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Science and Technology 1 > Exposure Factors > Flashcards

Exposure Factors Flashcards

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1
Q

How are x-rays produced?
(3)

A

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

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2
Q

What does mAs mean?

A

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

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3
Q

What does the mAs do?

A

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

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4
Q

What does kV mean?

A

Voltage

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5
Q

What does the kV do?

A

It controls the penetrating power of the x-ray beam

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6
Q

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

A

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

Because less of the x-rays are attenuated

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7
Q

What is contrast?

A

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

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8
Q

What does it mean if we have high contrast?

A

It’s easy to tell the difference between adjacent structures

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9
Q

What does it mean if we have low contrast?

A

It’s hard to tell the difference between adjacent structures

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10
Q

How many shades of grey can humans distinguish?

A

30 shades

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11
Q

Is this high, medium or low contrast?

A

High contrast

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12
Q

Is this high, medium or low contrast?

A

Medium contrast

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13
Q

Is this high, medium or low contrast?

A

Low contrast

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14
Q

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

A

60 kV

2 mAs

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15
Q

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

A

Less contrast

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16
Q

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

A

More contrast

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17
Q

What are x-rays used to detect?

A

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

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18
Q

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

A

Low contrast images

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

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19
Q

Is a hand or chest x-ray higher contrast?

A

Hand x-ray is higher contrast

20
Q

What is contrast dependent on?
(4)

A

X-ray beam quality/kV

Thickness

Density

Atomic number

21
Q

Is a higher or lower kV more penetrating?

A

Higher kV

22
Q

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

A

Thicker

23
Q

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

A

More dense

24
Q

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

A

Higher

25
Q

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

A

100 kV

1/2 mAs

26
Q

What does AEC stand for?

A

Automatic
Exposure
Control

27
Q

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

A

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

28
Q

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

A

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

29
Q

What does density of the image mean?

A

How many photons get through the patient

30
Q

What is the density of the image a measure of?

A

Image darkening (how many photons get through the patient)

More photons = more dense

31
Q

What is the density of the image proportional to?

A

The number of photons that are received by the detector

32
Q

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

A

The current in the x-ray tube (mAs)

Source image distance (SID)

Th material being imaged

33
Q

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

A

Density

34
Q

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

A

The density decreases

35
Q

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

A

There’s an increase in density

36
Q

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

A

Low density

Because its been under-exposed

37
Q

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

A

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

38
Q

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

A

If there’s a drastic under-exposure error made

39
Q

What causes a high density image?
(4)

A

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

40
Q

How do we know if an image has good contrast?

A

If we can make out the details in the image

41
Q

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

A

180cm

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

42
Q

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

A

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

43
Q

What does kV affect?

A

Contrast

44
Q

What does mAs affect?

A

Density

45
Q

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

A

85 kV

16 mAs

46
Q

What is the normal kV and mAs of the knee?

A

63 kV

2.5 mAs

47
Q

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

A

0 kV

1.6 mAs

Science and Technology 1 (17 decks)

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