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Medical Imaging and Radiation. Question:. X rays have trouble going through lead because lead is a metal lead is heavy lead atoms have many electrons lead is a hard material. Observations About Medical Imaging and Radiation. They manage to work right through your skin

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question
Question:

X rays have trouble going through lead because

  • lead is a metal
  • lead is heavy
  • lead atoms have many electrons
  • lead is a hard material
observations about medical imaging and radiation
Observations AboutMedical Imaging and Radiation
  • They manage to work right through your skin
  • Imaging involves radiation of various sorts
  • Some imaging radiation is itself hazardous
  • Radiation can directly make you well, sick, or neither
  • Some radiation involves radioactivity
  • Some radiation involves accelerators
x rays
X rays
  • Short-wavelength electromagnetic waves
  • An x-ray photon has lots of energy
    • Enough to do much chemical damage to molecules
    • Enough to knock particles out of atoms
  • Produced by energetic events
    • Rapid acceleration of a charge
    • Radiative transition in a highly excited atom
bremsstrahlung x rays
Bremsstrahlung X rays
  • When a fast-moving electron swings around a heavy nucleus, it accelerates rapidly
  • Electron emits much of its energy as X-ray photon
characteristic x rays
Characteristic X-rays
  • When a colliding electron excites an atom to high-energy state, that atom can then radiate an x-ray photon
producing x rays
Producing X-rays
  • Accelerate electrons to 10kV - 100kV
  • Let electrons hit heavy atoms
  • Some x-rays emitted via bremsstrahlung and some as characteristic X rays
  • X-ray tube filters awaylowest energy photons,which is good becausethey’re useless and burn
x rays and matter
X rays and Matter
  • X rays interact with atoms
    • Rayleigh scattering
    • Photoelectric effect
  • Rayleigh scatteringmakes the sky blueand deflects X rays
photoelectric effect
Photoelectric Effect
  • X ray causes radiativetransition in an atom andejects an electron
  • Electron’s energy is thedifference between thex-ray photon’s energy andthe energy needed to remove electron from atom
  • Effect is most likely when electron energy is low
  • Effect is strongest in many-electron atoms
x ray imaging
X-ray Imaging
  • Any atom that blocks X rays casts a shadow
    • Many-electron atoms produce shadows
    • Few-electron atoms cast essentially no shadows
  • All atoms Rayleigh scatter X rays, causing haze
  • X-ray imaging observes shadows of large atoms
  • Haze can be filtered by collimating structures
ct scanning
CT Scanning
  • Many separate X-ray imagesproduce CT database
    • X rays from different angles mixshadows differently
  • Computer can recreate original3-D from database
  • Computer typically plotscross sections
question1
Question:

X rays have trouble going through lead because

  • lead is a metal
  • lead is heavy
  • lead atoms have many electrons
  • lead is a hard material
gamma rays
Gamma Rays
  • Higher photon energy than X rays
  • Sources
    • Radiative transitions in nuclei
    • Particle accelerators (high-energy bremsstrahlung)
gamma rays and matter
Gamma-rays and Matter
  • Gamma rays interact with individual charges
    • Compton scattering
    • Pair production
radiation therapy
Radiation Therapy
  • Gamma rays are highly penetrating in tissue
  • Little Rayleigh scattering and photoelectric effect
  • Most gamma ray events are Compton or Pair Prod.
  • Either event damages many molecules
  • Gamma radiation kills cells
  • Approaching tumors from many angles minimizes collateral damage to healthy tissue
mri imaging
MRI Imaging
  • Hydrogen nuclei are protons
  • Protons are magnetic
  • In a magnetic field, spin-up and spin-down protons have different energies
  • Radio wave photons can flip the proton spins
  • By controlling the energy differences between spin-up and spin-down and adjusting the radio waves, you can locate hydrogen in a person