11. Digital Radiography Continued

Question 1

Define post-processing:

Answer 1

Anything that can be added, changed, ect. to a digital image after it has been acquired. (annotation, adding text)

Question 2

What is the biggest controlling factor for how the processed image will turn out?

Answer 2

The algorithm is the biggest controlling factor

Question 3

Define windowing:

What happens when we use windowing?

Answer 3

-Adjusting the brightness and contrast (adjusting the values on the pixels)
-As we adjust the windowing, the system reassigns pixels different values that correspond to different brightness’s

Question 4

Define window Level (WL) (Leveling):

Answer 4

• Controls image brightness

Question 5

As you increasing the WL, what happens to brightness typically?

Answer 5

The higher the WL the brighter the image

Question 6

Define window width:

Answer 6

• Controls image contrast

Question 7

As you increase WW, what happens to contrast?

Answer 7

Decrease or reduce image contrast

More shades of grey

Question 8

As you decrease WW, what happens to contrast?

Answer 8

increase image contrast

Question 9

Describe in detail how a narrowing in WW increases contrast:

Answer 9

Narrowing the WW will decrease the amount of values represented on Image contrast, which increases contrast due to the fact it can only produce only a few values

Question 10

Why would we want to magnify an image?

2

Answer 10

1.To see fractures
2.Looking at detail within image.

Question 11

Describe image inversion

Answer 11

Areas that were dark are bright and bright areas become dark

Question 12

Define DQE:

Answer 12

○ How efficient the detector is at converting the incident x-ray energy into an image signal (electronic signal)

Question 13

How is mAs affected by DQE?

Answer 13

The higher the DQE of a detector, the less mAs that is required for a specific projection

Question 14

What are 2 disadvantages of digital radiography?

Answer 14

1. Lower spatial resolution compared to film radiography
2. Dose creep

Question 15

Describe the concept of dose creep:

Answer 15

○ The system is really good at fixing over exposures compared to under exposures. As a result, we tend to use mAs values that are higher than required.

Question 16

Describe what it means to have a high DQE value compared to a low DQE:

Answer 16

A higher DQE value would mean that less radiation is required to achieve identical image quality compared to the detector with the lower DQE.

Question 17

List some things that contribute to the overall quality of the image:

Answer 17

1. SC
2. IC
3. Noise (Quantum model/mottle)
4. Signal
5. SR
6. Pixel size

Question 18

What is SC affected by?

Answer 18

Scatter Radiation-anything that affects scatter affects subject contrast

Question 19

What is SC determined by?

five

Answer 19

• Differential Absorption
• Atomic Number
• Physical/Mass Density
• Thickness Differences
• kVp

Question 20

Without SC, can we have IC?

Answer 20

No

Question 21

What is the purpse of the algorithm?

Answer 21

Puts our brightness values and contrast where we would like them to be for a specific projection

Question 22

What is noise (QM) determined by?

Answer 22

Determined by the number of photons hitting the detector to create the signal/image. Not every pixel will recieve the same amount of photons.

QM can be called nosie

Question 23

What is noise caused by?

Answer 23

1.X-ray photons (inadequate technique)
2.Attenuation

Question 24

With a Low kVp and mAs, what happens to singal and noise?

Answer 24

• Few photons hit the detector, we get a low signal, more noise

Question 25

With a high kVp and mAs, what happens to singal and noise?

Answer 25

Many photons hit the detector, we get a high signal, less noise

Question 26

Remnant radiation can also be called what?

Answer 26

Signal

Question 27

What happens to singal when more photons hit the IR?

Answer 27

Signal increases

Question 28

With more more lp/mm, what does that mean in terms of spatial frequency?

Answer 28

* More lp/mm = greater spatial frequency

Question 29

Do all CR IP’s have the same pixel size?

Answer 29

Not always. If there is a difference, typically the smaller IP’s have smaller pixels compared to the larger IP sizes.

Question 30

Where is the display pixel/display matrix located?

Answer 30

Is located on the monitor, and varies depending on the monitor that we use

Question 31

Define pixel pitch:

Answer 31

The distance from the centre of one pixel to the centre of the pixel next to it.

Question 32

In relation to SR, how does pixel pitch increase the SR?

Answer 32

The smaller the pixel pitch, the smaller the pixel, and therefor the better the spatial resolution.

Question 33

In relation to SR, how does pixel size, and pixel density increase SR?

Answer 33

The greater the pixel density, the smaller the pixels and therefore the better the spatial resolution

Question 34

For the same field of view, the greater the number of pixels the greater the what?

Answer 34

The greater the pixel density, the greater the SR

Question 35

As you increase decrease PP and increase PD, what happens to SR?

Answer 35

Increases SR

Question 36

What type of aquesition offers a higher DQE? What is the benefit of that?

Answer 36

Indirect aquesition-lower technqies per exposure

Question 37

Why does direct aquesition have better SR?

Answer 37

Because there is less spreading of the e- as they are being pulled down the amorphus se

Question 38

When comparing DQE’s of different detectors or systems, how should it be compared and why?

Answer 38

It must be compared at the same kVp so the beam has the same penetration.

Question 39

What is SNR?

Answer 39

How much noise can be tolerated in the image

Question 40

As SNR increases, what happens to noise?

Answer 40

Noise decreases ## Footnote * Different detectors will require more or less exposure to the IR to achieve the same SNR

Question 41

Why is it important to record more values than we can see?

Answer 41

* Through post processing, we can amplify those differences

Question 42

Can we record 75 different values?

Answer 42

Yes provided we have a great enough bit depth

Question 43

Define matrix

Answer 43

Number of pixels

Question 44

If you decrease the FOV, does pixel size decrease?

Answer 44

No

Question 45

How can you increase the SR with FOV, and matrix size?

Answer 45

For two different IR's the smaller the FOV and the larger the matrix size, the better the SR

Question 46

Define QM:

Answer 46

**Not enough information is getting to the IR, **not fixable through post-processing, flux in the amount of photons for unit area, will always be there.

Question 47

What is the relationship between QM and SR?

Answer 47

QM has nothing to do with SR

Question 48

Does mAs control contrast resolution?

Answer 48

* mAs doesn’t affect contrast, however when we reach the saturation point, it does affect contrast.

Question 49

Why cant we fix saturation through post processing?

Answer 49

* A specific area saw more photons than the other area. As we increase our technique, the system can no longer tell a difference between two different pixels since they have both taken the maximum amount of radiation.