X-Ray Production

1
X-Ray Production

• BMP 205
• Lecture 3
• Mike McNitt-Gray Ph.D.
• Some images scanned from A.B. Wolbarst, Physics
  of Radiology
• Bushberg et. al., Essential Physics of Medical
  Imaging

2
OutlineCh 5 of Bushberg

• X-ray Production
• Tube
• Anatomy
• Operation
• Generator
• Function
• Waveform
• Beam Production
• Quality
• Quantity
• Heat

3
X-Rays

• Discovered by Wilhelm Roentgen 1895

4
X-Rays

• Do occur naturally
• All medical X-ray generated by machine
• With X-ray Tube and High Voltage Generator
• Conversion of Electrical Energy to
  Electromagnetic Radiation

5
X-ray Production

• Fancy lightbulb high voltage vacuum tube
• Cathode e- source and Anode target
• electrons accelerated (high voltage) across
  vacuum
• Suddenly decelerated (smacked) into high Z target
• Conservation of Energy Electron kinetic energy
  (1/2mv2) converted into heat and E-M Radiation
• (1 efficient)

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X-Ray production (e- bombardment of high Z
target)
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X-Rays

• Bremsstrahlung
• Characteristic

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Bremsstrahlung Production
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Bremsstrahlung Spectrum
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Characteristic Production
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Binding Energies
units keV
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Total Spectrum
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X-ray Tube
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X-Ray Tubes (Inserts)
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Vacuum enclosure

• Vacuum enclosure pyrex glass or grounded Al
• High vacuum no air molecules to impede or cause
  secondary ionizations
• insulator and or grounded for high voltage
  applications
• able to handle high temperatures and thermal
  expansion

16
Siemens Straton Tube for CT
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Siemens Straton Tube for CT
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Siemens Straton Tube for CT
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kV, mA, mAs

• kV a measure of the voltage applied across the
  tube (from the anode to the cathode)
• mA the measure of electron flow from cathode to
  anode (e.g., tube current)
• mAs time integrated tube current

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Generator Console
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X-ray Beam kV and mAs

• mA or mAs
• affects quantity of x-rays
• kV
• Affects x-ray beam energy and penetrability
  (quality)
• Also affects efficiency of production (quantity)

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Basic Operation

• Cathode is source of electrons
• Heat up a filament, which emits electrons
  (thermionic emission)
• Electrons liberated from filament flow through
  the vacuum of tube when a positive voltage is
  applied to anode (wrt cathode)
• Adjustments in filament current change
  temperature of filament to control tube current
• Electrons hitting anode produce bremmstrahlung
  and ..????

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Cathode
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Focusing Cup Bias
Grid controlled tubes can pinch off the electron
flow.
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Space Charge Effect

• thermionic emission results in an electron cloud
  (space charge)
• sufficiently large cloud gt repel further
  emissions

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Space Charge Effect

• Applied high voltage across the tube
• tube current flows no cloud buildup
• no repelling of further emissions
• Accelerates electrons from cathode to anode
• Emission versus space charge limited output
• voltage dependent

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Space Charge Limited Output
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X-Ray Tube Anode

• High Z related to efficiency of X-ray
  production
• Tungsten (W) Z74
• high melting point 3370? C
• reasonably good heat conductor
• alloy w/ Rhenium (10) for structural strength

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Anode

• Two Types
• Stationary
• Rotating
• Rotating anodes prevent heat buildup
• 1 energy converted into x-rays (heat)
• rotating anode prevents heating 1 spot
  continuously

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Stationary Anode
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Anode Rotating

• Rotating higher heat capacity
• greater surface area
• 3600 10000 rpm
• stator/rotor induction motor
• Molybdenum stem

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Anode and Focal Spot
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Anode Angle and Field Coverage
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Line Focus Principle

• Problem of competing needs
• Want small focal spot for high resolution
  (penumbra)
• Want large focal spot for high heat capacity
• due to low efficiency of X-ray production

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Line Focus Principle

• Angled anode face (12 - 20 degrees) allows
• larger actual focal spot size
• smaller effective focal spot size
• Decreasing angle
• decreases heat capacity but
• increases resolution

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Anode Heel Effect
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Anode Heel Effect
e-
cathode
anode
Higher Intensity Softer Beam larger apparent
size
Lower Intensity Harder Beam smaller apparent
size
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Anode Heel Effect

• Intensity gradient from self absorbtion of anode
  (heel)
• As great as 30 along anode-cathode
• Anode side smaller apparent fs, harder beam,
  lower intensity
• put thicker/denser anatomy at cathode end
• becomes more pronounced with
• short SID
• large field size
• small anode angle

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Adjustable Collimator with light localizer
Coincidence of light x-ray field
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Generator

• Heart and Brain of X-ray System

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X-Ray Generators

• Converts electrical power from building
  electrical grid into form that can be used by
  X-ray Tube
• All grid regulated to 60Hz Alternating Current
• Single phase supply 110 Volt AC
• Three phase 220 Volt

42
Tube requirements

• Needs DC (is in fact a vacuum tube diode)
• Can only conduct in one direction
• cathode negative with respect to anode
• Because of thermionic emission
• Therefore need rectifiers convert AC to DC
• Need high voltage for X-ray production
• ½ mv2 into h?
• 110 Volts vs 110 thousand volts
• Therefore need transformers (changes voltage)

