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Resident Physics Lectures
Christensen, Chapter 2A
X-Ray Tube Construction
George DavidAssociate ProfessorDepartment of RadiologyMedical College of Georgia
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X-Ray Tube ComponentsHousing
Visible part of tube
Glass Enclosure(insert)VacuumElectrodes
Cathode Filament
Anode Target
*
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X-Ray Tube
Converts Energy FROM
electrical energyTo
Heat > 99% of incident energy Bad! Ultimately destroys tubes
X-Rays < 1% of incident energy Good! Our desired product
*
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Tube HousingShields against leakage
radiationlead linedleakage limit
100 mR / hour when tube operated at maximum continuous current for its maximum rated kilovoltage
*
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Tube Housing (cont.)
Shields against high voltageelectrically groundedhigh voltage cable receptacles
(wells)
housing filled with oilcoolselectrical insulation
all air removedbellows
on end of tube allows oil to expand
when hot.
OilVacuum
Insert
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Inside the Glass Insert
FilamentSimilar to light bulbGlows when heated
TargetLarge (usually) tungsten block
target filament
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X-Ray Tube PrincipleFilament heated
electrons gain energyelectrons freed (“boiled” off)Thermionic emission
--
*
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X-Ray Tube Principle
Positive (high) voltage applied to anode relative to filamentelectrons accelerate toward anode target
Gain kinetic energyelectrons strike target
electrons’ kinetic energy converted to heat x-rays
+
*
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keV = kilo-electron volt
energy of an electronKinetic energyHigher energy electron
moves fasterElectrons can be
manipulated by electric fieldsAcceleratedSteered
+
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Requirements to Produce X-Rays
Filament VoltageHigh Voltage
+
filamentanode
filamentvoltagesource
highvoltagesource
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Cathode (filament)Coil of tungsten wire
similar to light bulb filamentTungsten advantages
high melting pointlittle tendency to vaporizelong life expectancy
Tungsten disadvantagesnot as efficient at emitting
electrons as some other materials
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Cathode (filament)
Cathode is source of electronsfilament heated by electric
current~ 10 volts ~ 3-5 amps
filament current is not tube current
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X-Ray Production(cont.)
X-Rays are produced in the x-ray tube by two distinct processesCharacteristic Characteristic
radiationradiationBremsstrahlungBremsstrahlung
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Characteristic RadiationInteraction of high speed incident
electron with orbital electron of target
#1: Electron from filament removes inner-shell orbital electron from atom
#2: electrons from higher energy shells cascade down to fill vacancies
#3: characteristic x-ray emitted
-
-
-
+
+
~
~
+~
K
L
-
-#1
#2
#3
Electron from
Filament
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Characteristic RadiationConsists only of discrete x-
ray energies corresponding to energy difference between electron shells of target atom
Specific energies characteristic of target material
for tungsten 59 keV corresponds to the difference in energy between K and L shells
-
-
-
++
~~
+~
K
L
-
Energy
#
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Bremsstrahlunginteraction of moving electron from filament
with nucleus of target atomsPositive nucleus causes moving electron to
change speed / directionKinetic energy lostEmitted in form of Bremsstrahlung x-ray
-
-
-
++
~~
+~
K
L
-
Electron from
Filament
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Bremsstrahlung (cont.)Bremsstrahlung means braking braking
radiationradiationMoving electrons have many
Bremsstrahlung reactions small amount of energy lost with each
-
-
-
++
~~
+~
K
L
-
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Bremsstrahlung (cont.)Energy lost by moving electron is random &
depends ondistance from nucleuscharge (Z) of nucleus
Bremsstrahlung Energy Spectrum0 - peak kilovoltage (kVp) applied to x-ray tubemost Bramsstrahlung photons have low energylowest energy photons don’t escape tube
easily filtered by tube enclosures or added filtration
Energy
#
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Output Beam SpectrumOutput photon beam made
up of Characteristic RadiationCharacteristic Radiation
characteristic of target material several discrete energies
BremsstrahlungBremsstrahlung continuous range of energies
0 - kVp setting most photons have low energy
SpectrumSpectrum depicts fraction of beam at each energy
value combination of Bremsstrahlung and
characteristic radiationEnergy
#
Energy
#
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Tube Current (mA)
rate of electron flow from filament to targetElectrons / second
Measured in milliamperesmilliamperes (mA)
Limited byfilament emission (temperature / filament current)space charge (see next slide)
