X-Ray Interactions With Matter

Explain Bremsstrahlung radiation (review from target interactions)

• incoming electron is slowed by force field of the nucleus

• electron continues in a different direction with reduced energy

• loss of kinetic energy given off as an x-ray

  • a wide variety of x-ray energies are emitted

  • beam is said to be polyenergetic/heterogenous

Explain characteristic radiation (review from target interactions)

• incoming electron ejects electron from k-shell (of tungsten)

• outer shell electron drops into the vacancy

• x-ray energy emitted is equal to the difference in binding energy of the electrons involved

• a cascade is created as electrons from outer shells drop into the vacancies in the inner shells

  • specific x-ray energy can be calculated because the binding energy of the tungsten shells is known

Short wavelength photons have ___ energy and ___ penetrability

high energy and high penetrability

Long wavelength photons have ___ energy and ___ penetrability

low energy and low penetrability

Define attenuation

the reduction in the number of x-ray photons in the beam, and subsequent loss of energy as the beam passes through matter

X-ray photons interact with matter and lose energy in 4 different ways:

1. absorbed

2. attenuated

3. scattered

4. transmitted

___ beam creates the image

remnant (exit)

What is a latent image?

an image that is stored until you do something with it (needs to be processed)

stored in the IR

All interactions between x-rays and matter take place at the ___ level

atomic

Low energy x-rays interact at the level of ___

Moderate energy x-rays interact at the level of ___

High energy x-rays interact at the level of ___

low: whole atom

moderate: orbital electrons

high: nuclei

Interactions that produce x-rays in the diagnostic range occur in the ___ energy range and involve___

occur in the moderate energy range and involve orbital electrons

Electron binding energy is equal to ___

the force with which the electron is held in the shell

Protons have a ___ charge

Neutrons have a ___ charge

Electrons have a ___ charge

P: +

N: 0

E: -

The closer the electron is to the nucleus, the ___ (tighter/looser) it is bound in its orbital shell

tighter

The farther away from the nucleus that the electron is, the __ (greater/lesser) the total energy of the electron

greater

List the 5 basic x-ray interactions with matter

1. photoelectric absorption

2. coherent scattering

3. Compton scattering

4. pair production

5. photodisintegration

Which 2 interactions are part of the remnant beam?

photoelectric absorption and Compton

Photoelectric (PE) absorption/effect occurs when ___

energy of the incident photon is slightly greater than binding energy of k shell (which varies with different tissues of the body)

In PE absorption/effect, what happens with the incident photon?

it ejects an inner shell electron that becomes a photoelectron

the incident photon is then completely absorbed by the patient (delivers patient dose)

PE effect occurs at which orbital shell?

K-shell

How does PE effect end up creating characteristic radiation?

when K-shell electron is ejected, vacancy is filled by L or M shell electron (this energy gives off secondary radiation equal to the difference in binding energy of the two shells)

The probability of photoelectric interaction ___ as the energy of the incident photon increases

decreases

The probability of photoelectric interaction ___ as the atomic number of absorber/matter increases

increases

For a photoelectric interaction, the energy of the incident electron must be greater than ___

the binding energy of the inner shell electron

A photoelectric interaction is more likely to occur with electrons that have ___ binding energies (atomic #)

high

What type of interaction is primarily responsible for the subject contrast in a latent image?

photoelectric

Coherent scatter occurs with energies ___

<10 keV

In coherent scatter, a ___ photon interacts with an electron

low energy

Thompson coherent scatter causes temporary excitation on ___

a single electron

Rayleigh coherent scatter causes temporary excitation on ___

the entire atom

The secondary photon from coherent scatter travels in ___ direction with ___ energy as incident photon

different direction; same energy

Is coherent scatter significant for Dx? Explain

no; doesn’t affect the image

In Compton scatter, what happens with the incident photon?

it ejects an outer-shell electron, then continues in a different direction with less energy as scatter

In Compton scatter, what is the ejected outer shell electron called?

Compton electron or recoil electron

What direction are Compton scattered electrons ejected in?

any direction (0-180^o)

The more forward the direction of the Compton scattered electron, the ___ energy it retains

more

Compton scattered electrons that travel in a forward direction are more likely to ___

reach the IR (have a serious impact on image quality)

What is the keV diagnostic range for compton scatter?

50-140 keV

Compton scatter has a ___ impact on contrast. What does it cause to our scale of contrast?

negative and reduces contrast and causes a longer scale of contrast

Do Compton interactions add any useful diagnostic information?

no

What is backscatter radiation?

scatter that is deflected back toward the source (only a small number of photons backscatter)

What is the primary source of occupational exposure?

backscatter

What energies does pair production occur at?

>1.02 MeV

Pair production occurs when a high energy photon ___

interacts with the nucleus of an atom

What type of imaging uses pair production?

PET

List and explain the electrons created in pair production

• negatron

  • is absorbed

• positron

  • extremely volatile

  • combines with negative electron

  • releases 2 positrons moving in opposite directions

Pair production creates ___ reactions

annihilation

What energies does photodisintegration occur at?

>10 MeV

Does photodisintegration occur during x-ray imaging?

no

What happens in photodisintegration?

high energy photon excites the nucleus, and then the nucleus emits a fragment

At 50 kVp, what is the ratio of photoelectric and Compton scatter?

At 120 kVp, what is the ratio of photoelectric and Compton scatter?

50 kVp: 50/50

120 kVp: 25% photoelectric 75% Compton

An increase in energy has a(n) ___ interaction within the patient

decreased

Photoelectric absorption occurs with ___ energies

low

Compton scatter occurs with ___ energies

high

As kV increases, the total number of transmitted photons ___, and contrast ___

transmitted photons increases

contrast decreases

PE interactions ___ contrast, while Compton interactions ___ contrast

PE: create contrast

Compton: degrade contrast

Absorption occurs with what 3 things?

1. low kVp

2. high atomic number

3. contrast medium

High kVp produces a ___ percentage of scatter

high

What type of interaction is shown?

Bremsstrahlung

What type of interaction is shown?

characteristic

What type of interaction is shown?

photoelectric absorption

What type of interaction is shown?

coherent scatter

What type of interaction is shown?

Compton scatter

What type of interaction is shown?

pair production

What type of interaction is shown?

photodisintegration