Contrast-Noise

Question 1

What is contrast?

Answer 1

The ability to distinguish small attenuation differences between tissues

Question 2

How is contrast manifested in an image?

Answer 2

Through different grey scales

Question 3

What grey scale is associated with low contrast?

Answer 3

Long grey scale

Question 4

What grey scale is associated with high contrast?

Answer 4

Short grey scale

Question 5

What is a long grey scale?

Answer 5

Many different greys within an image with small differences between them

Question 6

What is short grey scale?

Answer 6

Less greys within an image with large differences between them

Question 7

What type of wavelength produces a short grey scale?

Answer 7

Longer wavelength

Question 8

What type of wavelength produces a long grey scale?

Answer 8

Short wavelength

Question 9

What is differential absorption also known as?

Answer 9

Subject contrast

Question 10

What is differential absorption?

Answer 10

The ratio between absorption of one tissue and adjacent tissue

Question 11

What are two extremes in tissue that demonstrate high contrast?

Answer 11

Bone and tissues since they attenuate the beam at different rates

Question 12

What is the relationship between subject contrast and photoelectric/compton ratio?

Answer 12

Directly proportional

Question 13

How do you calculate subject contrast?

Answer 13

High Attenuation Factor/ Low Attenuation Factor

Question 14

How is attenuation factor calculated?

Answer 14

Photoelectric interaction + Compton interactions

Question 15

If the differences in attenuation between two tissues is greater how is subject contrast affected?

Answer 15

Subject contrast is increase

Question 16

If the differences in attenuation between two tissue is smaller how is subject contrast affected?

Answer 16

Subject contrast decreases

Question 17

What attenuation properties directly affect subject contrast?

Answer 17

Tissue thickness, physical density of the tissue and average atomic number

Question 18

Which attenuation property has the most impact on subject contrast

Answer 18

Average atomic number

Question 19

Which attenuation property has the least impact on subject contrast?

Answer 19

Tissue density

Question 20

What is the relationship between atomic number and attenuation?

Answer 20

Exponential, small change in atomic # produces a big difference in contrast

Question 21

How does tissue thickness affect attenuation?

Answer 21

For every 4 cm of added tissue thickness, the attenuation of the beam is doubled

Question 22

If a tissue is 4cm thicker than the average, how much more mAs is needed?

Answer 22

Double the mAs

Question 23

If a tissue is 8 cm thicker than the average, how much more mAs is needed?

Answer 23

Quadruple the mAs

Question 24

How should contrast be kept in an image?

Answer 24

At an optimal level

Question 25

How can contrast be adjusted in post processing?

Answer 25

Through windowing

Question 26

What does an image that is overly dark indicate?

Answer 26

That the part/object was over penetrated with little attenuation

Question 27

What is image noise?

Answer 27

An unwanted often grainy appearance on a radiograph

Question 28

What are the different forms of noise?

Answer 28

Scatter and electronic

Question 29

What is quantum mottle?

Answer 29

A form of image noise that occurs from a lack of penetration due to low kVp or mass

Question 30

What is the predominent type of noise in digital imaging?

Answer 30

Quantum mottle

Question 31

How does noise affect contrast?

Answer 31

The grainy appearance of noise reduces contrast

Question 32

What level should noise be kept at in images?

Answer 32

At a minimal level

Question 33

What is the signal to noise ratio?

Answer 33

The total exposure minus the scatter

Question 34

What is SNR used to compare?

Answer 34

One exposure to another

Question 35

How can an image be made better in regards to SNR?

Answer 35

Increase signal or decrease noise