x-ray tube construction

milliampere (mA) control factors

quantity and density

time (s) control factors

quantity and density

kilovoltage (kV) control factors

quality, penetrating power, energy, density and contrast

contrast

difference between black and white

density

overall degree of darkness

x-ray room external structure

ceiling, floor to ceiling, C arm

ceiling structure

• most common

• freely moves in all directions

floor-to-ceiling structure

• multi-directional

• requires more effort to move

C arm structure

• nearly limitless in tube position

• robotic and fluid in motion

• angiography

leakage radiation

anything that escapes protective housing

protective housing

• isotropic

• protects from damage caused by rough handling

• provides mechanical support

• contains oil to insulate from shock and dissipate heat

• lead-lined which reduces radiation

x-ray tube material

pyrex glass or metal

pyrex x-ray tube

• withstands large amounts of heat

• has a vacuum that has a more efficient x-ray production and longer tube life

metal x-ray tube

prevents tube arcing and better tube life

cathode

negative electrode that contains filament

cathode filament material

tungsten

cathode filament wire coils

toaster and x-ray filament

thermionic emission

electrons are boiled off, resulting in a space charge

what does vaporized tungsten lead to?

tube failure

dual focus cathode filaments

small filament and large filament

small filament

200 mA or less, produces better detail

large filament

300 mA or more, has larger body parts and can withstand more heat

focusing cup

has a negative charge that creates a force that compresses electron beam to small area of anode

space-charge effect

prevents subsequent electrons from being boiled off due to electrostatic repulsion

electrostatic repulsion

same charges repel

anode

positive electrode that contains stationary or rotating anodes

stationary anode

• has a high tube current and does not require power

• mostly seen in dental and portable x-ray units

pitting

caused by damage to protective coating on anode or cathode, which leads to tube failure

rotating anode RPM

3,600 to 10,000 RPM

rotating anode track

area of disc that the electrons strike, and are converted to x-rays

rotating anode layers

tungsten/rhenium, molybdenum, and graphite

rotating anode pros

• better for heat dissipation

• higher tube currents with shorter exposure time

• increased heat loading capabilities

why is copper used in an x-ray tube?

helps dissipate heat

tungsten atomic number

#74, which has higher efficiency

thermal conductivity

better at heat dissipation

tungsten melting point

3410° C / 6170° F, minimize occurences of pitting