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Science and Technology 1 > Introduction To CR And Digital Imaging Detectors > Flashcards

Introduction To CR And Digital Imaging Detectors Flashcards

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1
Q

What is the intensity of x-rays impacted by?
(2)

A

The thickness of the material

The type of material

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2
Q

When there’s a thicker material, is attenuation more or less likely to occur?

A

More likely

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3
Q

What is noise?

A

Unwanted fluctuations in signal

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4
Q

What are the 2 types of noise we can get?

A

Electronic noise

Quantum noise

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5
Q

What is quantum noise about?

A

Poisson distribution

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6
Q

What is Poisson distribution?

A

The probability of the photons being attenuated or absorbed. (In the same situation, it’s expected that they’d have the same attenuation, but due to fluctuations they don’t)

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7
Q

How does scatter affect an image?

A

It makes the difference between photons passing through material and no material smaller, which reduces the contrast of the image

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8
Q

If there’s more photons, is there more or less noise?

A

Less noise, so better image quality

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9
Q

If there’s less photons, is there more or less noise?

A

More noise

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10
Q

What is a penumbra?

A

A shadow

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11
Q

Is it more ideal to have a small or large focal spot?

A

Small focal point. We want to get the spread of x-rays coming from a small source

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12
Q

What are the features of a perfect detector?
(4)

A

It fully absorbs all of the transmitted photons

It has a good spatial resolution and doesn’t add to the blurring of the penumbra

It operates in standard conditions, so normal day to day life, e.g. it doesn’t only work in extremely low temperatures

It’s durable and cheap

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13
Q

What are the 2 types of radiography?

A

Computed radiography (CR)

Digital radiography (DR)

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14
Q

Where is computed radiography (CR) used?

A

In dental systems

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15
Q

What happens in computed radiography (CR)?
(4)

A

Radiation is absorbed by an imaging plate

(There’s no fixed detector- there’s only an imaging plate that can move)

The energy from the absorption is trapped, forming a latent image

When we put the imaging plate in the reader, the release of the trapped energy is converted into the image

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16
Q

What are examples of CR readers and cassettes?
(3)

A

Agfa

Fuji

Carestream (Kodak)

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17
Q

For the exam, do we need to know equations?

A

NO!!!!!

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18
Q

For the exam, will we ever be asked any maths questions?

A

NO!!!!!

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19
Q

Label the CR plate/detector:

A
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20
Q

What are the 3 layers in a CR plate/detector?

A

Conductive layer

Reflective layer

Protective layer

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21
Q

What happens to the CR plate/detector?

A

The protective layer gets removed so that the conductive layer can be exposed to the detector

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22
Q

What is the conductive layer in the CR plate made out of?

A

A crystalline structure

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23
Q

What happens in the CR plate?
(6)

A

The x-ray photons enter the protective layer

The photons interact with the crystal in the reflective layer

The photons then interact with the electrons in the conductive layer.

The electrons get a lot of energy and become excited

The electrons want to get rid of the energy

When the electrons lose the energy, they go to the valence band

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24
Q

What can happen to the electrons in the CR plate?

A

The electrons can get trapped in the crystal structure of the forbidden zone and get held in place there. However, they can only get trapped if they find a trap in the structure. But they don’t have enough energy to leave the trap

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25
Q

What can we do to release the trapped electrons from the crystal structure in the CR plate?
(2)

A

We can give the electrons more energy to escape the trap- this is how readers use energy to produce images

This produces light, and the light is proportional to the energy that the electrons absorbed, which can produce an image

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26
Q

What’s an example of how we can give the trapped electrons in the CR plate energy?

A

We can use a laser, as that’ll give the electrons energy

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27
Q

Where do the electrons get trapped in the CR plate?

A

In the forbidden zone

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28
Q

What is the difference in energy between the conduction of x-rays and the valence band dependent on?

A

It’s dependent on what the crystal is made out of

29
Q

Which band is more stable- the valence band or the conduction band?

A

Valence band

30
Q

Where is the ideal place that all electrons in the CR plate want to go?

A

The valence band

31
Q

How does a reader produce an image?
(3)

A

It converts the light it detects into an electrical charge

The electrical charge is converted from a charge to a digital signal

This produces an image

32
Q

What are problems with using a reader to produce an image?
(3)

A

Not all trapped radiation is released

The longer the laser dwells, the more trapped electrons are released, which increases the sensitivity

After the plate is read, a white light exposes the whole plate so the electrons can all be released and the plate can be used again. This doesn’t contribute to the image

33
Q

Why do the electrons in the trap in the CR plate degrading by 25% in the first 8 hours of being there?