43
Transformer
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Single Phase
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3 Phase
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Voltage Ripple
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I a kVp2
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One vs. Three Phases
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Other generators

• Battery storage
• Capacitor discharge
• Constant potential gradient (CPG)
• Tetrodes (high voltage vacuum tubes) control kV
  and exposure time directly on high voltage side
• Flat waveform but expensive
• High freq nearly as good

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Summary

• Production
• Bremsstrahlung and Characteristic
• Quality and Quantity
• Tube and Generator
• Operation
• Imaging

51
Additional Detail Slides
52
Transformers

• Two separate coils of wire wrapped around closed
  core
• Many configurations
• Electrical supply connected to 1?
• Output device to 2?
• Step up or step down

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Laws of Transformers

• 1) Voltage in two circuits proportional to number
  of turns in the two coils
• 2) Power (Energy) is conserved
• As Power (watts) is voltage x current
• Therefore as voltage increases by turns ratio,
  current decreases

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Bushberg
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Autotransformer

• Unique single winding design
• Self inductive
• 1? 2? defined by number of turns enclosed by
  taps
• Variable number of turns from taps allows voltage
  control at relatively low potential
• Feeds primary of high voltage transformer and
  filament transformer
• Can be both step up and down

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Filament circuit

• Step down transformer drops voltage
• 10 V _at_ 3-5 A
• Filament current (A) indirectly controls tube
  current (and output X-ray intensity)

57
High Voltage Circuit

• Step up transformer
• gt 500 fold voltage increase
• Immersed in dielectric
• Secondary side of autotransformer
• Fixed of transformer windings
• Grounded at center (mA meter)
• So for 100 kVp, potential on one side is 50,000
  V other is 50,000 V
• Less of an insulation problem

58
Rectification

• Converts AC (needed by transformer) to DC (needed
  by tube)
• Conduct current in one direction only
• Vacuum tubes (old style) large, bulky, and
  burnout
• Solid state semiconductor diodes
• Made of N-P semiconductors
• Conduct only on forward bias

59
Diode Bridge (Wheatstone bridge)

• Four diode arrangement to allow current to flow
  in one direction through tube regardless of
  polarity of secondary side of high tension
  transformer
• Full wave rectified generator
• 2x as efficient as self (half) wave rectified
• But inefficient compared to high freq CPG
  generators

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Generator Efficiency

• Single Phase 100 ripple w/ half or full wave
  rectified
• High voltage varies between 0 and max
• For single phase, average voltage is R.M.S.

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Three phase generators

• Recall AC power avail. in 3?
• 3 voltage peaks per 1/60 sec
• 3?, 6 pulse
• High volt transform rectify
• 13.5 ripple
• 3?, 12 pulse
• 2 different winding config on 2
• Delta and wye
• Another 30 phase shift for 2 halves of output,
  peaks fill troughs
• 3.5 voltage ripple

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Medium/High Frequency

• Transformer efficiency V ?NA
• By increasing frequency, cross sectional area
  reduced for same power (50kW in tube head!)
• Frequency of invertor ranges from 5-100 kHz!
• Feedback loop controlled during exposure if kV
  drops off, increase invertor frequency kV
  increases
• Timer accuracy
• Shorter exposures
• (lt10 ms)

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Generator Type / High Voltage Waveform
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Tube Limits Rating Charts

• Tube insert has power/load limit
• Function of heat produced in exposure
• HU kVp x mA x time x correction factor
• single phase generator less efficient
• Correction factor cpg generator 1.4
• 70 kVp x 100 mA x 0.1 sec 700 HU (single phase)
• Joules watts x seconds
• 1 W 1 V x 1 A 1000 V x 0.001 A keV x mA!
• assume constant voltage, so divide by correction
  factor!
• 70 kVp / 1.4 x 100 mA x 0.1 sec 500 J (single
  phase)
• For cpg is 700 Joules

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Question What is highest kVp can safely use to
get 35 mAs (350 mA 100ms)?
67
Question What is highest kVp can safely use to
get 35 mAs (350 mA 100ms)? Answer Should not
exceed 100 kVp
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Falling Load

• Integrates area under tube rating curve
• Applies highest mA in shortest time, reduces mA
  as exposure continues
• Expensive, not used as much with todays high
  output tubes

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Generator Efficiency Implications

• Single phase seldom at peak voltage, so set
  higher kVp
• Three phase higher average kVp
• Less ripple means more mR/mAs (shorter exposure
  time)
• 5 mR/mAs single vs. 10 mR/mAs three phase
• Ripple based on some multiple of 60 Hz
• High frequency more common now, smaller and
  cheaper than CPG

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Generator Power Rating

• Tube power handling should match generator output
• Rated in kilowatts under load (kVp x mA) _at_ 100
  kVp
• 80 kW generator can produce 800 mA at 100 kVp
  (simultaneously)
• Polydoros 80s, Medio CP80
• Small clinic may have 20kW, 200 mA at most
• Angio/Cardio generators 100 kW and greater
• CT not necessarily high instantaneous, but tube
  and generator sustain for long periods