IO

X-Ray Tube Notes

Chapter 5: The X-ray Tube

Introduction

  • The radiographer is responsible for the safe operation and proper maintenance of the x-ray unit.
  • Appropriate operation and maintenance stems from the knowledge of how it works.

General Tube Construction: Housing

  • A protective housing provides solid mechanical support.
    • Lead-lined structure
    • Oil bath
    • Cooling fans
    • Electrical insulation
    • Large cables
    • Absorbs stray photons
  • Cautions:
    • The housing can become rather hot with continuous use.
    • The high-voltage cables should not be used as handles to maneuver the tube head assembly.

X-ray Tube

  • The general-purpose x-ray tube is an electronic vacuum tube that consists of:
    • An anode
    • A cathode
    • An induction motor
    • Encased in a glass or metal enclosure
  • The main purpose of the enclosure is to maintain a vacuum within the tube to prevent electrical arcing.
  • Two varieties of enclosures:
    • Glass envelope: Generally made of borosilicate glass because it is heat resistant.
    • Metal envelope: Provides a constant electric potential between the electron stream from the cathode and the enclosure, thereby avoiding the arcing problem and extending tube life.
  • Both types have a specially designed target window for the desired exit point of the x-rays produced.

Anode

  • The positive end of the tube.
    • Provides the target for electron interaction to produce x-rays.
    • Serves as an electrical and thermal conductor.
    • Some of the electrons interact with the target to produce x-rays and the rest continue on as current flows through the x-ray circuit.

Stationary Anode

  • A tungsten button embedded in a copper rod.
  • The target does not move.
  • Disadvantage: the rapidly building heat can damage the tube, which limits use.

Rotating Anode

  • A rotating tungsten-coated molybdenum disc mounted on a copper shaft with a molybdenum core.
  • Advantage: the rotating anode spreads the heat produced during x-ray production over a larger surface area.

Materials

  • Copper: Used as part of the shaft because it has excellent thermal and electrical conductive properties.
  • Molybdenum: Used as the disc base and core because it has a low thermal conductivity and it is a light but strong alloy.
  • Tungsten: Used because it has a very high melting point and a high atomic number (74), improving the efficiency of x-ray production.
  • Rhenium: May be added to the tungsten to increase thermal capacity and tensile strength.

Induction Motor

  • The anode is rotated using an induction motor.
    • The two major parts of this motor are the stator and the rotor.
      • The stator is made up of electromagnets arranged in pairs around the rotor.
      • The rotor is made of an iron core (iron bars embedded in the copper shaft) surrounded by coils.
  • The induction motor is operated through mutual induction.
    • The stators are energized in opposing pairs and induce an electric current and magnetic field.
    • This induced field opposes that of the stator pair and the rotor turns to correct that orientation.
    • Just as the two fields align, the next pair of stators is energized and a new electric current and magnetic field are induced, causing the rotor to turn again.
    • This process causes the rotor to turn continuously.
  • An induction motor allows for the rotation of the anode in a vacuum without engineering a motor into the vacuum.
    • Capable of speeds of 3400 revolutions per minute (rpm) for general-purpose tubes and 10,000 rpms for specialty tubes.

Line-Focus Principle

  • By angling the face of the anode target, a large actual focal spot size can be maintained and a small effective focal spot size can be created.
  • When the angle of the target face is less than 45 degrees, the effective focal spot will be smaller than the actual.
  • The target angles are 7 to 18 degrees for a general-purpose tube (12 degrees is the most common).
  • The smaller the anode angle, the smaller the effective focal spot will be while maintaining a large actual focal spot area.
  • The smaller the effective focal spot, the sharper the image will be.

Anode Heel Effect

  • The angle causes the intensity of the x -ray beam to be less on the anode side because some of the x-rays are absorbed in the target heel.

Cathode

  • The negative end of the tube.
    • Provides the source of electrons needed for x-ray production
    • Made up of the filaments and the focusing cup

Filaments

  • Dual-focus tubes are general-purpose tubes with two filaments
  • Each filament is a coil of wire usually 7 to15 mm long and 1 to 2 mm wide.
  • They are usually made of tungsten with 1% to 2% thorium added.
  • The filaments are parallel to one another in the focusing cup and share a common ground wire.

Focusing Cup

  • Made of nickel and surrounds the filaments’ back and sides, leaving the front open and facing the anode target
  • Receives a strong negative charge from the secondary circuit that forces the electrons together into a cloud as they are boiled off of the filament through electrostatic repulsion

Principles of Operation

  • At the operating console, the radiographer selects the desired exposure factors.
  • When the exposure switch is first pressed, some of the electricity is diverted to the induction motor of the x-ray tube to bring the rotor up to speed.
  • Inside the x-ray tube, the induction motor turns the anode at approximately 3400 rpm.
  • The selected filament is energized until the desired thermionic emission is achieved.
  • The second phase initiates the x-ray production process.
  • The process from rotor preparation to exposure lasts only a few seconds.
  • The actual exposure is measured in milliseconds.
  • When the exposure switch is pressed, the voltage from the autotransformer passes to the step-up transformer.
  • The voltage then passes through a rectifier bank before passing to the anode and cathode of the x-ray tube.
  • During the preparation phase, some power from the autotransformer was diverted to the filament circuit where it passes through a rheostat to a step-down transformer, then to the selected filament within the cathode focusing cup.
  • The current heats the filament to the point of incandescence and electrons are boiled off of the filament by thermionic emission.
  • The focusing cup forms them into a cloud called a space charge.
  • Once the space charge reaches a size commensurate with the current used, it becomes difficult for additional electrons to be emitted; this is called the space-charge effect.
  • The three things needed to produce x-rays are now present:
    • A large potential difference to give kinetic energy to the filament electrons (provided by kVp)
    • A vehicle on which kinetic energy can ride (a quantity of electrons provided by mAs)
    • A place for interaction (the target of the anode)
  • As they penetrate the target surface, filament electrons interact with the atoms of tungsten, generating heat and x-rays.

Quality Control and Extending Tube Life

  • Most of the factors that can shorten x-ray tube life are within the radiographer’s control:
    • Frequent use of very high or maximum exposure factors
    • Use of lower but very long exposure factors
    • Overloading the filament
  • Three processes of heat transfer are at play:
    • Conduction of heat by heat-tolerant materials
    • Radiation of heat energy from anode to oil bath
    • Convection of heat into the room by cooling fans
  • Protective circuits prevent the use of unsafe exposure techniques and heat overloads.
  • Even with safety measures, the radiographer must understand anode thermal capacity and keep in mind that x-ray production is an inefficient process.

Heat Units

  • A measure of the amount of heat stored in a particular device
  • Calculated by multiplying kVp ´ mA ´ s ´ c
  • If multiple exposures are made using a given technique, the answer from this formula is multiplied by the number of exposures
    • Heat Units = kVp \times mA \times s \times c

Extending Tube Life

  • To extend tube life, simple procedures and guidelines should be followed.
    • Follow the machine-specific warm-up steps completely and routinely.
    • Do not prep the rotor excessively.
    • Do not routinely use extremes of exposure factors.