IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
RADIATION PROTECTION IN
DIAGNOSTIC AND
INTERVENTIONAL RADIOLOGY
L 6: X Ray production
IAEA
International Atomic Energy Agency
Introduction
A review is made of:
• The main elements of the X Rays tube:
cathode and anode structure
• The technology constraints of the anode
and cathode material
• The rating charts and X Ray tube heat
loading capacities
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Topics
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•
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•
Basic elements of an X Ray source assembly
Cathode structure
Anode structure
Rating chart
X Ray generator
Automatic exposure control
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Overview
• To become familiar with the technological
principles of the X Ray production
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 6: X Ray production
Topic 1: Basic elements of an X Ray source
assembly
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International Atomic Energy Agency
Basic elements of the X Ray source
assembly
• Generator : power
circuit supplying the
required potential to
the X Ray tube
• X Ray tube 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, > 99% of electron
energy is dissipated as heat
• Metal tube housing surrounding glass (or
metal) X Ray tube (electrons are traveling in
vacuum)
• Shielding material (protection against extrafocal spot radiation from anode)
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X Ray tube components
housing
1: mark of focal spot
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cathode
1: long tungsten filament
2 : short tungsten filament
3 : real size cathode
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 6: X Ray production
Topic 2: Cathode structure
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International Atomic Energy Agency
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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Example of a cathode
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Cathode structure (I)
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Modern tubes have two filaments
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a long one : higher current/lower resolution
a short one : lower current/higher resolution
Coulomb interaction causes the electron beam
to diverge on the way to the anode
•
•
larger area of target used
focal spot increased  lower image resolution
Focusing of electrons is crucial !
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 6: X Ray production
Topic 3: Anode structure
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International Atomic Energy Agency
X Ray tube characteristics
• Anode mechanical constraints
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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
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•
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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 pits
caused by
electron beam
being stationery
on the anode
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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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Anode heel effect (I)
• Anode angle (from 7° to 20°) induces a
variation of the X Ray output in the imaging
plane parallel to 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
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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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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 6: X Ray production
Topic 4: Rating Chart
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International Atomic Energy Agency
Heat loading capacities
• A procedure generates an amount of heat depending on:
•
•
• kV used, tube current (mA), length of exposure
• type of voltage waveform
• number of exposures taken in rapid sequence
Heat Unit (HU) [joule] :
unit of potential x unit of tube current x unit of time
The heat generated by various types of X Ray
circuits are:
• 1 phase units :
• 3 phase units, 6 pulse :
• 3 phase units, 12 pulse:
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HU = kV x mA x s
HU = 1.35 kV x mA x s
HU = 1.41 kV x mA x s
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X Ray tube rating chart (I)
• Tube cooling characteristics and focal spot size
•  {mA - time} relationship at constant kV
• intensity decreases with increasing exposure time
• intensity increases with decreasing kV
• Note: higher power  reduced exposure time 
reduced motion unsharpness
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X Ray tube rating chart (II)
• Manufacturers
combine
heat
loading
characteristics and information about the
limits of their X Ray tubes in graphical
representations called Tube Rating Charts
• Example:
• Tube A: a 300 mA, 0.5 s, 90 kV procedure would
damage the system operated from a 1-phase half wave
rectified generator (unacceptable)
• Tube B: a 200 mA, 0.1 s, 120 kV procedure comply with
the technical characteristics of the system operated from
a 3-phase fully rectified generator (acceptable)
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X Ray tube rating chart (III)
X Ray tube A
1 f half-wave rectified
3000 rpm 90 kV
1.0 mm effective focal spot
700
Tube current (mA)
600
500
400
300
Unacceptable
200
100
0.01
0.05
0.1
0.5
1.0
5.0
10.0
Exposure time (s)
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X Ray tube rating chart (IV)
Tube current (mA)
700
X Ray tube B
3f full-wave rectified
10.000 rpm 125 kV
1.0 mm effective focal spot
600
500
400
Unacceptable
300
200
Acceptable
100
0.01
0.05
0.1
0.5
1.0
5.0
10.0
Exposure time (s)
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Anode cooling chart (I)
• Heat generated is stored in the anode and dissipated by
radiative cooling to the x-ray tube, oil, and housing
• A typical cooling chart has:
• input curves (heat units stored as a function of time)
• anode cooling curve
• The following graph shows that:
• a procedure delivering 500 HU/s can go on
indefinitely
• if it is delivering 1000 HU/s it has to stop after 10 min
• if the anode has stored 120,000 HU, it will take  5
min to cool down completely
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Anode cooling chart (II)
Heat units stored (x 1000)
240
Maximum Heat Storage Capacity of Anode
220
200
180
160
140
120
100
80
60
40
20
1
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5
6
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10
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12
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Elapsed time (min)
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 6: X Ray production
Topic 5: X Ray generator
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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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X-ray generator (III)
• Peak voltage value has an influence on the
beam hardness
• It has to be related to medical question
• What is the anatomical structure to investigate ?
• What is the contrast level needed ?
• For a thorax examination : 140 - 150 kV is suitable to
visualize the lung structure
• While only 65 kV is necessary to see bone structure
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Tube potential wave form (I)
• Conventional generators
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single f 1-pulse (dental and some mobile systems)
single f 2-pulse (double rectification)
three f 6-pulse
three f 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)
• HF : combines the advantages of constant
potential and conventional generator
• reproducibility and consistency of tube voltage
• high frame rate possible
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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 6: X-ray production
Topic 6: Automatic Exposure Control (AEC)
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International Atomic Energy Agency
Automatic exposure control
• Optimal choice of technical parameters (kV,
mA) to optimize patient dose and image
quality
• 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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IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
Part 6: X-ray production
Topic 7: X-ray equipment operation and mode
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X-ray equipment operation mode and
application (II)
Radiography and Tomography
• Single and 3 f 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 f generator (inverter
technology) - continuous output of 300 - 500 W
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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)
capable of pulsing at 30, 15, 7.5 fps or less
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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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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Summary
• The x-ray system:
• provides the required source of power
• delivers an appropriate X Ray spectrum
• assures the optimum adjustment of exposure to
optimize image quality
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Where to Get More Information
• The Essential Physics of Medical Imaging. JT
Bushberg, JA Seibert, EM Leidholdt, JM
Boone. Lippincott Williams & Wilkins,
Philadelphia, 2011
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