X-ray Tube (Cathode)

1. TOPIC 1
13/9/2018
X-RAY IMAGING SYSTEM
Dr. Nik Noor Ashikin Nik Ab Razak
School of Physics
Universiti Sains Malaysia
nnashikin@usm.my

2. 1.2 CATHODE
1.2.1 Filament
1.2.2 Focusing Cup
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3. 9/3/2018 3
2.3 Cathode
CATHODE
Filament Focusing Cup
NNANAR

5. 1.2.1 Filament
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6. 1. Negative Electrode
2. Source of electron heated by an electric current
3. Mounted within a negatively charged focusing cup
4. Made from tungsten wire of 0.2–0.3 mm diameter and
operates at around 2700 k
FILAMENT: Properties
1.2.1

7. FILAMENT : ADVANTAGES OF TUNGSTEN
• High melting point (3370o C).
• Little tendency to vaporize.
• Ductility & Stability
• Malleability and strength
• A trace of thorium in the tungsten wire increases the efficiency of the electron emission and
prolongs the life of filament)
• Good thermionic emitter
• Rugged and able to be drawn into the thin wire required
Disadvantages:-
Not an efficient electron emitting material.
1.2.1

8. • A very high current (few amperes ) is passed
through the filament and heats the metal causing
the outer electrons of the tungsten atoms boiled
off, and ejected from the surface of the coil.
• Emission of electrons resulting from absorption
of thermal energy is k/a thermionic emission.
• The electrons are liberated at a rate that increases
with the filament current
EDISON EFFECT
The electron cloud surrounding the
filament, which is produced by thermionic
emission
FILAMENT: THERMIONIC EMISSION
1.2.1

9. 9
2.3.1a Thermionic Emission
1
• High operating temperature (>2500 0K)
2
• High work function
3
• Low emission efficiency
Disadvantages Thermionic Emission
NNANAR
2.3.1.1 Thermionic Emission
FILAMENT : THERMIONIC EMISSION
1.2.1

10. 2.3.1 Filament
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Space
Charge
• Electrons emitted from the tungsten
filament form a small cloud in front
of the filament. This collection of
negatively charged electrons forms
space charge.
Space
Charge
Effect
• Tendency of space charge to limit
emission of other electrons from
filament.
FILAMENT :Space charge
1.2.1

11. FILAMENT : EQUILIBRIUM STATE
• As the electrons leave the filament, it
acquires a positive charge attracting some
electrons back to itself.
• Number of electrons returning to filament
is equal to number of electrons being
emitted.
• As a result, space charge remains
constant with actual number depending on
filament temperature.
1.2.1

12. • Two electrical currents flow in an x-ray tube:
• The filament current = flow of electrons through the filament to raise its
temperature and release electrons.
• The tube current = flow of released electrons from the filament to the anode
across the x-ray tube (varies from a few to several hundred milliamperes)
FILAMENT : Filament Current & Tube Current
1.2.1

13. FILAMENT : TUbe Current
•Is the number of electrons flowing from cathode to anode per
second.
•Measured in milli amperes(mA)
•The tube current is unidirectional - from cathode to anode.
•x-ray tubes do not exceed 1000 mA because of the space
charge effect
1.2.1

14. FILAMENT : saturation Current
•At a given filament current, x-ray tube current rises w/ increasing
kVp to max value. further increases kVp does not result in higher
mA since all available e- were used, not reached at low kVp b/c
space charge limitation.
•*at levels about the saturation current, an increase in kVp will not
increase the tube current as all available electrons have been
used
1.2.1

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1.2.1

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1.2.1

17. • Contain single filament/ double filaments/ sometimes 3
filaments
DOUBLE FILAMENT ARRANGEMENT:-
• They are placed side by side or one above the other.
• Only one filament is used for any fixed x ray exposure.
FILAMENT : MODERN DAY X-RAY TUBES
1.2.1

18. 2.3.1.2 Dual Focal Tube
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Modern tubes have two filaments:
examples:
1.2 mm and 2.0 mm
0.6 mm and 1.2 mm
0.3 mm and 2.0 mm
Long One :
higher current, lower resolution, larger exposure
Short One :
lower current, higher resolution
1.2.1

