should also be noted:  Firstly, the photoelectron can cause ionizations along its track.  Secondly, X-ray emission can occur when the vacancy left by the photoelectron is filled by an electron from an outer shell of the atom.
  • 30. 3. COMPTON EFFECT Disadvantages of Compton reaction : Scatter radiation : Almost all the scatter radiation that we encounter in diagnostic Radiology comes from Compton scattering. In the diagnostic energy range, the photon retains most of its original energy. This creates a serious problem, because photons that are scattered at narrow angles have an excellent chance of reaching an x- ray film & producing fog. Exceedingly difficult to remove – ► cannot be removed by filters because they are too energetic. ► cannot be removed by grids because of narrow angles of deflection. It is also a major safety hazard. Even after 90˚ deflection most of its original energy is retained. Scatter radiation as energetic as the primary radiation. Safety hazard for the radiologist, personnel and the patient.
  • 31. 4. PAIR PRODUCTION No importance in diagnostic radiology. What happens in Pair production ? A high energy photon interacts with the nucleus of an atom. The photon disappears & its energy is converted into matter in the form of two particles  An electron  A positron (particle with same mass as electron, but with +ve charge.) Mass of one electron is 0.51 MeV. 2 electron masses are produced. So the interaction cannot take place with photon energy less than 1.02 MeV.
  • 32. 4. PAIR PRODUCTION Positron annihilation. What happens to the Positron ? Slowly moving Positron combines with a free electron to produce two photons of radiation. 2 mass units are converted, giving a total energy of 1.022 MeV. To conserve momentum, two photons each with 0.511 MeV energy are ejected in opposite direction.
  • 33. 5. PHOTODISINTEGRATION A photon with extremely high energy ( 7-15 MeV), interacts directly with the nucleus of an atom. May eject a neutron, proton or on rare occasions even an alpha particle. No diagnostic importance. We rarely use radiation >150 KeV in diagnostic radiology. What happens in Photodisintegration ? A high energy photon encounters the nucleus of an atom. Part of the nucleus which may be a neutron, a proton, an alpha particle or a cluster of particles, is ejected.
  • 34. RELATIVE FREQUENCY OF BASIC INTERACTIONS Coherent scattering : About 5% . Minor role throughout the diagnostic energy range. Compton scattering : Dominant interaction in water. Water is used to represent tissues with low atomic nos. such as air, fat and muscle. Photoelectric reaction : usually seen in the contrast agents because of their high atomic numbers.  Bone is intermediate between water & the contrast agents. At low energies, Photoelectric reactions are more common, while at high energies, Compton scattering is dominant.
  • 35. RELATIVE FREQUENCY OF BASIC INTERACTIONS
  • 36. X-ray photon Photoelectric Compton scatter Pair production% energy absorption % % 10 keV 95 5 0 25 keV 50 50 0 (Mammography 60 keV 7 93 0 (Diagnostic) 150 keV 0 100 0 0 94 6 4 MeV 10 MeV 0 77 23 (Therapy) 24 MeV 0 50 50
  • 38. Scatter Radiation  Definition  A type of secondary radiation composed of photons of lower energy than the photons that produced them and which travel in a different direction.  The term scatter radiation is synonymous with secondary radiation in the context of x-rays
  • 39. Scatter radiation & Contrast - overview  Radiographic images are maps of radiation attenuation. Bones attenuate the most, air in lungs the least.  Good radiograph : maximum contrast difference between different tissues. X-Ray beam enters body. Large number of interactions producing scatter radiation. Image contrast reduced depending on scatter radiation content reaching film.
  • 40. CONTRAST REDUCTION Assumed that the object shown here is not penetrated and would produce 100% contrast if no scatter radiation.
  • 41. Sources of scattered radiation  Transmitted scatter constitutes greater portion of scattered radiation and originates from the patient under examination.  Scatter from cassette  Side scatter Side scatter originates from walls, or objects on the source side of the film  Reflection scatter or Back scatter It is often called backscatter when it comes from objects behind the film.  Undercut Undercut occurs due to scattering within the film
  • 42. Factors affecting scatter radiation  Field size  Kilo voltage (kVp)  Anatomical volume (Part thickness)
  • 43. Factors affecting scatter radiation  Scatter radiation is maximum with high kvp technique, large field , and thick parts----  Unfortunately, this is what we usually deal with in diagnostic radiology.  The only variable we can control is kvp , but we have less control .
  • 44. Factors affecting scatter radiation  Field Size  Most important factor in the production of scatter radiation.  A small x ray field usually called Narrow beam irradiates less tissue and generates fewer scattered photons. Contrast Improvement by Reducing X-Ray Beam Size
  • 45. Factors affecting scatter radiation  Kvp The effect of kvp on the production of scatter radiation is probably not as important as part thickness , and as field size.
  • 46. Factors affecting scatter radiation KvP Photoelectric Less Low Kvp effect scattering Higher Compton More Kvp effect scattering
  • 47. Factors affecting scatter radiation  Part thickness  Scatter radiation is directly proportional to the part thickness.  The operator has no control over this parameter.
  • 48. Effects of scatter radiation  Reduction of contrast: Scattered photons  Carry no useful information  Contribute to film blackness(film fog)  Increased patient dose  Increased risk to personnel
  • 49. Control of scatter radiation Backscatter : „B‟ is industry code. Lead „B‟ behind cassette to assess backscatter. If the letter "B" shows as a "ghost" image on the film, a significant amount of backscatter radiation is reaching the film. Control of backscatter radiation by : Backing film in the cassette with a sheet of lead that is at least 0.010 inch thick. Industry practice : 0.005" lead screen in front and a 0.010" screen behind the film.
  • 50. Prevention of scatter radiation Different techniques are used to keep the scatter radiation from reaching the films.  X ray filters  X ray beam restrictors  Grids (most important)
  • 51. SUMMARY Only two interactions are important in diagnostic radiology, the Photoelectric effect & Compton scattering.  The Photoelectric effect is the predominant interaction with low energy radiation & high atomic no. absorbers. It generates no significant scatter radiation & produces high contrast in the x-ray image.  But, unfortunately it exposes the patient to a great deal of radiation. Compton scattering  is the most common interaction at higher diagnostic energies.  responsible for almost all scatter radiation.  radiographic image contrast is less compared to photoelectric effect.  Coherent scattering is numerically unimportant.  Pair production & Photodisintegration occur at energies above the useful energy range.
  • 52. SUMMARY  Scatter Radiation  secondary radiation composed of photons of lower energy than the photons that produced them and which travel in a different direction.  Factors affecting it :  Field size  Kilo voltage (kVp)  Anatomical volume (Part thickness)  No useful information, causes film fog and increases patient exposure.
  • 53. Thank you Have A nice day THANK YOU

Notas del editor

  1. The smaller the field larger the escape angle.