Imaging

Imaging

1. X-rays

   • Continuous spectrum
     • Characteristic peaks
       • Bombarding electrons can knock an electron out of an inner shell (if it has sufficient energy). Electrons from higher states drop down to fill the vacancy, emitting x-ray photons with precise energies determined by the electron energy levels = characteristic x-rays
       • Depends on anode material
     • Depends on accelerating voltage
   • Interactions
     • Compton Effect
       • Scattered photon changes direction with a longer wavelength (lower energy), as most of the incident photon energy is transferred to an electron
       • 10 keV -1 MeV
     • Photoelectric Effect
       • All photon energy is transferred to the electron
       • < 10 keV
     • Pair production
       • All photon energy is transformed into mass energy and kinetic energy of an electron and positron
       • > 1 MeV
   • Production of X-rays
     • 1. Electrons produced by heating of the cathode (-)
     • 2. Accelerated towards the anode by a potential difference of 10-100 keV
     • 3. Electrons interact with atoms in the anode to produce X-rays
     • 4. Evacuated tube (no air molecules to interact with X-rays)
     • 5. Electrons are accelerated due to electric field of nuclei in the anode
     • 6. Electrons are decelerated around the nucleus, producing Bremsstrahlung ("braking") radiation
       • Close to nucleus = high braking = high energy Far from nucleus = low braking = low energy
         • Range of photon energies
   • Keeping the Anode cool
     • Bevelled edge
     • Tungsten = good thermal conductor, high melting point

2. Single Photon Emission CT

   • Uses injected radioisotopes

3. Positron Emission Tomography (PET)

   • Positron emitters used to label molecules which are to be introduced into the body

4. MAXIMUM Contrast

   • Low X-ray energy
     • Reduces Compton scattering
     • Greater difference between attenuation coefficients
     • Higher probability of photons being absorbed
   • C = I1 - I2 / I1
   • I/Io = e-^(ux)

5. Attenuation

   • High attenuation coefficient = More absorption and scattering
   • Bone = high attenuation Water = low attenuation

6. MAXIMUM Resolution

   • Bevelled edge on anode
     • Minimise area of bevelled edge to reduce penumbra, achieving crisper shadows
   • Grid
     • Filters Compton scattered X-rays before reaching the film

7. MINIMUM Dose

   • Aluminium Absorber
     • Reduces useless low energy photons
   • Increase distance to X-ray source
   • Collimator
     • Produces in-phase X-rays by focusing beam

8. Computed Tomography (CT)

   • Deduces a cross-sectional attenuation map, by comparing intensities along a line

9. Angiography

   • Assisted attenuation coeffcients
     • Iodine-rich compound increases attenuation of blood
     • Barium meal to the digestive system