Xray Interaction with matter

1. C-Slide 1
X-Ray Interaction
with Matter &
Human Biology
Dr.R.INDHUMATHI
I YEAR PG
DEPT OF RADIODIAGNOSIS

2. INTRODUCTION
n X ray photons are created by the interaction of
energetic electrons with matter at atomic
levels.
n These photons end their lives by transferring
their energy to electrons contained in matter.
C-Slide 2

3. C-Slide 3
Interaction in The body
begin at the atomic
level
Atoms
Molecules
Cells
Tissues
Organ structures

4. C-Slide 4
1. It can penetrate the
section of matter without
interacting.
2. It can interact with the
matter and be completely
absorbed by depositing its
energy.
3. It can interact and be
scattered or deflected
from its original direction
and deposit part of its
energy.

5. C-Slide 5
Patient Interactions
**Photoelectric**
Classic Coherent
Scatter
**Compton Scattering**
Pair Production
Photodisintegration

6. C-Slide 6

7. Coherent Scattering
n Also called: CLASSICAL SCATTERING
UNMODIFIED SCATTERING
ELASTIC SCATTERING
C-Slide 7

8. COHERENT SCATTERING
n It is type of interaction in which radiation
undergoes a change in direction without a
change in wavelength.
n is a pure scattering interaction and deposits no
energy in the material
n It accounts for less than 5% of total interaction.
C-Slide 8

9. PROBABILITY OF CLASSICAL
SCATTERING
 Increases with
 A] low atomic number materials
( soft tissue more likely than bone )
 B] lower photon energies
( 5 kev more likely than 10 kev)
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10. COHERENT SCATTERING
n Low energy x-ray photons interacts with
electron of the atom .
Absorption of radiation by the atom
n Vibration of the atom
n Emission of radiation as the atom returns to
its undisturbed state
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11. Coherent Scattering
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12. Coherent Scattering
n The wavelength is equal to the incident x-ray
n The only difference is the direction of travel
n Energy in = Energy out - Only changes is
direction
n
n No ionization takes place ( only type).
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13. SUB CLASSIFICATION
 THOMPSON SCATTERING:
if interaction occurs with single
electron.
 RAYLEIGH SCATTERING:
if interaction occurs with all the
electrons of whole atom.
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14. Use in diagnostic radiology
Since its quantity is too small no value in
diagnostic radiology.
scattered radiation produced contributes
only to film fog.
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15. Compton scattering
 A compton interaction is one in which only a
portion of the energy is absorbed and the
resultant photon is scattered with reduced
energy.
 Occurs throughout the diagnostic imaging
range
 Most common interaction b/w x rays and body
tissues
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16. Probability of crompton scatter
n 1] the probability of compton interaction is
directly proportional to the number of outer
shell electrons
n 2] compton effect decreases with increase in
photon energy.
C-Slide 16

17. Compton Scattering
an incident photon with relatively high energy
Interacts with atom
it ejects the outer shell
electron from its orbit
the incident photon thus gets
deflected by electron
travels in a new direction as
scatter radiation. C-Slide 17

18. Compton Scattering
this reaction produces an ion pair
that is a) A positive atom
b) A negative electron
( called as recoil electron)
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19. C-Slide 19

20. Compton Scattering
n Energy of incident photon is distributed as
a) part of it goes to the recoil electron
as kinetic energy.
b) rest is retained by the deflected
photon.
C-Slide 20

21. Compton scattering
n Two factors determine the amount of energy
A] its initial incident energy
B] its angle of deflection off the recoil
electron.
C-Slide 21

22. Applications in diagnostic radiology
n Incident energy:
the higher the energy of the photons
the more difficult they are to deflect
n For example
with x ray energies of 1Mev most
scattered photons deflect in a forward
direction.
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23. C-Slide 23

24. Compton scattering
 the lower the energy radiation fewer
photons scatter forward and more
scatter back at an angle of 180
 in the diagnostic energy range upto 150 kev
the photon retains most of its original
energy and very little is transferred to the
recoil electron.
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25. Compton scattering
n Angle of deflection
at narrow angles of deflection scattered
photons retain almost all their original
energy.
a) they have an excellent chance of
reaching an x ray film and producing
fog
C-Slide 25

26. Compton scattering
b) they are exceedingly difficult to removed
by grids because their angle of deflection is too
small and also by the filters because they are
too energetic
this causes major radiation hazard to
personnel
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27. Compton scattering
n For example in flouroscopic procedure
n the scattered radiation produced is almost as
energetic as the primary beam
n This creates a real safety hazard for the
flouroscopist and other personnel in exposure
rooms.
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28. Scatter from the Patient
during Fluoroscopy
C-Slide 28

29. Photoelectric interaction
It is a type of photon electron interaction in
which a photon transfers all of its energy to an
electron located in one of the atomic shells.
Most useful interaction in diagnostic radiology.
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30. Photoelectric Effect or Absorption
n Inner-shell ionization
n The photon is not scattered it is totally
absorbed
n The only disadvantage is that it exposes the
patient to great deal of radiation.
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31. Photoelectric effect
n An incident photon with a little more energy
than the binding energy of k shell electron
Ejects the electron from orbit
The photon disappears giving up all its
energy to electron.
C-Slide 31

