1. Resident Physics Lectures
Christensen, Chapter 2A
X-Ray Tube Construction
George David
Associate Professor
Department of Radiology
Medical College of Georgia

2. X-Ray Tube Components Housing Glass Enclosure (insert)*
Housing
Visible part of tube
Glass Enclosure (insert)
Vacuum
Electrodes
Cathode
Filament
Anode
Target

3. X-Ray Tube Converts Energy FROM To Heat X-Rays electrical energy*
Converts Energy
FROM
electrical energy To
Heat
> 99% of incident energy
Bad! Ultimately destroys tubes
X-Rays
< 1% of incident energy
Good! Our desired product

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

5. Tube Housing (cont.) Shields against high voltage
electrically grounded
high voltage cable receptacles (wells)
housing filled with oil
cools
electrical insulation
all air removed
bellows
on end of tube
allows oil to expand when hot.
Vacuum
Oil
Insert

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

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

8. 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

9. keV = kilo-electron volt
energy of an electron
Kinetic energy
Higher energy electron moves faster
Electrons can be manipulated by electric fields
Accelerated
Steered +

10. Requirements to Produce X-Rays
Filament Voltage
High Voltage
anode
filament
filament
voltage
source +
high
voltage
source

11. Cathode (filament) Coil of tungsten wire Tungsten advantages
similar to light bulb filament
Tungsten advantages
high melting point
little tendency to vaporize
long life expectancy
Tungsten disadvantages
not as efficient at emitting electrons as some other materials

12. Cathode (filament) Cathode is source of electrons
filament heated by electric current
~ 10 volts
~ 3-5 amps
filament current is not tube current

13. X-Ray Production(cont.)
X-Rays are produced in the x-ray tube by two distinct processes
Characteristic radiation
Bremsstrahlung

14. Characteristic Radiation
Interaction of high speed incident electron with orbital electron of target
#1: Electron from filament removes inner-shell orbital electron from atom
#2: electrons from higher energy shells cascade down to fill vacancies
#3: characteristic x-ray emitted L K - + ~ + ~ + ~ #1 -
Electron from Filament - - #2 - #3

15. Characteristic Radiation
Consists only of discrete x-ray energies corresponding to energy difference between electron shells of target atom
Specific energies characteristic of target material
for tungsten 59 keV corresponds to the difference in energy between K and L shells - + ~ K L
Energy #

16. Electron from Filament
Bremsstrahlung
interaction of moving electron from filament with nucleus of target atoms
Positive nucleus causes moving electron to change speed / direction
Kinetic energy lost
Emitted in form of Bremsstrahlung x-ray - + ~ K L
Electron from Filament -

17. Bremsstrahlung (cont.)
Bremsstrahlung means braking radiation
Moving electrons have many Bremsstrahlung reactions
small amount of energy lost with each - + ~ K L

18. Bremsstrahlung (cont.)
Energy lost by moving electron is random & depends on
distance from nucleus
charge (Z) of nucleus
Bremsstrahlung Energy Spectrum
0 - peak kilovoltage (kVp) applied to x-ray tube
most Bramsstrahlung photons have low energy
lowest energy photons don’t escape tube
easily filtered by tube enclosures or added filtration #
Energy

19. Output Beam Spectrum Output photon beam made up of Spectrum # Energy #
Characteristic Radiation
characteristic of target material
several discrete energies
Bremsstrahlung
continuous range of energies
0 - kVp setting
most photons have low energy
Spectrum
depicts fraction of beam at each energy value
combination of Bremsstrahlung and characteristic radiation
Energy # #
Energy

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

21. Beam Intensity Product of Units Depends on # 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

22. Intensity & Technique + beam intensity proportional to mA
beam Intensity ~ proportional to kVp2
filament
voltage
source +
high
voltage
source

23. Space Charge + Electrons leave filament*
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 + -

24. Kilovoltage & Space Charge
raising kilovoltage gradually overcomes space charge
Higher fraction of electrons make it to anode as kilovoltage increases
At high enough kilovoltage saturation results
All electrons liberated by filament reach target
Raising kilovoltage further has no effect on # electrons reaching anode + -
Tube Current (mA)
Saturation
Voltage
kVp

25. Saturation Voltage + + + + +- + + - + -
kilovoltage at which a further increase does not increase tube current
100% of electrons already going to target
Tube current said to be emission limited
tube current can only be increased by increasing filament temperature
Tube Current (mA)
Saturation
Voltage
kVp

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

27. Focusing Cup + negatively charged focuses electron stream to target
overcomes tendency of electrons to spread because of mutual repulsion +
Focusing
Cup

28. 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

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

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

31. 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
thins the filament
filters the x-ray beam
increases possibility of arcing
electrons attracted to glass instead of target +

32. Cross Section of X-Ray Tube
Dunlee Web Site:

33. Cross Section of X-Ray Tube
Dunlee Web Site:

34. Line Focus Principle + Focal spot steeply slanted
7-15 degrees typical
Focal spot looks small from patient’s perspective
Imaging size
Looks large from filament
better heat capacity +
Actual FS
Apparent FS
Patient

35. Line Focus Principle + Actual (true) focal spot
as seen from filament
Apparent (effective, projected) focal spot
as seen from tube port or patient +
Actual FS
Apparent FS
Patient

36. Target Angle Angle between target & perpendicular to tube axis
Typically 7 – 15 degrees +
Target Angle, Q

37. Line Focus (cont.) Apparent FS = Actual FS X sin Q + Actual FS
Target Angle, Q
Apparent FS = Actual FS X sin Q

38. Target Angle Large Same apparent focal spot size! + + Small
poorer heat ratings
better field coverage
Small
optimizes heat ratings
limits field coverage
Large Target Angle
(Small Actual Focal Spot)
Small Target Angle
(Large Actual Focal Spot) + +
Same apparent focal spot size!

39. Heel Effect
Intensity of x-ray beam significantly reduced on anode side
beam goes through more target material exiting the anode x - - -
cathode side
anode side

40. Anodes Stationary Rotating Target is annular track
spreads heat over large area of anode
speeds
3600, 9600 rpm
Faster = much better heat ratings

41. Rotating Anode Advantages Disadvantages better heat ratings
More complex ($)
Rotor drive circuitry
motor windings in housing
bearings in insert

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

43. 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
cine
grid +