DR . MANISHA SAXENA PG FIRST YEAR OMR

1. RADIATION PHYSICS

2. Composition of matter
 Anything that occupies space and has weight is
called matter.
 Atom is basic unit of all matter.
 Atom is a fundamental unit of matter that can not
be subdivided .

3. Bohr model
 In atomic physic, the Bohr model , introduced
by NEIL BOHR in 1983 depicts atom as small ,
positively charged nucleus surrounded by
electrons that travel in circular orbits around
the nucleus held by electrostatic forces .

4. Quantum mechanical model

7.  The number of protons present in nucleus is called the atomic
number [ Z]
 The total numbers of protons and neutrons in the nucleus is
called atomic mass [A]
 When the number of protons is equal to the number of
electrons atom is known to be in stable state .
 The electrons in the orbit are maintained by the electrostatic
force between positively charged nucleus and negatively
charged electrons on one hand balanced by the centrifugal
force of the revolving electrons .

8. Electrostatic force
Force between electrons and nucleus.
Centrifugal force
pulls electron from nucleus

9.  Balance between electrostatic force and
centrifugal force is required to keep electron in
its orbit.

10. Binding Energy
Electrons are maintained in their orbit by
electrostatic force, or attraction, between the
positive nucleus and negative electrons.This is
known as Binding energy.
It is related to atomic number.
For example,Tungsten atom has following binding
energies-
K shell - 70 kev
L shell - 12 kev
M shell - 3 Kev

11. Radiation
Emission or propagation of energy through
space or substance in the form of wave or
particle
1. Ionizing radiation
2. Non ionizing radiation
Ionizing radiation are defined as radiation
capable of producing ions by removing or
adding electron to an atom . It is of 2 types-
1. Electromagnetic radiation.
2. Particulate radiation.

13. Electromagnetic radiation
 The movement of energy through space as
combination of electric and magnetic fields.
 Travels at speed of light
3 x 108meters/second
186,000 miles/second

14. Energy
Ability to penetrate
Shorter wavelength- higher energy
Higher frequency - higher energy

19. Particulate radiation
 Unstable atom break up releasing -
 Alpha particles .
 Beta particles .
 Gamma rays .
The capacity of particulate radiation to ionize
atoms depends on its mass, velocity and charge.
The rate of loss of energy from a particle as it
moves along its track through matter is called
LET(linear energy transfer)

20.  The greater the physical size of the particle, the
higher its charge and lower the velocity, the greater
its LET.
 Alpha particle- High mass
High charge Densely ionising
Low velocity (high LET)
Beta particle- Light mass
Lower charge Less ionising
( low LET)

22. Theories of electromagnetic
radiation( Dualistic theory)
 Quantum theory – small discrete bundles of
energy called photon.
 Explains photoelectric effect and X ray production.
 Wave theory – Explains refraction ,reflection ,
diffraction ,polarization and interference.

23. Radiation history
 History of dental
radiography begins
with the discovery
of the X rays
 WilhemConrad
Roentgen
discovered X rays on
November 8, 1895.

24.  Working with a cathode ray in his laboratory,
William Morgan observed a fluorescent glow
of crystals on a table near his tube.
 Roentgen named as X RAYS as maths symbol
of unknown.
 He first radiograph his wife’s hand.
 Dr. C Edmund Kells is known as FATHER OF
DENTAL RADIOLOGY. He used radiograph in
RCT.
 William David Coolidge is related with tube
design as hot cathode tube.

26. X Ray characteristics
 EMR of wavelength 10 A to .01A
 Travel in straight line.
 Cannot be focused to a point.
 Differentially absorbed
 Cause fluorescence
 Harmful to living being
 High energy waves
 Invisible
 No mass

27.  Cannot be refracted , reflected or deflected.
 Do not require medium to propagate.
 Obeys inverse square law.
 Causes ionization.
 Can produce an image on film.
 Induce color change in solutions.

28. Types of X rays
 X rays are weightless packages of pure
energy without electrical charge.
 Grenz / Super soft X rays – 1 – 2 A.
 Soft X rays – 1 - .5 A
 Medium X rays - .5 - .1A
 Hard X rays - .1 A

29. X ray machine

30. Component of X ray machine
 Head Tube Cathode
Anode
Power supply (transformer)
 Control Panel
 Extension arm

32. Cathode
 Consist of filament and focusing cup.
 Filament made up of tungsten wire about 2mm
in diameter and 1 cm in length.
 When heated emits electrons .
 Filament lies in focusing cup made of
molybdenum is parabolic in shape.
 This shape electro statically directs electron in
narrow beam to focal spot on anode.

