Attenuation As x-rays pays through matter, the exit beam will contain less photons than the entrance...

AttenuationAttenuation

As x-rays pays through matter, the exit beam will contain less photons than the entrance beam. This reduction in the quantity of photons is termed ATTENUATION

Dependent upon:Dependent upon:

•Energy of the radiation

•Atomic number of the absorber

•Thickness of the absorber

•Density of the absorber

X-ray Interactions X-ray Interactions with matter IIwith matter II

Compton EffectCompton EffectAlso known as:Also known as:

•Modified scattering

or

•Incoherent scattering

Compton EffectCompton Effect

•Occurs within the patient

•Predominates at higher x-ray energies

•Ionizing interaction between an x-ray photon and a loosely bound outer shell electron

•Incident photon is diverted from its original path after ejecting an outer shell electron of the target atom.

Compton EffectCompton Effect•The ejected electron may be refered to as: Compton electron

Secondary electronRecoil electron

•The incident x-ray photon continues in a different direction as a scattered photon with less energy yet, the scattered photon will usually retain most of its energy.

•Most of the energy is diverted between the scattered photon and the ejected Compton electron.

•The scattered photon and Compton electron have enough energy to continue ionizing interactions within the patient

Compton EffectCompton EffectImportance to Radiography Is In A Importance to Radiography Is In A Negative SenseNegative Sense

• Scattered x-rays produce no useful information in radiography

• The scattered photon has enough energy to reach the image receptor and reduce radiographic contrast by fogging the image.

• Creates a serious exposure hazard especially in fluoroscopy

• Source of most of the occupational radiation exposure that radiologic technologists receive.

!

Compton EffectCompton EffectImportance to Radiography Is In A Importance to Radiography Is In A Negative SenseNegative Sense

• The incident photon may be deflected from its original path anywhere from 0 to 180 degrees

• At a 0 degree deflection angle, no energy is transferred because the incident photon is still traveling in its original direction.

• At a 180 degree deflection angle, more energy is transferred to the recoil electron and less remains with the scattered photon.

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Compton EffectCompton EffectImportance to Radiography Is In A Importance to Radiography Is In A Negative SenseNegative SenseBackscatter Radiation:

• This occurs when the incident photon is deflected back towards its source, it is traveling in a direction opposite to the original incident photon. This happens when the deflection angle approaches 180 degrees.

Note: Most photons will scatter in a more forward, especially when incident photon energy increases. (Higher kVp)

!

Compton ScatteringCompton Scattering

ProbabilityProbabilityThe probability of the occurrence of Compton scattering relative to the photoelectric effect increases as the energy of the x-ray photon increases.

This means that:Relative to the photoelectric effect, there will be more Compton scattering at 90 kVp than at 60 kVp.

Compton EffectCompton EffectKey Points to RememberKey Points to Remember

Most Likely To OccurWith loosely bound outer-shell electrons

As x-ray energy increasesIncreased Compton relative to the photoelectric effect scattering

As atomic number of absorber increases

No effect on Compton scattering

As mass density of absorber increases

Proportional increase in Compton scattering

Summary

Compton EffectCompton EffectMathematical ApplicationMathematical Application

E E i i = E = E s s + E+ E b b + E+ E KE KE

E i = Energy of the incident photon

E s = Energy of the Compton scattered photon

E b = Electron binding energy of the Compton

electron

E KE = Kinetic energy of the ejected Compton

electron

Compton EffectCompton EffectMathematical ApplicationMathematical Application

E i = Energy of the incident photon

E s = Energy of the Compton scattered photon

E b = Electron binding energy of the Compton

electron

E KE = Kinetic energy of the ejected Compton

electron

Compton EffectCompton EffectMathematical ApplicationMathematical Application

E E i i = E = E s s + E+ E b b + E+ E KE KE

E i = Energy of the incident photon

E s = Energy of the Compton scattered photon

E b = Electron binding energy of the Compton

electron

E KE = Kinetic energy of the ejected Compton

electron

-

- K

L

-

-Incident X-ray Photon with 30 KeV

Barium Atom

ShellBinding Energy

In keV

K 37

L 6

M 1.3

N 0.3

O 0.04

P

-N

O

M

Compton Scattered Photon

Compton Electron

-

- K

L

-

-Incident X-ray Photon with 60 KeV

Barium Atom

ShellBinding Energy

In keV

K 37

L 6

M 1.3

N 0.3

O 0.04

P

Secondary X-ray Photon created with 36.7 KeV

Ex = K b - N b

Ex = 37 keV - 0.3 keV

Ex = 36.7 keV

E x = energy of secondary

photon

K b = K shell binding energy

N b = N shell binding energy

Photoelectron with a kinetic energy of 23 KeV

EKE = K i - K b

EKE = 60 keV - 37keV

EKE = 23 keV

E KE = kinetic energy ofphotoelectron

K i = kinetic energy of incident

photon

K b = binding energy of k shell

-

Photoelectric EffectPhotoelectric Effect

• Important to diagnostic radiography including mammography (23 to 150 kVp)

• Most important mode of interaction between x-ray photons and the atoms of the patient

• In the human body, this energy transfer results in increased patient dose and contributes to biologic damage of tissues

• Responsible for contrast on a radiographic image(No photoelectric effect = no radiographic contrast)

Photoelectric EffectPhotoelectric Effect

• Occurs within the patient

• Predominates at lower x-ray energies

• Ionizing interaction with an inner shell electron

• Low energy incident x-ray photon is completely absorbed by an inner shell electron

• The electron shell absorbing the incident x-ray photon’s energy ejects its electron (photoelectron) leaving a “hole” where the former electron was.

