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Radiology Physics > Ch. 13 X-ray Interactions In Matter > Flashcards

Ch. 13 X-ray Interactions In Matter Flashcards

1
Q

What is an incident x-ray photon?

A

A photon from the tube; primary radiation entering an object.

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2
Q

What does attenuation mean in the context of x-ray interactions?

A

A reduction in the number of x-ray photons as the beam passes through matter.

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3
Q

What can happen when an incident photon passes into matter?

A

The photon can be absorbed, pass right through, interact and scatter, or interact with the nucleus.

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4
Q

What happens to an absorbed photon in tissue?

A

It shows as white on the image and is associated with lower energy photons.

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5
Q

What does it mean if a photon passes right through the tissue?

A

It appears as black on the image with no interaction, typically involving high energy photons.

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6
Q

What is ‘transmission’ in x-ray imaging?

A

The process where photons pass through tissue without interaction.

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7
Q

What is ‘scatter’ in x-ray imaging?

A

Photons interact and move through tissues, resulting in gray areas on the image.

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8
Q

What occurs when an incident photon interacts with the nucleus of an atom?

A

It can kill the cell, but this is not at the diagnostic level; relevant to radiation therapy.

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9
Q

What is Eb (electron binding energy)?

A

The amount of energy needed to remove an electron from orbit.

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10
Q

How does electron binding energy relate to the atomic number?

A

Eb is greater the closer the electron is to the nucleus and is greater in higher atomic number atoms.

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11
Q

What is the electron binding energy for tungsten?

A

69.5 KeV (K shell, Z# 74).

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12
Q

What is the electron binding energy for calcium?

A

4 KeV (K shell, Z# 20).

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13
Q

What is the electron binding energy for soft tissue?

A

roughly ~0.5 KeV.

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14
Q

Name the five interactions between an incident photon and matter.

A
  • Photoelectric Absorption
  • Compton Scattering
  • Coherent Scattering
  • Pair Production
  • Photodisintegration
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15
Q

What occurs when an incident photon interacts with the atoms of the body?

A

The incident photon changes direction (scatter) and may interact with the atom or be completely absorbed (whites)

This is part of the diagnostic interactions that occur at the diagnostic wheel.

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16
Q

What happens during photoretic absorption?

A

An incident x-ray photon interacts with an inner shell electron, ejecting it from orbit and completely absorbing the photon energy

This occurs in patient tissue and results in the ionization of the atom.

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17
Q

What is the term for the ejected electron during photoelectric absorption?

A

Photoelectron

This term describes the electron that is ejected when the incident photon energy exceeds the binding energy.

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18
Q

What are the basic rules governing the interaction of x-ray photons with atoms?

A
  1. The incident x-ray photon energy must be greater than the binding energy of the inner shell electron.
  2. The ejected electron is termed ‘photonection’.
  3. Binding energies of inner shell electrons are close to each other

These rules help determine the likelihood of ionization.

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19
Q

What is characteristic cascading?

A

It occurs when an atom tries to fill the hole left by an ejected electron, resulting in the release of energy and production of a secondary x-ray photon

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20
Q

What is coherent scattering?

A

also known as classical or unmodified scattering

An interaction between very low energy incident photons and atoms in the body

This includes Thompson scattering and Rayleigh scattering.

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21
Q

What occurs during Thompson scattering?

A

A very low energy photon interacts with a single electron in the atom

This interaction causes the electron to vibrate but involves no energy transfer.

22
Q

What happens during Rayleigh scattering?

A

A very low energy photon interacts with all electrons in the atom, causing them to vibrate at the same frequency

This results in the release of a secondary photon and no energy transfer.

23
Q

What is the energy threshold for coherent scattering to occur?

A

10 keV or lower

Coherent scattering is not diagnostically significant but does occur.

24
Q

Fill in the blank: The energy released by each characteristic cascading produces a _______.

A

Characteristic photon

This secondary radiation contributes to the dose received by the patient.

25
Q

True or False: No ionization occurs during coherent scattering.

A

True

Coherent scattering does not result in ionization or significant diagnostic value.

26
Q

What is Compton Scattering?

A

A phenomenon where an incident photon interacts with a loosely bound outer shell electron of an atom, causing ionization.

27
Q

During Compton Scattering, what happens to the incident photon?

A

It ejects the outer shell electron, proceeds in a different direction, and at a lower energy.

28
Q

What is the relationship between the energy of the incident photon and the scattered photon in Compton Scattering?

A

There is a direct relationship; the incident photon energy is divided between the recoil electron and the scattered photon.

29
Q

What is the significance of the recoil electron in Compton Scattering?

A

The recoil electron can fill other holes in atoms after being ejected.

30
Q

What are the three types of scatter in imaging?

A
  • Forward scatter
  • Side scatter
  • Back scatter
31
Q

What happens if too much Compton scattering occurs in imaging?

A

It can lead to a foggy image, reducing image quality.

32
Q

What is needed to clean up fog in imaging caused by Compton scattering?

A

A grid.

33
Q

What is Pair Production?

A

A process where energy is converted to matter, occurring at high energies (≥ 1.02 MeV).

34
Q

What happens during Pair Production?

A

A high energy incident photon loses all its energy near a strong nuclear field, creating a pair of electrons (matter): a negatron and a positron.

35
Q

What occurs to the negatron created during Pair Production?

A

It is absorbed by other atoms.

36
Q

What happens to the positron in Pair Production?

A

It combines with a free electron, resulting in the creation of photons that travel in opposite directions.

37
Q

What is the annihilation reaction?

A

The process where matter is converted back to energy when a positron combines with a free electron.

38
Q

Does Pair Production occur in diagnostic X-rays?

A

No, it does not occur in diagnostic X-rays.

39
Q

Fill in the blank: Compton scattering can affect the _____ of the image.

A

quality

40
Q

What is photodisintegration?

A

Occurs when an interaction with an extremely high energy photon, greater than 10 MeV, excites the nucleus of an atom and causes it to emit a nuclear fragment.

This does not occur in diagnostic imaging.

41
Q

What is the effect of increasing kVp on photoelectric absorption?

A

As kVp increases, photoelectric absorption decreases.

This is because higher energy photons are less likely to interact with tissues.

42
Q

What is the effect of increasing kVp on Compton scattering?

A

As kVp increases, Compton scattering increases.

Higher energy photons result in more scatter.

43
Q

What percentage of the x-ray beam is attenuated at about a 10 cm thick object at 50 kVp?

A

99% of the beam will be attenuated.

Only a small amount of the beam exits to create the image.

44
Q

What are the conditions for photoelectric absorption to occur?

A
  • Lower energy photons, range from 40-70 kVp
  • Introduction of high atomic number elements

High atomic number elements, such as iodine and barium, enhance absorption.

45
Q

What type of image results from the Compton effect?

A

Leads to lower contrast images with many shades of gray.

This results in less black and white contrast.

46
Q

What is the relationship between kVp and patient dose?

A

Higher kVp reduces patient dose.

Higher energy photons are less absorbed, leading to lower exposure.

47
Q

Fill in the blank: The Compton effect results in a _______ image.

A

low contrast

48
Q

Fill in the blank: A high contrast image is characterized by more _______ and _______.

A

black & white

49
Q

What are the two factors that control what happens in radiological imaging?

A
  • Body composition
  • KVp

KVp refers to kilovolt peak, which influences the energy and penetrating ability of X-rays.

50
Q

Define attenuation in the context of radiology.

A

The reduction of photons as it passes through matter

Attenuation is synonymous with absorption in radiological terms.

Radiology Physics (6 decks)

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