suchithra ks junior resident oral medicine and radiology government dental college calicut kerala

1. Spontaneous remission of a
squamous cell carcinoma of the
floor
of the mouth
C A S E R E P O R T
Good morning

2. P R E S E N T E D B Y
SUCHITHRA K.S
1 S T M D S
D E P T . O F O R A L M E D I C I N E A N D
R A D I O L O G Y
X-RAY MACHINE

3. CONTENTS
 Introduction
 History
 X ray machine
 Ideal requirements
 Classification
 Tubehead
 Circuitry
 Recent advances
 Conclusion
 References

4. Introduction
 X- rays belong to a group of radiations called electromagnetic
radiation.
 X rays are produced by energy conversion when a fast moving
stream of electrons is suddenly decelerated in the “target” anode
of an x ray tube.
 The x ray tube is made of Pyrex glass that encloses a vacuum
containing two electrodes. Electrons are produced by the heated
tungsten filament(cathode) and accelerated by a high potential
difference to hit the target anode, where x rays are produced.

5. DISCOVERY OF X RAYS
 Wilhelm Conrad Roentgen, a German
Physicist, discovered X-rays on
November 8, 1895
 He termed these rays as ‘X Rays’ after the
mathematical symbol for the unknown
– ‘X’ which were ultimately, called
‘Roentgen Rays’.
 1st radiograph the human body-
Bertha Roentgen’s hand(exposure time15 min)

6. HISTORY OF DENTAL RADIOLOGY
YEAR DISCOVERY PERSON
1895 Discovery of X rays W.C. Roentgen
1896 First Dental
Radiograph
O. Walkhoff
1896 First Dental
Radiograph in
US(skull)
W.J. Morton
1896 First Dental
Radiograph in US(live)
C.E. Kells
1901 First paper on dangers
of radiation
W.H. Rollins
1904 Introduction of
Bisecting Technique
W.A. Price
1913 First dental text H.R. Raper

7. HISTORY OF DENTAL RADIOLOGY
YEAR DISCOVERY PERSON
1913 First pre-wrapped
dental films
Eastman Kodak
Company
1913 First X ray tube W.D. Coolidge
1920 Concept of paralleling
Technique
F. MacCormack
1920 First machine made film
packets
Eastman Kodak
Company
1923 First dental X ray
machine
Victor X ray
Corporation of Chicago
1925 Introduction of
Bitewing technique
H.R.Raper

8. HISTORY OF DENTAL RADIOLOGY
YEAR DISCOVERY PERSON
1933 Concept of Rotational
panoramic proposed
Dr. Hisatugu Numata
1947 Introduction of Long
cone paralleling
technique
F.G. Fitzgerald
1948 Introduction of
Panoramic Radiography
Dr. Yrjo Veli Paatero
1955 Introduction of D speed
film
1957 First variable
kilovoltage Dental Xray
machine
General Electric
1960 First Panoramic Xray
machine Marketed
S.S White & Co

9. HISTORY OF DENTAL RADIOLOGY
YEAR DISCOVERY PERSON
1978 Introduction of dental
Xeroradiography
1981 Introduction of E speed
film
1987 Introduction of
Intraoral digital
Radiography
1987 Denta scan designed
1998 CBCT European market
2000 Introduction of ‘F’
speed films
2001 CBCT US Market

10. Ideal requirements
The equipment should be:
 Safe and accurate
 Capable of generating X-rays in the desired energy range and with
adequate mechanisms for heat removal
 Small
 Easy to maneuver and position
 Stable, balanced and steady once the tube head has been positioned
 Easily folded and stored
 Simple to operate
 Robust.

11. TUBE HEAD
 It is a tightly sealed, heavy metal housing that contains the
X ray tube that produces dental X rays.

12. Components of tube head
• protect the X ray tube and grounds
the high voltage component
Metal
housing
• filament, copper block and the
target
X ray tube
• step-up the mains voltage of 240
volts to the high voltage (kV)
required across the X-ray tube
Step-up
transformer
• step down the mains voltage of 240
volts to the low voltage current
required to heat the filament
Step-down
transformer

13. Components of tube head
• minimize leakageLead shield
• heat removal, insulation.
(metal bellows)Surrounding oil
• remove harmful low energy
(soft) X-rays
Aluminium
filtration
• shape and limit the beam sizeCollimator
• indicating the direction of the beam and setting the ideal
distance from the focal spot on the target to the skin
Spacer cone or
PID