Photoelectric EffectPhotoelectric Effect

• An electron from an outer shell moves in to fill the vacancy

• As an electron cascades to fill the vacancy, a secondary photon is created (characteristic photon)

• Energy of the secondary/characteristic photon created is the difference between the binding energies of the orbital shells involved in the cascade effect

• More likely to occur in absorbers of high atomic number (e.g., bone, positive contrast media)

Photoelectric EffectPhotoelectric Effect

• In human tissue, the energy of the secondary/characteristic photon created is very low and will be absorbed locally within the irradiated object.

• More likely to occur in absorbers of high atomic number (e.g., bone, positive contrast media)

Photoelectric EffectPhotoelectric Effect

Important Fact:Important Fact:A photoelectric interaction cannot occur unless the incident x-ray photon has an energy equal to or greater than the electron-binding energy of the electron which it interacts with.

A barium k shell electron with a binding energy of 37 keV cannot be removed by a 36 keV incident x-ray photon.

Photoelectric EffectPhotoelectric EffectMathematical ApplicationMathematical Application

E E i i = E = E b b + E+ E KE KE

E i = Energy of the incident photon

E b = Electron binding energy

E KE = Kinetic energy of the ejected electron

-

- K

L

-

-

Incident X-ray Photon with 60 KeV

Barium Atom

ShellBinding Energy

In keV

K 37

L 6

M 1.3

N 0.3

O 0.04

P

Secondary X-ray Photon created with 36.7 KeV

Ex = K b - N b

Ex = 37 keV - 0.3 keV

Ex = 36.7 keV

E x = energy of secondary

photon

K b = K shell binding energy

N b = N shell binding energy

Photoelectron with a kinetic energy of 23 KeV

EKE = K i - K b

EKE = 60 keV - 37keV

EKE = 23 keV

E KE = kinetic energy ofphotoelectron

K i = kinetic energy of incident

photon

K b = binding energy of k shell

-

- K

L

-

-

Incident X-ray Photon with 60 KeV

Barium Atom

ShellBinding Energy

In keV

K 37

L 6

M 1.3

N 0.3

O 0.04

P

Secondary X-ray Photon created with 36.7 KeV

Ex = K b - N b

Ex = 37 keV - 0.3 keV

Ex = 36.7 keV

E x = energy of secondary

photon

K b = K shell binding energy

N b = N shell binding energy

Photoelectron with a kinetic energy of 23 KeV

EKE = K i - K b

EKE = 60 keV - 37keV

EKE = 23 keV

E KE = kinetic energy ofphotoelectron

K i = kinetic energy of incident

photon

K b = binding energy of k shell

Photoelectric EffectPhotoelectric Effect

ProbabilityProbabilityThe probability is a function of:

•The x-ray energy

•The atomic number of the atom the photon interacts with.

Photoelectric EffectPhotoelectric EffectProbability:The probability of the photoelectric effect is inversely proportional to the third power of the x-ray energy.

This means that:As photon energy increases, the probability of a photoelectric interaction markedly decreases

Or conversely stated:

As photon energy decreases, the probability of photoelectric interaction markedly increases

31

E

Photoelectric EffectPhotoelectric EffectProbability:The probability of the photoelectric effect is directly proportional to the third power of the atomic number of the absorbing material.

This means that:There will be more photoelectric interactions in lead than in aluminum. More in bone than in soft tissue.

3Z

-

- K

L

-

-

Incident X-ray Photon with 60 KeV

Barium Atom

ShellBinding Energy

In keV

K 37

L 6

M 1.3

N 0.3

O 0.04

P

Secondary X-ray photon created with 36.7 KeV (characteristic)

Ex = K b - N b

Ex = 37 keV - 0.3 keV

Ex = 36.7 keV

E x = energy of secondary photon

K b = K shell binding energy

N b = N shell binding energy

Photoelectron with a kinetic energy of 23 KeV is produced

EKE = K i - K b

EKE = 60 keV - 37keV

EKE = 23 keV

E KE = kinetic energy of photoelectron

K i = kinetic energy of incident photon

K b = binding energy of k shell

M

N

-

- K

L

-

-

Incident X-ray Photon with 60 KeV

Barium Atom

ShellBinding Energy

In keV

K 37

L 6

M 1.3

N 0.3

O 0.04

P

Secondary X-ray Photon created with 36.7 KeV

Ex = K b - N b

Ex = 37 keV - 0.3 keV

Ex = 36.7 keV

E x = energy of secondary

photon

K b = K shell binding energy

N b = N shell binding energy

Photoelectron with a kinetic energy of 23 KeV

EKE = K i - K b

EKE = 60 keV - 37keV

EKE = 23 keV

E KE = kinetic energy ofphotoelectron

K i = kinetic energy of incident

photon

K b = binding energy of k shell

M

N

Pair ProductionPair Production

• Occurs within the patient

• Requires minimum incident photon energy of 1.02 MeV

• Incident photon comes close to the strong nuclear field and loses all of its energy in the interaction

• Energy of incident photon is converted into a pair of electrons:

- one negatron- one positron (antimatter)

Pair ProductionPair Production

• Negatron is quickly absorbed by another atom

• Positron (antimatter) is attracted to a negative electron and both undergo an annihilation reaction

• Annihilation of the positron and electron results in the production of two photons each possessing an energy of 0.51 MeV.

• This interaction does not occur in diagnostic radiography because of the high energy required to create the positron and negatron!

-+

-

- K

L

-

-

Incident X-ray Photon

with 1.02 MeV

-

Photon with 0.51 MeV

Photon with 0.51 MeV

Annihilation ReactionAnnihilation Reaction

Pair ProductionPair Production