01 - X-ray Production and Interactions

atoms

composed of multiple subatomic particles:

1. protons: positively charged, mass of 1.007 AMU

2. neutrons: no charge, mass of 1.008 AMU

3. electrons: negatively charged, mass of 0.0005 AMU

   • orbit around nucleus in orbitals that have different energy levels

• protons and neutrons make up the nucleus of an atom

atomic number

Z, equal to amound of protons

atomic mass

A, equal to number of protons and neutrons

ion

atom that has either a positive or negative charge

isotope

same atomic number (# of protons), but different atomic mass (protons + neutrons)

types of radiation

1. particulate radiation → alpha particles (2P + 2N), beta particles (1 e)

2. electromagnetic radiation → gamma rays, x-rays, microwave, visible light

sources of radiation

• natural → radon (natural gas), terrestrial (rocks), cosmic (from space), internal (within body)

• man-made → computed tomography, radiographs

radioactive decay

when unstable atoms spontaneously emit an alpha particle, beta particle, or gamma ray in attempt to be more stable

• N is approximately P for small atoms

• as atomic number increases, N/P ratio increases and results in an unstable atom

  • atoms above a certain atomic number will be unstable due to having too many neutrons

electromagnetic radiation

produced when velocity of charged particle is altered

• x-rays produced when electrons interact with atoms, slowing them down

• x-rays are ionizing radiation because it has enough energy to knock electrons off atoms and. turn them into ions

artificial x-ray generation

1. cathode is heated by power supply to release electrons

   • negatively charged to repel electrons away

2. anode. contains tungsten target where electrons hit

   • high voltage accelerates electrons towards positively charged anode

3. when electrons hit target, the energy is converted into heat (99%) and x-rays

components in x-ray machines

• focusing cup: helps ensure electron beam is directed at tungsten target

• power supply: provides a high voltage between cathode and anode, and low voltage to heat up cathode

• current selector: measure in mA and indicates number of electrons sent from cathode to anode per second

• voltage selector: measured in kVp and indicates maximum voltage of current

• timer: controls duration of exposure, measured in seconds

• rectifier: decreases variance in voltage by using capacitors to make it a steady stream of voltage to allow for better image quality

focal spot

area on tungsten target where electron beam from cathode hits and produces x-rays

• anode should be angled for increased sharpness due to focal spot appearing smaller

• larger surface area for ehat dissipation

• will get a perfect image if all electrons in beam go to a single focal spot and image receptor

electron volt (eV)

change in energy of an electron moving across one volt

• energy of x-ray photon measured in keV and usually range between 0 and 70 keV

• more photons with lower photon energies and very rarely photons at high energies

Bremsstrahlung (breaking) radiation

energy emitted when directly hitting or passing near nucleus

• electron slows down because positively charged protons slow electrons and change its trajectory

• energy is released as electromagnetic wave, where resulting radiation is a whole spectrum of energies → conservation of energy

• for direct hit → energy of incident electron = energy of resulting photon

• for slowed electron → difference in kinetic energy = energy of resulting photon

characteristic radiation

incident electron interacts with and ejects inner shell electron

• valence electron from outer shell fills vacancy in inner shell

• outer shell electron is at a higher energy state, so difference in energy is released in form of a photon

• resulting radiation is at very specific levels of energy → characteristic of target material