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
Requirements to Produce X-Rays

• Filament Voltage
• High Voltage

filament
anode
filament voltage source

high voltage source
10
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

11
Cathode (filament)

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

12
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)

13
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

14
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

Tube Current (mA)
Saturation Voltage
kVp
15
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

16
Focal Spot

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

17
Focusing Cup

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

Focusing Cup
18
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

19
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

20
Cross Section of X-Ray Tube
Dunlee Web Site http//www.dunlee.com/new_tube_an
atomy.html
21
Cross Section of X-Ray Tube
Dunlee Web Site http//www.dunlee.com/new_target.
html
22
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
23
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
24
Target Angle

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

25
Line Focus (cont.)

• Apparent FS Actual FS X sin Q

26
Target Angle (cont.)

• 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!
27
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
28
Anodes

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

29
Rotating Anode

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

30
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
• low energy characteristic radiation

31
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