Resident Physics Lectures

1
Resident Physics Lectures

• Christensen, Chapter 2A
• X-Ray Tube Construction

George David Associate Professor Department of
Radiology Medical College of Georgia
2
X-Ray Tube Components

• Housing
• Visible part of tube
• Glass Enclosure(insert)
• Vacuum
• Electrodes
• Cathode
• Filament
• Anode
• Target

3
X-Ray Tube

• Converts Energy
• FROM
• electrical energy
• To
• Heat
• gt 99 of incident energy
• Bad! Ultimately destroys tubes
• X-Rays
• lt 1 of incident energy
• Good! Our desired product

4
Tube Housing

• Shields against leakage radiation
• lead lined
• leakage limit
• 100 mR / hour when tube operated at maximum
  continuous current for its maximum rated
  kilovoltage

5
Tube Housing (cont.)

• Shields against high voltage
• electrically grounded
• high voltage cable receptacles (wells)
• housing filled with oil
• cools
• electrical insulation
• all air removed
• bellows
• on end of tube
• allows oil to expandwhen hot.

Vacuum
Oil
Insert
6
Inside the Glass Insert

• Filament
• Similar to light bulb
• Glows when heated
• Target
• Large (usually) tungsten block

7
X-Ray Tube Principle

• Filament heated
• electrons gain energy
• electrons freed (boiled off)
• Thermionic emission

-
-
8
X-Ray Tube Principle

• Positive (high) voltage applied to anode relative
  to filament
• electrons accelerate toward anode target
• Gain kinetic energy
• electrons strike target
• electrons kinetic energy converted to
• heat
• x-rays

9
keV kilo-electron volt

• energy of an electron
• Kinetic energy
• Higher energy electron moves faster
• Electrons can be manipulated by electric fields
• Accelerated
• Steered

10
Requirements to Produce X-Rays

• Filament Voltage
• High Voltage

filament
anode
filament voltage source

high voltage source
11
Cathode (filament)

• Coil of tungsten wire
• similar to light bulb filament
• Tungsten advantages
• high melting point
• little tendency to vaporize
• long life expectancy
• Tungsten disadvantages
• not as efficient at emitting electrons as some
  other materials

12
Cathode (filament)

• Cathode is source of electrons
• filament heated by electric current
• 10 volts
• 3-5 amps
• filament current is not tube current

13
X-Ray Production(cont.)

• X-Rays are produced in the x-ray tube by two
  distinct processes
• Characteristic radiation
• Bremsstrahlung

14
Characteristic Radiation

• Interaction 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

L
K
-






1
-
Electron from Filament
-
-
2
-
3
15
Characteristic Radiation

• Consists 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

16
Bremsstrahlung

• interaction of moving electron from filament with
  nucleus of target atoms
• Positive nucleus causes moving electron to change
  speed / direction
• Kinetic energy lost
• Emitted in form of Bremsstrahlung x-ray

Electron from Filament
-
17
Bremsstrahlung (cont.)

• Bremsstrahlung means braking radiation
• Moving electrons have many Bremsstrahlung
  reactions
• small amount of energy lost with each

18
Bremsstrahlung (cont.)

• Energy lost by moving electron is random
  depends on
• distance from nucleus
• charge (Z) of nucleus
• Bremsstrahlung Energy Spectrum
• 0 - peak kilovoltage (kVp) applied to x-ray tube
• most Bramsstrahlung photons have low energy
• lowest energy photons dont escape tube
• easily filtered by tube enclosures or added
  filtration

Energy
19
Output Beam Spectrum

• Output photon beam made up of
• Characteristic Radiation
• characteristic of target material
• several discrete energies
• Bremsstrahlung
• continuous range of energies
• 0 - kVp setting
• most photons have low energy
• Spectrum
• depicts fraction of beam at each energy value
• combination of Bremsstrahlung and characteristic
  radiation

Energy
20
Tube Current (mA)

• rate of electron flow from filament to target
• Electrons / second
• Measured in milliamperes (mA)
• Limited by
• filament emission (temperature / filament
  current)
• space charge (see next slide)

