X-ray Interaction with Matter

1
X-ray Interaction with Matter

• Electromagnetic Radiation interacts with
  structures with similar size to the wavelength of
  the radiation.
• Interactions have wavelike and particle like
  properties.
• X-rays have a very small wavelength, no larger
  than 10-8 to 10-9.

2
X-ray Interaction with Matter

• The higher the energy of the x-ray, the shorter
  the wavelength.
• Low energy x-rays interact with whole atoms.
• Moderate energy x-rays interact with electrons.
• High energy x-rays interact with the nuclei.

3
Five forms of x-ray Interactions

• Classical or Coherent Scattering
• Compton Effect
• Photoelectric Effect
• Pair production
• Photodisintegration

4
Two Forms of X-ray Interactions Important to
Diagnostic X-ray

• Compton Effect
• Photoelectric Effect

5
Classical or Coherent Scattering

• Low energy x-rays of about 10 keV interact in
  this manner.
• Incident photon interacts with the atom.
• There is a change in direction.

6
Classical or Thompson Scattering

• There is no loss of energy and no ionization.
• Photon scattered forward.
• Because these are low energy x-rays, they are of
  little importance.

7
Classical Scattering

• At 70 kVp only a few percent of the x-rays
  undergo this form of scattering.
• Classic Scatter may contribute to the graying of
  the image called film fog.

8
Compton Effect

• Moderate energy x-ray photon through out the
  diagnostic x-ray range can interact with outer
  shell electron.
• This interaction not only changes the direction
  but

9
Compton Effect

• reduced its energy and ionizes the atom as well.
  The outer shell electron is ejected. This is
  called Compton Effect or Compton Scattering.

10
Compton Scattering

• The x-ray continues in an altered direction with
  decreased energy.
• The energy of the Compton-scattered x-ray is
  equal to the difference between the energy of the
  incident x-ray and the energy imparted to the
  electron.

11
Compton Scattering

• The energy imparted to the electron is equal to
  its binding energy plus the kinetic with which it
  leaves the atom.
• During Compton-scattering most of the energy is
  divided between the scattered photon and the
  secondary electron.
• The Secondary Electron is called a Compton
  Electron.

12
Compton Scattering

• The scattered photon and secondary electron will
  retain most of its energy so it can interact many
  times before it losing all of its energy.

13
Compton Effect

• The scattered photon will ultimately be absorbed
  photoelectrically.
• The secondary electron will drop into a hole in
  the outer shell of an atom created by an ionizing
  event.
• Compton-scattered photons can be deflected in any
  direction.

14
Compton Effect

• A zero angle deflection will result in no energy
  loss.
• As the angle approaches 180 degrees, more energy
  is transferred to the secondary electron.
• Even at 180 degrees, 66 of the energy is
  retained.

15
Compton Effect

• Photons scattered back towards the incident x-ray
  beam are called Backscatter Radiation.
• While important in radiation therapy, backscatter
  in diagnostic x-ray is sometimes responsible for
  the hinges on the back of the the cassette to be
  seen on the x-ray film

16
Compton Effect

• The probability of Compton Effect is about the
  same for soft tissue or bone.
• This decreases with increasing photon energies.
• Compton scatter decreases with increased kVp.

17
Features of Compton Scattering

• Most likely to occur
• As x-ray energy increases

• With outer-shell electrons
• With loosely bound electrons.
• Increased penetration through tissue w/o
  interaction.
• Increased Compton relative to photoelectric
  scatter.
• Reduced total Compton scattering.

18
Features of Compton Scatter

• As atomic number of the absorber increases
• As mass density of absorber increases

• No effect on Compton Scatter
• Proportional increase in Compton Scatter.

19
Photoelectric Effect

• X-rays in the diagnostic range can undergo
  ionizing interactions with inner shell electron
  of the target atom.
• It is not scattered but totally absorbed.

20
Photoelectric Effect

• The Photoelectric Effect is a photon absorption
  interaction.

21
Photoelectric Effect

• The electron removed from the target atoms is
  called a photoelectron.
• The photoelectron escapes with kinetic energy
  equal to the difference between the energy of the
  incident x-ray and the binding energy of the
  electron.

