Resident Physics Lectures Christensen, Chapter 4 Basic Interactions Between X-Rays and Matter George David Associate Professor Medical College of Georgia.

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?


K: 100L: 50M: 20

Photon in

Photon energy: 15


NO

NO

NO


K: 100L: 50M: 20

Photon in

Photon energy: 25



K: 100L: 50M: 20

Photon in

Photon energy: 25


NO

NO

YES


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


K: 100L: 50M: 20

Photon in

Photon energy: 55



K: 100L: 50M: 20

Photon in

Photon energy: 55


NO

YES

YES


K: 100L: 50M: 20

Photon in

Photon energy: 105



K: 100L: 50M: 20Photon energy: 105


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


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

**


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

**


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


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



-

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)


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


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


Compton

Photoelectric

Photon Interaction Probabilities

Photoelectric Pair Production

COMPTON

Z protons

10

100

E energy (MeV)

0.01 0.1 1.0 10 100

Resident Physics Lectures Christensen, Chapter 4 Basic Interactions Between X-Rays and Matter George David Associate Professor Medical College of Georgia.

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