33. Anode
 It consist of tungsten target embedded I copper
stem.
 Purpose is to convert kinetic energy of electrons
to X ray photons.
 99% energy is converted into heat.
 Copper at as thermal conductor to remove heat
from tungsten to reduce the risk of target
melting.
 Focal spot is area to which electrons are
directed.

34. Why tungsten
An ideal target material-
 High atomic number – 74
 High melting point – 3422c
 High thermal conductivity – 173W/m/K
 Low vapor pressure

35. Types of anode
 Stationary anode-
Target is placed at an angle of 20 degrees to
central ray of x ray beam.
Effective focal spot is 1mm by 1mm.
Actual focal spot is 1mm by 3mm.
Small focal spot allows better heat dissipation
while maintaining the image quality.

36. Rotating Anode

37. Evacuated Glass envelope
X ray tube is evacuated-
 To prevent collision of fast moving electrons
with gas molecules which would significantly
reduce their speed.
 Vacuum prevents oxidation or burnout of the
filament.

38. Power supply
 Two transformer and X ray tube lie in head of
X ray machine.
 Filament transformer -3-5 kvp
 High voltage transformer – 65,000-
100.000kvp
 It perform 2 function
1. Provide low voltage current to heat the X
ray tube filament.
2. Generate high potential difference to
accelerate electrons from cathode to anode.
An electrical insulating material like oil
surrounds the transformer.

39. X ray tube control
 Tube current – Flow of e-
Cathode Anode
Intraoral x ray units – 7 to 10 mA
 Tube voltage – High voltage is required
between cathode and anode to generate X
rays.
 Intraoral ,panoramic and cephalometric units
operate between 50 -90 kvp.
 CBCT – 90 – 120 kvp.

40.  AC - half rectified or self rectified.
Electrons do not flow during negatative cycle
 DC – Electrons flow during entire cycle .It yields
X ray with narrow spectrum of energy . It helps
by reducing exposure time n patient motion.

41.  Timer –
The electronic timer controls the length of
time that high voltage is applied to tube and
thus time during which X rays are produced.
Exposure time is expressed as number of
pulses in an exposure.
A setting of 30 pulses means that there will
be 30 pulses of radiation equivalent to .5
second exposure.
Rule of thumb
Point to remember that vary a setting by
2kvp/cm of tissue thickness.

42. Tube Rating and Duty Cycle
The heat buildup at the anode is measured in
heat units(HU)
HU = kvp× mA× seconds
Heat storage capacity for anodes of dental -
20KHU.
Tube Rating-
Longest exposure time the tube can be energized
for range of voltage(kvp) and tube current(MA)
without damaging target.
Duty Cycle-
Frequency with which successive exposure can be
made without overheating the anode.

43. Production of X ray
 When high speed electrons collide with anode
it produces X rays.The kinetic energy of
electrons are converted into photons by-
1. Bremsstrahlung radiation- Primary source
2. Characteristic Radiation.

45. Bremsstrahlung radiation
 Primary source of radiation.
 “braking radiation” in German.
 Produced by sudden stopping or slowing of high
speed electrons by tungsten nuclei.
 Direct hit –All kinetic energy of electron is converted
into single X ray photon.
Energy of photon(kev) = Energy of electron(kvp)
 Near miss – Electron is attracted by nuclei and path
is altered towards nucleus and losses its velocity

46.  Bremsstrahlung interaction generate X ray
photon with continuous spectrum of energy.
Characteristics Radiation –
 Contribute only small fraction of photons.
 When incident electrons ejects an inner electron
from tungsten.
 Electron from outer orbital quickly fill void in
inner orbit.
 Photon is emitted with energy equivalent to
difference in binding energies of two orbital.
 Energy of characteristics radiation - discrete

47. Factors controlling X ray Beam
1. Mill amperage setting (tube current)
2. Exposure time
3. Tube voltage
4. Filtration
5. Collimation

48. Effect of current and time
 Increasing the mAs, increases the number of
photons .
 It does not affect the penetrating power of
the photons.
 Increase mA will increase overall blackness of
the film.
 Quantity of radiation = current × time.
 Beam quantity – Number of photons

49. Effect of Kilo voltage
 Increasing Kv , increases the penetrating
ability of X ray photons.
 More X rays get through to darken the film.
 High Kv produces darker images but poor
contrast.
 Beam quality – mean energy of X ray beam.