22
Photoelectric Effect

• Low anatomic number target atoms such as soft
  tissue have low binding energies.
• Therefore the photoelectric electron is released
  with kinetic energy nearly equal to the incident
  x-ray.
• Higher atomic number target atoms will have
  higher binding energies.

23
Photoelectric Effect

• Therefore the kinetic energy of the photoelectron
  will be proportionally lower.
• Characteristic x-rays are produced following a
  photoelectric interaction to those produced in
  the x-ray tube.
• These characteristic x-rays are also secondary
  radiation and acts like scatter.

24
Photoelectric Effect

• The probability of a photoelectric interaction is
  a function of the photon energy and the atomic
  number of the target atom.
• A photoelectric interaction can not occur unless
  the incident x-ray has energy equal to or greater
  than the electron binding energy.

25
Photoelectric Effect

• The probability of photoelectric interaction is
  inversely proportional to the third power of the
  photon energy.
• The probability of photoelectric interaction is
  directly proportional to the third power of the
  atomic number of the absorbing material

26
Effective Atomic Numbers

• Human Tissue
• Muscle
• Fat
• Bone
• Lung
• Other Material
• Air
• Concrete
• Lead

• Effective Atomic
• 7.4
• 6.3
• 13.8
• 7.4
• 7.6
• 17
• 82

27
Photoelectric Effect

• A probability of interaction to the third power
  changes rapidly.
• For the photoelectric effect this means that a
  small variation in atomic number or x-ray energy
  results in a large changes in chance of an
  interaction.
• This is unlike Compton interactions.

28
Features of the Photoelectric Effect

• Most likely to occur

• With inner-shell electrons
• With tightly bound electrons.
• When the x-ray energy is greater than the
  electron-binding energy.

29
Features of the Photoelectric Effect

• As the x-ray energy increases

• Increased penetration through tissue without
  interaction.
• Less photoelectric effect relative to Compton
  effect.
• Reduced absolute absorption.

30
Features of the Photoelectric Effect

• As the atomic number of the absorber increases
• As mass density of the absorber increases

• Increases proportionally the cube of the Z.
• Proportional increase in photoelectric effect.

31
Pair Production

• If the incident x-ray has sufficient energy, it
  may escape the electron cloud and come close
  enough to the nucleus to come under the influence
  of the strong electrostatic field of the nucleus.

32
Pair Production

• The interaction with the nucleus strong
  electrostatic field causes the photon to
  disappear and in its place appear two electrons.

33
Pair Production

• One is positively charged and called a positron
  while the other remains negatively charged. This
  is called Pair Production.

34
Pair Production

• It take a photon with 1.02 MeV to undergo Pair
  Production.
• Therefore it is not important to diagnostic x-ray.

35
Photodisintegration

• High energy x-ray photons with energies above 10
  MeV can escape interaction with both the
  electrons and nucleus electrostatic fields.

36
Photodisintegration

• It is absorbed into the nucleus that excites the
  nucleus resulting in the release of a nucleon or
  other nuclear material. This is referred to as

37
Photodisintegration

• Photodisintegration. Like pair production, the
  high energy needed to cause this makes it
  unimportant to diagnostic radiography.

38
Differential Absorption

• Only Compton and Photoelectric Effects are
  important interactions that the x-ray may have
  with matter in the diagnostic spectrum.
• More important than the x-rays resulting from
  these effects are a third type, those transmitted
  through the body without interacting.

39
Differential Absorption

• Those that make it through the body contribute to
  the radiograph.
• It should be clear than Compton Scatter X-rays
  contribute no useful information.
• The film does not recognize the scattered x-rays
  as representing an interaction of the straight
  line from the target.

40
Differential Absorption

• These scattered x-rays result in film fog, a
  generalized dulling of the image on the
  radiograph by film densities not representing
  diagnostic information.
• To reduce this type of fog, we use techniques and
  apparatus to reduce the amount of scatter
  reaching the film.

