resident physics lectures christensen, chapter 4 basic interactions between x-rays and matter george...
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Resident Physics Lectures
Christensen, Chapter 4
Basic Interactions Basic Interactions Between X-Rays Between X-Rays and Matterand Matter
George DavidAssociate ProfessorMedical College of GeorgiaDepartment of Radiology
Basic InteractionsCoherent ScatteringPair ProductionPhotodisintegrationPhotoelectric EffectPhotoelectric EffectCompton ScatteringCompton Scattering
Photon Phateabsorbed
completely removed from beamceases to exist
scatteredchange in directionno useful information carriedsource of noise
NothingPhoton passes unmolested
X
*
– covers valid information with distracting or obscuring garbage
Caution!ImageNoise
– covers valid information with distracting or obscuring garbage
Caution!ImageNoise
Coherent ScatteringAlso called
unmodified scatteringclassical scattering
TypesThomson
photon interacts with single electronRayleigh
photon interacts with all electrons of an atom
Coherent ScatteringChange in directionNo change in
energy frequencywavelength
No ionizationContributes to scatter as film
fogLess than 5% of interactions
insignificant effect on image quality compared to other interactions
Pair Production Processhigh energy photon interacts with nucleusphoton disappearselectronelectron & positronpositron (positive electron)
createdenergy in excess of 1.02 MeV given to
electron/positron pair askinetic energy.
-
-
-
++
~~
+~-
+
***
Positron PhatePositron undergoes ANNIHILATION
REACTIONTwo 0.511 MeV photons createdPhotons emerge in exactly opposite
directions
Pair ProductionThreshold 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
-
-
-
++
~~
+~-
+
Photodisintegrationphoton causes ejection of part of
atomic nucleusejected particle may be
neutronprotonalphaparticle cluster
-
-
-
++
~~
+~
?
*
PhotodisintegrationThreshold photon energy for
occurrencenuclear binding energy
typically 7-15 MeV
Threshold is above diagnostic energiesdoes not occur in diagnostic radiology
Photoelectric Effectphoton interacts with bound (inner-
shell) electronelectron liberated from atom
(ionization)photon disappears
Electron outPhoton in -
**
PHOTOELECTRIC EFFECT
Photoelectric EffectExiting 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
Electron outPhoton in
M to L
L to K-
****
Photoelectric Interaction Probability
inversely proportional to cube of photon energylow energy event
proportional to cube of atomic numbermore likely with inner (higher) shells
tightly bound electrons
1P.E. ~ ----------- energy3
P.E. ~ Z3
Photoelectric EffectInteraction much more likely for
low energy photonshigh atomic number elements
1P.E. ~ ----------- energy3
P.E. ~ Z3
Photoelectric Effect
Photon Energy Threshold> binding energy of orbital electron
binding energy depends onatomic number
higher for increasing atomic numbershell
lower for higher (outer) shells
most likely to occur when photon energy & electron binding energy are nearly the same
Photoelectric ThresholdBinding Energies
K: 100L: 50M: 20
Photon in
Photon energy: 15
Which shells are candidates for photoelectric interactions?
Photoelectric ThresholdBinding Energies
K: 100L: 50M: 20
Photon in
Photon energy: 15
Which shells are candidates for photoelectric interactions?
NO
NO
NO
Photoelectric ThresholdBinding Energies
K: 100L: 50M: 20
Photon in
Photon energy: 25
Which shells are candidates for photoelectric interactions?
Photoelectric ThresholdBinding Energies
K: 100L: 50M: 20
Photon in
Photon energy: 25
Which shells are candidates for photoelectric interactions?
NO
NO
YES
Photoelectric ThresholdBinding Energies
K: 100L: 50M: 20
Photon in
Photon energy: 22
Which photon has a greater probability for photoelectric interactions with the m shell?
Photon energy: 25
A
B
1P.E. ~ ----------- energy3
Photoelectric ThresholdBinding Energies
K: 100L: 50M: 20
Photon in
Photon energy: 55
Which shells are candidates for photoelectric interactions?
Photoelectric ThresholdBinding Energies
K: 100L: 50M: 20
Photon in
Photon energy: 55
Which shells are candidates for photoelectric interactions?
NO
YES
YES
Photoelectric ThresholdBinding Energies
K: 100L: 50M: 20
Photon in
Photon energy: 105
Which shells are candidates for photoelectric interactions?
Photoelectric ThresholdBinding Energies
K: 100L: 50M: 20Photon energy: 105
Which shells are candidates for photoelectric interactions?
YES
YES
YES
Photoelectric Threshold
• Photoelectric interactions decrease with increasing photon energy
BUT …
1P.E. ~ ----------- energy3
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
Photon Energy
InteractionProbability
K-shell interactions
possible
L-shell interactions
possible
L-shell binding energy
**
K-shell binding energy
Photoelectric Threshold• causes step increases in interaction
probability as photon energy exceeds shell binding energies
Photon Energy
InteractionProbability
L-edge
K-edge
Characteristic Radiation
Occurs any time inner shell electron removed
energy statesorbital 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 fullcharacteristic x-rays released whenever
electron falls to lower energy state
M to L
L to K
characteristicx-rays
**
Characteristic Radiation
only iodine & barium in diagnostic radiology have characteristic radiation which can reach image receptor
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 scatterscatter
photoelectric interactions deposit most beam energy that ends up in tissue alwaysalways use highest kVp technique consistent with
imaging contrast requirements
Compton ScatteringSource of virtually all scattered
radiationProcess
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 inPhoton out
-
***
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 ScatteringIncident photon energy split between
electron & emerging photonFraction of energy carried by emerging
photon depends onincident photon energyangle of deflection
similar principle to billiard ball collision
Electron out(recoil electron)
Photon in Photon out
-
Compton Scattering & Angle of Deflectionhigher incident energy = less photon
deflectionhigh energy (1MeV) photons primarily scatter forwarddiagnostic energy photons scatter fairly uniformly
forward & backwardat diagnostic energy photons lose very little energy during
Compton Scattering
higher deflection = less energy retainedphotons having small deflections retain
most incident incident energy Electron out(recoil electron)
Photon in Photon out
deflectionangle
-
Compton Scattering & Angle of Deflection
Photons having small deflections retain most incident incident energy
Photons will scatter many times, losing a little energy each time.
--
-
-
Compton ScatteringFormula
= 0.024 (1-cos )
where= change in wavelength (A) for
photon = angle of photon deflection (0-180
degrees) recoil electron
Photon in Photon out
Angle
-
0o results in no change in wavelength
180o results in maximum change in wavelength
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 energydecrease much less pronounced than for
photoelectric effect
Photon Energy
InteractionProbability
Compton
Photoelectric
Photon Interaction Probabilities
Photoelectric Pair Production
COMPTON
Z protons
10
100
E energy (MeV)
0.01 0.1 1.0 10 100