X-ray Tube Components

X-ray Tube Components

• 2 primary internal components: Cathode and Anode
• Secondary external components: Glass enclosure, Cathode assembly, and Anode assembly

Cathode Assembly

• Focusing cup:
  • Focuses the beam to the anode
  • Prevents electron beam from repelling itself
  • Targets a spot on the anode
  • Effectiveness depends on size, shape, and position of filaments in the cup
• Filament:
  • Receives electrical supply (mA)
  • kVp determines the voltage

Anode Assembly

• Anode:
  • Tungsten-outer layer (focal track)
  • Internal layers: generally graphite, for dissipating heat and lower mass density
  • Bevelled edge forms anode angle at the target surface (focal track)
  • Angled at 5-20 degrees for x-ray photons to exit the tube
• Anode stem:
  • Connects rotor and high tension cables
• Rotor:
  • Rotates at 3000-10000 rev/minute in high capacity x-ray tube
• Anode disc:
  • Tungsten focal track
  • Prevents repetitive targeting of electrons on anode

Focal Spot Size

• Fine:
  • Used for smaller body parts
  • Better spatial resolution
  • Used for fine details
• Broad:
  • Used for large body parts
  • More heat is generated

Anode Functions

• Electrical conductor
• Thermal dissipater
• Mechanical support for the target

X-ray Formation

• Select voltage, current (mAs to filament), and focus
• mA sends current to filament, starting the process of sending electrons to anode
• Apply high voltage, creating potential difference between anode and cathode, pulling electrons to anode
• Focusing cup targets electrons at the anode

X-ray Spectrum

• Beam typically consists of a continuous spectrum of x-ray photon energies
• Bremsstrahlung:
  • Diagnostic energy range: 30-150kVp
• Characteristic (peak):
  • 69kVp (for tungsten target)

Factors Affecting X-ray Spectrum

• Bremsstrahlung:
  • Electron beam travels close to nucleus
  • Proton causes electron to change direction and lose speed and energy
  • Loss of energy reappears as bremsstrahlung x-rays
• Characteristic x-rays:
  • Electron beam hits the tungsten atom, causing an inner electron to be ejected (ionisation)
  • An outer electron fills the gap, releasing energy as x-ray photons

Properties of X-rays

• Short wavelength and high frequency
• Higher the energy, shorter the wavelength
• Wavelength and energy are heterogeneous
• Electromagnetic radiation
• Travels through air in straight lines
• Divergent beam once emitted from x-ray tube
• Energy transfer:
  • Electrical: in x-ray machine (control at console)
  • Electromagnetic: when x-rays are produced at the anode
  • Chemical: conversion to electrical signal at image receptor/detector

Medical Imaging

• X-rays pass through patient and are captured on the imaging receptor
• Creates a latent image, which is then processed through chemical energy to produce a 2D x-ray image