Wilhelm Conrad Roentgen used a Crookes-Hittorf tube to produce the first x-rays in 1895. Early x-ray tubes had no shielding and emitted radiation in all directions, posing major hazards. Modern tubes are designed with shielding and safety features to overcome these problems. Key components of x-ray tubes include the cathode, which emits electrons via thermionic emission, the anode target, which converts the electrons' kinetic energy to x-rays, and various designs like rotating anodes to dissipate heat and improve performance.

1. - Dr. Bipin Joseph

2. The X-Ray Tube
Development
 Wilhelm Conrad
Roentgen used a
Crookes-Hittorf tube
to make the first x-ray
image.
 There was no
shielding so x-rays
were emitted in all
directions.

3. The X-Ray Tube
Development
 This is the variety of
tube designs
available in 1948.

4. The X-Ray Tube Development
 Two major hazards plagued
early radiography.
 Excessive radiation exposure
 Electric Shock
 Moderns tubes are designed
to overcome these problems
 The modern tube is based on the Coolidge
tube

7. Collimator

8. Manual
controls

9. Transformer

10. X –ray
tube

11. Newer X ray
tube

12. The
improvement

13. X-Ray Tube Components
 Housing
 Visible part of tube
 Glass
Enclosure
(insert)
 Vacuum
 Electrodes
 Cathode
 Filament
 Anode
 Target
*

14. X-Ray Tube
 Converts Energy
 FROM
 electrical energy
 To
 Heat
 > 99% of incident energy
 Bad! Ultimately destroys tubes
 X-Rays
 < 1% of incident energy
*

15. Inside the Glass Insert
 Filament
 Similar to light bulb
 Glows when heated
 Target
 Large (usually) tungsten block
target filament

16. Requirements to Produce X-Rays
 Filament Voltage
 High Voltage
+
filamentanode
filament
voltage
source
high
voltage
source

17. X-Ray Tube Principle
 Filament heated
 electrons gain energy
 electrons freed (“boiled” off)
 Thermionic emission
-
-
*

18. Space Charge
 Electrons leave filament
 filament becomes positive
 Negative electrons stay close
 Electron cloud surrounds filament
 Cloud repels new electrons from filament
 Limits electron flow from cathode to anode
+ -
-
-
*

19. X-Ray Tube Principle
 Positive (high) voltage applied
to anode relative to filament
 electrons accelerate toward anode target
 Gain kinetic energy
 electrons strike target
 electrons’ kinetic energy converted to
 heat
 x-rays
+
*

20. Cathode
 Fillament +
 Focussing cup +
 Connecting wires
 Tungsten filament- 0.2mm
diameter wire is coiled to
spring of 0.2cm diameter and
1 cm length

21. Cathode (filament)
 Coil of tungsten wire
 Tungsten advantages
 high melting point 3370’C
 little tendency to vaporize
 long life expectancy
 Tungsten disadvantages
 not as efficient at emitting
electrons as some other
materials

22. Cathode (filament)
 1-3% Thorium added for better
emission
 Cathode is source of electrons
 filament heated by electric current
 ~ 10 volts
 ~ 3-5 amps
 filament current is not tube current

24. Filament (cont.)
 Large Filament normally left on at
low “standby” current
 boosted before exposure (prep or first trigger)
 With time tungsten from hot filament
vaporizes on glass insert- sunburn
 thins the filament
 filters the x-ray beam
 increases possibility
of arcing
 electrons attracted to
glass instead of target
+

26. Focusing Cup
 Negatively charged
 Focuses electron
stream to target
 overcomes tendency of
electrons to spread
because of mutual
repulsion
+
Focusing
Cup

29. Focal Spot
 portion of anode struck by electron
stream
 Focal spot sizes affects and limits
resolution
+

30. Focal Spots
 Most tubes have 2 filaments & thus 2
focal spots
 only one used at a time
 small focus
 improved resolution
 large focus
 improved heat ratings
 Electron beam strikes larger portion of
target

