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3. Radiology > 3.3 - X-ray Interaction with Matter > Flashcards

3.3 - X-ray Interaction with Matter Flashcards

1
Q

What do we need to produce radiographic images?

A

we need a source of x-rays which are directed at pt and then when X-Ray reaches pt there is interaction (If none we dont get an image) - must be variable interaction with diff tissues.

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2
Q

Where do x-ray photons pass from and to?

A

tube and some go through the pt to reach the image receptor

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3
Q

What can happen to x-ray photons transversing tissue? 4

A

• Pass through unaltered – NO ENERGY LOSS
• Change direction with no energy loss – SCATTER
o The photon comes out, exposes the image receptor but does not create a useful image
• Change direction losing energy – SCATTER AND ABSORPTION
o This gives a radiation dose to the patient
• Be stopped depositing all energy within the tissue – ABSORPTION
o All energy transferred to the material and x-ray doesn’t come out other end

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4
Q

What is attenuation?

A

reduction in the number of photons (x-rays) within the beam

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5
Q

Why does attenuation occur?

A
  • It occurs as a result of absorption and scatter
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6
Q

What affect the number of photons reaching the image receptor?

A

attenuation

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7
Q

What colour if all photons reach the film?

A

black as no material to interact with

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8
Q

What colour if there is partial attenuation?

A

Grey

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9
Q

What colour if there is complete attenuation?

A

white

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10
Q

What is atom composed of?

A

Nucleus - protons and neutrons (+ and 0) both with Amu of 1

orbiting electrons in shells which have charge of -1 and mass of 0

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11
Q

What is the atomic number?

A

No of protons an atom has (Z)

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12
Q

What is the atomic mass?

A

No of protons and neutrons (A)

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13
Q

Where are electrons kept?

A

In electron shells

k - 2
l - 8
m - 18
n - 32
o

2 nsquared

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14
Q

What holds electrons in place?

A

Electrostatic force (attraction of the positive nucleus)

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15
Q

What is binding energy?

A

binding energy is the smallest amount of energy required to remove a particle from a system of particles

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16
Q

What electrons have highest binding energy?

A

Those closest to the nucleus

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17
Q

What are he types of x-ray and electron interactions that occur inside the tube?

A

Continus spectrum

characteristic spectrum

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18
Q

What are the general aims for continuous and characteristic spectrum target interactions?

A

we want electron to interact with atom at target and then this will lead to production of x-ray photons

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19
Q

What Is the continuous spectrum target interaction?

A

This is where incoming electrons from the filament pass close to the target nucleus and are slowed down and deflected as the negative electrons get drawn into positive nucleus resulting in loss of kinetic energy which is transferred into photons which are released from the X-ray tube

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20
Q

What does continuous spectrum target interaction produce?

A

Lots of photons with low energy however These are useless we need high energy

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21
Q

What is the characteristic spectrum target interaction?

A

This is where incoming electrons calling with the inner shell electron and either displace it to another shell or remove it entirely. This results in an unstable atom so electrons reshuffle by filling the space and as this happens energy is released in specific values as they go from one shell to the next and there is a release of energy

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22
Q

What are the interactions that occur in patients tissues?

A

Photoelectric effect

Compton effect

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23
Q

What is the photoelectric effect simplified?

A

Absorption

24
Q

What is the Compton effect?

A

Absorption and scatter

25
What is the photoelectric effect?
This involved inner shell electrons and is where photons come in and there is complete absorption of x-ray photon for this to happen energy of incoming photons must be just greater than energy of binding electron
26
What does incoming electron interact with in photoelectric effect?
Inner shell electron (usually k shell) and only happens if photon has energy greater than the binding energy of electron resulting in x-ray photon completely disappearing
27
What is most of photons energy used for in photoelectric effect?
Used to overcome the binding energy of the electron, the rest gives the electron kinetic energy and causes electron to be ejected
28
What is the photoelectron?
This is when the photons energy is greater than that of the electron binding energy and most of energy is used to overcome binding energy but the rest gives the electron energy to be ejected and the electron that is ejected is the photoelectron
29
What happens when the photoelectron leaves?
Now an empty space in electron shell and atom is pos - wants to be stable so atom regains stable state by electrons from outer shell dropping to fill the void
30
What is the difference in energy between the two electron shells emitted as?
head/light
31
What happens to the outer voids in photoelectric effect?
they are filled by free photoelectric electrons from another atom overall resulting in complete absorption of photon energy so photon doesn't reach film
32
What is characteristic radiation?
This is the spare energy as electrons go between different shells and is in form of light or heat
33
Wha is photoelectric effect mathematically proportional to?
To the cube of atomic number - this results in large differences in photoelectric absorption
34
What is the Compton effect?
This involves outer shell electrons and results in scatter and partial absorption of photons it is where energy of incoming photons is way greater than binding energy of the electron causing electron to come from outer shell and photon is left with lots of energy still. the ejected electron takes some of the photons energy as kinetic energy and is known as the recoil electron atom now unstable so atom captures free electron the photon following the collision has lower energy so longer wavelength and is known as the scatter photon the recoil electron can interact with other atoms in tissue and scatter photons dependent on the energy and position of bound electron involved
35
What is a scatter photon?
This is where photon has way more energy than electron and ejects the electron and photon is left with less energy so longer wavelength and undergoes a change in direction
36
What is recoil electron?
This is the electron that id displaced and it can interact with other atoms in tissue and the scatter photon depending on energy and position of bound electron
37
What can scatter photons do?
Travel in any direction
38
What can high energy scatter photons do?
Travel forward
39
What can low energy scatter photons do?
Travel backwards
40
What is probability of Compton effect happening?
proportional to density of material - very dense material with lot of electrons packed closely together are more likely to have Compton effect occur
41
What is the issue with the Compton effect?
If scattered photons are produced before receptor is reached and scattered backwards then no issue but if they are produced beyond the image and scattered to the receptor then they can affect the receptor by causing darkening
42
Why can scatter photons cause problems?
Their path is randomly altered os if they are produced beyond image receptor and are scattered back then they can cause fogging, reduced contrast and affect image quality
43
How do we reduce scatter?
Via collimation
44
What does collimation do?
Reduces area irradiated therefore reduces volume irratated therefore reduces number of scattered photons produced as well as reducing pts dose
45
What happens to all x-rays?
Collimated to reduce area of beam at pt and therefore reduce dose
46
How do we collimate x-rays?
In film packets we have lead foil which prevents back scattered photons from oral tissues reaching the film so any back scatter photons absorbed and prevented from degrading image
47
Why dont digital receptors have this lead back?
They are more sensitive to x-rays at low dose reduced exposure time collimation
48
When are absorption of photons most likely?
High atomic number object thicker photon energy lower
49
What is radiographic contrast?
This is the difference in density of light and dark areas of radiograph. Allows us to see sharp junctions between diff materials
50
What does photoelectric absorption result in?
Deposition of all photon energy within tissue
51
What does photoelectric absorption do?
Increases pt dose but needed for image quality to get diagnostic image
52
What does Compton scatter result in?
Deposition of some photon energy within tissue - so adds to dose with no diagnostic benefit and may inc dose to operators
53
What happens if we lower x-ray kvp?
we would increase photoelectric interactions and contrast between tissues but dose to pt would also increase (no need)
54
What does a lower tube potential difference produce?
lower energy photons
55
What dose a high tube kVp produce?
Higher energy photons
56
What happens if we increase kVp on image quality and pt dose?
We have inc in Compton effect photoelectric interactions reduced reduced contrast reduce dosage absorbed by pt but can't be reduced so much that no diagnostic image

3. Radiology (12 decks)

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