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.

14. PHOTOELECTRIC EFFECT

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

37. Scatter Radiation

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