ORAL RADIOLOGY

1. PRODUCTION OF
X-RAYS

2. CONTENTS
 X-ray machine
 Production of X-rays
 Factors controlling the X-ray beam
 Tube current
 Tube voltage
 Exposure time
 Filtration
 Collimation
 Inverse square law
 Interaction of X-rays with matter
 Coherent scattering
 Photoelectric effect
 Compton effect
 References

3. QUESTIONS
 X-ray machine (Manipal 2011 )
 Draw the labeled diagram of x-ray tube & expalin the
functions of each component (DA 2014)
 Filtration & collimation of diagnostic x-ray machine ( Amrita
2014 )
 Production of x-rays. (PAHER 2016 ,Manipal 2010 )
 Factors controlling the X-ray beam (DA 2016 )

4.  X-rays are produced by the sudden deceleration or stoppage of
a rapidly moving stream of electrons at a metal target in a high
vaccum tube.
 X-ray tube is an important part of any X-ray machine.
 Dental X-ray machine is made up of three parts or components:
i. Control panel
ii. Extension arm
iii. Tube head

5. CONTROL PANEL
 Control panel of the dental X-ray
machine contains-
a. An on and off switch & an
indicator light.
b. An exposure button .
c. Control device( time, kvp, mA
selectors) to regulate the X-ray
beam.

6. EXTENSION ARM
 It contains electrical wires that extend
from control panel to the tube head.
 It also allows the movement and
positioning of the tube head .

7. TUBE HEAD
It is a tightly sealed, heavy metal housing that contains:
-Metal body that surrounds the x-ray tube
-Insulating oil
-Tube head seal
-X-ray tube
-Aluminium disks
-Lead collimator
-Position indicating
device

8.  Metal Housing: This is the metal body of the tube head that
surrounds the X-ray tube and transformer and is filled with oil, it
protects the X-ray tube and ground the high voltage component.
o Insulating Oil: It is that which surrounds the X-ray tube and
transformer inside the tube head, it prevents over heating by
absorbing the heat created by the production of X-rays

9.  Tube Head Seal: Aluminum or leaded glass of the tube
head that permits the exit of X-rays from the tube head, it
seals the oil in the tube head and acts as a filter to the X-
ray beam
X-ray Tube : It is the main X-ray generating system.

10. Aluminum disks – the sheet of 0.5mm thick aluminum is
placed in the path of the X-ray beam.
 It filters the non-penetrating, longer wavelength X-rays which
results in a higher energy & more penetrating useful beam,
which is less harmful to patients.
 2 types of filtration
- inherent filtration
- added filtration

11. INHERENT FILTRATION
 It takes place when the primary beam passes through the
glass window of the x-ray tube, the insulating oil and
the tube head seal.
 It is approximately 0.5 to 1mm thickness of aluminum.

12. ADDED FILTRATION
 Placement of aluminum disks in the path of the x-ray beam
between the collimator and the tube head seal.
 Aluminum disks may be added in 0.5 mm increments.
 Total filtration (inherent + added filtration) is regulated by
the state and federal law (in USA).

13.  Dental machines operating:
 At or below 70 kvp require a minimum total filtration of
1.5 mm of aluminum thickness.
 Above 70 kvp require a minimum total filtration of 2.5
mm of aluminum thickness.

14. LEAD COLLIMATOR
 It is a lead plate with a central hole that fits directly over
the opening of the metal housing where the x-ray exits.
 Used to restrict the size and shape of the x-ray beam and
thus reduce exposure to the patients.
 Collimator is of two types:
 Fixed
 Adjustable

15.  In the dental X-ray machine usually the fixed
 collimators are used, they may either have a round or
rectangular opening.
 A rectangular collimator restricts the size of the X-ray
beam to an area slightly larger than a size of 2 intraoral
(normal adult intraoral periapical films) and thus
significantly reduces the patient exposure.

16.  A circular collimator produces a cone shaped beam that
is 2.75 inches in diameter and is considerably larger than
the size of two intraoral periapical films, and thus leads
to an increased skin dose to the patient.

17. POSITION INDICATING DEVICE
 Open end lead cylinder
 Extend from the opening of metal housing of the tube head also
called as “ cone”.
 It appears as an extension of the tube head; it aims and shapes
the X-ray beam.
 There are three types of PIDs-
i. Conical
ii. Rectangular
iii. round

18.  Both rectangular and round PIDs are available in two
lengths:
i. Short(8 inches)
ii. Long(16 inches)

19. X-RAY TUBE
 Design introduced by W.Coolidge in 1913
 An x-ray tube is composed of a cathode and an anode situated
within an evacuated glass envelope or tube Electrons stream
from a filament in the cathode to a target in the anode, where
they produce x rays.

