"Resident Physics Lectures"
Resident Physics Lectures • Christensen, Chapter 4 Basic Interactions Between X-Rays and Matter, Grid Attenuation and George David Associate Professor Medical College of Georgia Filtration Department of Radiology Photons atoms interations • What happen when photons interact with human tissue? * Photon Phate • absorbed completely removed from beam X ceases to exist • scattered change in direction no useful information carried source of noise • Nothing Photon passes unmolested Image Noise Example Caution! Image Noise Basic Interactions • Coherent Scattering • Compton Scattering • Photoelectric Effect • Pair Production • Photodisintegration What is important in this lecture • How the interaction happen? • When happen? • The interaction affect on the image quality? Interaction depends on • Photon energy e.v • Atom atomic number Z - ~ + ~ + + ~ - - Photon Interaction Probabilities 100 Photoelectric Pair Production Z protons COMPTON 10 0.01 0.1 1.0 10 100 E energy (MeV) Basic Interactions • Coherent Scattering D • Compton Scattering D • Photoelectric Effect D • Pair Production T • Photodisintegration T • D=Diagnostic radiology • T=Treatment radiology Photoelec Compton Pair Coherent tric effect scattering productio scattering n Source Photon Photon Photon photon interacts interacts interacts interacts With inner With outer With with all electrons electrons nucleus electrons of an atom Photon low medium high energy D OR T D D T Coherent Scattering • Also called unmodified scattering classical scattering • Types Thomson » photon interacts with single electron Rayleigh » photon interacts with all electrons of an atom Coherent Scattering • Change in direction • No change in energy frequency wavelength • No ionization • Contributes to scatter as film fog • Less than 5% of interactions insignificant effect on image quality compared to other interactions *** Pair Production Process • high energy photon interacts with nucleus • photon disappears • electron & positron (positive electron) created • energy in excess of 1.02 MeV given to electron/positron pair as kinetic energy. - - ~ + ~ + + ~ + - - Positron Phate • Positron undergoes ANNIHILATION REACTION • Two 0.511 MeV photons created • Photons emerge in exactly opposite directions Pair Production • Threshold energy for occurrence: 1.02 MeV » energy equivalent of rest mass of 2 electrons • Threshold is above diagnostic energies does not occur in diagnostic radiology - - ~ + ~ + + ~ + - - * Photodisintegration • photon causes ejection of part of atomic nucleus • ejected particle may be neutron proton alpha - particle cluster ~ + ~ + + ~ ? - - Photodisintegration • Threshold photon energy for occurrence nuclear binding energy » typically 7-15 MeV • Threshold is above diagnostic energies does not occur in diagnostic radiology ** Photoelectric Effect • photon interacts with bound (inner-shell) electron • electron liberated from atom (ionization) • photon disappears Photon in Electron out - PHOTOELECTRIC EFFECT **** Photoelectric Effect • Exiting electron kinetic energy incident energy - electron’s binding energy • electrons in higher energy shells cascade down to fill energy void of inner shell » characteristic radiation M to L Photon in Electron out - L to K Photoelectric Interaction Probability • inversely proportional to cube of photon energy low energy event • proportional to cube of atomic number • more likely with inner (higher) shells tightly bound electrons 1 P.E. ~ ----------- P.E. ~ Z3 energy3 Photoelectric Effect • Interaction much more likely for low energy photons high atomic number elements 1 P.E. ~ ----------- P.E. ~ Z3 energy3 Photoelectric Effect • Photon Energy Threshold > binding energy of orbital electron • binding energy depends on atomic number » higher for increasing atomic number shell » lower for higher (outer) shells • most likely to occur when photon energy & electron binding energy are nearly the same Photoelectric Threshold • Binding Energies K: 100 Photon energy: 25 L: 50 M: 20 1 P.E. ~ ----------- A energy3 Which photon has a greater Photon in probability for photoelectric interactions with the m shell? B Photon energy: 22 Photoelectric Threshold 1 P.E. ~ ----------- energy3 • Photoelectric interactions decrease with increasing photon energy BUT … ** Photoelectric Threshold • When photon energies just reaches binding energy of next (inner) shell, photoelectric interaction now possible with that shell shell offers