32. PHOTOELECTRIC EFFECT
n The electron which is now free of its energy
flies off into space as a photoelectron
n The vacancy in the atom is filled by the
electron from L shell
n Resulting in the formation of characteristic
radiation
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33. PHOTOELECTRIC EFFECT
When k shell is filled by an outer shell electron
from the same atom
The atom is left with the deficiency of one
electron
Remains as a positive ion
C-Slide 33

34. PHOTOELECTRIC EFFECT
n The photoelectric effect always yields three
end products
1] characteristic radiation
2] a negative ion ( photoelectron)
3] a positive ion ( an atom defecient one
electron)
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35. C-Slide 35

36. PROBABILITY OF PE EFFECT
n It depends on two factors
n 1] incident photon energy
n 2] atomic number
C-Slide 36

37. Probability of photoelectric interaction
n For the PE to occur incident x ray must have
energy greater than binding energy of the inner
shell electron.
n For example
the BEof k shell electron in iodine is 33 kev,
hence sharp increase in interaction of photons
occurs when x ray photon energy exceeds 33
kev.
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38. Binding Energy is very important
C-Slide 38

39. PROBABILITY OF PHOTOELECTRIC
EFFECT
n The PE is proportional to the atomic number
.the more tightly bound an electron the greater
the probability of the PE if energy is greater
than the BE.
n The photoelectric effect is more likely to
occur in absorbers of high atomic number
(eg, bone, positive contrast media)
C-Slide 39

40. APPLICATION OF PE INTERACTION
n Produces radiographic images with excellent
quality.
n The PE does not produce scatter.
n Enhances natural tissue contrast.
n The PE magnifies the difference in tissue
composed of different elements such as bone
and soft tissue.
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41. C-Slide 41
PHOTOELECTR
IC
ABSORBTION
IS WHAT GIVES
US THE
CONTRAST
ON THE FILM

42. Pair production
n It is a type of photon matter interaction in
which high energy photons( energy > 1.02Mev)
n Interacts with nucleus of the atom
the photons diappear
n Its energy is converted into matter in the form
of two particles
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43. Pair production
electron positron ( 0.51 Mev)
(a particle with the
same mass as
electron but with
positive charge)
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44. C-Slide 44

45. Probability of pair production
n It happens when photon energy is in excess of
1.022 Mev.
n Increases with increase in atomic number.
C-Slide 45

46. photodisintegration
 In photodisintegration ,part of the nucleus of
an atom is ejected by a high energy photon
 The ejected portion may be a neutron, a
proton, an alpha particle, or a cluster of
particles
 The photon must have sufficient energy to
overcome nuclear binding energies of the order
of 7 to 15Mev.
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47. Photodisintegration
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48. Diagnostic radiology
n Since pair production does not occur with
photon energies less than 1.02 Mev and
n Photodisintegration does not occur with
energies less than 7 Mev .
n Both of these interaction is of no use in
diagnostic radiology because of its energy
range is only upto 150 kev.
C-Slide 48

49. Important X-ray Interactions
n Of the five interactions only two are important to
radiology
• Photoelectric effect or photoelectric
absorption
• Compton scatter
n Which two tube interactions are important?
C-Slide 49

50. Compton scatter
n Contributes to no useful information
n Is independent of the atomic number of tissue.
The probability of Compton is the same for
bone atoms and for soft tissue atoms
n The probability for Compton is more dependent
on kVp or x-ray energy
C-Slide 50

51. Compton Scatter
n Results in image fog by optical densities not
representing diagnostic information
C-Slide 51

52. Photoelectric Absorption
n Provides information to the IR because
photons do not reach the IR
n This represents anatomic structures with
high x-ray absorption characteristics;
radiopaque structures; tissue with high
atomic number; or tissue with high mass
density
C-Slide 52

53. Compton vs. Photoelectric
n Below 60 kVp Photoelectric absorption is
predominant above 60 kVp Compton scatter
begins to increase.
C-Slide 53

54. 3 Types of x-rays are important for IMAGE
FORMATION
n DIFFERENTIAL ABSORPTION = the difference
between those x-rays absorbed and those
transmitted to the IR
n Compton scatter (no useful information)
n Photoelectric absorption (produces the light
areas on the image)
n Transmitted x-rays (produces the grey/dark
areas on the image)
C-Slide 54

55. Differential absorption
n The probability of radiation interaction is a
function of tissue electron density/ atomic
number, tissue thickness/density, and x-ray
energy (kVp).
n Dense material like bone and contrast dye
attenuates more X-rays from the beam than
less dense material (muscle, fat, air).
n The differential rate of attenuation provides the
contrast necessary to form an image.
n Table 10-10
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56. Differential Absorption
n Increases as the kVp is reduced
n Approximately 1% of photons that interact with
the patient (primary beam) reach the IR. Of that
1% approximately 0.5% interact to form the
image
C-Slide 56

57. Differential Absorption
n The difference in x-ray interactions
n Fundamental for image formation
n Occurs because of Compton Scattering,
Photoelectric absorption, and X-ray
transmission
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58. Differential Absorption
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59. Thank u
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