MOD 1 - Xray Tube Components

Cathode

• creates free electrons thru the process of thermionic emission

• negatively charged

thermionic emission

• high amperage current is circulated thru the thungsten filament within the cathode → gets very hot → it creates a cloud of free electrons (space charge)

• controller of this process is mA

Dual Focus Tubes

two cathode filaments one small and large

small filament

• used for small mA = small heat capacity

• used for small body parts bc it can only be used for small exposures

• creates small, tight electron beam which becomes a narrow, tight x-ray beam = high spatial resolution

large filament

• used for large mA = large heat capacity

• used for large body parts that require large exposure

• creates large, wide -e beam which becomes a large, wide x-ray beam = low spatial resolution

Focusing Cup

purpose is to focus the -e beam with its negative charge, as -e would try to spread apart and repel each other in space

diagnostically useful kV range

20-150kV

kV purpose

accelerates the electrons at the cathode to the anode

kV and xray quality

increased kV increases -e energy and gives higher speed / more kinetic energy that can penetrate deeper into the body

anode

• absorbs electrons and creates x-rays

• positively charged

functions of the anode

• to conduct the electrons back to the generator through the cables

• dissipate heat

• produce xrays! (NOTE kinetic electron energy makes 99% heat and only 1% x-ray photons)

  • thermal infrared energy

  • electromagnetic energy

Rotating Anode Benefit

• rotating disc → allows a new part of the focal spot to be hit by the electrons for every exposure → longer anode life

• high heat capacity of the x-ray tube

Components of the Rotating Anode

Induction Motor = what rotates the anode

• Stator = surrounded by electromagnets

• Rotor = rotating tube shaft that directly attached to the anode disk

• Bearings = low friction spheres that allow for free rotation of the rotor

Focal Spot

• the target area on the anode where electrons are directly hit

• made of tungsten and alloyed with rhenium

Glass Envelope

• creates an air free vacuum around the cathode and the anode

• important as it protects the tube from oxidation and corrosion

Tube Housing

purpose is to prevent x-rays from being emitted thruout the exam room by absorbing all photons except those aimed at the PT

Xray Production Requirements

1. Source of -e

   • at the cathode thru thermionic emission

2. Acceleration

   • done by applying kVp

3. Deceleration

   • done as -e slam into anode