31. Focal Spot Size & Resolution
The larger the focal
spot the more it will
blur a tiny place on
the patient.

32. Focal Spot Size & Heat
The larger the area the
electron beam hits, the
more intense the beam
can be without melting the
target

33. Tube Current (mA)
Rate of electron flow from filament to target
Electrons / second
Measured in milliamperesmilliamperes (mA)
Limited by
filament emission (temperature / filament current)
space charge (see next slide)
+

34. Stationary
Anode

35. Target Angle
Angle between target &
perpendicular to tube axis
Typically 6-20 degrees
+
Target Angle, Θ

36. Target Angle
 Small
 optimizes heat ratings
 limits field coverage
+
Large Target Angle
(Small Actual Focal Spot)
+
Small Target Angle
(Large Actual Focal Spot)
• Large
– poorer heat ratings
– better field coverage

37. Anode angle

38. Anode angle
 Remember
 If anode angle is small
 Then focal spot is also small
 So good radiographic detial
 And small area of exposure

39. Anod
e

40. The Target
 Tungsten-Rhenium is used as the target for
the electron beam.
 Rhenium is used to increase the surface
properties to minimise the pitting and cracking
of the target

41. The Rotating Anode
 The rotating
anode allows the
electron beam to
interact with a
much larger
apparent target
area.
 The heat is not
confined to a
small area.

42. Rotating anode

43. Rotating anode

44. Rotating Anode
 Advantages
 Better heat ratings
 Disadvantages
 More complex ($)
 Rotor drive circuitry
 motor windings in housing
 bearings in insert

45. Rotating Anode
 Larger diameter
 Better heat ratings
 heavier
 requires more support
 costly
 Materials
 usually tungsten
 high melting point
 good x-ray production
 molybdenum (and now Rhodium) for mammography
(sometimes)
 low energy characteristic radiation

46. Molybdenum and graphite as
insulator

47. The anode stem
 The anode stem is made from
molybdenum(2600’)
 It is made appropriately thin as to
minimize the heat conduction towards
the rotor

48. Lubricatio
n
 Not oil
 Not graphite
 Silver is the best available in high vaccum,
bearing wear is negligible

49. Safety circuit
 Delay of 0.5-1second for anode to reach
full speed

50. Breaking The Rotating Anode
 When the anode is spinning at the correct
speed, the exposure can be made.
 After the exposure is completed, it slows
by reversing the motor.
 This is necessary to avoid excessive wear
and tear of the bearings

51. Heel effect

52. Beam Intensity
 Product of
 # photons in beam
 energy per photon
 Units
 Roentgens (R) per unit time
 Measure of ionization rate of air
 Depends on
 kVp
 mA
 target material
 filtration

53. Grid-controlled tubes
Grid used to switch tube on/off
grid is third electrode
relatively small voltage
controls current flow
from cathode to anode
Negative grid voltage repels electrons from filament
Grid much closer to filament than target
Applications
speedy switching
required
+
grid

55. Tube Housing
 Shields against leakage
radiation
 lead lined
 leakage limit
 100 mR / hour when tube
operated at maximum continuous
current for its maximum rated
kilovoltage
*

56. Tube Housing (cont.)
 Shields against high
voltage
 Housing filled with oil
Oil
Vacuum
Insert

57. Off focal radiation

58. Collimator
Aluminium and copper sheet to remove low energy X rays

59. Dissipation of heat from the target
 Even with the anode rotating, some melting
occurs.
 The heat must be rapidly dissipated from the
target.
 The anode dissipates heat by radiating
towards the glass envelop

60. Tube cooling

61. Newer X ray tube

62. Super Rolatix ceramic X ray
tube

63. Super rolatix ceramic X ray
tube

64. Summary
 Earliest X ray tubes
 Basic principle
 Cathode
 Focusing cup
 Stationary anode
 Anode angle
 Rotating Anode
 Anode stem
 Grid control
 Tube housing
 Collimator
 Tube cooling
 Super rolatix tube

65. Thank you
Wilhelm Conrad Roentgen