xray

1. The X-Ray Tube
The X-Ray Tube
S. Guilbaud
S. Guilbaud
Education Director
Education Director
School of Radiologic Technology
School of Radiologic Technology

2. S. Guilbaud, Education Director
X-Ray Tube
X-Ray Tube
• Electrical device used for the
Electrical device used for the
generation of x-rays.
generation of x-rays.
• This is accomplished by the
This is accomplished by the
acceleration of electrons and then
acceleration of electrons and then
suddenly decelerating them.
suddenly decelerating them.
• The energy of the x-rays is
The energy of the x-rays is
dependent on the kinetic energy of
dependent on the kinetic energy of
the electrons.
the electrons.

3. S. Guilbaud, Education Director
X-Ray tube components
X-Ray tube components
• Glass envelope
Glass envelope
• Cathode
Cathode
• Anode
Anode
• Protective housing
Protective housing

4. S. Guilbaud, Education Director
Glass envelope
Glass envelope
• Made of Pyrex glass
Made of Pyrex glass
– Able to withstand tremendous heat
Able to withstand tremendous heat
• Tube maintains a vacuum.
Tube maintains a vacuum.
• Tube window
Tube window
– A segment of glass that is thinner
A segment of glass that is thinner
than the rest of the glass envelope.
than the rest of the glass envelope.
– Contributes to inherent filtration.
Contributes to inherent filtration.
• 0.5mm Al equivalency.
0.5mm Al equivalency.

5. S. Guilbaud, Education Director
Cathode
Cathode
• Negatively charged electrode.
Negatively charged electrode.
• Two primary parts:
Two primary parts:
– Filament
Filament
– Focusing cup
Focusing cup

6. S. Guilbaud, Education Director
Cathode

7. S. Guilbaud, Education Director
Focusing cup
Focusing cup
• Metallic shroud containing the
Metallic shroud containing the
two filaments.
two filaments.
– Usually made from nickel.
Usually made from nickel.
• Contains a negative charge.
Contains a negative charge.
– Designed to repel electrons.
Designed to repel electrons.
– Designed to condense electron
Designed to condense electron
beam to small area on on focal
beam to small area on on focal
track.
track.

8. S. Guilbaud, Education Director
Focusing cup
Focusing cup
• Four factors
Four factors
determine the
determine the
effectiveness of the
effectiveness of the
cup.
cup.
– Size & shape.
Size & shape.
– Charge
Charge
– Filament size &
Filament size &
shape.
shape.
– Position of filament
Position of filament
w/in cup.
w/in cup.

9. S. Guilbaud, Education Director
Filament
Filament
• Small coil of thoriated tungsten.
Small coil of thoriated tungsten.
• Modern x-ray tubes contain two filament.
Modern x-ray tubes contain two filament.
– They correspond to the focal spot sizes.
They correspond to the focal spot sizes.
• When machine is turned on, small amount of
When machine is turned on, small amount of
current flows through to heat filament.
current flows through to heat filament.
• Tube current is adjusted by controlling the
Tube current is adjusted by controlling the
filament current.
filament current.

10. S. Guilbaud, Education Director
Anode
Anode
• Positively charged electrode.
Positively charged electrode.
• Two types.
Two types.
– Stationary anode.
Stationary anode.
– Rotating anode.
Rotating anode.

11. S. Guilbaud, Education Director
Stationary Anode
Stationary Anode
• Made of
Made of
tungsten target
tungsten target
embedded in a
embedded in a
large copper bar.
large copper bar.
• Usually used in
Usually used in
dental x-ray
dental x-ray
machine.
machine.

12. S. Guilbaud, Education Director
Rotating anode
Rotating anode
• Constructed of
Constructed of
tungsten target
tungsten target
(focal track).
(focal track).
– High melting point
High melting point
3400
34000
0
Celsius.
Celsius.
• Molybdenum
Molybdenum
– Surrounds tungsten
Surrounds tungsten
target area.
target area.
– Assists in
Assists in
dissipating heat.
dissipating heat.
• Graphite
Graphite
– Serves as mount for
Serves as mount for
molybdenum and
molybdenum and
tungsten target.
tungsten target.
– Also assists in
Also assists in

13. S. Guilbaud, Education Director
Rotating anode
Rotating anode
• Provides greater
Provides greater
target area and
target area and
greater heat
greater heat
dissipation.
dissipation.
• Affords the ability
Affords the ability
to attain greater
to attain greater
exposure loads by
exposure loads by
providing a larger
providing a larger
area for the
area for the
electron beam to
electron beam to
interact with the
interact with the
target.
target.

14. S. Guilbaud, Education Director
Rotating anode
Rotating anode
• The heating
The heating
capacity is
capacity is
further
further
enhanced with
enhanced with
an increased
an increased
RPM (3400).
RPM (3400).