21
Beam Intensity

• Product of
• photons in beam
• energy per photon
• Units
• Roentgens (R) per unit time
• Measure of ionization rate of air
• Depends on
• kVp
• mA
• target material
• filtration

22
Intensity Technique

• beam intensity proportional to mA
• beam Intensity proportional to kVp2

filament voltage source

high voltage source
23
Space Charge

• Electrons leave filament
• filament becomes positive
• Negative electrons stay close
• Electron cloud surrounds filament
• Cloud repels new electrons from filament
• Limits electron flow from cathode to anode

24
Kilovoltage Space Charge

• raising kilovoltage gradually overcomes space
  charge
• Higher fraction of electrons make it to anode as
  kilovoltage increases
• At high enough kilovoltage saturation results
• All electrons liberated by filament reach target
• Raising kilovoltage further has no effect on
  electrons reaching anode

25
Saturation Voltage


-

-


-

• kilovoltage at which a further increase does not
  increase tube current
• 100 of electrons already going to target
• Tube current said to be emission limited
• tube current can only be increasedby increasing
  filament temperature

26
Focal Spot

• portion of anode struck by electron stream
• Focal spot sizes affects and limits resolution

27
Focusing Cup

• negatively charged
• focuses electron stream to target
• overcomes tendency of electrons to spread because
  of mutual repulsion

Focusing Cup
28
Focal Spots

• Most tubes have 2 filaments thus 2 focal spots
• only one used at a time
• small focus
• improved resolution
• large focus
• improved heat ratings
• Electron beam strikes larger portion of target

29
Focal Spot Size Resolution
The larger the focal spot the more it will blur a
tiny place on the patient.
30
Focal Spot Size Heat
The larger the area the electron beam hits, the
more intense the beam can be without melting the
target
31
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 filament
• filters the x-ray beam
• increases possibilityof arcing
• electrons attracted toglass instead of target

32
Cross Section of X-Ray Tube
Dunlee Web Site http//www.dunlee.com/new_tube_an
atomy.html
33
Cross Section of X-Ray Tube
Dunlee Web Site http//www.dunlee.com/new_target.
html
34
Line Focus Principle

• Focal spot steeply slanted
• 7-15 degrees typical
• Focal spot looks small from patients perspective
• Imaging size
• Looks large from filament
• better heat capacity

Actual FS
Apparent FS
Patient
35
Line Focus Principle

• Actual (true) focal spot
• as seen from filament
• Apparent (effective, projected) focal spot
• as seen from tube port or patient

Actual FS
Apparent FS
Patient
36
Target Angle

• Angle between target perpendicular to tube axis
• Typically 7 15 degrees

37
Line Focus (cont.)

• Apparent FS Actual FS X sin Q

38
Target Angle

• Small
• optimizes heat ratings
• limits field coverage

• Large
• poorer heat ratings
• better field coverage

Large Target Angle (Small Actual Focal Spot)
Small Target Angle (Large Actual Focal Spot)


Same apparent focal spot size!
39
Heel Effect

• Intensity of x-ray beam significantly reduced on
  anode side
• beam goes through more target material exiting
  the anode

x
-
-
-
cathode side
anode side
40
Anodes

• Stationary
• Rotating
• Target is annular track
• spreads heat over large areaof anode
• speeds
• 3600, 9600 rpm
• Faster much better heat ratings

41
Rotating Anode

• Advantages
• better heat ratings
• Disadvantages
• More complex ()
• Rotor drive circuitry
• motor windings in housing
• bearings in insert

42
Rotating Anode

• Larger diameter
• Better heat ratings
• heavier
• requires more support
• Materials
• usually tungsten
• high melting point
• good x-ray production
• molybdenum (and now Rhodium) for mammography
  (sometimes)
• low energy characteristic radiation

43
Grid-controlled tubes

• Grid used to switch tube on/off
• grid is third electrode
• relatively small voltagecontrols current
  flowfrom cathode to anode
• Negative grid voltage repels electrons from
  filament
• Grid much closer to filament than target
• Applications
• speedy switchingrequired
• cine

grid