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FRCR Part 1 Physics > Computed and Digital Radiography (Erin) > Flashcards

Computed and Digital Radiography (Erin) Flashcards

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1
Q

Describe xray fluorescence

A

the IMMEDIATE emission of light

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2
Q

Describe xray phosphorescence

A

the DELAYED emission of light

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3
Q

Is fluorescence of phosphorescence used in CR?

A

phosphorescence

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4
Q

In CR:
What type of phosphor are the imaging plates coated with

A

Barium fluorohalide (BaFBr)

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5
Q

In CR:
What happens to electrons in the phosphor plate during exposure?

A

The electrons in the phosphor are excited to a higher energy state
Temporarily trapped in this higher excited energy state
Form the LATENT image

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6
Q

In CR:
After exposure what happens to the imaging plate

A

Put into a read to release the stored xray data
And digital xrays are produced

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7
Q

In CR:
During readout what colour laser is the imaging plate exposed to?

A

Red Laser light

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8
Q

In CR:
What colour light is emitted by the imaging plate to be detected by the PM tube?

A

Blue light

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9
Q

In CR:
How is the imaging plate data erased

A

Exposing it to bright white light

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10
Q

In CR:
Is a larger or smaller crystal size used for a standard imaging plate

A

Larger

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11
Q

In CR:
Is a larger or smaller crystal size used for a high resolution imaging plate

A

Smaller

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12
Q

In CR:
Is a thicker or thinner layer of phosphor crystal used for a high resolution imaging plate

A

Thinner

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13
Q

In CR:
Is a thicker or thinner layer of phosphor crystal used for a standard resolution imaging plate

A

Thicker

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14
Q

In CR:
Does a standard imaging plate contain a light reflection layer?

A

Yes

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15
Q

In CR:
Does a high resolution imaging plate contain a light reflection layer?

A

No

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16
Q

In CR:
Which has a better xray absorption efficiency; standard imaging plates or high resolution imaging plates?

A

Standard imaging plates (40%)

High resolution imaging plates lower efficiency - i.e need larger xray doses

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17
Q

In CR:
What is the role of the PM tube?

A

Collects blue light photons and converts them to an electrical current

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18
Q

In CR:
How is each signal sample stored?

A

In an individual pixel

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19
Q

In CR:
How is the signal output from the photomultiplier tube samples/digitized?

A

The signal output from PM is sampled at regular intervals by an ADC to produce a digital image

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20
Q

In CR what do we mean by sampling pitch?

A

Distance between each sample

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21
Q

In CR:
What is the sampling pitch equal to?

A

Sample pitch = pixel size

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22
Q

In CR:
How is sample frequency (number of samples per mm) calculated?

A

1/pixel size

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23
Q

In CR:
What is the effect of increased sample frequency on spatial resolution?

A

Increases SR

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24
Q

In CR:
What is the effect of a large imaging plate on pixel size and spatial resolution

A

Larger pixel size
Lower spatial resolution

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25
Q

In CR:
What is the effect of a small imaging plate on pixel size and spatial resolution?

A

Small pixels
Better spatial resolution

26
Q

In CR:
What is the typical spatial resolution?

A

2-3lp/mm

27
Q

In CR what is the highest spatial resolution?

A

5lp/mm

28
Q

In CR:
what 4 factors determined spatial resolution

A

Size of readout laser beam
Thickness of phosphor layer
Pixel size
Sampling rate

29
Q

In CR:
What effect does smaller diameter readout laser beam have on SR?

A

Improved SR

30
Q

In CR:
What effect does larger diameter readout laser beam have on SR?

A

Decreased SR

31
Q

In CR:
What effect does smaller pixels have on SR?

A

Improved SR

32
Q

In CR:
What effect does larger pixels have on SR?

A

Decreased SR

33
Q

In CR:
What effect does smaller phosphor crystals have on SR?

A

Improved SR

34
Q

In CR:
What effect does larger phosphor crystals have on SR?

A

Decreased SR

35
Q

In CR:
What effect does a thinner phosphor layer have on SR?

A

Improved SR

36
Q

In CR:
What effect does a thicker phosphor layer have on SR?

A

Decreased SR

37
Q

What type of detectors are used in digital radiography?

A

Flat panel detectors

38
Q

What are the two types of flat panel detectors

A

Indirect FPD
Direct FPD

39
Q

In CR:
What is detective quantitive efficiency (DQE) a measure of?

A

Sensitivity of detector

40
Q

What is the difference between indirect and direct DR?

A

Indirect: xray photons -> light photons -> electrical signal

Direct: xray photons -> electrical signal

41
Q

What type of phosphor is typically used in indirect DR?

A

Caesium iodide
Or
Gadolinium Oxysulphide

42
Q

Indirect DR:
In the scintillation later what is the caesium iodide phosphor typically coated in

A

Amorphous silicon

43
Q

In indirect DR?
What is the typical SR?

A

2.0-3.5lp/mm

44
Q

In indirect DR:
How is detector size changed?

A

Detector size is usually fixed
But different sizes are available

45
Q

In indirect DR:
What colour photons does the phosphor release?

A

Green

46
Q

In Indirect DR:
Where are the light photons detected?

A

Detected by active matrix which is separated into pixels

47
Q

In indirect DR:
What does each pixel contain

A

Photodiode (amplifies signal)
Charged storage capacitor (stores signal of latent image)

48
Q

In indirect DR:
What is the role of the TFT switch

A

Latent image read out and transferred to TFT switches that produce voltage signal that is digitised and converted into the image

49
Q

In indirect DR:
What is fill factor and how is it calculated

A

TFT and charge storage take up a small area of the pixel
Fill factor = sensitive area/overall area

50
Q

In direct DR:
What is the photoconductor made of?

A

Amorphous Selenium

51
Q

Direct DR:
Describe the conversion of xrays to an image

A
  1. Xrays interact with a-Se photoconductor
  2. electrical charge carriers (negative electrons and positive holes) are created in the a-Se photoconductor
  3. Electrons are attracted by a strong positive electric field to a transistor array where they are collected
  4. When the exposure ends the charge stored in each transistor is read out and fed into a computor
  5. Voltage signal is digitised
52
Q

In Direct DR:
What does pixel size depend on?

A

Size of transistors

53
Q

Describe Moire pattern artefact

A

When stationary xray anti-scatter grid is used and there is interference between the linear structure of the grid and the regular pixel array of the digitised image

54
Q

Describe ghost image artefact

A

In CR
Due to carry over image content from a previous exposure

55
Q

Which is used more commonly for general radiography - direct or indirect FPD?

A

Indirect

56
Q

Why are indirect FPD used more commonly (over direct FPD)? (3)

A
  1. CSI Good absorber of xrays at 70-90kvp so lower patient doses
  2. Cheaper than direct
  3. Greater stability than direct FPD
57
Q

Which FPD are preferred for mammography (direct or indirect)?

A

Direct

58
Q

Why are direct FPD preferred for mammography?

A

Good absorber of low energy xrays

59
Q

Which has a better spatial resolution?
Direct or indirect FPD?

A

Direct

60
Q
A

FRCR Part 1 Physics (20 decks)

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