+
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Beam IntensityProduct of
# photons in beamenergy per photon
UnitsRoentgens (R) per unit timeMeasure of ionization rate of air
Depends onkVpmAtarget materialfiltration
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Intensity & Technique
beam intensity proportional to mAbeam Intensity ~ proportional to kVp2
+ filamentvoltagesource
highvoltagesource
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Space ChargeElectrons leave filament
filament becomes positive Negative electrons stay close
Electron cloud surrounds filamentCloud repels new electrons from filamentLimits electron flow from cathode to anode
+ ---
*
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Kilovoltage & Space Chargeraising kilovoltage
gradually overcomes space chargeHigher fraction of electrons
make it to anode as kilovoltage increases
At high enough kilovoltage saturationsaturation resultsAll electrons liberated by
filament reach target
Raising kilovoltage further has no effect on # electrons reaching anode
+ ---
++++
Tub
e C
urre
nt (
mA
)
SaturationVoltage
kVp
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Saturation Voltage
kilovoltage at which a further increase does not increase tube current100% of electrons already
going to targetTube current said to be
emission limitedemission limitedtube current can only be
increasedby increasing filament temperature
+ ---
++++
Tub
e C
urre
nt (
mA
)
SaturationVoltage
kVp
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Focal Spotportion of anode struck by electron
streamFocal spot sizes affects and limits
resolution
+
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Focusing Cupnegatively chargedfocuses electron
stream to targetovercomes tendency of
electrons to spread because of mutual repulsion
+
FocusingCup
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Focal SpotsMost tubes have 2 filaments & thus 2
focal spotsonly one used at a timesmall focus
improved resolutionlarge focus
improved heat ratingsElectron beam strikes larger portion of
target
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Focal Spot Size & Resolution
The larger the focal spot the more it will blur a tiny place on
the patient.
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Focal Spot Size & HeatThe larger the area the electron beam hits, the more intense the beam
can be without melting the target
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Filament (cont.)Large Filament normally left on
at low “standby” current boosted before exposure (prep or first trigger)
With time tungsten from hot filament vaporizes on glass insert
thins the filamentfilters the x-ray beam increases possibility
of arcing electrons attracted to
glass instead of target
+
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Dunlee Web Site:http://www.dunlee.com/new_tube_anatomy.html
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Dunlee Web Site:http://www.dunlee.com/new_target.html
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Line Focus PrincipleFocal spot steeply slanted
7-15 degrees typicalFocal spot looks small from
patient’s perspectiveImaging size
Looks large from filamentbetter heat capacity
+
Actual FS
Apparent FS
Patient
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Line Focus PrincipleActual (true) focal spot
as seen from filament
Apparent (effective, projected) focal spotas seen from tube port
or patient
+
Actual FS
Apparent FS
Patient
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Target AngleAngle between target & perpendicular to tube axis
Typically 7 – 15 degrees
+
Target Angle,
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Line Focus (cont.)
Apparent FS = Actual FS X sin
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Target AngleSmall
optimizes heat ratingslimits field coverage
+
Large Target Angle(Small Actual Focal Spot)
+
Small Target Angle(Large Actual Focal Spot)
• Large– poorer heat ratings
– better field coverage
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Heel EffectIntensity of x-ray beam
significantly reduced on anode side
beam goes through more target material exiting the anode
anode side
x
cathode side
---
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AnodesStationary
RotatingTarget is annular trackspreads heat over large area
of anodespeeds
3600, 9600 rpm Faster = much better heat ratings
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Rotating Anode
Advantagesbetter heat ratings
DisadvantagesMore complex ($)
Rotor drive circuitry motor windings in housing bearings in insert
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Rotating Anode
Larger diameterBetter heat ratingsheavier
requires more support$$$
Materialsusually tungsten
high melting point good x-ray production
molybdenum (and now Rhodium) for mammography (sometimes) low energy characteristic radiation
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Grid-controlled tubesGrid used to switch tube on/off
grid is third electroderelatively small voltage
controls current flowfrom cathode to anode Negative grid voltage repels electrons from
filament Grid much closer to filament than target
Applicationsspeedy switching
required cine +
grid