14. Position indicating devices
 The spacer cone or beam-indicating device (BID) or Position
indicating device is a device for indicating the direction of the
beam and setting the ideal distance from the focal spot on the
target to the skin.
 The ideal focus to skin (fsd) distances:
— 200 mm for sets operating above 60 kV
— 100 mm for sets operating below 60 kV
 The long PID is preferred because less
divergence of X ray beam occurs.
 The rectangular type is most effective in
reducing patient exposure.
1.Conical
2. Rectangular
3. Round
1. Short (8 inches)
2. Long (16 inches)

15. X ray tube
 All dental and medical x-ray tubes are called Coolidge tubes
because they follow the original design of W. C. Coolidge
introduced in 1913.
 The X ray tube in dentistry measures approximately several
inches long by one inch in diameter.(12-18 cm in length and 9 cm
in breadth, made of pyrex glass/borosilicate glass to withstand
high temperatures)
 Crookes tube first

16. Components of x ray tube
LEADED GLASS HOUSING
 It prevents X ray from escaping in all directions(radiation
leakage)
 It has a ‘window’ that permits the X ray beam to exit the tube
and directs the X ray beam towards the Aluminium disc, lead
collimator and PID
 Used for earthing.

17.  The effectiveness of the tube housing in limiting leakage radiation
must meet the specifications listed in the National Council on
Radiation Protection and Measurements Reports No.49,which states
that,
“The leakage radiation measured at a distance of 1 meter from the
source shall not exceed 100mR in an hour when the tube is operated
at its max continuous rated current for the maximum rated tube
potential.”

18.  Usually x ray tube has a glass envelope(pyrex glass) that encloses
vacuum containing two electrodes.
 Sun-burning or Sun- tanning of tube – Vaporization of the filament
occurs over a period of time. When the particle vaporize (turn into
gaseous form), they solidify on the glass of the X ray tube, which is
called ‘sun-burning or sun-tanning of the tube.
This reduce the output of the X ray tube, destruction of the vacuum and
integrity of the tube, resulting in ‘arcing’ and ultimate tube failure.

19. metal or ceramic x-ray tubes
 This leads to the development of a high performance x ray tube by
Philips Medical Systems under the trade name of “ceramic super
rolatix” tube.
 Metal or ceramic tube has a metal casing instead of the usual glass
envelope, and has three ceramic insulators.
 Ceramic insulators are used to insulate the high voltage parts of the
x ray tube from the metal tube envelope. Aluminium oxide is a
commonly used ceramic insulator.
 Advantages
1. Less off-focus radiation
2. Longer tube life with high tube currents.
3. Higher tube loading.

20. METAL OR CERAMIC TUBES

21. Why vacuum inside???
 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.
 Also, the electrons that were being accelerated towards the anode
would collide with the gas molecules and cause the secondary electrons
to be ejected ,resulting in a wide variation in the tube current.
 The purpose of vacuum in the modern x ray tube is to allow the number
and speed of the accelerated electrons to be controlled independently.

22.  Connecting wires must be sealed into the glass wall of the x ray
tube.
 During operation, both the glass and the connecting wires are
heated to high temperatures. Because of differences in coefficient
of expansion, most metals expand more than glass when heated
resulting in glass-metal seal to break & would destroy the
vacuum in the tube.
 So, special alloys having approximately the same coefficients of
linear expansion as pyrex glass, are generally used in x ray tubes.

23. cathode
Two components:
a) Filament
b) Focusing cup
Filament – source of electron.
It is a coil of tungsten wire about 2 mm in diameter and 1 cm or less in
length
The filament is heated to incandescence by the flow of current from the
low-voltage source and emits electrons
Focusing Cup - the filament lies in a focusing cup, a negatively charged
concave reflector made of molybdenum
The parabolic shape of the focusing cup focuses the electrons into a
narrow beam directed at a small rectangular area on the anode ,the
focal spot
FOCUSING
CUP
FOCAL
SPOT

24. Focusing cup
 Modern x ray tubes may be
supplied with a single or more
commonly, a double filament.
 Only one filament is used for any
given exposure.
 Two additional filament
arrangements such as triple focus
and stereoscopic angiographic tube
is also available.

25. Thermionic emission
 The filament is heated to incandescence through a range of
temperatures by varying voltage (10V), across the filament from a step
down transformer in a low voltage circuit.
 The hot filament emits electrons that are separated from the outer orbits
of tungsten atoms at a rate proportional to its temperature by a process
called ‘thermionic emission’.

