2. Dual characteristics of X-ray
X-rays belong to a group of radiation called electromagnetic radiation .
Electromagnetic radiation has dual characteristic, comprises of both
Wave concept : Propagated through space in the form of waves. Waves
of all types have associated wavelength and frequency Relationship :
c=velocity of light
The wavelength of diagnostic X-rays is very short around 0.1 to 1A.
Wave concept explains why it can be reflected.
3. Methods of Interactions
v Photons : absorbed / scattered.
v Attenuation : Reduction of intensity. Difference in attenuation gives
the radiographic image.
v Absorbed : completely removed from the x-ray beam & cease to exist.
v Scattered : Random course. No useful information. No image only
darkness. Adds noise to the system.
v Film quality affected : “film fog”.
v 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.
v Remaining 99% of the x-rays ---
Scattered / Absorbed.
4. ATOMIC STRUCTURE
vX-ray photons may interact either with orbital electrons or
with the nucleus. In the diagnostic energy range, the
interactions are always with orbital electrons.
vThe molecular bonding energies ,however are too small to
influence the type and number of interactions .
vThe most important factor is the atomic make up of a tissue
and not its molecular structure.
5. Atomic structure
Ø K shell : 2 electrons
Ø L shell : 8 electrons
Ø 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”.
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. )
7. 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.
8. BASIC INTERACTIONS BETWEEN X-RAYS AND
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-
- COHERENT SCATTERING
- COMPTON SCATTERING
B: PHOTON DISAPPEARANCE
- PHOTOELECTRIC EFFECT
- PAIR PRODUCTION
9. 1. COHERENT SCATTERING
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
( also called “ Classical scattering”)
Thomson scattering : Single electron involved in the interaction.
Rayleigh scattering : Co-operative interaction of all the electrons.
10. 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
11. 1. COHERENT SCATTERING
ØNo ionization --- why??? because, no energy transfer.
Only change of direction.
ØOnly effect is to change direction of incident photon.
ØLess than 5%. Not important in diagnostic radiology.
Produces scattered radiation but of negligible quantity.
12. 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
The atom is ionised.
15. 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,
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”.
16. Characteristic radiation
How does this happen ?
Ø 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.
Ø 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.
Ø 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
18. 2. PHOTOELECTRIC EFFECT
Thus the Photoelectric effect yields three end products :
Ø Characteristic radiation
Ø A -ve ion ( photoelectron )
Ø A+ve ion (atom deficient in one electron )
19. 2. PHOTOELECTRIC EFFECT
Probability of occurrence :
The incident photon energy > binding energy of the
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
20. 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
21. 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
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
22. K-shell electron binding energies of elements
important in diagnostic radiology
Atom Atomic number K-shell binding energy(keV)
23. 2. PHOTOELECTRIC EFFECT :
Applications in diagnostic radiology :
Excellent radiographic images :
Ø No scatter radiation.
Ø Enhances natural tissue
contrast. Depends on 3rd
power of the atomic no., so it
magnifies the difference in
tissues composed of different
elements, such as bone & soft
Ø Lower energy photons : total
absorption. Dominant upto 500
Maximum radiation exposure.
Ø All the energy is absorbed by the
patient whereas in other reactions
only part of the incident photon’s
energy is absorbed.
24. 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
26. 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
3. COMPTON EFFECT
27. 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
28. 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
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.
29. 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.
30. 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
A positron (particle with same mass as electron, but with +ve
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
31. 4. PAIR PRODUCTION
What happens to the
Slowly moving Positron
combines with a free electron
to produce two photons of
2 mass units are converted,
giving a total energy of 1.022
To conserve momentum, two
photons each with 0.511 MeV
energy are ejected in opposite
32. 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.
33. RELATIVE FREQUENCY OF BASIC
Ø Coherent scattering : About 5% .
Minor role throughout the diagnostic energy range.
Ø Compton scattering : Dominant interaction in water.
Water is used to represent tissues with low atomic nos. such as air,
fat and muscle.
Ø 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.
37. Scatter Radiation
§ 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
38. Scatter radiation & Contrast - overview
q Radiographic images are maps of radiation attenuation. Bones
attenuate the most, air in lungs the least.
q Good radiograph : maximum contrast difference between different
X-Ray beam enters body.
Large number of interactions producing scatter radiation.
Image contrast reduced depending on scatter radiation content
40. Sources of scattered radiation
q Transmitted scatter
constitutes greater portion of scattered radiation and originates
from the patient under examination.
q Scatter from cassette
q Side scatter
Side scatter originates from walls, or objects on the source side of
q Reflection scatter or Back scatter
It is often called backscatter when it comes from objects behind the
Undercut occurs due to scattering within the film
42. 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
The only variable we can control is kvp , but we
have less control.
43. Factors affecting scatter radiation
q 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
44. Factors affecting scatter radiation
The effect of kvp on the production of scatter radiation
is probably not as important as part thickness, and as
45. Factors affecting scatter radiation
q Part thickness
§ Scatter radiation is directly proportional to the part
§ The operator has no control over this parameter.
46. Effects of scatter radiation
q Reduction of contrast: Scattered photons
§ Carry no useful information
§ Contribute to film blackness(film fog)
q Increased patient dose
q Increased risk to personnel
47. Control of scatter radiation
Backscatter : ‘B’ is industry
Lead ‘B’ behind cassette to
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.
48. Different techniques are used to keep the
scatter radiation from reaching the films.
X ray filters
X ray beam
Grids (most important)
Prevention of scatter radiation
Only two interactions are important in diagnostic radiology, the Photoelectric effect &
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 radiation.
The 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
Ø secondary radiation composed of photons of lower energy than
the photons that produced them and which travel in a different
Ø Factors affecting it :
q Field size
q Kilo voltage (kVp)
q Anatomical volume (Part thickness)
Ø No useful information, causes film fog and increases patient