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X-ray Interaction with Matter

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X-ray Interaction with Matter Electromagnetic Radiation interacts with structures with similar size to the wavelength of the radiation. Interactions have wavelike and ... – PowerPoint PPT presentation

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Title: 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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