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Radiologic Physics > Chapter 11 > Flashcards

Chapter 11 Flashcards

1
Q

Primary factors of Exposure Technique

A

kVp - kilovoltage peak
mA - milliamperage
Time - seconds
mAs - milliamperage seconds

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2
Q

What mA is used for small filament?

A

100mA

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3
Q

Benefits of using small filament

A

Will use the small focal spot giving you sharpness and good spatial resolution

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4
Q

What mA is used for large body parts?

A

300 mA

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5
Q

What happens when large filament is used?

A

Will use large focal spot, giving you large penumbra, crappy spatial resolution - but good enough for large body part

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6
Q

What type of relationship does mA and s have

A

Inverse Proportional Relationship

Numbers can be adjusted to come out with the same dose (Resiprocity Law)

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7
Q

What kind of relationship is kVp and Receptor Expsoure (RE)

A

Direct - Increasing kVp will affect RE by making xrays stronger

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8
Q

Relationship between kVp and scatter

A

Increasing kVp, increased scatter

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9
Q

How do you get rid of scatter?

A

Grid

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10
Q

How does scatter affect contrast?

A

Lowers contrast

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11
Q

How does changing kVP affect xray transmission?

A

A higher kVP increases the number of xrays reaching the IR, therefore increasing transmission

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12
Q

What is the Inverse Square Law?

A

Relationship between distance of the xray beam and intensity

Initial Intensity (final distance)^2
——————— = ————————
Final intensity (Initial distance)^2

If distance doubles, divide dose by 4
If distance halves, times the dose by 4

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13
Q

What are the standards xray SIDs?

A

40 inch and 72 inch

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14
Q

What is the Direct Square Law?

A

Used when you change SID in order to maintain an optimum exposure image

Relationship between mAs and distance

Initial mAs Intl Dis ^2
————— = —————-
Final mAs Fin Dis ^2

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15
Q

What happens when SID increases/decreased (spatial resolution and magnification)?

A

Increases:
Spatial resoltuion increases and magnification decreases

Decreases:
Spatial resolution decreases and magnification increases

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16
Q

How do you fix a noisy image?

A

mAs

17
Q

Grid conversion factors

A

No grid –> 1
5:1 –> 2
6:1 –> 3
8:1 –> 4
10:1 –> 5
12:1 –> 5
16:1 –> 6

18
Q

Grid conversion equation

A

New mAs New grid factor
————- = ———————–
Old mAs Old grid factor

19
Q

Where does mA come from?

A

Step down transformer

20
Q

What is attenuation?

A

Decrease in intensity as the photon passes through matter

21
Q

What is the gray blanket on an image?

A

Scatter

22
Q

What is the relationship between kVp and attenuation?

A

Inverse

23
Q

What happens if the grid is tilted more than 3 degrees from perp

A

Grid cutoff

24
Q

What does it mean if a problem has the IR speed?

A

No grid was used.

25
Q

What is a good magnification factor?

A

1.1 to 1.3

26
Q

What is the line focus prinicple?

A

The relationship between the actual focal spot on the anode surface and the effective focal spot.

Decreasing the size of the anode angle, decreases the effective focal spot

27
Q

Magnification Factor Equation

A

SID/SOD

28
Q

Relationship between field size and collimation

A

Inverse

A large field size - decrease collimation

A smaller field size - increase collimation

29
Q

What are the effects of a large field size?

A

Increases the amont of tissue irradiated

Increasing scatter

Increasing the amount of radiation reaching the IR

Lowering contrast

30
Q

What are the effects of using tube filtration?

A

Decreases x-ray quantity

Filtration absorbs low-energy photons that do not contribute to the image.

Reduces patient dose by removing these photons

31
Q

Relationship between mAs and part thickness

A

Thinner parts - need less mAs

Thicker parts - need more mAs

32
Q

Guideline for thickness and mAs

A

For every change in part thickness of 4-5 cm, adjust the mAs by a factor of 2

33
Q

What exposure technique factors affect spatial resolution?

A

Focal Spot Size
SID
OID
Central Ray
Patient thickness
Motion

34
Q

What exposure technique factors affect contrast?

A

kVp
OID
Grid use
Collimation
Tube filtration
Patient thickness

35
Q

What exposure techniques affect RE?

A

kVp
mAs
Grid
OID
SID
CR Angle
Collimation
Tube filtration
Patient Thickness

36
Q

How does SID affect prenumbra?

A

The greater the SID, the less prenumbra

Radiologic Physics (9 decks)

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