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Physics Applied to Radiology RADI R250 -- Fall 2003

Physics Applied to Radiology RADI R250 -- Fall 2003. Chapter 13. Particle vs. Photon Interactions. particles ( a, b , etc.) interactions based on electric charge on particle mass of particle photons ( g, c , etc) interactions based on energy of photon type of matter. Probability.

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Physics Applied to Radiology RADI R250 -- Fall 2003

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  1. Physics Applied to RadiologyRADI R250 -- Fall 2003 Chapter 13

  2. Particle vs. Photon Interactions • particles (a, b, etc.) • interactions based on • electric charge on particle • mass of particle • photons (g, c, etc) • interactions based on • energy of photon • type of matter

  3. Probability • “chance” of event happening • can be mathematically expressed • example: • The probability of a woman experiencing breast cancer in her lifetime is 1:9 • x-ray interactions are chance events • relative predictions can be made • energy of the photons • type of matter the x rays are passing through • cannot predict how one photon will interact

  4. X-ray Interaction Probability • probability depends on energy (i.e. l ) of photon depends on type of matter • energy keV l*interacts with • low <10 long l* whole atom • inter.10-1000 middle l* electron cloud • high >1MeV short l nucleus • *lof Dx x rays »10-8 to 10-9m Higher energy (short l) photons more likely to make it into the atom before interacting.

  5. Types of Photon Interactions • Transmitted through matter unchanged • Change direction with no energy loss • Classical Scattering • Change direction and lose energy • Compton Scattering • Deposit all energy in the matter • Photoelectric Effect • Pair Production • Photodisintegration

  6. Classical Scattering Also called coherent or elastic • low photon energy (< 10 keV)enters atom • atom excited by photon • releases (radiates) photon of same keV & l • new photon travels in different direction from original photon but usually forward (small scatter angle)

  7. Classical Scattering (cont.) • subclassifications of classical scattering • Rayleigh Scattering • if interaction occurs with whole atom • Thompson Scattering • if interaction occurs with shell e- • Results 1. photon D direction with no E loss Ei = E¢li=l¢ 2. does not ionize matter 3. may cause slight film fogging (forward direction)

  8. Classical Scattering (cont.) • Probability of classical scattering • increases with: 1) low Z materials • soft tissue more likely than bone 2) lower photon energies • 5 keV more likely than 10 keV

  9. Photoelectric Effect (Total absorption) intermediate photon energy (Dx range) • photon ejects an inner shell e- • photoelectron emitted with some KE • photon loses all energy Ei = EBE + EKE • characteristic radiation emitted when hole filled • “cascade of photons” until atom is neutral again

  10. PE Energy example An 85 keV photon interacts with a K-shell electron whose BE is 62 keV. What will the ejected electron’s KE? Ei = EBE+ EKE EKE = Ei- EBE = 85 keV- 62 keV=23 keV If an L-shell electron whose BE is 940 eV fills the K-shell hole, what would be the characteristic x-ray energy? Exray= D E = -.94 keV - (-62 keV) = 61.06 keV

  11. Photoelectric Effect (cont.) • Results 1. photon disappears 2. atom is ionized • ion pair = atom+ + photoelectron 3. characteristic photon(s) emitted • secondary radiation • biologic tissue produces very low Exray • contrast agents (Ba & I) produce higher Exray that may result in film fog • cascade effect

  12. L-shell K-shell r photon E (keV) Photoelectric Effect (cont.) • Probability (r) • ¯ sharplyas photon E ­ • “edge” at BE with ­ r • r µ 1/E3 • ­ with ­Z (table 8-2, pg 201) • ­ Z = ­ ofinner shell e- • ­ Z = ­ BE • r µ Z3

  13. Compton Scattering (partial absorption) • intermediate photon energy (Dx range) • photon ejects outer shell e- • acts like particle colliding with e- • e- (Compton or recoil e-) ejected from atom with some KE • photon loses energy E¢ = Ei- [EBE + EKE]

  14. Compton ScatteringEnergy Example A 48 keV photon interacts with an outer shell electron bound by 72 eV. If the electron is emitted with a KE of 4.2 keV, what is the scattered photon’s energy? E¢ = Ei- [EBE + EKE] =48keV-[.072keV + 4.2keV] =43.728 = 44 keV

