Computed and Digital Radiography (Erin)

Question 1

Describe xray fluorescence

Answer 1

the IMMEDIATE emission of light

Question 2

Describe xray phosphorescence

Answer 2

the DELAYED emission of light

Question 3

Is fluorescence of phosphorescence used in CR?

Answer 3

phosphorescence

Question 4

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

Answer 4

Barium fluorohalide (BaFBr)

Question 5

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

Answer 5

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

Question 6

In CR:
After exposure what happens to the imaging plate

Answer 6

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

Question 7

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

Answer 7

Red Laser light

Question 8

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

Answer 8

Blue light

Question 9

In CR:
How is the imaging plate data erased

Answer 9

Exposing it to bright white light

Question 10

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

Answer 10

Larger

Question 11

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

Answer 11

Smaller

Question 12

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

Answer 12

Thinner

Question 13

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

Answer 13

Thicker

Question 14

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

Answer 14

Yes

Question 15

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

Answer 15

No

Question 16

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

Answer 16

Standard imaging plates (40%)

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

Question 17

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

Answer 17

Collects blue light photons and converts them to an electrical current

Question 18

In CR:
How is each signal sample stored?

Answer 18

In an individual pixel

Question 19

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

Answer 19

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

Question 20

In CR what do we mean by sampling pitch?

Answer 20

Distance between each sample

Question 21

In CR:
What is the sampling pitch equal to?

Answer 21

Sample pitch = pixel size

Question 22

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

Answer 22

1/pixel size

Question 23

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

Answer 23

Increases SR

Question 24

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

Answer 24

Larger pixel size
Lower spatial resolution

Question 25

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

Answer 25

Small pixels Better spatial resolution

Question 26

In CR: What is the typical spatial resolution?

Answer 26

2-3lp/mm

Question 27

In CR what is the highest spatial resolution?

Answer 27

5lp/mm

Question 28

In CR: what 4 factors determined spatial resolution

Answer 28

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

Question 29

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

Answer 29

Improved SR

Question 30

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

Answer 30

Decreased SR

Question 31

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

Answer 31

Improved SR

Question 32

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

Answer 32

Decreased SR

Question 33

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

Answer 33

Improved SR

Question 34

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

Answer 34

Decreased SR

Question 35

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

Answer 35

Improved SR

Question 36

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

Answer 36

Decreased SR

Question 37

What type of detectors are used in digital radiography?

Answer 37

Flat panel detectors

Question 38

What are the two types of flat panel detectors

Answer 38

Indirect FPD Direct FPD

Question 39

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

Answer 39

Sensitivity of detector

Question 40

What is the difference between indirect and direct DR?

Answer 40

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

Question 41

What type of phosphor is typically used in indirect DR?

Answer 41

Caesium iodide Or Gadolinium Oxysulphide

Question 42

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

Answer 42

Amorphous silicon

Question 43

In indirect DR? What is the typical SR?

Answer 43

2.0-3.5lp/mm

Question 44

In indirect DR: How is detector size changed?

Answer 44

Detector size is usually fixed But different sizes are available

Question 45

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

Answer 45

Green

Question 46

In Indirect DR: Where are the light photons detected?

Answer 46

Detected by active matrix which is separated into pixels

Question 47

In indirect DR: What does each pixel contain

Answer 47

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

Question 48

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

Answer 48

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

Question 49

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

Answer 49

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

Question 50

In direct DR: What is the photoconductor made of?

Answer 50

Amorphous Selenium

Question 51

Direct DR: Describe the conversion of xrays to an image

Answer 51

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

Question 52

In Direct DR: What does pixel size depend on?

Answer 52

Size of transistors

Question 53

Describe Moire pattern artefact

Answer 53

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

Question 54

Describe ghost image artefact

Answer 54

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

Question 55

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

Answer 55

Indirect

Question 56

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

Answer 56

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

Question 57

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

Answer 57

Direct

Question 58

Why are direct FPD preferred for mammography?

Answer 58

Good absorber of low energy xrays

Question 59

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

Answer 59

Direct