41
Differential Absorption

• X-rays that undergo photoelectric interaction
  provide diagnostic information to the image
  receptor.
• Since they do not reach the film, these x-rays
  are representative of anatomic structures with
  high x-ray absorption characteristics. These
  structures are said to

42
Differential Absorption

• Be Radiopaque.
• The other x-rays that penetrate the body and are
  transmitted without interaction are said to be
  Radiolucent. Radiolucent matter appears as high
  density or dark areas on the radiograph.

43
Differential Absorption

• Radiopaque
• Radiolucent

• Appears Bright
• Appears Dark

44
Differential Absorption

• The radiographic image is the result of the
  difference between those x-rays absorbed
  photoelectrically and those not absorbed at all.
• This characteristic is called differential
  absorption.

45
Differential Absorption

• Except at very low kVp, most x-rays that interact
  do so by the Compton effect this is one reason
  why radiographs are not as sharp as photographs.
• As a rule of thumb, less than 5 of x-rays
  incident on the patient reaches the film and less
  than one half of these interact with the film.

46
Differential Absorption

• The radiographic image results from less than 1
  of the x-rays emitted from the tube.
• Therefore careful control of the x-ray beam is
  necessary to produce high quality radiographs!

47
Differential Absorption

• Differential Absorption increases as the kVp is
  lowered but lowered kVp results in a higher
  patient radiation exposure.
• A compromise is needed for each examination.

48
Differential Absorption

• Notice how much of the x-rays are absorbed
  photoelectrically in bone compared to the soft
  tissue.

49
Differential Absorption

• The photoelectric absorption of bone is about 7
  times greater than in soft tissue regardless of
  the energy.

50
Differential Absorption

• As kVp is increased fewer interaction occur so
  more x-rays are transmitted without interaction.
• Compton Scatter is independent of the atomic
  number of the absorbing material and is inversely
  proportional to the x-ray energy.

51
Differential Absorption

• At low energies the majority of the x-rays
  interactions are photoelectric, where as at high
  energies, Compton scattering predominates.
• As kVp is increased, more x-rays reach the film
  so lower output (lower mAs) is required.

52
Differential Absorption

• To image small differences in soft tissue, one
  must use low-kVp in order to get maximum
  differential absorption.
• This is the principle for mammography.

53
Differential Absorption

• High kVp can be used for examinations of bony
  structures since the crossover for photoelectric
  and Compton scattering is about 40 keV. This
  lowers patient exposure.

54
Dependence on Mass Density

• We know that we could image bone even if the
  differential absorption were not atomic number
  related because bone has a higher mass density
  than soft tissue.
• The interaction between x-rays and soft tissue is
  proportional to the mass density of the tissue.

55
Mass Densities of Materials Important in
Radiography

• Human Tissues
• Muscle
• Fat
• Bone
• Lung

• Mass Density
• 1.00
• 0.91
• 1.85
• 0.32

56
Mass Densities of Materials Important in
Radiography

• Contrast Media
• Barium
• Iodine
• Air
• Other
• Concrete
• Lead

• Mass Density
• 3.5
• 4.93
• 0.001293
• 2.35
• 11.35

57
Contrast Examinations

• In Medical radiography. To better image soft
  tissue structures such as internal organs,
  contrast media are used.
• The primary items are Barium with an atomic
  number of 56 and iodine which has an atomic
  number of 53.
• Air can be combined with the contrast.

58
Exponential Attenuation

• An interaction such as photoelectric effect is
  called an absorbing process because x-ray
  disappear.
• All interactions in which the x-ray photon is
  only partially absorbed such as the Compton
  effect is called a scattering process. Pair
  reduction, Photodisintegration and Classic
  scatter are scattering processes.

59
Exponential Attenuation

• The total reduction in the number of x-rays
  remaining in an x-ray beam following penetration
  through a given thickness of matter is called
  attenuation.
• X-rays are attenuated exponentially which means
  they have do have a fixed range of matter.

60
Exponential Attenuation

• They are reduced in number by a given percentage
  for each incremental thickness of the absorber.

61
End of Lecture

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