IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
RADIATION PROTECTION IN
DIAGNOSTIC AND
INTERVENTIONAL RADIOLOGY
L 6: X Ray production
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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X Ray tubes
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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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X Ray tube components
housing
1: mark of focal spot
cathode
1: long tungsten filament
2 : short tungsten filament
3 : real size cathode
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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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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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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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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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Anode characteristic
1 : anode track
2 : anode track
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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
Increased
apparent
focal spot size
Film
Film
THE SMALLER THE ANGLE
THE BETTER THE RESOLUTION
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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
film.
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
dissipation.
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
side.
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
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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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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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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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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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Tube potential wave form (I)
• Conventional generators
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•
•
•
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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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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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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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
Collimator
Beam
Soft
Air tissue
Bone
Patient
Table
Grid
AEC detectors
Cassette
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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
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•
•
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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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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
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•
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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
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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
•
•
•
•
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 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.