  15. Compton Scattering (cont.) 0°Ð 180°Ð • as Ð ­, energy loss ­ • scattered photon has ¯ E E90°Ð < E 30°Ð • lowest E come from 180°Ð (backscatter) angle of scatter (pg. 204) scatter E L 30°Ð 90°Ð scatter E H scatter E M • high E photons scatter with smaller Ð • high E photons more likely to forward scatter • low E photons more likely to backscatter photon E H photon E L small Ð large Ð

  16. Compton Scattering (cont.) • Results 1. photonD direction with ¯ E in DX range direction is usually forward E > E¢l<l¢ 2. atom is ionized ion pair = atom+ + recoil e- 3. source of: personnel / patient exposure film fog (¯contrast)

  17. Compton Scattering (cont.) • probability of Compton interactions • ­ with ­ density of matter (NOT Z) • ­ # of e- per unit volume • ­ in matter containing abundant hydrogen • H contains 2x # of e-/gm compared to other matter • ­ with ¯ photon E • mathematical probability P µ e- density / photon energy

  18. Compton & PE vs. kVp • Beam consisting of 10,000 photons kVp transmitted PE Compton 50 3000 4000 3000 100 7000 1000 2000 • As energy increases both PE & Compton decrease • PE decreases more rapidly than Compton • PE decreased to 25% while Compton only 67%

  19. Pair Production(Total absorption) e+ high photon energy Ei > 1.022 MeV (e- mass = .511 MeV) • photon interacts with nuclear forcefield • uses 1.022 MeV to produce pair of electron like particles e+ (positron) & e- (negatron) • photon ceases to exist E = 1.022 MeV + Ee+KE + Ee-KE e-

  20. Pair Production (cont.) • annihilation radiation e+ • e+ can’t exist without KE • e+ + e- combine & are destroyed • matter converted back to energy • 2 photons of .511 MeV emitted .511 MeV .511 MeV e-

  21. Pair Production (cont.) • Results • photon disappears • electron & positron created • e- after loss of KE = free e- • e+after loss of KE cannot exist • annihilation radiation produced • e+ + e- = 2 (.511 MeV) photons

  22. Pair Production (cont.) • probability • energy must be ³ 1.022 MeV • ­ with ­ Z (larger nuclear force field)

  23. Photodisintegration (Total Absorption) • high photon energy Ei> 10 MeV • photon absorbed by the nucleus • nucleus excited • ejects particles and photons to return to ground state

  24. Photodisintegration (cont.) • results • photon disappears • nucleus changes form • Becomes a different nuclide

  25. Attenuation & Transmission • Photon Attenuation(figure 13-15, page 175) • removal of photons • all interactions total or partial absorbtion • Photon Transmission • incident photons that do not interact total beam = attenuated# + transmitted # 10,000 xrays=4350 absorbed+5650 transmitted

  26. Differential Absorption (DA) • attenuation & transmission in tissue that results in the image formation • attenuation that enables image production • image due to relative PE vs. transmission • Compton interactions have NO image value • important when subject contrast is low • tissues that are similar in 1) density 2) atomic # (Z)

  27. Factors that Influence DA 1) Beam Energy: DA é as E ê • due to é probability of PE at low E • but also é in pt dose (PE = total absorption) 2) Atomic Number: DA é as Z é • probability of PE é with éZ • Z has no effect on Compton see Bushong table 13-6, pg 174

  28. Substance Z fat 6.3 muscle 7.4 lung 7.4 air 7.6 bone 13.8 iodine 53 barium 56 aluminum 13 concrete 17 lead 82 similar DA based on similar Z ???? good DA compared to above contrast media with excellent DA even in low concentration non biological materials DA vs. Atomic Number

  29. Factors that Influence DA (cont.) 3) Mass Density of matter DA é as density é • quantity of matter per unit volume • do not confuse with image density • é matter / volume = é interactions • both PE & Compton • tissue densities vary more widely than Z see Bushong table 13-5, pg 173

  30. DA vs. Z & Tissue Density Substance Z Density fat 6.3 0.91 muscle 7.4 1.00 lung 7.4 0.32 air 7.6 0.0013 bone 13.8 1.85 iodine 53 4.92 barium 56 3.5 aluminum 13 2.7 concrete 17 2.35 lead 82 11.35

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