Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Medical Imaging Radiation I More suggested reading: Naked to the Bone: Medical Imaging in the Twentieth Century (Paperback)by Bettyann Kevles E=mc2: A Biography of the World’s Most Famous Equation by David Bodanis Radiation is: Energy that travels through space and matter We are interested in electromagnetic radiation: X-ray waves Visible waves Radio waves Gamma-rays (...) Electromagnetic wave Monochromatic radiation, electric and magnetic field can be represented as: x,t o cost kx This is the solution of the one dimensional wave equation 2 1 2 2 2 2 x c t The electromagnetic wave: ADD steve EM wave The electromagnetic wave: Period f=1/T Wavelength The electromagnetic wave: red=300nm blue=900nm EM radiation Wavelength [m] Frequency [Hz] E h f [eV] Energy [ev] h 4.1333e -15 eV sec Plank' s constant Energy eV Electron volt [eV]: is the kinetic energy gained when a single electron is accelerated between two plates that differ in potential by 1V. Before leaving the negatively charged plate, the electron has potential energy of 1eV. - - + + + + 1eV=1.6x10-19 J 1Joule [J]=1kg m2 s-2 EM radiation Wavelength [m] c 1 f sec Frequency [Hz] Energy [eV] m c 310 sec 8 Speed of light in vacuum EM radiation Wavelength [m] Frequency [Hz] Energy [ev] E hc eV h 4.1333e -15 eV sec m c 310 sec 8 Why is knowing the wavelength important? EM spectrum Wavelength [m] Wavelength and size of an object! Is object large or small compared to the wavelength? Frequency,Energy, Wavelength are related 10 10 wavelength [nm] Energy [keV] 105 10 E 0 E h f 10 -5 10 -10 10-15 0 1 eV sec sec h 4.1333e -15 0.5 1 1.5 frequency [Hz] 2 2.5 3 x 10 22 eV sec Frequency,Energy, Wavelength are related 10 25 frequency [Hz] Energy [keV] 10 20 10 15 10 10 10 5 10 0 10 -5 f 10-10 0 E 1000 2000 3000 4000 5000 6000 wavelength [m] 7000 E 8000 9000 10000 hc eV c 1 f sec The photon The smallest amount of EM radiation possible, fundamental particle Has no rest mass Move at speed of light c, (c/n in media) Travel in straight line (bends at interfaces) E hc eV E h f eV sec sec1 c 1 f sec The atom 50e- 72e- 98e- 32e- Electron 2e- 8e- 18e- K L M N O P Orbitals Nucleus Bohr model Binding Energy (BE) Energy binding electron to atom A photon will need an energy > than binding energy to remove an electron from a atom Nomenclature - binding energies are negative (eV) Ionization Energy = - BE, energy necessary to remove 1 electron from the atom. Binding Energy Stronger bound (KeV) Less strong bound Weak bound (eV) Valence electrons, # of electrons in outer orbital, determines chemical properties of atom The atom 2e- 8e- Z-Atomic Number, # of protons N-Neutron number, # of neutrons Mass Number, Am = Z + N 18e- 22 K L M Na 11 Atomic Mass, actual mass of the atom Protons Neutrons Oxygen-16 Atomic Mass -> 15.9949 amu Mass Number ->16 Excitation Photon Absorption E = E3-E2 Electrons want to be as close as possible to the nucleus BREAK !! Relaxation Emission E = E2-E1 Photon -Visible -IR -X-Ray Vacancy emission shorter or longer ?? DEPENDS ON ATOMIC NUMBER I.E BE Radiation II Ch. 3 of, The essential physics of medical imaging, Bushberg et al. We focus on X-rays and Gamma-rays production and interaction with matter X-Rays (g-Rays) interactions Scattering and Absorption Absorption - All energy of incident photon is absorbed by a material, the photon is destroyed Scattering - Photon path is altered by a “scattering event”, loss of energy can occur (inelastic scattering) or not (elastic scattering) Transmission - No interaction Absorption Photon detector Scattering Photon detector Transmission Photon detector X-rays, g-rays interactions Rayleigh scattering (coherent) Compton Scattering Photoelectric effect Pair production Rayleigh scattering Photon excites the ALL ATOM Low energy X-rays (15-30 keV) Photon energy makes all electrons oscillate in phase A photon is emitted in a different direction NON IONIZING It’s noise in X-ray imaging 12% of photons <30 keV 5% of photons >70 keV Rayleigh scattering Scattered photon Incident photon What is important to note here Rayleigh scattering Polarized radiation Isotropic radiation Compton scattering Inelastic scattering Dominates X-Rays scattering from 26keV to 30MeV in soft tissue Photon interact with valence electrons Electron is ejected from shell generating an ion Compton scattering is noise in X-Rays imaging Safety hazard! Compton scattering Incident photon Compton scattering Compton electron Ee- Esc=Eo-EeIncident photon Eo 1 1<2 Eo>Esc q 2 Scattered photon Esc Compton scattering E sc Eo Eo 1 (1 cos(q )) 511keV a=mec2=511keV Higher Eo generate more forward scattering photons (smaller q) Compton scattering Back scattering Forward scattering Photoelectric effect All incident photon energy is absorbed Often interaction between photon and electrons in K shell An electron in the K shell is ejected Ee-=Eo-Eb Lower binding energy electron fills the empty orbital - electron cascade Emitted energy can be Auger or X-Rays Photoelectric effect Incident photon Photoelectric effect Incident photon Photoelectric effect 2 Incident photon Photoelectric effect 3 Incident photon X-rays <2 <3 Auger Electron Photoelectric effect 2 Incident photon Auger Electron Photoelectric effect 2 Incident photon Auger Electron Photoelectric effect 2 X-rays Incident photon Photoelectric absorption Atomic Number 3 Z 3 Eo Photoelectric cross section likelihood of p.e. absorption to occur Photon energy Photoelectric effect Photoelectric absorption process is most likely for Eo IK, L, M,... (resonance) Photoelectric absorption cross section decreases strongly with photon energy ( Ep-3) as photon energy increases relative to IK, L, M,... Photoelectric absorption cross section increases strongly with Z (~ Z3) because I Z Photoelectric absorption in K shell usually dominates Photoelectric effect Absorption edge 33.3keV is 6 times most likely to have photoelectric interaction than 33.1keV in iodine atom K edge 40 60 80 x103 Pair production & photodisintegration Require high energy photons >1MeV Interaction with nuclei Pair production photon is absorbed by nucleus the energy is converted into an electron and positron Electron (511keV) positron (511keV) Pair production threshold 1.02MeV Photodisintegration, photon absorbed by nucleus, nucleons are ejected by nucleus m/r [cm2/g] Z=6 Legend: : Photoelectric absorption s: Compton scatter p: Pair production sr: Raleigh scatter Z = 53 Z = 82 X-Rays generation White radiation, Bremsstrahlung (Brake) -Inelastic interaction with nuclei -Loss of kinetic energy -Xray (E) = lost kinetic E electron Coulombic interaction X-Ray -High kinetic energy -Forward radiation -Emission Z2 (Atomic number) # of protons White radiation, Bremsstrahlung -Smaller a produce larger X-ray -Broad range of wavelengths X-Ray a