50. Filtration
 It removes low energy photons.
 2 types-
1. Inherent filtration.- Glass tube, insulating oil
2. Added filtration. – Aluminum disk.
Total filtration = Inherent +Added.
Dental X ray specification by US federal-
1.5mm of Al. - 70 kvp.
2.5mm of Al - Higher voltage.

51. Collimation
 Metallic barrier with aperture in the middle
used to shape and restrict the size of X ray
beam and volume of tissue irradiated.
 and used in dental radiography.
 Circle of 2.75 inches(7cm)

53. INTERACTION OF X RAYS WIH MATTER
 The x ray beam enters the face of a patients ,
interacts with soft and hard tissues and strikes
a digital sensor or film.
 Intensity of the beam is essentially uniform for
the center of the beam outwards as the beams
goes through the patients , it is reduced in
intensity Beam attenuation.
 HVL -Thickness of material required to reduce
one half the number of photons .
 Density

54. 3 means of beam attenuation in a X ray beam-
1. Photoelectric absorption.
2. Compton scattering.
3. Coherent scattering.

55. Photoelectric absorption
 Critical in diagnostic imaging as it is basis of
image radiographic formation.
 Incident photon interacts with an electron in
an inner orbital of an atom in the patient.
 Photon loses all its energy to electron and
ceases to exist.
 Electron/recoil/photoelectron absorb energy
and overcome the binding energy.
 Most biologic molecules have low atomic
number so most of the energy of incident
photon is absorbed photoelectron.

56.  Photoelectron travel only short distances in
the absorber give up their energy.
 Occur in 1s orbital as they are electron dense.
 Constitute about 27% of interaction.
 PE interaction causes ionization of the atom
because of electron loss.
 Electron deficiency is instantly filled by 2s ,2p
electron.
 Characteristic radiation generated are
absorbed by patient.

58. Compton scatter
 Photon interacts with an outer orbital electron
 Constitute about 57% of interaction.
 Photon collides with outer electron causing it to
recoil from point of impact.
 Path of incident electron is deflected and
scattered in new direction
 It causes additional ionizations and when
reaching the receptor results in degradation of
image.
 The probability is independent of atomic
number.

60. Coherent scattering
 Also known as Rayleigh, classical or elastic
scatter.
 Accounts for 7% of total interaction.
 Low energy(<10kev) photon interacts with
whole atom.
 Atom momentarily excited.
 Photon ceases to exist.
 Excited atom quickly returns to ground state
and generate another X ray photon with the
same energy as incident photon.

63. Practical implication of PEE
 Differential absorption by tissues and object
provide radiographic contrast.
 Atomic number of compact bone Z= 13.8 and
that of soft tissue Z= 7.4, the probability of PE
interaction is about 6.5 times more for bone.
 Differential PE absorption of X ray photons in
enamel, dentin ,pulp ,bone and soft tissue is
different degree of radiopacity on image.
 Cause ionization and potential for biological
damage.

64. Beam hardening
 X ray beam of polychromatic energies passes
through an object, results in selective
attenuation of lower energy photons.
 Mean energy of beam is increased (hardened)
 Metal filters pre harden the beam.

65. Dorsimetry
 Exposure – Measure of the capacity of X rays
to ionize the air.
 Roentgen was used in SI system.
 1 R = 2.58 × 10 8 Coulombs/kg.
 Replaced by Air Kerma.
 Kinetic energy released in matter.
 Energy of photons transfer to electrons.
 1 R= 8.77mGy.

66.  Absorbed dose-
Measure of total energy absorbed by any type
of ionizing radiation per unit mass of any type
of matter.
 Traditionally rad (radiation absorbed dose)
was used.
 SI unit is gray.
Gy= 1J/kg.
 1GY = 100rad.

67. Equivalent dose
 Use to compare the biologic effects of
different types of radiation on tissue or
organ.
 SI unit is Sievert.
 For X rays 1Sv = 1 Gy.
Effective Dose
 Used to estimate the risk in humans.
Radioactivity
 Decay rate of sample of radioactive
material.
 SI unit is Becquerel(Bq).

68. References
 White and Pharoah
 Freny R karjodkar