19. •Tube with 3 filaments
•Stereoscopic angiographic tube
-In this tube, 2 focal spots are widely separated producing
stereoscopic film pair when 2 films are exposed.
-Used in angiography.
FILAMENT : HIGHLY SPECIALISED X-RAY TUBES
1.2.1

20. •When x-ray is turned on and no exposure is made, (as in
fluoroscopy) stand by current heats the filament at low current
(5mA).
•When exposures are needed, automatic circuit will raise filament
current to required value and lower it to stand by after exposure.
FILAMENT : AUTOMATIC FILAMENT
BOOSTING CURCUIT
1.2.1

21. 1.2.2 Focusing Cup
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22. FOCUSSING CUP: Properties
• The filament is embedded in a metal shroud called the
focusing cup
• Made of NICKEL
- High melting point (due to tremendous amount of heat that is
generated at the cathode)
- Relatively poor thermionic emitter
• It is maintained at same negative terminal as that of filament
1.2.2

23. • The specially designed cup cause the electron
stream to converge to the target area on the
anode
• Prevents bombardment of unacceptably large
target area
• All electrons accelerated from cathode to
anode are electrically negative, the beam
tends to spread out due to electrostatic
repulsion, and some electrons can even miss
the anode completely
FOCUSSING CUP: Functions
1.2.2

24. 1.3 Glass Envelope
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25. • Is a sealed evacuated tube made up of borosilicate (to withstand the
tremendous heat generated & maintains vacuum inside the tube)
• mounted from the anode and cathode.
• The glass must be a good electrical insulator, or a substantial current will
flow through it when a potential difference is applied between the anode
and cathode
GLASS ENVELOPE: Properties
1.3

26. • Enclosed the tube with the vacuum
• Accelerated electrons  collide with gas molecules  secondary electrons
(less speed) wide variation in tube current and energy of x ray produced.
• The purpose of the vacuum in the modern x-ray tube is to allow the number
and speed of accelerated electrons to be controlled independently.
• The shape and size of these x-ray tubes are specially designed to prevent
electric discharge between electrodes.
GLASS ENVELOPE : Functions
1.3

27. 1. On long term use, tungsten vaporizes and form thin coat on inner surface of
glass wall of x-ray tube. This alters the electric potential of the tube, allowing
tube current to stray and interact with the glass envelope; the result is arcing
and tube failure
2. If tube becomes glassy, x-ray production will fall off and tube will fail
3. A failing vacuum, resulting from leakage or degassing of the materials,
causes increased ionization of the gas molecules, which slows down the
electrons. further, a current of positive ions flowing back could impair or
destroy the cathode filament
GLASS ENVELOPE : problems
1.3

28. 2.4 Glass Envelope (SOLUTION)
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 Improvement in tube design incorporates
metal rather than glass
- Metal maintain a constant electric potential
between the electron of the tube current and
the envelope.Therefore they have longer life
and less likely to fail
GLASS ENVELOPE : solution
1.3

29. Made of aluminum oxide. They insulate high voltage parts of x-ray
tube from metal envelope.
 Less off focus radiation
 Longer tube life with high tube currents
 Higher tube loading
 Adequate electrical safety
 Compact size
GLASS ENVELOPE : CERAMIC INSULATORS
1.3

30. 1.4 Tube Housing
303/9/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak

31. • Consists of metal case made up of Aluminum
alloy lined on the inside by a layer of lead
which protects and supports the glass x-ray
tube insert.
• The tube housing is packed with industrial
grade oil to provide electrical and thermal
insulation.
TUBE HOUSING: properties
1.4

32. 2.5 Tube housing
2.5 Tube Housing
• Tube housing provides an efficient
radiation barrier where in the x-rays
produced in the x-ray tube are
attenuated in all the directions except
at the tube port.
• Provides shielding for the high
voltages required to produce x rays
TUBE HOUSING: function
1.4

33. *X-rays that escape through protective
housing are they contribute no diagnostic
info and result in unnecessary exposure
*Leakage radiation must not exceed
100mR/hr
(1 Gy t) at 1 meter
TUBE HOUSING: LEAKAGE RADIATION
TUBE HOUSING
1.4