new candidate target electrons L-shell interactions possible Interaction Probability L-shell binding K-shell energy interactions possible K-shell binding energy Photon Energy Photoelectric Threshold • causes step increases in interaction probability as photon energy exceeds shell binding energies Interaction Probability L-edge K-edge Photon Energy ** Characteristic Radiation • Occurs any time inner shell electron removed • energy states orbital electrons seek lowest possible energy state » innermost shells M to L L to K ** Characteristic Radiation • electrons from higher states fall (cascade) until lowest shells are full characteristic x-rays released whenever electron falls to lower energy state M to L characteristic x-rays L to K Characteristic Radiation • only iodine & barium in diagnostic radiology have characteristic radiation which can reach film-screen Photoelectric Effect Why is this important? • photoelectric interactions provide subject contrast variation in x-ray absorption for various substances • photoelectric effect does not contribute to scatter • photoelectric interactions deposit most beam energy that ends up in tissue always use highest kVp technique consistent with imaging contrast requirements *** Compton Scattering • Source of virtually all scattered radiation • Process incident photon (relatively high energy) interacts with free (loosely bound) electron some energy transferred to recoil electron » electron liberated from atom (ionization) emerging photon has - » less energy than incident » new direction Electron out (recoil electron) Photon out Photon in Compton Scattering • What is a “free” electron? low binding energy » outer shells for high Z materials » all shells for low Z materials - Electron out (recoil electron) Photon in Photon out Compton Scattering • Incident photon energy split between electron & emerging photon • Fraction of energy carried by emerging photon depends on incident photon energy angle of deflection » similar principle to billiard ball collision - Electron out (recoil electron) Photon in Photon out Compton Scattering Probability of Occurrence • independent of atomic number (except for hydrogen) • Proportional to electron density (electrons/gram) fairly equal for all elements except hydrogen (~ double) Compton Scattering Probability of Occurrence • decreases with increasing photon energy decrease much less pronounced than for photoelectric effect Interaction Probability Compton Photoelectric Photon Energy Photon Interaction Probabilities 100 Photoelectric Pair Production Z protons COMPTON 10 0.01 0.1 1.0 10 100 E energy (MeV) Resident Physics Lectures • Christensen, Chapter 5 Attenuation George David Associate Professor Medical College of Georgia Department of Radiology Beam Characteristics • Quantity number of photons in beam 1, 2, 3, ... ~ ~ ~ ~ ~ Beam Characteristics • Quality energy distribution of photons in beam 1 @ 27 keV, 2 @ 32 Energy Spectrum keV, 2 at 39 keV, ... ~ ~ 10 20 30 40 50 Energy 60 70 80 ~ ~ ~ ~ ~ ~ Beam Characteristics • Intensity weighted product of number and energy of photons depends on 324 mR » quantity » quality ~ ~ ~ ~ ~ ~ ~ ~ Beam Intensity • Can be measured in terms of # of ions created in air by beam • Valid for monochromatic or for polychromatic beam 324 mR - ~ + Attenuation Coefficient • Parameter indicating fraction of radiation attenuated by a given absorber thickness • Attenuation Coefficient is function of absorber photon energy Linear Attenuation Coef. • Why called linear? distance expressed in linear dimension “x” • Formula N = No e -mx where N = number of incident photons o N = number of transmitted photons e = base of natural logarithm (2.718…) N N m = linear attenuation coefficient (1/cm); property of o energy material x = absorber thickness (cm) x Linear Attenuation Coef. Larger Coefficient = More Attenuation • Units: 1 / cm ( or 1 / distance) N = No e - m x • Properties reciprocal of absorber thickness that reduces beam intensity by e (~2.718…) » ~63% reduction » 37% of original intensity remaining as photon beam energy increases » penetration increases / attenuation decreases » attenuating distance increases » linear attenuation coefficient decreases • Note: Same equation as used for radioactive decay Polychromatic Radiation • X-Ray beam contains spectrum of photon energies highest energy = peak kilovoltage applied to tube mean energy 1/3 - 1/2 of peak » depends on filtration X-Ray Beam Attenuation • reduction