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Science and Technology 1 > Scatter And Grids > Flashcards

Scatter And Grids Flashcards

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1
Q

What 3 things can happen to an x-ray beam?

A

It can be transmitted

It can be absorbed

It can be scattered

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2
Q

What does it mean if an x-ray beam is transmitted?

A

The x-ray photons pass straight through the patient and interact with the image reception, which produces a black image

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3
Q

What does it mean if an x-ray beam is absorbed?

A

The x-ray photons are completely absorbed by the patient and don’t reach the image receptor. The image is then given contrast

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4
Q

What does it mean if an x-ray beam is scattered?

A

The x-ray photons collide with the atoms in the patients and then travel in different directions

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5
Q

Out of the 3 things that can happen to an x-ray beam, which are important for image production?
(2)

A

Transmitted

Absorbed

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6
Q

Out of the 3 things that can happen to an x-ray beam, which one is unwanted and degrades the image?

A

Scattered

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7
Q

What happens in scatter?
(2)

A

Some incoming photons aren’t absorbed.

They lose energy during interactions with the atoms within the tissues

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8
Q

Why is it important to reduce scatter?
(2)

A

Degrades the image (image looks noisy)

Increases dose to the patient

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9
Q

What are the 2 types of scatter?

A

Coherent scatter

Compton scatter

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10
Q

Does coherent scatter occur at very high or low energies?

A

Very low energies

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11
Q

What happens in coherent scattering?
(3)

A

The incoming photon interacts with the atom

The photon doesn’t have enough energy to release any electrons from their bond state

The photon is absorbed by the atom and re-emitted in a random direction using the same energy

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12
Q

What are the features of coherent scatter?
(4)

A

The energy of the photons coming in is equal to the energy of the photons coming out

There’s no energy loss

Occurs with low energy x-rays typically below the diagnostic range

Could occur within the diagnostic range of x-rays, but only in a very small amount

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13
Q

What happens in Compton scattering?
(6)

A

The incoming photon interacts with an orbital electron (electron on the outside)

The electron is removed from its shell

The electron is ejected from the atom, as is known as a Compton electron

The photon loses energy in the process and is deflected in a new direction

Both the photon and the electron have enough energy to interact with he tissues within the patient or exit the patient

If a scattered photon interacts with the image receptor, it doesn’t provide any useful information

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14
Q

What are the factors that affect scatter?
(3)

A

kVp

Thickness of body part

Field size (collimation)

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15
Q

What are the factors that affect scatter?
(3)

A

kVp

Thickness of body part

Field size (collimation)

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16
Q

How does kVp affect scatter?
(2)

A

Increasing the kVp will increase scatter

As the energy of the beam increase, more Compton scatter effects will occur and cause more collisions

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17
Q

How does thickness of body part affect scatter?
(3)

A

Increasing the thickness of the body part will increase scatter

The photons will have more distance to travel through the body part

Increasing the amount of Compton scatter events

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18
Q

What does thicker body parts do to the kV?

A

They increase the kV

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19
Q

What has to happen since increasing the thickness of a body part increases scatter?

A

A Bucky has to be used for larger body parts- the Bucky contains the grid and the grid contains scatter

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20
Q

How does field size (collimation) affect scatter?
(3)

A

Increasing the field size will increase scatter

More tissue is exposed to the x-ray beam

This increases the amounts of scatter events

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21
Q

What is the difference between characteristic x-rays and brehmstrahlung, and Compton scattering and photoelectric effect?

A

Characteristics x-rays and brehmstrahlung occurs in the x-ray tube

Compton scattering and photoelectric effect occurs in the patient

22
Q

How does reducing the kVp reduce the scatter?
(3)

A

Lowering the kVp will reduce the energy of the x-ray beam

This will reduce the amount of Compton scatter effects occurring

Therefore, decreasing the kVp will decrease scatter

23
Q

What can be used to reduce the thickness of the body part?

A

Compression

24
Q

How does reducing the thickness of the body part affect scatter?

A

Reducing the thickness reduces the amount of tissues that the photons must pass through, resulting in less Compton scatter events

25
Q

When is reducing the thickness of the body part used?

A

In mammography- the breasts are compressed

26
Q

How does reducing the field size affect scatter?
(2)

A

Reducing the field size of the x-ray beam will result in a smaller area that’s exposed to the patient

This reduces the amount of scatter that can occur

27
Q

What are other scatter reducing techniques?
(2)

A

Air gap technique

Grids

28
Q

How does the air gap technique reduce scatter?

A

Creating a gap between the patient and the image receptor will prevent some of the scattered photons from reaching the receptor

29
Q

What kVp is the air gap technique most effective for?

A

Projections requiring a kVp below 90

30
Q

Why isn’t the air gap technique effective for projections with a kVp above 90?

A

With a kVp above 90, there’s too many interactions occurring, so it doesn’t have much of an impact on reducing the scattered photons

31
Q

What is a limitation of the air gap technique?

A

The increase in the object to image receptor distance will result in image unsharpness and magnification. The source to image distance should be decreased to compensate for this

32
Q

What is the main way to reduce scatter?

A

Grids

33
Q

When are grids used?

A

For dense areas of the body (over 10cm thick), such as the abdomen, pelvis and skull

34
Q

What do grids do?

A

They reduce the effects of scatter

35
Q

Where are the grids found?

A

In the Buckys

36
Q

What are the functions of a grid?
(3)

A

Transmit the primary beam

Absorb the scatter

Improve the image quality

37
Q

How is a grid constructed?
(2)

A

It’s made up of parallel strips of a highly attenuating material, such as lead

These strips are uniformly spaced out and are separated by an interspace material such as aluminium

38
Q

How do grids work?
(2)

A

Primary photons will pass through the grid’s plates

While secondary photons that cause scatter will be absorbed by the strips

39
Q

What are the types of grids?
(2)

A

Parallel

Focused

40
Q

What would happen if the grid was at an angle?

A

The primary beam would also be absorbed by the grid

41
Q

What is a parallel grid?
Where is it used?
(2)

A

The lead strips run parallel to each other

It’s used mainly in fluoroscopy and parallel imaging

42
Q

What is a focused grid?
(2)

A

The lead strips re angled to match the divergence of the x-ray beam

This allows more of the primary beam to be transmitted than parallel grids

43
Q

Which grid is most common?

A

Focused grid

44
Q

What does the SID mean?

A

Source to image distance

= distance from x-ray tube to detector

45
Q

Why is it important to that the correct grid is used for the SID when using a focused grid?

A

Because the incorrect SID could result in the lead strips absorbing the primary beam, which results in grid cut off

46
Q

When can grid cut off happen?

A

If the grid is misaligned or the SID is incorrect grid, cut off can happen

47
Q

What happens when grid cut off happens?
(2)

A

Less of the primary beam will reach the detector

This results in an increase in noise on the image (which causes the whiteness in the image)

48
Q

What are the factors to consider when using a grid?
(3)

A

The mAs should be increased (because more photons will be absorbed)

The grid must be perpendicular (at right angles) to the x-ray beam

The grid must be the correct way up (so that the lead strips are angled correctly)

49
Q

What happens if we use a grid on a thinner area compared to a thicker area?
(2)

A

We have to increase the mAs, which increases the dose to the patient

Whereas on a thicker area, increasing the dose mAs and dose has less of an impact as the material is thicker

50
Q

What is a virtual grid?

A

It’s software that replicates a grid. It uses algorithms to remove scatter. It recognises and removes the scatter, but it’s nit as good as usual an actual grid.

Science and Technology 1 (17 decks)

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