A

Because normal heating and other things are happening to the electrons whilst in the trap

34
Q

What does the resolution of CR depend on?

A

The thickness of the phosphor layer

The size of the layer

The use of a reflective layer

35
Q

What will happen if we use a thicker phosphor layer in CR?

A

The image will be higher sensitivity but poorer resolution (more blurry)

36
Q

What type of conversion does CR use?

A

Indirect conversion

37
Q

What type of conversion does DR use?
(2)

A

Indirect conversion

Direct conversion

38
Q

How does DR use indirect conversion?

A

It uses an indirect detector

39
Q

How is an indirect detector used in DR?
(5)

A

X-rays are converted into light by the material

This causes multiple light photons

This causes the spread of light

The light is turned into a current

The current is tuned into a signal

40
Q

Do we want the scintillator to be small or big?
Why?

A

As small as possible

To prevent blurring

41
Q

What is the disadvantage of having a small scintillator?

A

It causes less absorption of photons- so we have to decide which is more important: resolution or attenuation?

42
Q

What is the scintillator made out of?

A

Caesium iodide doped with thallium

43
Q

What are the features of the scintillator?
(3)

A

The caesium iodide crystals are grown in columns and act as a light guide

The caesium iodide and thallium have a K edge of 36 keV and 33keV

The caesium iodide and thallium have atomic numbers of 55 and 53

44
Q

Why is a benefit that the caesium iodide crystals are grown in columns and act as a light guide?

A

Because this minimises the spread of light

45
Q

Why is it a benefit that the caesium iodide and thallium have a K edge of 36 KeV and 33 KeV?

A

Because t causes good absorption

46
Q

Why is it a benefit that the caesium iodide and thallium have atomic numbers of 55 and 53?

A

Because it causes good photoelectric effects to occur

47
Q

What is the oldest method of indirect conversion DR?

A

CCD indirect detectors

48
Q

What are CCD indirect detectors mainly used for?
(2)

A

Mammography

Dental

49
Q

What is the current method of indirect conversion CR?

A

Flat panel indirect detect

50
Q

How does the flat panel indirect detector work (DR)?
(4)

A

The x-rays interact with the scintillator

This produces visible light

The photodiodes convert this to a current

The current is then converted into a signal

51
Q

How is direct conversion done in DR?

A

Using direct detection

52
Q

How does direct detection work in DR?
(4)

A

The x-rays interact with the material

This produces electrons

The electrons are directed towards the detector

This prevents blurring

53
Q

What are limitations of direct detection in DR?

A

It uses amorphous-selenium, which has a lower atomic number than caesium. So it’s less likely to absorb x-rays

54
Q

When would direct detection in DR be useful?
Why?

A

Mammography

Because they use low k-electrons

55
Q

What does each detector consist of?

A

An array of detector elements

56
Q

What does each detector element consist of?
(2)
Why?

A

Photoconductor
Electronics

To read signal

57
Q

What do smaller detector elements have to do?
Why?

A

Sacrifice the photoconductor area

Because the electronic size is fixed

58
Q

What happens if we increase the number of detector elements?

A

The detector efficiency changes, causing a better resolution but less absorption of x-rays, due to having a smaller photoconductor size

59
Q

Compare direct and indirect conversion in DR:
(3)

A

Direct: uses a semi-conductor material

Indirect: uses a scintillator material

Direct: produces an image with better resolution (since it doesn’t have a light stage)

Indirect: produces an image with poorer resolution (since it has a light stage

Direct: it has a poorer sensitivity at high energy (it can’t absorb as much energy)

Indirect: it has a greater sensitivity at diagnostic (it can absorb more energy)

60
Q

What does absorption efficiency mean?

A

How well does a detector absorb the photons that interact with it?

61
Q

What does conversion efficiency mean?

A

It’s the conversion of what you’re detecting from light/starting molecule

62
Q

What does capture efficiency mean?

A

How good the image produced is from the signal we’re getting

63
Q

What are the characteristics of a radiation detector?
(5)

A

Absorption efficiency

Conversion efficiency

Capture efficiency

Dose efficiency

Reproducibility

64
Q

How does the detector material impact absorption?

A

If you want to produce an image with the highest optimum, you need a higher absorption

65
Q

Is the mA is doubled, how many electrons will interact with the target?

A

Twice as may electrons

66
Q

What does intensity mean?

A

The energy passing through an area at a time

67
Q

Instead of increasing the tube current, what else can we do?

A

We can expose the patient for a longer duration

68
Q

What is the problem with longer exposure times?

A

The image is at risk of motion artefacts- voluntary and involuntary

Science and Technology 1 (17 decks)

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