in beam Lower Energy Higher Energy intensity by absorption (photoelectric) deflection (scattering) • Attenuation alters beam quantity quality » higher fraction of low energy photons removed » Beam Hardening Half Value Layer (HVL) N = No e -mx • absorber thickness that reduces beam intensity by exactly half • Units of thickness • value of “x” which makes N equal to No / 2 HVL = .693 / m Half Value Layer (HVL) • Indication of beam quality • Valid concept for all beam types Mono-energetic Poly-energetic • Higher HVL means more penetrating beam lower attenuation coefficient Factors Affecting Attenuation • Energy of radiation / beam quality higher energy » more penetration » less attenuation • Matter density atomic number electrons per gram higher density, atomic number, or electrons per gram increases attenuation Polychromatic Attenuation • Yields curved line on semi-log graph line straightens with increasing attenuation slope approaches that of monochromatic beam at the peak energy • mean energy increases with attenuation 1 beam hardening Fraction .1 Transmitted Polychromatic .01 Monochromatic .001 Attenuator Thickness Photoelectric vs. Compton • Fractional contribution of each determined by photon energy atomic number of absorber • Equation m = mcoherent + mPE + mCompton Small Photoelectric vs. Compton m = mcoherent + mPE + mCompton • As photon energy increases Both PE & Compton decrease Interaction Probability PE decreases faster » Fraction of m that is Compton increases Compton » Fraction of m that is PE decreases Photoelectric Photon Energy Photoelectric vs. Compton m = mcoherent + mPE + mCompton • As atomic # increases Fraction of m that is PE increases Fraction of m that is Compton decreases Interaction Probability Photoelectric Atomic Pair Number of Production Absorber Compton Photon Energy • PE dominates for very low energies Interaction Probability Photoelectric Atomic Pair Number of Production Absorber Compton Photon Energy • For lower atomic numbers – Compton dominates for high energies Interaction Probability Photoelectric Atomic Pair Number of Production Absorber Compton Photon Energy • For high atomic # absorbers – PE dominates throughout diagnostic energy range Attenuation & Density • Attenuation proportional to density difference in tissue densities accounts for much of optical density difference seen radiographs • # of Compton interactions depends on electrons / unit path which depends on » electrons per gram » density Relationships • Density generally increases with atomic # different states = different density » ice, water, steam • no relationship between density and electrons per gram • atomic # vs. electrons / gram hydrogen ~ 2X electrons / gram as most other substances as atomic # increases, electrons / gram decreases slightly Applications • As photon energy increases subject (and image) contrast decreases differential absorption decreases » at 20 keV bone’s linear attenuation coefficient 6 X water’s » at 100 keV bone’s linear attenuation coefficient 1.4 X water’s 100 90 80 70 60 50 Bone 40 Water 30 20 10 0 20 keV 100 ke Applications Photo- electric Pair Production Compton • At low x-ray energies attenuation differences between bone & soft tissue primarily caused by photoelectric effect » related to atomic number & density Applications Photo- electric Pair Production Compton • At high x-ray energies attenuation differences between bone & soft tissue primarily because of Compton scatter » related entirely to density Applications • Difference between water & fat only visible at low energies effective atomic # of water slightly higher » yields photoelectric difference electrons / cm almost equal » No Compton difference Photoelectric dominates at low energy Scatter Radiation • NO Socially Redeeming Qualities no useful information on image detracts from film quality exposes personnel, public • represents 50-90% of photons exiting patient Abdominal Photons • ~1% of incident photons on adult abdomen reach film • fate of the other 99% mostly scatter » most do not reach film absorption Scatter Factors • Factors affecting scatter field size thickness of body part kVp An increase in any of above increases scatter. Scatter & Field Size • Reducing field size causes significant reduction in scatter radiation II II Tube Tube X-Ray X-Ray Tube Tube Field Size & Scatter • Field Size & thickness determine volume of irradiated tissue • Scatter increase with increasing field size initially large increase