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Resident Physics Lectures

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

3 X-Ray Tube Converts Energy FROM To Heat X-Rays electrical energy
* Converts Energy FROM electrical energy To Heat > 99% of incident energy Bad! Ultimately destroys tubes X-Rays < 1% of incident energy Good! Our desired product

4 Tube Housing Shields against leakage radiation lead lined
* 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 expand when hot. Vacuum Oil Insert

6 Inside the Glass Insert
Filament Similar to light bulb Glows when heated Target Large (usually) tungsten block filament target

7 X-Ray Tube Principle Filament heated electrons gain energy
* 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 anode filament filament voltage source + high voltage source

11 Cathode (filament) Coil of tungsten wire Tungsten advantages
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 - + ~ K L Energy #

16 Electron from Filament
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 - + ~ K L Electron from Filament -

17 Bremsstrahlung (cont.)
Bremsstrahlung means braking radiation Moving electrons have many Bremsstrahlung reactions small amount of energy lost with each - + ~ K L

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 don’t escape tube easily filtered by tube enclosures or added filtration # Energy

19 Output Beam Spectrum Output photon beam made up of Spectrum # Energy #
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 # # 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 Units Depends on # 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
* 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 + - Tube Current (mA) Saturation Voltage kVp

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 increased by increasing filament temperature Tube Current (mA) Saturation Voltage kVp

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 possibility of arcing electrons attracted to glass instead of target +

32 Cross Section of X-Ray Tube
Dunlee Web Site:

33 Cross Section of X-Ray Tube
Dunlee Web Site:

34 Line Focus Principle + Focal spot steeply slanted
7-15 degrees typical Focal spot looks small from patient’s 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 + Target Angle, Q

37 Line Focus (cont.) Apparent FS = Actual FS X sin Q + Actual FS
Target Angle, Q Apparent FS = Actual FS X sin Q

38 Target Angle Large Same apparent focal spot size! + + Small
poorer heat ratings better field coverage Small optimizes heat ratings limits 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 area of anode speeds 3600, 9600 rpm Faster = much better heat ratings

41 Rotating Anode Advantages Disadvantages better heat ratings
More complex ($) Rotor drive circuitry motor windings in housing bearings in insert

42 Rotating Anode Larger diameter Materials 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 voltage controls current flow from cathode to anode Negative grid voltage repels electrons from filament Grid much closer to filament than target Applications speedy switching required cine grid +

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