26. • Edison effect
The electron cloud surrounding the filament produced by the
thermionic emission.
• Electrons emitted from the tungsten filament form a small cloud in the
immediate vicinity of the filament. This collection of negatively
charged electrons forms what is called the space charge.
• The tendency of the space charge to limit the emission of more
electrons from the filament is called the space charge effect. The
number of electrons in the space charge remains constant.

27. Anode
 The anode consists of a tungsten target embedded in a copper stem.
 The purpose of the target in an x-ray tube is to convert the kinetic
energy of the colliding electrons into x-ray photons
 This is an inefficient process with more than 99% of the electron
kinetic energy converted to heat
 Two types of Anodes:
 1. Stationary or fixed anodes
 2. Rotating anodes

28. Stationary anode
 2 or 3 mm thick tungsten plate embedded in a large mass of
copper.
 Tungsten plate is square or rectangular in shape, with each
dimension usually being greater than 1 cm.
 The anode angle is usually 15 to 20 degree.

29. Why tungsten ?
 High atomic number (74) -A target made of a high atomic number
material is most efficient in producing x rays
 High melting point(3380 C) - heat is generated at the anode
 High thermal conductivity - readily dissipating its heat into the copper
stem
 Low vapor pressure - maintain the vacuum in the tube at high operating
temperatures

30. Why Copper is used ?
 Thermal conductor - removes heat from the tungsten, thus
reducing the risk of the target melting

31. Rotating anode
Used to produce x ray tubes capable of withstanding the heat generated
by large exposures
 The electrons strike successive areas of the target, widening the focal
spot by an amount corresponding to the circumference of the beveled
disk, thus distributing the heat over this extended area
 The focal spot of a stationary tube is now a focal track in rotating anode
machines
 Used in tomographic ,cephalometric
units ,medical computed tomography
x-ray machines

32. Structure
 The anode of rotating anode tube consists of a large beveled disc of
tungsten, or an alloy(90%tungsten and 10%rhenium) of tungsten
disk (The angle of bevel vary from 6 to 20 degree, to take advantage of
the line-focus principle), which theoretically rotates at a speed about
3600 rpm when exposure is being made
 The magnetic field produced by the
Stator coils induces a current in the
copper rotor of the induction motor,
and this induced current provides the
power to rotate the anode assembly.

33. Electron beam striking an area
(assume a 7mm high&2mm wide area)
 Stationary anode - 7×2 = 14 sqmm
 Rotational anode – track of 7mm wide that extends
around the periphery
 The typical disc diameter measure 75, 100 or 125 mm.

34. Factors to be considered
 Heat dissipation - the stem is made up of Molybdenum (high melting point
(2600c) & poor heat conductivity)
 Length of Molybdenum Stem – inertia – bearings at each end of the anode axle
 Lubricant - vaporize and destroy the vacuum - the use of metallic lubricants
(especially silver)
 Roughening of the focal track - very rapid heating and cooling of the surface -
an alloy of about 90% tungsten and 10% rhenium
 Speed of rotation –
1. Length of the anode stem is made as short as possible to decrease the
inertia of anode
2. The anode assembly rotates on two sets of bearings, which are placed as
far as possible
3. The inertia is reduced by decreasing the weight of the anode by a
compound anode disc in which the largest part of the disc is made of molybdenum
(specific gravity 10.2), which is lighter than tungsten( 19.3).

35. Methods of heat dissipation
1. Conduction : through the Copper stem
2. Convection : through the oil surrounding the tube
3. Radiation : through the radiator device attached to the copper
stem
4. Rotating Anode

36. Focal spot and line focus principle
 The focal spot is the area on the target to which the focusing cup directs
the electrons and from where x rays are produced.
 Most of the energy of the electrons is converted in to heat, with less
than 1% being converted into x rays.
 The sharpness of image increases as the size of the focal spot
decreases, but heat/area increases.
 This conflicting need of a large focal spot to allow great heat loading
and a small focal area to produce good radiographic detail, were
resolved in 1918 with the development of line focus principle.

37.  The target is inclined about 20
degrees (ANGLE OF
TRUNCATION) to the central
ray of the x-ray beam. This
causes the effective focal spot
to be approximately 1 × 1 mm,
as opposed to the actual focal
spot (1 × 3 mm)

38.  The anode angle differs
according to individual tube
design and may vary from 6 to
20 degree.
 For general diagnostic
radiography done at 40-in focus
film distance, the anode angle
is usually no smaller than 15
degree.
 Focal spot size is expressed in
terms of the apparent or
projected focal spot, sizes of
.3,.6.1.0,1.2mm are commonly
employed.