in scatter with increasing field size saturation reached (at ~ 12 X 12 inch field) » further field size increase does not increase scatter reaching film » scatter shielded within patient Thickness & Scatter • Increasing patient thickness leads to increased scatter but • saturation point reached scatter photons produced far from film shielded within body kVp & Scatter • kVp has less effect on scatter than than field size thickness • Increasing kVp increases scatter more photons scatter in forward direction Scatter Management • Reduce scatter by minimizing field size » within limits of exam thickness » mammography compression kVp » but low kVp increases patient dose » in practice we maximize kVp Scatter Control Techniques: Grid • directional filter for photons • Increases patient dose Scatter Control Techniques: Air Gap • Gap intentionally left between patient & image receptor • Natural result of magnification radiography Patient • Grid not used Air • (covered in detail in Patient Gap chapter 8) Grid Film Cassette Resident Physics Lectures • Christensen, Chapter 8 Grids George David Associate Professor Department of Radiology Medical College of Georgia Purpose • Directional filter for Focal Spot photons “Good” • Ideal grid photon passes all primary photons » photons coming from focal spot Patient blocks all secondary photons “Bad” photon » photons not coming from XGrid focal spot Film Grid Construction • Lead ~ .05“ thick upright strips (foil) • Interspace material between lead strips maintains lead orientation materials » fiber » aluminum » wood Interspace Lead Grid Ratio • Ratio of interspace height to width Lead Interspace h w Grid ratio = h / w Grid Ratio • Expressed as X:1 • Typical values 8:1 to 12:1 for general work 3:1 to 5:1 for mammography • Grid function generally improves with higher ratios h w Grid ratio = h / w Lines per Inch • # lead strips per inch grid width • Typical: 103 25.4 W Lines per inch = ------------ W+w w = thickness of interspace (mm) W = thickness of lead strips (mm) w Grid Structure Grid Patterns • Orientation of lead strips as seen from above • Types Linear Cross hatched » 2 stacked linear grids » ratio is sum of ratios of two linear grids » very sensitive to positioning & tilting » Rare; only found in specials Grid Styles • Parallel • Focused Parallel Grid • lead strips parallel • useful only for small field sizes large source to image distances Focused Grid • Slightly angled lead strips • Strip lines converge to a point in space called convergence line • Focal distance Focal range distance from convergence line to grid plane • Focal range Focal working distance range distance » width depends on grid ratio » smaller ratio has greater range Grid Cassette • Grid built into cassette front • Sometimes used for portables formerly used in mammography • low grid ratios • focused Ideal Grid • passes all primary radiation Reality: lead strips block some primary Interspace Lead Ideal Grid • block all scattered radiation Reality: lead strips permit some scatter to get through to film Interspace Lead Grid Performance Measurements • Primary Transmission (Tp) • Bucky Factor (B) • contrast improvement factor (K) Resident Physics Lectures • Christensen, Chapter 6 Filters George David Associate Professor Department of Radiology Medical College of Georgia Energy Spectrum • X-ray beams from tubes Polychromatic » Brehmstrahlung » Characteristic spectrum of energies from 0 – kVp set on generator • average beam energy 1/3 to 1/2 of peak (kVp) kVp (as set on generator) Unfiltered Beams • most energy deposited in first few centimeters of tissue lowest energy photons selectively removed • energy of low energy photons contributes to dose does not contribute to image Patient » photons don’t reach film film Ideal Filtration • absorption characteristics absorbs all low energy radiation absorbs no high energy radiation • high atomic number desirable increases photoelectric absorption of low energy photons Filter’s Function • shape beam’s energy spectrum Filter • selectively attenuate low energy photons less low energy radiation incident on patient energy deposited in filter, not in patient Film Filtration Locations • x-ray tube and housing inherent filtration • metal sheets placed in beam path placed between tube and collimator or in collimator Filter Usually aluminum added filtration • collimator mirror* • table (for under-table tube fluoro) Lamp * not mentioned in book X-Rays Light Tabletop Tabletop