THE XRAY TUBE

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THE X-RAY TUBE
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Protective Housing

• Houses the x-ray tube
• Isolates the high voltages
• Provides a means to cool the tube
• Controls leakage and scatter radiation
• Mechanical support

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ISOLATES HIGH VOLTAGE

• Protects the radiographer from electrical shock

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TUBE COOLING

• Contains oil between the x-ray tube and the metal
  housing
• Conducts heat away from the x-ray tube

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CONTROLS LEAKAGE ANDSCATTERED RADIATION

• Isotropic emission
• X-rays are emitted in all directions
• Window
• The area in the tube housing which permits x-rays
  to pass through (useful beam)
• Helps to focus x-rays in one direction
• Leakage
• Any radiation leaving the tube housing from
  outside the window
• Must be kept to less than 100mR/hr at 1m

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Mechanical Support

• Provides a support for the x-ray tube
• Protects the tube from damage due to rough
  handling

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GLASS OR METAL ENVELOPE

• Made of Pyrex glass or metel
• Pyrex glass can withstand tremendous heat
• Surrounds the anode and cathode
• Is considered a vacuum tube
• Does not allow a gas build up within the envelope

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The Modern X-Ray Tube
Reprinted from Principles of Imaging Science and
Protection, Thompson, et al., pg. 5, 1994, with
permission from Elsevier.
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INTERNAL COMPONENTS

• Cathode
• Anode

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CATHODE

• The negative side of the x-ray tube
• Composed of
• Filament
• Coli of wire used to boil off electrons
• Focusing cup
• Focuses the electron cloud so that X-rays are
  emitted more evenly

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ANODE

• The positive side of the x-ray
• Functions include
• Electrical conductor
• Mechanical support for the target
• Good thermal radiator

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TARGET

• The area of the anode struck by the electrons
  from the cathode

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TARGET MATERIAL IS BASED ON 3 CHARACTERISTICS

• Atomic number
• Must be high so that it results in high
  efficiency in x-ray production
• Thermal conductivity
• Must be able to conduct heat away from the target
• High melting point
• Must be able to withstand high temperatures

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TWO TYPES OF ANODES

• STATIONARY
• ROTATING

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STATIONARY ANODE

• Used in dental and some portable units
• Does not rotate
• All electrons hit the same area on the anode
• Are not adequate for high technique procedures
• Composed of tungsten embedded in copper

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A Stationary Anode
Copper Block
Tungsten Target Plate
The above photo shows the tungsten (W) target
plate mounted on its copper block.
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ROTATING ANODE

• The anode rotates making the target area larger
  so that it does not wear out as quickly
• Allows for higher tube currents and exposure
  times
• Uses an induction motor to turn the anode

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Related Terms

• Focal spot the area of the target (anode) from
  which x-rays are emitted
• Actual focal spot size the area on the anode
  target that is exposed to electrons from the tube
  current
• Effective focal spot area projected onto the
  patient and the image receptor

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LINE FOCUS PRINCIPLE

• By angling the target, the effective focal spot
  size is made much smaller that the actual area of
  electron interaction

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TARGET DESIGN

• Designed with an angle so that the effective
  focal spot is smaller than the actual focal spot
• Some have 2 angles and are called biangle targets
  that produce 2 focal spots for the tube.

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HEEL EFFECT

• Due to the construction of the target, the x-ray
  intensity on the cathode side of the tube is
  greater than on the anode side.
• The smaller the focal spot the greater the heel
  effect

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EXTRAFOCAL RADIATION

• Undesirable radiation caused from electrons
  interacting with the target and bouncing in an
  unwanted direction causing scatter

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CAUSES OF TUBE FAILURE

• Excessive heat
• Maximal radiographic techniques
• Tube arch

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Methods of Heat transfer in the tube

• Radiation the filament heats up and radiates
  electrons to the anode
• Conduction the oil surrounding the tube
  conducts heat away from the tube / anode conducts
  heat away from the tube
• Convection uses a fan to move heat away from
  the tube

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RATING CHARTS

• Radiographic rating chart
• Gives a range of safe techniques for a specific
  tube
• Anode cooling chart
• Specifies a thermal capacity of the anode and its
  heat dissipation characteristics
• Housing cooling chart
• Gives the characteristics of the tube housing
  cooling abilities

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HEAT UNITS

• Single phases
• HU kVp x mA x s
• Three phases and high frequency
• HU kVp x mA x S x 1.4

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THE END