RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

1
RADIATION PROTECTION INDIAGNOSTIC
ANDINTERVENTIONAL RADIOLOGY
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• L 6 X Ray production

2
Introduction

• A review is made of
• The main elements of the of 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

3
Topics

• Basic elements of an X Ray source assembly
• Cathode structure
• Anode structure
• Rating chart
• X Ray generator
• Automatic exposure control

4
Overview

• To become familiar with the technological
  principles of the X Ray production

5
Part 6 X Ray production
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 1 Basic elements of an X Ray source
  assembly

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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
cathode
1 long tungsten filament 2 short tungsten
filament 3 real size cathode
1 mark of focal spot
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Part 6 X Ray production
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 2 Cathode structure

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Cathode structure (I)

• Cathode includes filament(s) and associated
  circuitry
• tungsten material preferred because of its high
  melting point (3370C)
• 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 1500C so
  that 2700C emission temperature can be reached
  within a second

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Example of a cathode
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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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Part 6 X Ray production
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 3 Anode structure

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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 cups 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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Anode angle (II)
Angle
?
 
Angle
?
Actual focal spot size
Actual focal spot size
Incident electron beam width
Incident electron beam width
Increased apparent focal spot size
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 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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Part 6 X Ray production
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 4 Rating Chart

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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 HU kV x mA x s
• 3 phase units, 6 pulse HU 1.35 kV x mA x s
• 3 phase units, 12 pulse 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
700 600 500 400 300 200 100
70 kVp
50 kVp
Tube current (mA)
Unacceptable
90 kVp
120 kVp
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)
700 600 500 400 300 200 100
X Ray tube B 3f full-wave rectified 10.000 rpm
125 kV 1.0 mm effective focal spot
70 kVp
50 kVp
Tube current (mA)
90 kVp
Unacceptable
125 kVp
Acceptable
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 through the cooling circuit
• 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)
Maximum Heat Storage Capacity of Anode
240 220 200 180 160 140 120 100 80 60 40
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1000 HU/sec
Imput curve
500 HU/sec
350 HU/sec
Heat units stored (x 1000)
250 HU/sec
Cooling curve
1 2 3 4 5 6 7 8 9 10
11 12 13 14
Elapsed time (min)
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Part 6 X Ray production
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• 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
• The ripple r of a generator has to be as low as
  possible
• r (U - Umin)/U x 100

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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

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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 (lt2 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 lt2 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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Part 6 X-ray production
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• Topic 6 Automatic Exposure Control (AEC)

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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
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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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Part 6 X-ray production
IAEA Training Material on Radiation Protection in
Diagnostic and Interventional Radiology

• 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 ? 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

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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
• output 40 kW _at_ 0.6 focus size for spot film
• output 70 kW _at_ 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 _at_ 0.4 focus size
• output 80 kW _at_ 0.8 focus size
• frame rate up to 120 fr/s

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Summary

• The main parts of the system contributing to the
  desired X Ray production
• provide the required source of power
• deliver an appropriate X Ray spectrum
• ensure the optimum adjustment of exposure to
  warrant the image quality

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Where to Get More Information

• Equipment for diagnostic radiology, E. Forster,
  MTP Press, 1993
• IPSM Report 32, part 1, X-ray tubes and
  generators
• The Essential Physics of Medical Imaging,
  Williams and Wilkins. Baltimore1994
• Manufacturers data sets for different X Ray tubes