IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
L 6: X Ray production
Basic elements of the X Ray source
• Generator : power
circuit supplying the
required potential to
the X Ray tube
• X Ray tube and
collimator: device
producing the X Ray
6: X Ray production
X Ray tubes
6: X Ray production
X Ray tube components
• Cathode: heated filament which is the source
of the electron beam directed towards the
• 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
• Shielding material (protection against
scattered radiation)
6: X Ray production
X Ray tube components
1: mark of focal spot
1: long tungsten filament
2 : short tungsten filament
3 : real size cathode
6: X Ray production
Cathode structure (I)
Cathode includes filament(s) and associated
• 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
6: X Ray production
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 !
6: X Ray production
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
6: X Ray production
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
6: X Ray production
Anode characteristic
1 : anode track
2 : anode track
6: X Ray production
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.
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
Anode angle (II)
 Angle
Incident electron
beam width
‘ Angle
Actual focal
spot size Incident electron
beam width
Apparent focal spot size
Actual focal
spot size
focal spot size
6: X Ray production
Line focus principle
• Was incorporated into xray 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
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.
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
Line focus principle
• The unfortunate bi-product of the line-focus
principle is the “anode heel effect”
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
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.
Using the anode heel effect
• The following anatomical parts may be
imaged using the anode heel effect:
• Thoracic vertebrae
• Humerus
• Femur
• Tibia & fibula
• Forearm
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
6: X Ray production
Anode heel effect (II)
• The heel effect is not always a negative
• It can be used to compensate for
different attenuation through parts of the
• For example:
• thoracic spine (thicker part of the patient
towards the cathode side of the tube)
• mammography
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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
• Balance depends on organ movement (fast moving
organs may require larger focus)
6: X Ray production
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)
6: X Ray production
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
• Since the dose at the image plane can be expressed
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
6: X Ray production
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”
6: X Ray production
Tube potential wave form (II)
Single phase single pulse
kV ripple (%)
Single phase 2-pulse
Three phase 6-pulse
Three phase 12-pulse
Line voltage
0.01 s
0.02 s
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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
6: X Ray production
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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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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Automatic exposure control
X Ray tube
Air tissue
AEC detectors
6: X Ray production
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 : 50 - 70 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 300 - 500 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
6: X Ray production
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 : 40 kW @ 0.6 focus size for spot film
output : 70 kW @ 1.0 for fluoroscopy (high resolution)
priority given to contrast
automatic settings of kV
• Cardiac angiography
• Three phase generator - continuous output  1kW
• output : 30 kW @ 0.4 focus size
• output : 80 kW @ 0.8 focus size
• frame rate : up to 120 fr/s
6: X Ray production
Protective housing
Protective housing
• X-ray tube is always mounted inside a leadlined protective housing that is designed to:
• Prevent excessive radiation exposure.
• Prevent electric shock to the patient and
operator (technologist).
Protective housing
• Incorporates specially designed high-voltage
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 aircool 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.