Midterm1 Review

MOD 1

Control Panel

25kV to 150kV is typically used in diagnostic work → impacts quality
25 to 1200mA is the range of tube current → impacts quantity
line compensation: prevents voltage fluctuations (can reduce IQ and risk pt dose) and adjusts it to keep it constant.
AEC: device that measures the quantity of radiation that reaches the image receptor, which will automatically terminate the exposure when the image receptor has received the required radiation intensity
APR: a set of pre-programmed exposure factors that allows for quick access to average/recommended exposure factors

Generator

main role of generator is to supply a steady high power source with little voltage ripple to the x-ray tube → consistent beam of electron energies striking the anode → consistent x-ray beam energy
high frequency power with near constant potential voltage

X-ray Tube

filaments are made of thoriated tungsten
Large Filament – for larger body parts – more heat dissipation - less spatial resolution (vice versa for small)
rotating anode = rotating disc anodes which allows a new part of the focal spot to be hit by the electrons for every exposure → longer anode life
- three functions:
  - to conduct the electrons back to the generator through the cables
  - provide a base to support the target ring/focal spot and also to dissipate heat
  - produce xrays! (NOTE kinetic electron energy makes 99% heat and only 1% x-ray photons)
    - thermal infrared energy
    - electromagnetic energy

target/ focal spot = made of tungsten for its high atomic number and rhenium for strength
glass/vacuum envelope: not vacuum = the tube becomes inefficient, producing fewer photons (gassy)
protective tube housing: the part we move and adjust
filtration:
- inherent - found within the xray tube
- added - Al sheets between the tube and collimator exit glass mirrors and plastic windows within the collimator

characteristics:

-e only ionizes/ejects only if the photon energy is greater than the binding energy → -e from higher shell will drop down to fill vacancy (characteristic cascade) → the difference between the two binding energy is released as characteristic radiation
only -e at K-shell (inner) produce diagnostic photons, high binding energy = lower energy state -e

brems:

Half Value Layer

represents the penetration ability the x-ray beam has through differing thicknesses of different materials (objects)

inverse square law: to see the intensity change as SID changes
- doubled SID = quartered intensity
direct square law: to counteract the intensity lost

diverging beam contributes to shape & size distortions

*mag increases = OID increase, SID decrease * (think regarding diverging beam)

shape distortion occurs of the beam divergence and the position of the part (should be perpen to beam)

anode effects

line focus principle
- smaller anode surface angle = smaller effective focal spot = better IQ
anode heel effect

focal spot blurs

photon scatter

increased by increased kV and field size
- coherent: low energy photons interact with atom → release scattered xray → increase noise very slightly
- compton scatter: outer shell -e with incident xray → scattered xray and ionization of atom → may reach the receptor but doesn’t contribute to IQ = radiation fog → use grid to absorb scatter
- photelectric effect: inner shell -e with incident xray → photoelectron emitted instead of scatter xray (cascade effect) → therefore ionization occurs but not scatter → this photoelectron gets absorbed in tissues

elimination of expensive film and chemical-heavy wet processing equipment
latent image: incident x-ray photon that excites the photostimulable phosphors (PSP)
PMT converts light signals to analog light signals
ADC converts analog signal to digital

mag mode in fluoro