16.2 Medical Imaging & Radiation

16.2 Medical - 1 Key Concepts 1 X-rays imaging 2 Radiation therapy 3 MRIs Observations About Medical Imaging and Radiation They manage to work

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Unformatted text preview: 7/31/2008 1 Key Concepts: 1. X-rays imaging 2. Radiation therapy 3. MRIs Observations About Medical 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 make you well, sick, or neither ¡ Some radiation involves radioactivity ¡ Some radiation involves accelerators Electromagnetic Spectrum X-rays and gamma rays are light! 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 ¡ When a fast-moving electron swings around a heavy nucleus, it accelerates rapidly ¡ Electron emits much of its energy as X-ray photon Photons (light) can be absorbed or emitted by an atom, only at certain values corresponding to energy differences between electron energy levels. 7/31/2008 2 Thursday ¡ Review ¢ bring your questions ¡ Lab ¢ check grades online ¡ Grade Sheet ¢ All grades, unless you have a make up lab on Thursday Clicker Question 1 In general, X rays have A. higher energy than gamma rays B. the same energy than gamma rays C. lower energy than gamma rays Characteristic X-rays When a colliding electron excites an atom to high- energy state, that atom can then radiate an x-ray photon X-ray Spectrum Note the characteristic X-rays are sharp lines on the broad Bremstrahlung background 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 away lowest energy photons, which is good because they£re useless and burn X rays and Matter ¡ X rays interact with atoms ¢ Rayleigh scattering ¢ Photoelectric effect ¡ Rayleigh scattering makes the sky blue and deflects X rays 7/31/2008 3 Photoelectric Effect ¡ X ray causes radiative transition in an atom and ejects an electron ¡ Electron¢s energy is the difference between the x-ray photon¢s energy and the 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 ¡ 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 ¡ Many separate X-ray images produce CT database £ X rays from different angles mix shadows differently ¡ Computer can recreate original 3-D from database ¡ Computer typically plots cross sections *C omputerized ( A xial) T omography Gamma Rays ¡ Higher photon energy than X rays ¡ Sources £ Radiative transitions in nuclei £ Particle accelerators (high-energy...
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This note was uploaded on 02/07/2010 for the course PHY 1405 taught by Professor Russell during the Fall '07 term at Baylor.

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16.2 Medical - 1 Key Concepts 1 X-rays imaging 2 Radiation therapy 3 MRIs Observations About Medical Imaging and Radiation They manage to work

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