20.  For the x-ray tube to function, a power supply is
necessary to
(1) Heat the cathode filament to generate electrons
(2) Establish a high-voltage potential between the anode
and cathode to accelerate the electrons toward the
anode.

21. CATHODE
 It is principally composed of two parts:
a. Filament
b. Focussing cup
 It is the source of electrons in the tube, made up of a tungsten
wire
Diameter – 0.2cm
Width – 1-2 cm
Thickness- 0.1-0.2 mm
Length – 7- 15 mm

22.  It is mounted on two stiff wires that support it and carry the
electric current.
 These two mounting wires lead through the glass envelope and
connect to both the high- and low-voltage electrical sources.
 The filament is heated to incandescence by the flow of current
from the low-voltage source and emits electrons at a rate
proportional to the temperature of the filament

23. FOCUSING CUP
 It is a negatively charged concave reflector cup of molybdenum
or nickel and houses the filament.
 The focal spot - The focusing cup parabolic shape
electrostatically focuses the electrons emitted by the filament
into a narrow beam, directed at a small rectangular area in the
anode.

24.  The electrons move in this direction because they are both
repelled by the negatively charged cathode and attracted to the
positively charged anode.
 The x-ray tube is evacuated to prevent collision of the fast-
moving electrons with gas molecules, which would significantly
reduce their speed.
 The vacuum also prevents oxidation, “ burnout, ” of the filament.

25. ANODE
 It consist of a wafer thin tungsten plate embedded in a solid
copper stem.
Target:
• Function is to convert the kinetic energy of electrons into x-ray
photons
• Inefficient process & more than 99% of energy is converted
into heat.
Copper – Good thermal conductor, dissipates heat from tungsten ,
reduces risk of target melting

26. Why is Tungsten used?
oHigh atomic number (74)
oHigh melting point (3422 ̊ C) – can be raised to high
temperature without detriment
 High thermal conductivity(173 W ) thus readily dissipating its
heat into the copper stem.
 Low vapor pressure - low vapor pressure of tungsten at high
temperatures helps maintain the vacuum in the tube at high
operating temperatures.

27. Stationary
anode
Rotating
anode

28. STATIONARY ANODE
 Small plate of tungsten embedded in a large mass of Copper
 Helps in heat dissipation from small focal spot.

29. ROTATING ANODE
 A large disc of tungsten/alloys of tungsten, which theoretically
rotates at speed of about 3000 rpm
Used in tomographic or cephalometric units
Used in medical x-ray machines requiring high radiation
outputs

30. FOCAL SPOT
 Focal spot is the area on target to which the focusing cup
directs the electrons from the filament
 It is the region where X-rays & the heat are produced
 Heat generated per unit target area becomes greater as the focal
spot decreases in size.
 To take advantage of small focal spot along with proper heat
dissipation, target is placed at an angle with respect to electron
beam in the tube.

31. LINE FOCUS PRINCIPLE
o The Benson line focus principle developed in 1918 is a method
of reducing the effective focal spot size.
o Target is inclined about 17°-20° to central ray of x-ray beam
o Use of an anode with the target angulated such that the effective
focal spot is smaller than the actual focal spot size.

32. oProjection of focal spot perpendicular to electron beam is
effective focal spot
oActual focal spot is projected perpendicular from target
oActual focal spot = 1x 3 mm, Effective focal spot = 1x 1 mm

33. HEEL EFFECT:
• Intensity of the beam is not
uniform across the exposure
field
• Cathode side of the beam is
more intense than the anode
side because of self-absorption
of some of photons in the target
itself.

34.  Effect arises because x-rays are generated at a small
depth inside the target and target surface gets rough with
use, therefore attenuation.

35. ELECTRICAL CIRCUIT OF THE X-RAY UNIT
Electrical Current: Refers to the flow of electrons through the
conductor
Direct Current: When the electrons flow in one direction
through the conductor
Alternating Current: Electrical current in which electrons flow
in two, opposing directions

36.  Rectification is the process of converting AC to DC
 A rectifier essentially eliminates the –ve phase of the AC,
leaving the +ve phase to behave as DC

37.  Most dental x-rays, the amount of heat produced at the
anode does not give rise to excessive electrons, when the
current changes its direction, there are no electrons at the
anode to travel back to the cathode, this half of the cycle
is called inverse voltage, hence the dental x-ray tube is
called self or half wave rectifying.

38.  Amperage- is the measurement of the number of
electrons moving through a conductor.
 Current- is measured in amperage or milli amperage.
If the mA increases, the number of electrons passing
through the cathode filament increases.
 Voltage: Measurement of electrical force that causes
electrons to move from negative cathode to positive
anode.