15. S. Guilbaud, Education Director
Induction motor
Induction motor
Responsible for driving the rotating anode.
Responsible for driving the rotating anode.
Consists of two parts separated by the glass envelope.
Consists of two parts separated by the glass envelope.

16. S. Guilbaud, Education Director
Induction motor
Induction motor
• Works on the principle similar to the
Works on the principle similar to the
transformer.
transformer.
– Electromagnetic induction.
Electromagnetic induction.
– Current flowing in the stator develops a
Current flowing in the stator develops a
magnetic field.
magnetic field.
– Stator windings are sequentially
Stator windings are sequentially
energized so that the induced magnetic
energized so that the induced magnetic
field rotates on the axis of the stator.
field rotates on the axis of the stator.
– This causes the rotor to rotate.
This causes the rotor to rotate.

17. S. Guilbaud, Education Director
Line focus principle
Line focus principle
• The area of the
The area of the
x-ray tube anode
x-ray tube anode
from which the
from which the
x-ray photons
x-ray photons
are emitted.
are emitted.
• This is called
This is called
the actual focal
the actual focal
spot
spot

18. S. Guilbaud, Education Director
Line focus principle
Line focus principle
• The projection
The projection
perpendicular to
perpendicular to
the central ray,
the central ray,
which is its
which is its
apparent area
apparent area
from the position
from the position
of the film, is the
of the film, is the
effective focal
effective focal
spot.
spot.

19. S. Guilbaud, Education Director
Line focus principle
Line focus principle
• Was incorporated
Was incorporated
into x-ray tube
into x-ray tube
targets to allow a
targets to allow a
large area for
large area for
heating while
heating while
maintaining a small
maintaining a small
focal spot.
focal spot.
• The effective focal
The effective focal
spot is the area
spot is the area
projected onto the
projected onto the
patient and film.
patient and film.

20. S. Guilbaud, Education Director
Line focus principle
Line focus principle
• Focal spot sizes
Focal spot sizes
always make
always make
reference to the
reference to the
effective focal
effective focal
spot.
spot.
• The lower the
The lower the
target angle, the
target angle, the
smaller the
smaller the
effective focal
effective focal
spot size.
spot size.

21. S. Guilbaud, Education Director
Line focus principle
Line focus principle
• The advantage
The advantage
of the line-focus
of the line-focus
principle is that
principle is that
it provides the
it provides the
detail of a small
detail of a small
focal spot while
focal spot while
allowing a large
allowing a large
amount of heat
amount of heat
dissipation.
dissipation.

22. S. Guilbaud, Education Director
Line focus principle
Line focus principle
• The unfortunate bi-product of the
The unfortunate bi-product of the
line-focus principle is the “anode
line-focus principle is the “anode
heel effect”
heel effect”

23. S. Guilbaud, Education Director
Anode heel effect
Anode heel effect
• Construction
Construction
phenomenon that
phenomenon that
causes the x-ray
causes the x-ray
photons exiting the
photons exiting the
tube on the
tube on the
cathode side to
cathode side to
have a greater
have a greater
energy value than
energy value than
those exiting the
those exiting the
tube on the anode
tube on the anode
side.
side.

24. S. Guilbaud, Education Director
Anode heel effect
Anode heel effect
• More energy
More energy
absorption
absorption
occurs at the
occurs at the
anode heel
anode heel
resulting in less
resulting in less
energy value
energy value
from the
from the
incident photons
incident photons
at the anode
at the anode
heel.
heel.

25. S. Guilbaud, Education Director
Anode heel effect
Anode heel effect
• This is used to
This is used to
advantage when
advantage when
imaging
imaging
anatomical parts
anatomical parts
that are unequal
that are unequal
in thickness and
in thickness and
densities
densities
throughout their
throughout their
respective
respective
lengths.
lengths.

26. S. Guilbaud, Education Director
• The following anatomical parts
The following anatomical parts
may be imaged using the anode
may be imaged using the anode
heel effect:
heel effect:
– Thoracic vertebrae
Thoracic vertebrae
– Humerus
Humerus
– Femur
Femur
– Tibia & fibula
Tibia & fibula
– Forearm
Forearm

27. S. Guilbaud, Education Director
Using the anode heel effect
Using the anode heel effect

28. S. Guilbaud, Education Director
Anode heel effect
Anode heel effect
• The thicker portion of the
The thicker portion of the
anatomical part is placed
anatomical part is placed
beneath the cathode end of the
beneath the cathode end of the
x-ray tube.
x-ray tube.

29. S. Guilbaud, Education Director
Protective housing
Protective housing

30. S. Guilbaud, Education Director
Protective housing
Protective housing
• X-ray tube is always mounted
X-ray tube is always mounted
inside a lead-lined protective
inside a lead-lined protective
housing that is designed to:
housing that is designed to:
– Prevent excessive radiation
Prevent excessive radiation
exposure.
exposure.
– Prevent electric shock to the
Prevent electric shock to the
patient and operator (technologist).
patient and operator (technologist).

