Exposure Factors

Question 1

How are x-rays produced?
(3)

Answer 1

Electrons are emitted by thermionic emission

They’re accelerated with a high voltage towards a target

They slow down, and this deceleration emits x-rays

Question 2

What does mAs mean?

Answer 2

Current (the no. of electrons going into the x-ray tube)

Question 3

What does the mAs do?

Answer 3

It controls the quantity (amount) of radiation in the x-ray beam, which correlates with the number of photons in the beam, or its intensity

Question 4

What does kV mean?

Answer 4

Voltage

Question 5

What does the kV do?

Answer 5

It controls the penetrating power of the x-ray beam

Question 6

What happens if we increase the quality and intensity of the x-ray beam?
Why?

Answer 6

More x-rays get through the target, and more x-rays reach the detector

Because less of the x-rays are attenuated

Question 7

What is contrast?

Answer 7

The difference in density or grayness between areas of the radiographic image (the ability to tell the difference between different things)

Question 8

What does it mean if we have high contrast?

Answer 8

It’s easy to tell the difference between adjacent structures

Question 9

What does it mean if we have low contrast?

Answer 9

It’s hard to tell the difference between adjacent structures

Question 10

How many shades of grey can humans distinguish?

Answer 10

30 shades

Question 11

Is this high, medium or low contrast?

Answer 11

High contrast

Question 12

Is this high, medium or low contrast?

Answer 12

Medium contrast

Question 13

Is this high, medium or low contrast?

Answer 13

Low contrast

Question 14

What is the normal kV and mAs of a hand x-ray?

Answer 14

60 kV

2 mAs

Question 15

If there’s a high kV, is there more or less contrast?

Answer 15

Less contrast

Question 16

If there’s a low kV, is there more or less contrast?

Answer 16

More contrast

Question 17

What are x-rays used to detect?

Answer 17

Water- they detect water where there shouldn’t be any, which reveals abnormalities

Question 18

Are chest x-rays low or high contrast images?
Why?

Answer 18

Low contrast images

Because if they were high, we wouldn’t be able to see the lung linings

Question 19

Is a hand or chest x-ray higher contrast?

Answer 19

Hand x-ray is higher contrast

Question 20

What is contrast dependent on?
(4)

Answer 20

X-ray beam quality/kV

Thickness

Density

Atomic number

Question 21

Is a higher or lower kV more penetrating?

Answer 21

Higher kV

Question 22

Does a thicker or thinner material attenuate more x-rays?

Answer 22

Thicker

Question 23

Does a more or less dense material attenuate more x-rays?

Answer 23

More dense

Question 24

Does a higher or lower atomic number attenuate more x-rays?

Answer 24

Higher

Question 25

What is the normal kV and mAs of a chest x-ray?

Answer 25

100 kV 1/2 mAs

Question 26

What does AEC stand for?

Answer 26

Automatic Exposure Control

Question 27

Why does a high kV cause a lower contrast image? (4)

Answer 27

Increase in the kV increases the photon energy This is because the increase in the kV increases the speed of the electrons in the x-ray tube, which increases the photon energy This increases the penetrating power of the x-ray beam This will reduce contrast

Question 28

Why does a lower kV cause a higher contrast image? (3)

Answer 28

Lower kV reduces the penetrating power of the x-ray beam This results in less absorption and less transmission into the anatomy So a wider variety of of x-ray photons are coming out of the other side, causing a higher contrast

Question 29

What does density of the image mean?

Answer 29

How many photons get through the patient

Question 30

What is the density of the image a measure of?

Answer 30

Image darkening (how many photons get through the patient) More photons = more dense

Question 31

What is the density of the image proportional to?

Answer 31

The number of photons that are received by the detector

Question 32

What is the density of the image controlled by? (3)

Answer 32

The current in the x-ray tube (mAs) Source image distance (SID) Th material being imaged

Question 33

What do changes in the mAs have a direct effect on?

Answer 33

Density

Question 34

When the mAs decreases by half, what happens to the density?

Answer 34

The density decreases

Question 35

When the mAs is doubled, what happens to the density?

Answer 35

There’s an increase in density

Question 36

Does the image have a low or high density? Why?

Answer 36

Low density Because its been under-exposed

Question 37

What causes an image to have a low density? (4)

Answer 37

Too few photons are reaching the imaging receptor (lower mAs) This results in quantum noise This gives a grainy, mottled appearance The image lacks density

Question 38

In practice, when will a low density image only be seen?

Answer 38

If there’s a drastic under-exposure error made

Question 39

What causes a high density image? (4)

Answer 39

Too many photons reach the imaging receptor, causing the receptor to become saturated This increases the density This causes a poor definition between the soft tissue and the other tissues The dose to the patient is therefore too high

Question 40

How do we know if an image has good contrast?

Answer 40

If we can make out the details in the image

Question 41

What distance should the chest be from the detector? Why?

Answer 41

180cm Because it reduces magnification- as the photons travel through the air, the distance is higher so more photons are attenuated

Question 42

What causes the heart to appear larger if we do an AP chest x-ray?

Answer 42

The heart is more anterior, so it’ll be closer to the detector

Question 43

What does kV affect?

Answer 43

Contrast

Question 44

What does mAs affect?

Answer 44

Density

Question 45

What is the normal kV and mAs of an AP pelvis?

Answer 45

85 kV 16 mAs

Question 46

What is the normal kV and mAs of the knee?

Answer 46

63 kV 2.5 mAs

Question 47

What is the normal kV and mAs of a PA chest?

Answer 47

0 kV 1.6 mAs