39.  After entering the x-ray machine housing, the electrical current
is directed into 2 transformers.
 One of these transformers, step-up, increases the incoming
voltage into the range of thousands of volts or KV range (high-
voltage circuit)
 Other transformer, step-down, decreases the incoming voltage
to 10V (low voltage circuit or filament circuit)

40.  Filament circuit, using 10V, regulates the flow of electrical
current to the filament of the x-ray tube, & is controlled by the
mA settings
 Filament current regulates the tube current which consists of
electrons flowing from negative cathode to positive anode
 High voltage circuit then provides the high voltage required to
accelerate the electrons & to generate the x-rays & is regulated
by the kilovoltage settings

41. POWER SUPPLY
Primary functions are to :
 Provide low-voltage current to heat filament by use of step-
down transformer
 Generate high potential difference between anode and cathode
by use of high-voltage transformers

42. TRANSFORMERS
• Is a device that is used to either increase or decrease the
voltage in an electrical circuit
• 3 transformers are used to adjust the electrical circuit
-step-down transformer
-step-up transformer
-autotransformer

43. Step-down Transformer:
 Used by the filament circuit.
 It is used to decrease the voltage
from the incoming 110-220 line
voltage to 3-4V as required for the
filament circuit. (This transformer
has more coils in the primary coil
than in the secondary coil).

44. Step-up Transformer::
It is used to increase the voltage from the incoming 110-220 line
voltage to 65,000- 1,00,000 volts as required by the high voltage
circuit.

45. Autotransformer :
 Converts primary voltage from the input source into secondary
voltage
 Secondary voltage is regulated by kvp selector dial, selects
voltage from different levels on autotransformer and applies it
across primary winding of high-voltage transformer
 Accelerates electrons from cathode to anode and generate x-
rays

46.  A timing control device to control x-ray exposure time is
included in the primary circuit of the high voltage supply

47. TUBE RATING AND DUTY CYCLE
 X- ray tubes produces heat at the target while in operation.
 Heat buildup at the anode is measured in heat units(HU),
HU=kVp × mA × second
 Heat storage capacity for anodes of dental dignostic tubes
is approx. 20 kHU.
 Heat is removed by copper anode and then to the
surrounding oil and housing tube and by convection to the
atmosphere.

48. PRODUCTION OF X-RAYS
The kinetic energy of electrons is the tube current is converted
into x-ray photons at the focal spot of an X-ray tube by two
mechanisms :
1. Bremsstrahlung radiation
2. Characteristic radiation

49. BREMSSTRAHLUNG RADIATION
Bremstrahlung radiation, the primary source of X-ray photons
from an X-ray tube, is produced by
1.Direct hit of electron on nucleus in target – during this type of
collision all the kinetic energy of high speed electron will be
converted into single X-ray photon with maximum energy.

50.  The energy of the resultant photons in units of kilo
electron volts is numerically equal to the energy of the
electron that is in turn equal to the kilovoltage applied
across the x-ray tube at the time of its passage.

51. 2. By passage of electron near nucleus, which results in electron
being deflected and decelerated. This deceleration causes it to
lose some of its kinetic energy. This energy is given off in the
form of a photon of electromagnetic radiation, which has an
energy equal to that lost by the deflected electron i.e. photon
of lower energy.

52. CHARACTERISTIC RADIATION :
 Characteristic radiation occurs when
a bombarding electron of the tube
current displaces an electron from a
shell of a target atom, thereby
ionizing the atom.
 Incident electron ejects photoelectron
from inner orbit, creating vacancy.
 Inner vacancy is filled with electron
from outer orbit.

53.  Photon is emitted with energy equal
to difference in energy levels of two
orbits.
 Electrons from various orbits may be
involved, giving rise to other
photons.
 Energies of photons thus created are
characteristic of target atomic
composition (i.e. energy levels of
various electron orbital levels).

54. Factors Controlling the X-ray Beam
 Exposure Time (Timer)
 Tube current (mA)
 Tube Voltage Peak (kVp)
 Filtration
 Collimation
 Inverse square law

55. EXPOSURE TIME
 Exposure time(Timer) :
As exposure time increases, so does the total number of
photons, but the mean energy and maximal energy of the
beams the uncharged.

56. Tube current
Tube current (MA) : Quantity of radiation produced by an
x-ray tube (i.e. number of photons that reach the patient and
film) is directly proportional to tube current and time the
tube is operated.

57. Tube voltage
 Tube Voltage (kvp) : As kvp is increased (with mA held
constant), a corresponding increase occurs in mean energy of
the beam, the total number of photons emitted, and maximal
energy of photons
 Quantity of X-ray radiation is directly proportional to tube
current and exposure time
 Quality of X-ray radiation is directly proportional to tube
voltage

58. Filtration :
 The purpose of conventional filtration is to remove low
energy X-ray photons selectively from x-ray beam. This
results in decreased patient exposure with no loss of
radiologic information.