31. S. Guilbaud, Education Director
Protective housing
Protective housing
• Incorporates specially designed high-
Incorporates specially designed high-
voltage receptacles.
voltage receptacles.
• Provides mechanical support for the
Provides mechanical support for the
x-ray tube and protects it from
x-ray tube and protects it from
damage.
damage.
• Some tube housings contain oil in
Some tube housings contain oil in
which the tube is bathed.
which the tube is bathed.
• Some tube housings contain a
Some tube housings contain a
cooling fan to air-cool the tube.
cooling fan to air-cool the tube.

32. S. Guilbaud, Education Director
Protective housing
Protective housing
• When properly designed, they
When properly designed, they
reduce the level of leakage
reduce the level of leakage
radiation to
radiation to less than 100
mR/hr at 1 meter when
when
operated at maximum
operated at maximum
conditions.
conditions.

33. S. Guilbaud, Education Director
Tube rating charts
Tube rating charts

34. S. Guilbaud, Education Director
Tube rating chart
Tube rating chart
• A graph that indicates the
A graph that indicates the
maximum exposure values that
maximum exposure values that
may be made w/o damage to the
may be made w/o damage to the
tube.
tube.
• Each chart contains a family of
Each chart contains a family of
curves representing the various
curves representing the various
tube currents in mA.
tube currents in mA.

35. S. Guilbaud, Education Director
Tube rating chart
Tube rating chart
• The X axis and Y axis show
The X axis and Y axis show
scales of the two radiographic
scales of the two radiographic
parameters of kV and mA.
parameters of kV and mA.
– For a given mA, any combination of
For a given mA, any combination of
kVp and time that lies below the
kVp and time that lies below the
mA curve is safe.
mA curve is safe.

36. S. Guilbaud, Education Director
Tube rating charts
Tube rating charts

37. S. Guilbaud, Education Director
Anode cooling chart
Anode cooling chart
• Provides the
Provides the
thermal capacity
thermal capacity
of the anode and
of the anode and
its heat
its heat
dissipation
dissipation
characteristics.
characteristics.

38. S. Guilbaud, Education Director
Anode cooling chart
Anode cooling chart
• Thermal energy is
Thermal energy is
measured in British
measured in British
Thermal Units
Thermal Units
(BTU’s) where x-ray
(BTU’s) where x-ray
thermal energy is
thermal energy is
measured in Heat
measured in Heat
Units (HU).
Units (HU).
• Thus:
Thus:
– 1 kVp, 1 mA, 1 s = 1
1 kVp, 1 mA, 1 s = 1
HU.
HU.

39. S. Guilbaud, Education Director
Calculating Heat Units
Calculating Heat Units
• For a single phase unit,
For a single phase unit,
– HU = kVp x mA x s
HU = kVp x mA x s
• For a 3 phase 6 pulse unit,
For a 3 phase 6 pulse unit,
– HU = 1.35 x kVp x mA x s
HU = 1.35 x kVp x mA x s
• For a 3 phase 12 pulse unit,
For a 3 phase 12 pulse unit,
– HU = 1.41 x kVp x mA x s
HU = 1.41 x kVp x mA x s
• For a high frequency unit,
For a high frequency unit,
– HU = 1.44 x kVp x mA x s
HU = 1.44 x kVp x mA x s

40. S. Guilbaud, Education Director
Anode cooling chart
Anode cooling chart
• Determines the
Determines the
maximum heat
maximum heat
capacity of the
capacity of the
anode.
anode.
• Determines the
Determines the
length of time
length of time
required for
required for
complete cooling
complete cooling
following any level
following any level
of heat input.
of heat input.

41. S. Guilbaud, Education Director

42. S. Guilbaud, Education Director
References
References
Bushberg, et al.
Bushberg, et al. The Essential Physics of Medical
The Essential Physics of Medical
Imaging
Imaging, Williams & Wilkins, 1994.
, Williams & Wilkins, 1994.
Bushong, S.
Bushong, S. Radiologic Science for Technologists,
Radiologic Science for Technologists,
Physics, Biology, and Protection
Physics, Biology, and Protection, 7
, 7th
th
Edition, Mosby,
Edition, Mosby,
2000.
2000.
Carlton et al.
Carlton et al. Principles of Radiographic Imaging an
Principles of Radiographic Imaging an
Art and a Science
Art and a Science, 3
, 3rd
rd
Edition, Delmar, 2001.
Edition, Delmar, 2001.
Selman, J.
Selman, J. The Fundamentals of X-Ray and Radium
The Fundamentals of X-Ray and Radium
Physics
Physics, 8
, 8th
th
Edition, Charles Thomas, 1994.
Edition, Charles Thomas, 1994.