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DMI 261

DMI 261. Radiation Biology And Protection. Unit 2 RADIATION. Electromagnetic Spectrum The frequency range of electromagnetic radiation and the photon wavelength associated with them. Important to Radiography. Visible light Smallest segment of the EM spectrum

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DMI 261

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  1. DMI 261 Radiation Biology And Protection

  2. Unit 2 RADIATION • Electromagnetic Spectrum • The frequency range of electromagnetic radiation and the photon wavelength associated with them.

  3. Important to Radiography • Visible light • Smallest segment of the EM spectrum • Only part of the spectrum that we can directly sense • Radiographs viewed on viewboxes (visible light)

  4. Radio frequency • Very low energy, very long wavelength • Used for communication, TV and radio • Microwaves (shorter radiowaves), cell phones, highway radar, cooking • MRI uses radiowaves.

  5. Ionizing radiation – gamma & x-rays • High energy, short wavelength • Sources • X-rays – emitted from electron cloud in a man made tube • Gamma – emitted from nucleus of a radioactive atom • Both have similar energy and travel at the speed of light

  6. Characteristics of Ionizing Radiation • Electromagnetic radiation • High energy, high frequency, short wavelength • Travels at the speed of light • X and gamma are identical except for origin • No mass • Electrically neutral

  7. Characteristics of Ionizing Radiation • Interaction with matter causes electron excitation or ionization • Affects photographic film and fluorescent crystals • Behaves as a wave and a particle • Travels in straight lines • Produces chemical & biological changes • Produces secondary & scatter radiation

  8. Interactions of Radiation with Matter • Coherent Scattering • X-ray photon interacts with a target atom causing it to vibrate. • Electrons emit electromagnetic waves with the same energy as the incoming photon.

  9. Coherent Scattering(also called Rayleigh Scattering) • Occurs mostly below 30 kVp • Small change in direction occurs (scattering of x-ray photon) • Diagnostic radiology – some x-ray are scattered by classical scattering and may contribute to radiographic fog.

  10. Compton Scattering • Responsible for most scatter radiation produced during radiographic procedures • Forward scatter • Small angle scatter • Backscatter • Side scatter

  11. Compton Scattering important

  12. Compton Scattering • Probability in Diagnostic Radiology • As photon energy increases, probability increases • 100 kVp beam = significant interactions • Occurs along with photoelectric absorption • Can cause large amounts of scatter • Grids to absorb scatter (radiographic fog) • Lead aprons in fluoroscopy

  13. Compton Scattering is responsible for most of the scatter radiation produced during a radiologic procedure.

  14. Photoelectric Absorption • X-ray absorption interaction in which the photon is not scattered, but totally absorbed. important

  15. Photoelectric Absorption • Most important mode of interaction between x-ray photons and the atoms of the patient’s body for producing useful images. (photons #1 and 2)

  16. Auger Electrons • Outer shell electrons that are released (instead of characteristic radiation) when an electron moves from an outer to an inner shell during the photoelectric absorption interaction.

  17. Photoelectric Absorption • Probability in Diagnostic Radiology • Dominant interaction in diagnostic radiology • Between 30 and 150 kVp, it is the most important interaction for producing useful images.

  18. Photoelectric Absorption • Probability in Diagnostic Radiology • Probability increases as the effective atomic # increases: • Air 7.6 • Compact bone 13.8 • Barium 56 • Iodine 53 • Because it has a higher effective atomic #, bone will undergo more PE absorption than an equal amount of soft tissue or air.

  19. Photoelectric Absorption • Probability in Diagnostic Radiology • Probability increases as the energy of the x-ray photon decreases (longer wavelength) and the effective atomic # of the irradiated atoms (bone vs. soft tissue) increases. • Attenuation / Absorption / Contrast Media

  20. Pair Production • Incoming photon interacts with the nucleus of an atom and disappears • Only occurs with photons greater than 1.022 MeV

  21. Pair Production & PET • Annihilation reaction photons are products of radioactive decay of an unstable isotope. • Scanner detects this reaction and produces a cross-sectional image of the radioactivity • Commonly used isotopes: fluorine-18, carbon-11, nitrogen-13

  22. PET scan demonstration of epilepsy on the right side of the brain

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