59. TOTAL FILTRATION:
 Is sum of the inherent filtration plus any added external
filtration.
Governmental regulations require the total filtration in
path of a dental x-ray beam to be equal to equivalent of
1.5 mm of aluminium to 70 KVp 2.5 mm of aluminium
for all higher voltages

60. INHERENT FILTRATION :
 Consists of materials that X-ray photons encounter as
they travel from the focal spot on the target to form the
usable beam outside the tube enclosure.
 Inherent filtration of most X-ray machine ranges from
equivalent of 0.5 to 2 mm of aluminium.

61.  Examples of inherent filtration are :
 Glass wall of the X-ray tube.
 Insulating oil that surrounds many dental tubes.
 Barriers material that prevents oil from escaping
through the x-ray port.

62. EXTERNAL FILTRATION :
 Aluminium disks placed over the port in the head of the
x-ray machine.
 Aluminium wedge filter is a part of cephalostat.

63. COLLIMATION :
 A collimator is a metallic barrier with an aperture in the
middle used to reduce the size of the x-ray beam and
therefore the volume of irradiated tissue within the
patient lese of collimation also improves image quality.
 Dental x-ray beams are usually collimated to a circle 2¾
inches (7cm) in diameter.

64. TYPES OF COLLIMATORS :
Round Collimator : is a thick
plate of radiopaque material with
a circular opening centered over
the port in the x- ray head
.Round collimator are built into
open – ended aiming cylinders.

65. Rectangular Collimator : limits
the size of the beam to just
larger than the x- ray film,
thereby reducing patients
exposure

66.  Intensity of a x-ray beam is
inversely proportional to the
square of distance between the
source and the point of measure
 The relationship is follows
I1/I2 = (D2)2/(D1)
 Where I is Intensity and D is
distance
Inverse Square Law :

67. INTERACTION OF X-RAYS WITH MATTER
 Intensity of an X-ray beam is reduced by interaction with
the matter it encounters. X-ray photons are attenuated by
absorption and scattering of X-ray photons out of the
absorber as a result of interactions with the orbital
electrons of component atoms.
 There are three mechanisms –
1. Coherent scattering
2. Photoelectric effect
3. Compton effect

68. COHERENT SCATTERING (THOMPSON EFFECT OR
CLASSIC SCATTERING)
 Coherent scattering (also known as
classical, elastic , or Thompson
scattering) may occur when a low-
energy incident photon (less than 10
keV) passes near an outer electron
of an atom.
 The incident photon interacts with
the electron by causing it to become
momentarily excited at the same
frequency as the incoming photon .

69. o The incident photon ceases to exist.
o The excited electron then returns to
the ground state and generates another
x-ray photon with the same frequency
(energy) as in the incident beam.
o Usually the secondary photon is
emitted at an angle to the path of the
incident photon.
oThe net effect is that the direction of
the incident x-ray photon is altered.

70.  Coherent scattering accounts for only about 7% of the total
numberof interactions in a dental exposure Coherent scattering
contributes little to fi lm fog because the number of scattered
photons is small and their energy is too low for many of them
to reach the film or sensor.

71. COMPTON SCATTERING:
 Or inelastic scattering is an
interaction of photons with
free or loosely bound outer
shell electron.
 The photon gives some of its
energy to the electron and it,
itself continues in a new
direction, but with reduced
energy and hence with
increased wavelength.

72.  The ejected outer shell electron
is called compton or recoil
electron.
 If scattered through a small
angle, very small amount of
energy is lost to the outer
electron.

73.  The recoil electrons further
ionizing interactions with the
tissues, and gradually lose
energy along their tracts by
causing secondary radiations and
consequent biological damage.
 Due to their energy, rays can
emit photoelectrons from metals,
when allowed to fall on them.

74. PHOTOELECTRIC EFFECT:
It is a process of interaction of the incident photon and the
bound electron leading to emission of characteristic radiation.
It occurs when an incident photon collides with a bound
electron in the atom of the absorbing medium.
The incident photon ceases to exit and its energy helps to
eject a bound electron from its shell to become a recoil
electron or a photo electron.

75. The kinetic energy imparted to the recoil electron is equal to the
energy of the incident photon minus that required to overcome
the electron binding energy.
The orbital vacancy caused by the electron reshuffle and the
neutrality is obtained by attracting an electron from outside.
During this rearrangement characteristic radiation is emitted.

76. References
Text book of oral radiology -white & Pharoah 6thedition
Text book of oral radiology -Freny Karjodkar 1st edition
Text book of diagnostic radiology – cristensen’s 4th edition
 Text book of oral radiology -Ghom