1. RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY
L 6: X Ray production
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IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources

2. Basic elements of the X Ray source assembly
Generator : power circuit supplying the required potential to the X Ray tube
X Ray tube and collimator: device producing the X Ray beam
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3. X Ray tubes
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4. X Ray tube components
Cathode: heated filament which is the source of the electron beam directed towards the anode
tungsten filament
Anode (stationary or rotating): impacted by electrons, emits X Rays
Metal tube housing surrounding glass (or metal) X Ray tube (electrons are traveling in vacuum)
Shielding material (protection against scattered radiation)
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5. X Ray tube components housing cathode 1: long tungsten filament
2 : short tungsten filament
3 : real size cathode
1: mark of focal spot
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6. Cathode structure (I)
Cathode includes filament(s) and associated circuitry
tungsten material : preferred because of its high melting point (3370°C)
slow filament evaporation
no arcing
minimum deposit of W on glass envelope
To reduce evaporation the emission temperature of the cathode is reached just before the exposure
in stand-by, temperature is kept at ± 1500°C so that 2700°C emission temperature can be reached within a second
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7. Cathode structure (I) Modern tubes have two filaments
a long one : higher current/lower resolution
a short one : lower current/higher resolution
Coulomb interaction makes the electron beam divergent on the travel to the anode
lack of electrons producing X Rays
larger area of target used
focal spot increased  lower image resolution
Focalisation of electrons is crucial !
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8. X Ray tube characteristics
Anode mechanical constraints
Material : tungsten, rhenium, molybdenum, graphite
Focal spot : surface of anode impacted by electrons
Anode angle
Disk and annular track diameter (rotation frequency from 3,000 to 10,000 revolutions/minute)
Thickness  mass and material (volume)  heat capacity
Anode thermal constraints
Instantaneous power load (heat unit)
Heat loading time curve
Cooling time curve
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9. This conflict is solved by slanting the target face
Anode angle (I)
The Line-Focus principle
Anode target plate has a shape that is more rectangular or ellipsoidal than circular
the shape depends on :
filament size and shape
focusing cup’s and potential
distance between cathode and anode
Image resolution requires a small focal spot
Heat dissipation requires a large spot
This conflict is solved by slanting the target face
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10. Anode characteristic 1 : anode track 2 : anode track
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11. Induction motor Works on the principle similar to the transformer.
Electromagnetic induction.
Current flowing in the stator develops a magnetic field.
Stator windings are sequentially energized so that the induced magnetic field rotates on the axis of the stator.
This causes the rotor to rotate.

12. Line focus principle
The area of the x-ray tube anode from which the x-ray photons are emitted.
This is called the actual focal spot

13. THE BETTER THE RESOLUTION
Anode angle (II) 
Angle ‘  
Angle
Actual focal
spot size
Actual focal
spot size
Incident electron
beam width
Incident electron
beam width
Increased
apparent
focal spot size
Apparent focal spot size
Film
Film
THE SMALLER THE ANGLE
THE BETTER THE RESOLUTION
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14. Line focus principle
Was incorporated into x-ray tube targets to allow a large area for heating while maintaining a small focal spot.
The effective focal spot is the area projected onto the patient and film.

15. Line focus principle
Focal spot sizes always make reference to the effective focal spot.
The lower the target angle, the smaller the effective focal spot size.

16. Line focus principle
The advantage of the line-focus principle is that it provides the detail of a small focal spot while allowing a large amount of heat dissipation.

17. Line focus principle
The unfortunate bi-product of the line-focus principle is the “anode heel effect”

18. Anode heel effect
Construction phenomenon that causes the x-ray photons exiting the tube on the cathode side to have a greater energy value than those exiting the tube on the anode side.

19. Anode heel effect
More energy absorption occurs at the anode heel resulting in less energy value from the incident photons at the anode heel.
This is used to advantage when imaging anatomical parts that are unequal in thickness and densities throughout their respective lengths.