39. Heel effect
 The intensity of the x ray beam that leaves the x ray tube depends on the angle at
which the x rays are emitted from the focal spot. This variation is termed as the
‘heel effect’.
 The decreased intensity at the anode side is caused by the absorption of the x ray
photons by the target itself
 Significance :
1. The intensity of film exposure on the anode side
of the x ray tube is significantly less (thicker parts of
the body should be placed towards the cathode side)
2. The heel effect is less noticeable when larger
focus-film distances are used.
3. For equal target film distances, the heel
effect will be less for smaller films. (the intensity
of the x ray beam nearest the central ray is more uniform)

40. Heel effect

41. circuitry

42. X-ray generators
 An x ray generator is the device that supplies electric power to the x-
ray tube.
 The tube requires electrical energy for two purposes.
1. To boil electrons from the filament-filament circuit.
2. To accelerate these electrons from the cathode to the anode-
high voltage circuit.
• The generator has a timer mechanism ,a third circuit, which regulates
the length of exposure and a group of rectifiers for the high voltage
circuit.

43.  There are two basic circuits in a diagnostic x ray unit.
 One circuit contains the step up transformer and supplies the high
voltage to the x ray tube.
 The other circuit contains a step down transformer and supplies the
power that heats the filament of the x ray tube.
 A transformer called the “autotransformer” supplies the primary
voltage for both these circuits.

44. The autotransformer
 Single winding
 Self induction
 Can function as step up
or step down.

45. Filament circuit

46. Filament circuit
 Regulates current flow through the filament of the x ray tube.
 A current flow of 3 to 5 A with an applied voltage of about 10V are
typical values.
 The power to heat the x ray tube filament is provided by a small step
down transformer called the “filament transformer.”
 Precise control of filament heating is critical, because a small variation
in filament current results in a large variation in x-ray tube current.
 In addition, stabilizing and compensating circuits are also present.

47. High voltage circuit
 The circuit has two transformers, an autotransformer and a step up
transformer.
 The autotransformer is actually the kVp selector and is located in the
control panel.

48.  The step up transformer increases the voltage by a factor of 600 and the
potential difference across the secondary coil may be as high as 150000
V.
 Two meters are present in high voltage circuit,one to measure
KVp(“prereading peak kilovoltmeter”) and the other to measure mA.
 KVp-indicate the potential across the x ray tube
 mA-indicate the actual current flowing through the tube during an x ray
exposure.
 KVp meter is placed in the circuit between autotransformer and step up
transformer and mA, at the center of secondary coil where the
transformer is grounded. Both the meters can be located on the control
panel with a minimum of insulation and without serious risk of electric
shock.

49. Tube current
The tube current is the flow of electrons through the
tube; that is, from the cathode filament, across the
tube to the anode, and then back to the filament
Transformer
reduce voltage
to10 V
mA selector
adjust I to 10
mA
Regulates
filament
temperature
Regulates no of
electrons

50. Tube voltage
kVp selector
adjust primary
voltage
Secondary voltage
applied to step-up
transformers
110V becomes
60,000 to
100,000V
Increase energy of
electrons
Sufficient energy
for X ray
production
A high voltage is required between
the anode and cathode to give
electrons sufficient energy to
generate x rays

51. Rectification
 Process of changing alternating current into direct current and the
device that produces the change is called a rectifier.
 The high voltage transformer provides an alternating voltage for the x
ray tube.

52. Half-wave rectification
 Current flows only when anode is + ve & cathode – ve
 The half of the cycle where cathode is + ve & anode – ve ,where there
is no flow of electron is Inverse Voltage or Reverse bias.
 The circuitry in which the alternating high voltage is applied directly
across the x-ray tube, limits x-ray production to half the AC cycle and
is called self-rectified or half-wave rectified.
 Self rectification has two disadvantages-
 Prolonged exposure times.
 Destroy the filament

53.  Therefore, to protect the x ray tube and to improve the efficiency of the
x ray production, special rectifiers are incorporated into the high
voltage circuit.

