Contrast Ch. 27

Question 1

Contrast is

Answer 1

the difference between adjacent densities that enables the visibility of anatomy detail

Question 2

Which is better? Low contrast or high contrast?

Answer 2

Depends on the part being imaged

Question 3

When would you want low contrast?

Answer 3

When imaging an abdomen since there is a lot of soft tissue that needs to be visualized

Question 4

When would you want high contrast?

Answer 4

When you need to see bone in detail.

Question 5

Low contrast has more..

Answer 5

gray

Question 6

High contrast = _____ scale

Answer 6

short

Question 7

Low contrast = ______ scale

Answer 7

long

Question 8

Long scale has ____ shades of gray

Answer 8

more

Question 9

Short scale has ____ shades of gray

Answer 9

less

Question 10

Subject contrast is AKA

Answer 10

patient contrast

Question 11

Subject contrast has a difference in..

Answer 11

tissue thickness, density eff. atomic number & differences in amount of attenuation

Question 12

Does the amount of absorption affect subject contrast?

Answer 12

yes

Question 13

What has a medium amount of shades of gray?

Answer 13

muscle/fat

Question 14

Bone has a high atomic # and therefore..

Answer 14

more absorption, less shades of gray (more black and white)

Question 15

Air has a low atomic # and therefore…

Answer 15

low density, darker shades of gray

Question 16

What is the controlling factor for contrast in F/S?

Answer 16

kVp

Question 17

If kVp is the controlling factor for contrast in F/S, then it should also be the controlling factor for digital. T or F?

Answer 17

False!! kVp is an influencing factor for contrast in digital

Question 18

Does higher kVp increase or decrease contrast? (digital)

Answer 18

Decreases….higher kVp=lower mAs

Question 19

Does collimation increase or decrease contrast?

Answer 19

Appropriate collimation increases contrast and reduces scatter.

Question 20

Do grids increase or decrease contrast?

Answer 20

increases contrast and reduces scatter, gives fewer shades of gray

Question 21

Does the air gap technique increase or decrease contrast?

Answer 21

increases contrast b/c scatter doesn’t reach the IR.

Question 22

If there was an increase in lead content in a grid, would that increase or decrease contrast?

Answer 22

increase contrast cause it cleans up more scatter

Question 23

How many shades of gray can our eyes see?

Answer 23

about 32

Question 24

How many shades of gray are displayed on a computer screen?

Answer 24

about 4,000

Question 25

What does LUT stand for?

Answer 25

Look up table

Question 26

What does the LUT do?

Answer 26

it is a part of the software that gets appropriate contrast for the part being images (preset contrast)

Question 27

What could happen if you image a knee and run it in the LUT as a chest?

Answer 27

It will be overexposed and you will need to manually change it in the computer.

Question 28

What is display contrast?

Answer 28

ambient light in viewing area (on the computer's display)

Question 29

For digital...brightness is emission of light from where?

Answer 29

the computer monitor

Question 30

WL & WW do what?

Answer 30

adjust the range of densities visible

Question 31

Window level =

Answer 31

brightness

Question 32

Window width =

Answer 32

contrast

Question 33

Too much or too little mAs can do what to an image/contrast

Answer 33

over or under expose it which would decrease the contrast either way

Question 34

F/S IRs effect image contrast how?

Answer 34

the design, use of screens, film density & issues related to processing - especially reduced time and temperature of the developer - all reduce contrast

Question 35

What is film contrast?

Answer 35

the range of densities that the film is capable of recording

Question 36

Increasing film developer time, temp or replenishment rate will increase...

Answer 36

the chemical fog on the film & decrease contrast

Question 37

What is the controlling factor of contrast in digital?

Answer 37

software...window width

Question 38

The proper _____ is important for the desired differential attenuation through the patient for a given examination

Answer 38

kVp

Question 39

In post-processing, what can affect the final image contrast that is displayed?

Answer 39

the histogram and LUT

Question 40

What is the primary controller of subject contrast?

Answer 40

kVp

Question 41

Why does low kVp give high subject contrast

Answer 41

because more low energy photons are being absorbed by thicker body parts and penetrate thinner body parts

Question 42

As kVp increases, the % of compton interactions increase which means what?

Answer 42

more scatter, less contrast

Question 43

When overall body thickness increases or field size increases what happens?

Answer 43

the amount of scatter will increase and decrease subject contrast