Resident Physics Lectures

Christensen, Chapter 2A

X-Ray Tube Construction

George DavidAssociate ProfessorDepartment of RadiologyMedical College of Georgia

X-Ray Tube ComponentsHousing

Visible part of tube

Glass Enclosure(insert)VacuumElectrodes

Cathode Filament

Anode Target

*

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

*

Tube HousingShields against leakage

radiationlead linedleakage limit

100 mR / hour when tube operated at maximum continuous current for its maximum rated kilovoltage

*

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

Inside the Glass Insert

FilamentSimilar to light bulbGlows when heated

TargetLarge (usually) tungsten block

target filament

X-Ray Tube PrincipleFilament heated

electrons gain energyelectrons freed (“boiled” off)Thermionic emission

--

*

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

+

*

keV = kilo-electron volt

energy of an electronKinetic energyHigher energy electron

moves fasterElectrons can be

manipulated by electric fieldsAcceleratedSteered

+

Requirements to Produce X-Rays

Filament VoltageHigh Voltage

+

filamentanode

filamentvoltagesource

highvoltagesource

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

Cathode (filament)

Cathode is source of electronsfilament heated by electric

current~ 10 volts ~ 3-5 amps

filament current is not tube current

X-Ray Production(cont.)

X-Rays are produced in the x-ray tube by two distinct processesCharacteristic Characteristic

radiationradiationBremsstrahlungBremsstrahlung

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

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

#

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

Bremsstrahlung (cont.)Bremsstrahlung means braking braking

radiationradiationMoving electrons have many

Bremsstrahlung reactions small amount of energy lost with each

-

-

-

++

~~

+~

K

L

-

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

#

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

#

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)

+

Beam IntensityProduct of

# photons in beamenergy per photon

UnitsRoentgens (R) per unit timeMeasure of ionization rate of air

Depends onkVpmAtarget materialfiltration

Intensity & Technique

beam intensity proportional to mAbeam Intensity ~ proportional to kVp2

+ filamentvoltagesource

highvoltagesource

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

+ ---

*

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

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

Focal Spotportion of anode struck by electron

streamFocal spot sizes affects and limits

resolution

+

Focusing Cupnegatively chargedfocuses electron

stream to targetovercomes tendency of

electrons to spread because of mutual repulsion

+

FocusingCup

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

Focal Spot Size & Resolution

The larger the focal spot the more it will blur a tiny place on

the patient.

Focal Spot Size & HeatThe larger the area the electron beam hits, the more intense the beam

can be without melting the target

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

+

Dunlee Web Site:http://www.dunlee.com/new_tube_anatomy.html

Dunlee Web Site:http://www.dunlee.com/new_target.html

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

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

Target AngleAngle between target & perpendicular to tube axis

Typically 7 – 15 degrees

+

Target Angle,

Line Focus (cont.)

Apparent FS = Actual FS X sin

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

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

---

AnodesStationary

RotatingTarget is annular trackspreads heat over large area

of anodespeeds

3600, 9600 rpm Faster = much better heat ratings

Rotating Anode

Advantagesbetter heat ratings

DisadvantagesMore complex ($)

Rotor drive circuitry motor windings in housing bearings in insert

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

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