34. It is due to electron back scatter from
anode interacting with metal other than
the focal track and striking anode a
second time to produce X-rays.
Decreased by :
1. Placing collimator
2. Lead diaphragm as close to X-ray tube as
possible.
3. Using a metal enclosure – attracts off focus
radiation to the grounded metal tube.USEFUL
OFF-FOCUS
TUBE HOUSING: OFF FOCUS RADIATION
1.4

35. 1.4

36. COOLING MECHANISM OF X-RAY TUBE
Almost all energy put into x-ray tube is converted into heat and <1%
is converted into x-rays.
2.5 Tube Housing
HEAT
DISSIPATION
CONDUCTION
Through
solid parts of
anode.
CONVECTION
Through oil
surrounding
the tube
RADIATION :
Occurring through the
vacuum of the tube
which passes off the heat
to glass envelope or
from metallic housing
through air into the
atmospheric air.
1.4

37. * most heat generated in the x-ray tube is dissipated by radiation
from the anode
*RADIATION is the transfer of heat by emission of infrared radiation
*CONDUCTION is transfer of heat by touching from one area to
another
*CONVECTION is the transfer of heat by movement of liquid or gas
COOLING MECHANISM OF X-RAY TUBE
2.5 Tube Housing
1.4

38. Due to time allowed for rotor to accelerate to designed rpm; filament current increases to get correct x-ray tube
current
2. What happens when the exposure switch is first pressed?
1. Why is there a small delay when the rad tech pushes the exposure button of the
imager?
Phase 1
Some of the electricity is diverted to the induction motor of the x-ray tube to bring the rotor up to speed.
Approximately 3400 RPM
Phase 2
The second phase actually initiates the x-ray production process

40. 1.5 Power Ratings For X-ray Tubes
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41. 41
Dr. Nik Noor Ashikin Bt Nik Ab
2.6.2 Maximum Energy
2. Maximum-energy ratings are provided for the target, anode, and housing of an x-ray
tube. These ratings are expressed in heat units:
SINGLE-PHASE
THREE-PHASE
1. The heat storage capacity of a tube component is the total number of heat units
that may be absorbed without damage to the component. (diagnostic x-ray tubes range
from several hundred thousand to over a million heat units)
1.5
Power Ratings For X-ray Tubes

42. The area above each curve reflects combinations of tube current and exposure time that overload the x-ray tube and might damage the target.
1.5
Power Ratings For X-ray Tubes

44. 443/9/2018 Dr. Nik Noor Ashikin Bt Nik Ab Razak
EXAMPLE
An exposure of 80 kVp, 250 mA and 100 msec results in following
HU deposition on the anode:
• Single-phase generators: 80 × 250 × 0.1
= 2000 HU
• Three-phase generators: 80 × 250 × 0.1 × 1.35
= 2700 HU
• Constant-potential generators: 80 × 250 × 0.1 × 1.4
= 2800 HU
• For continuous x-ray production (fluoroscopy), the HU/sec is defined
• HU/sec = kVp × mA

45. 1. How many heat units are generated by an exposure of
70 kVp, 300 mA, and 0.1 second on a single phase full-
wave rectified unit?
2. How many heat units are generated by an exposure of
70 kVp, 400 mA and 0.1 second on a 3 phase 6-pulse
unit?
3. How many heat units are generated by an exposure of
100kVp, 400mA and 0.01 second on a 3 phase 6-pulse
unit?
4. How many heat units are generated by 3 exposures of
100kVp, 400mA and 0.01 second on a 3 phase 12-pulse
unit?

46. QUESTION
ANSWER
• SIX SEQUENTIAL SKULL FILMS
ARE EXPOSED WITH A THREE-
PHASE GENERATOR OPERATED AT
82 KvP, 120 mAs. WHAT IS
THE TOTAL HEAT (IN HU)
GENERATED?
Number of heat units/film =
1.35×82×120
= 13,284 HU
total HU = 6 × 13,284
HU
= 79,704 HU

47. SUMMARY
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49. Summary of X-ray tube component