20. Using the anode heel effect
The following anatomical parts may be imaged using the anode heel effect:
Thoracic vertebrae
Humerus
Femur
Tibia & fibula
Forearm

21. Anode heel effect (I)
Anode angle (from 7° to 20°) induces a variation of the X Ray output in the plane comprising the anode-cathode axis
Absorption by anode of X photons with low emission angle
The magnitude of influence of the heel effect on the image depends on factors such as :
anode angle
size of film
focus to film distance
Anode aging increases heel effect
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22. Anode heel effect (II) The heel effect is not always a negative factor
It can be used to compensate for different attenuation through parts of the body
For example:
thoracic spine (thicker part of the patient towards the cathode side of the tube)
mammography
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23. Focal spot size and imaging geometry
Focal spot finite size  image unsharpened
Improving sharpness  small focal spot size
For mammography focal spot size  0.4 mm nominal
Small focal spot size  reduced tube output (longer exposure time)
Large focal spot allows high output (shorter exposure time)
Balance depends on organ movement (fast moving organs may require larger focus)
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24. It supplies the X-ray tube with :
X-ray generator (I)
It supplies the X-ray tube with :
 Current to heat the cathode filament
 Potential to accelerate electrons
 Automatic control of exposure (power application time)
 Energy supply  1000  X-ray beam energy (of which 99.9% is dissipated as thermal energy)
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25. X-ray generator (II)
Generator characteristics have a strong influence on the contrast and sharpness of the radiographic image
The motion unsharpness can be greatly reduced by a generator allowing an exposure time as short as achievable
Since the dose at the image plane can be expressed as:
D = k0 . Un . I . T
U: peak voltage (kV)
I: mean current (mA)
T: exposure time (ms)
n: ranging from about 1.5 to 3
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26. Tube potential wave form (I)
Conventional generators
single  1-pulse (dental and some mobile systems)
single  2-pulse (double rectification)
three  6-pulse
three  12-pulse
Constant potential generators (CP)
HF generators (use of DC choppers to convert 50Hz mains into voltages with frequencies in the kHz range)  “Inverter technology”
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27. Tube potential wave form (II)
Single phase single pulse
kV ripple (%)
100%
Single phase 2-pulse
13%
Three phase 6-pulse 4%
Three phase 12-pulse
Line voltage
0.01 s
0.02 s
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28. The choice of the number of pulses (I)
Single pulse : low power (<2 kW)
2-pulse : low and medium power
6-pulse : uses 3-phase mains, medium and high power (manual or automatic compensation for voltage drop)
12-pulse : uses two shifted 3-phase system, high power up to 150 kW
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29. The choice of the number of pulses (II)
CP : eliminates any changes of voltage or tube current
high voltage regulators can control the voltage AND switch on and off the exposure
voltage can be switched on at any moment (temporal resolution)
kV ripple <2% thus providing low patient exposure
HF : combines the advantages of constant potential and conventional generator
reproducibility and consistency of tube voltage
high frame rate possible
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30. Automatic exposure control
Optimal choice of technical parameters in order to avoid repeated exposures (kV, mA)
Radiation detector behind (or in front of) the film cassette (with due correction)
Exposure is terminated when the required dose has been integrated
Compensation for kVp at a given thickness
Compensation for thickness at a given kVp
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31. Automatic exposure control
X Ray tube
Collimator
Beam
Soft
tissue
Bone
Air
Patient
Table
Grid
Cassette
AEC detectors
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32. X-ray equipment operation mode and application (II)
Radiography and Tomography
Single and 3  generators (inverter technology)
output : 30 kW at 0.3 focus spot size
output : kW at 1.0 focus spot size
selection of kV and mAs , AEC
Radiography and Fluoroscopy
Under couch equipment, three  generator (inverter technology) - continuous output of W
output : 50 kW at 1.0 focus size for spot film
output : 30 kW at 0.6 for fluoroscopy (high resolution)
priority given to contrast
automatic settings of kV
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33. X-Ray equipment operation mode and application (III)
Radiography and Fluoroscopy
Over couch equipment, three phase generator (inverter technology) - continuous output of at least 500 W
output : focus size for spot film
output : for fluoroscopy (high resolution)
priority given to contrast
automatic settings of kV
Cardiac angiography
Three phase generator - continuous output  1kW
output : focus size
output : focus size
frame rate : up to 120 fr/s
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34. Protective housing

35. Protective housing
X-ray tube is always mounted inside a lead-lined protective housing that is designed to:
Prevent excessive radiation exposure.
Prevent electric shock to the patient and operator (technologist).

36. Protective housing
Incorporates specially designed high-voltage receptacles.
Provides mechanical support for the x-ray tube and protects it from damage.
Some tube housings contain oil in which the tube is bathed.
Some tube housings contain a cooling fan to air-cool the tube.
When properly designed, they reduce the level of leakage radiation to less than 100 mR/hr at 1 meter when operated at maximum conditions.