X ray

• Photoelectric Effect


  When the Photon has the energy in the range 20kEV - 0.1mEV it can be absorbed by the electrons in the atom. The energy absorbed by the electron can overcome the electrostatic attraction between the electron and the Nucleus so it escapes the atom.

  
• Photoelectric Effect


  • Often used in hospital X ray imaging.
• Pair production


  The photon has high energy usually above 1.02 MEV. Photon absorbed by nucleus and its energy is used to create an electron and an positron.

  
• X rays


  • Propagate through oscillations of electric and magnetic Fields.
  • Travel through a vacuum at the speed of light.
  • Have a high frequency and short wavelength.
  • Transverse waves
  • EM radiation travel in packets of energy known as photons.

  
• X ray production in an X ray tube

  1. Cathode (heated metal filament) emits electrons through Thermionic emission.

  2. Electrons are accelerated towards the anode (positively charged) by a potential difference.

  3. Electrons are slowed down when they hit the anode and emit a continuous spectrum of X rays - Bremsstrahlung radiation.

  
• How Characteristic X rays are produced (C-X rays)

  1. When electrons in a higher energy shell drop down to fill vacancy for knocked out electrons C- X rays are produced.

  2. The wavelengths of the produced C- X rays depends on the energy level spacing of the target atoms (metal atoms of anode).

  
• Minimum Wavelength from an X ray Tube

  Maximum energy of a photon is equal to the maximum kinetic energy of an electron.

  
• Intensity


  Total energy hitting an area perpendicular to the beam per second per unit area.

  
• Intensity


  Depends on the current as the number of electrons hitting the target per second increases. So increasing current increases the intensity of the X-rays produced.

  
• Attenuation


  The decrease in intensity of EM radiation as it passes through matter.

  
• Linear attenuation coefficient


  The rate of energy loss by a photon Beam per unit thickness within a medium.

  
• Equation for Attenuation
• Thermionic emission


  Heat metal Filament (cathode) emits electrons
• 1. ray production


  1. Electrons accelerated towards anode using a large potential difference

  2. Electrons hit the anode and decelerate, releasing energy as heat and X-rays
• 1. ray production


  • Use Lead shielding so X-rays aren't released in all directions
  • Cool anode with oil or by rotating the metal to avoid same point in contact
  • 99% of the electron energy heats anode, 1% accounts for X-ray production
• Difference between X-rays and gamma photons

  1. rays are produced when electrons are decelerated, gamma photons are released from a nucleus of an atom

  
• Assumption - 100% of electrons' energy is transferred into photon
• Increase in current in X-ray tube

  Number of X-rays produced increases, but no effect on frequency or wavelength
• Photoelectric effect


  1. ray photon energy is absorbed by electron, causing the electron to leave the atom
• Compton effect


  High energy X-rays bounce an electron off the atom, causing the X-ray to be deflected in a different direction and lose some energy
• The bigger the angle the electron bounces off, the more energy the X-ray photon has lost
• Pair production


  An electron and positron pair are produced when an X-ray interacts with an atom
• Attenuation


  Decrease in intensity of electromagnetic radiation as it travels through matter or space
• Simple scattering


  An X-ray photon interacts with an electron in an atom, but lacks sufficient energy to remove the electron and scatters with no change in energy
• 1. ray absorption and attenuation coefficient

  Elements with a high atomic number are used to examine soft tissue as they have higher attenuation coefficients (e.g. iodine, barium)

  
• Attenuation depends on intensity of X-rays, thickness of material, and type of material
• Attenuation coefficient


  A measure of the absorption of X-ray photons by a substance

  
• CAT scanner operation


  1. Patient lies on flat bed and enters gantry

  2. One side of the ring is an X-ray tube and the other is detectors

  3. Fan-shaped beam of X-rays about 1-10mm thick irradiates thin slices of the patient as the ring rotates

  4. Every 360° rotation, the bed moves 1cm so the next image slice is obtained

  5. X-rays collected by detectors are sent as signals to a computer which produces the digital image

  
• CAT scanners


  • Deliver more data about the patient
  • Can differentiate between soft tissue of similar attenuation coefficients

  
• Medical radioactive tracers/radiopharmaceuticals


  • Used for diagnosis and medical therapy
  • Highly ionizing radiation (alpha, beta minus, beta plus)
• Technetium-99m (Tc-99m)


  • Gamma-emitting tracer produced from beta minus decay of Molybdenum-99
  • Has a long half-life, which can damage the patient, nurses, and other medical staff
• Reasons why Tc-99m is delivered to the patient: short half-life means high activity, so only a small dose is needed to form an image, and the patient is not exposed to radiation for long periods
• Gamma camera


  • Lead collimator detects radiation perpendicular to it, absorbing gamma rays that arrive at an angle
  • Scintillator gives out small flashes of light when hit by gamma photons, with 1/10 gamma rays interacting
  • Light guide guides the light to the photomultiplier
  • Photomultiplier creates a cascade of electrons which produce a voltage pulse

  
• Gamma camera can be used to analyze the function of an organ
• Inside photomultiplier tube


  For each electron incident on a dynode, four other electrons are produced, resulting in 4^n electrons for each dynode (N being the dynode number)
• PET scanner


  Array of detectors that detect the two gamma photons produced by the electron-positron annihilation of the positron from Fluorine-18 in FDG

  
• PET scanners


  • Non-invasive
  • Can be used to detect cancer and investigate the function of organs such as the brain
  • Expensive, and making F-18 and Carbon-11 for positron emission can be quite difficult
• Positron emission process


  Positrons released by radioactive Fluorine in FDG react with electrons to form two gamma photons that propagate in opposite directions, due to the law of conservation of momentum
• Description of PET scan


  1. Patient lies horizontal to a ring of gamma detectors consisting of photomultipliers and sodium iodide scintillators

  2. Gamma detectors detect the arrival of the two photons produced by the electron-positron annihilation

  3. Voltage signals from the detectors are directed to a computer, which calculates the previous positions of the photons in the body and creates an image of the body area under diagnosis

  
• Ultrasound


  Longitudinal waves with a frequency above 20kHz