54. Full wave rectification
 In Full-wave rectified , constant potential between anode
& cathode.
Mean energy of X ray beam is higher
Longer contrast scale
Patient receives less dose

55.  The principal disadvantage of pulsed radiation is that a considerable
portion of the exposure time is lost while the voltage is in valley
between two pulses. the time spent bombarding the target with low
energy electrons does little except to produce heat in the target and to
produce low energy x rays, which are absorbed in patient and raise
patient dose.
 This lead to the development of three phase generators, capacitor-
discharge generators, battery powered generators, medium frequency
and falling load generators…

56. Forms of rectification

57. Exposure timers
 A variety of ways to control the length of an x ray exposure
have been developed.
 There are four basic types of exposure timers-
 Mechanical timers
(rarely used today)
 Electronic timers
 Automatic exposure control
(phototimers)
o Pulse – counting timers

58. Electronic timers
 The electronic timer controls the length of time that high voltage is
applied to the tube and therefore the time during which tube current
flows and x rays are produced.
 The length of the x ray exposure is determined by the time required to
charge a capacitor through a selected resistance. The exposure button
starts the exposure and also starts charging the capacitor and is
terminated when the capacitor is charged to a value necessary to turn
on associated electronic circuits.

59. Subjecting the filament to continuous heating at normal operating current
shortens its life.
To minimize filament damage, the timing circuit first sends a current
through the filament for about half a second to bring it to the proper
operating temperature and then applies power to the high-voltage
circuit.

60. Phototimers
 Mechanical and electronic timers are subject to human error.
 Phototimers measure the amount of radiation required to produce the
correct exposure for a radiographic examination.
 The essential element in phototimers is a device that can detect
radiation and in response to this radiation, produce a small electric
current. There are three such devices-
 Photomultiplier detectors
 Ionization chambers
 Solid state detectors
 Phototimers can be located in front of the cassette, and are called
entrance types, or behind the cassette as exit types.

62. Tube Rating
 The total load that can be safely accepted by an x ray tube is a function
of the heat energy produced during the exposure.
 The rate at which heat is generated by an electric current is proportional
to the product of the voltage (kV) and the current (mA).
 Thus the total heat produced is a product of voltage and current and
exposure time. This energy is currently expressed in two different
systems.
 Heat units(an artificial system)(mA*kVp*sec)
 SI units(the watt-second or joule)

63.  The heat storage capacity for anodes of dental diagnostic tubes is
approximately 20KHU.
 Another parameter that measure x ray tube loading is kilowatt
rating. It is commonly used to express the ability of the tube to
make a single exposure of a reasonable duration(for 0.1 sec) and
always calculated for an x ray tube with a constant potential and
high- speed rotation.

64. Tube Rating Charts
 An x ray tube rating chart help us to calculate the approximate kW
rating.
 kW rating of a tube
that accept an exposure
of 70 kVp and 500 mA
at 0.1 sec is-
70kVp*500mA=35000w
=35kW
So this a 35-kW tube.

65.  Each x-ray machine comes with a tube rating chart that describe the
longest exposure time the tube can be energized for a range of voltages
(kVp) and tube current (mA) values without risk of damage to the
target from overheating.
 Anode heat-storage chart is used to determine the length of time the
tube must be allowed to cool before additional exposures can be made.
 Duty cycle relates to the frequency with which successive exposures
can be made. It must be long enough for heat dissipation and this
characteristic is a function of the size of the anode and the method used
to cool it.

66. Summary
 X rays are produced by energy conversion when a fast moving
stream of electrons is suddenly decelerated in the target of an x
ray tube. An x ray tube is a specially designed vacuum diode
tube. The target of an x ray tube is usually tungsten or an alloy
of tungsten. Heat production in the x ray tube is minimized by
using the line focus principle and a rotating anode.

67. An x ray generator supplies electrical energy to the x ray
tube and regulates the length of the radiographic exposure.
Single phase generators have half wave rectification. Three
phase generators may be six pulse or twelve pulse.
Transformers are given a rating that indicates the
maximum safe output of the secondary windings. Such ratings
are expressed as the kilowatt rating.

68. references
 Oral radiology – Principles and Interpretation - 6th Edition
Stuart C White & Michael J Pharoah
 Christensen's Physics of Diagnostic Radiology – 4th Edition
 Oral radiology – Principles and Interpretation - 5th Edition
Stuart C White & Michael J Pharoah
 Essentials of dental radiography and Radiology- 3rd
Edition,
Eric Whaites
 Fundamental physics of radiology –W.J. Meredith – 3rd
Edition
 Essentials of Oral & Maxillofacial Radiology – Freny R
Karjodkar