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Xray Physics > Midterms > Flashcards

Midterms Flashcards

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1
Q

Name 4 different ways to control scatter

A
  1. Use relatively low kVp
  2. Use compression
  3. Collimate
  4. Use grids
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2
Q

T or F

As we increase kVp we increase compton scatter

A

True

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3
Q

What is optimum kVp to control compton scatter?

A

70-90

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4
Q

At what kVp do we increase the probability of compton scatter?

A

60-125 kVp

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5
Q

T or F

Increased patient thickness improves contrast

A

False

Decreased thickness improves contrast

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6
Q

What is the advantage of a PA vs. an AP lumbar view?

A

Decreases shape distortion because the angle of the vertebrae match the angles of the beams

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7
Q

T or F

Decreased collimated area = increased compton scatter?

A

False

Increase collimated area increase compton scatter.

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8
Q

What is the rule when you are collimating significantly in regards to mAs? Why?

A

When collimating significantly increase mAs by 50%
- you have to increase at least 30% to see a difference. You are losing scatter when you collimate but you have to make up for it by increasing mAs

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9
Q

T or F

When you have 0 OID you will lose more X-rays and therefore less density and will have to increase mAs

A

False!

When you have no OID the same scatter on the object will be on the film. (no air gap)

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10
Q

What happens when you have increase OID?

A

Increased OID = increased divergent rays and therefore lose density and will therefore have to increase mAs by 50%

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11
Q

T or F

Tissue compression is the best device to absorb Compton Scatter

A

False!!!

Bucky Grids are the best device

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12
Q

What is the base of a Xray film made of?

A

Polyester plastic

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13
Q

What is the emulsion on the Xray film?

A

Silver halide crystals

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14
Q

T or F
The faster the “speed” of a film the more sensitive the film is and less x-ray is needed to produce the same radiographic density.

A

True

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15
Q

What is the purpose of the intensifying screens?

A

Reduce exposure

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16
Q

What is the most important factor for radiation protection?

A

Screens

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17
Q

If you go from a 200 speed to a 600 speed film what must you do to the mAs?

A

Divide by the factor of change. You multiplied the speed by a factor of 3 and will therefore have to reduce the mAs by 1/3

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18
Q

What is quantum mottle?

A

Graininess on a film

Considered visual noise

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19
Q

Why is quantum mottle or graininess a bad thing?

A

Interferes with our ability to to interpret info on the film

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20
Q

Why does the high speed systems result in increased quantum mottle?

A

Fast systems use relatively few X-rays to stimulate the screen. It is almost like you see the pattern of each packet and where it has interacted with the screen. Similar to having fewer pixels.

21
Q

What are the different types of grids?

A

Linear Parallel
Linear Focused
Crosshatch cutoff

22
Q

When should we use a radiographic grid?

A

When the body part is greater than 10 cm

23
Q

At what kVp should we consider using grids?

A

When using greater than 60 kVp

24
Q

What is the “grid material” made of?

A

lead

25
Q

What are the lines of radiolucent “interspace material” made of?

A

Plastic, aluminum, carbon fibre.

26
Q

How do grids work?

A

They transmit only the rays in line from the focal spot to the film. Obliquely angled rays are “cleaned up”

27
Q

When is a parallel linear grid recommended?

A

If both 40” and 72” SID are used.

28
Q

T or F

In a parallel linear grid, they are usually installed with the grid lines running parallel to the floor?

A

False!

Usually installed vertically in upright position in bucky cabinet.

29
Q

How are the strips angled in a focused linear grid in terms of the beam?

A

Angled to coincide with the divergent beam.

30
Q

T or F

A focused linear grid can be used at both 40” & 72” SID

A

False

They cannot!! Must be at a very specific SID because you have a very specific divergence.

31
Q

What grid is very efficient in removing scatter in both directions?

A

Crosshatch

32
Q

Why is crosshatch grids rarely used?

A

CR must be perpendicular to the centre of the grid.

Can’t angle the beam in ANY direction

33
Q

What is the term used for when you use the wrong grid and an undesirable absorption of the primary beam by a grid.

A

Grid Cutoff

34
Q

What is grid ratio?

A

The height of lead strips / width of the interspace strip

35
Q

T or F

Increase grid ratio - decrease contrast

A

False

it will increase contrast

36
Q

T or F

Increase grid ratio - increase lateral angle tolerance

A

False!

Decreases lateral angle tolerance

37
Q

T or F

As grid ratio increases patient exposure decreases

A

False!

As grid ratio increases patient exposure increases

38
Q

Define grid frequency

A

number of lines per inch or cm

39
Q

What happens when you increase the grid frequency in terms of visibility of lines?

A

decrease line visibility

40
Q

T or F

High grid frequency requires a mobile device

A

False!

Low grid frequency requires a mobile device.

41
Q

Name the three types of moving grids

A

Single stroke/manually cocked
Reciprocating
Oscillating

42
Q

T or F

A single stroke grid moves in two directions

A

False

Only in one direction

43
Q

How does a reciprocating grid work?

A

Motor driven back and forth during the exposure. Movement is achieved by electromagnets

44
Q

What happens from lateral de-centring of a grid?

A

CR off centre to the left or right and causes collision of primary x-rays with the lead strips.

45
Q

With what type of grids is lateral de-centering a problem?

A

Focused or crosshatch

46
Q

What happens when a grid is off level and what grids are affected most?

A

Either the grid is angled or the CR is angled against the direction of the strips. Collision of primary x-rays with grid strips.
- problem for ALL types of grids.

47
Q

What happens when a focused grid is used at improper SID

A

You will see the cutoff pattern and density will decrease on both lateral sides of the radiograph.

48
Q

Name the advantage and disadvantage or using an air gap technique instead of a grid?

A 
A = scatter escapes before it hits the film 
D = increased magnification and blur on image.

Xray Physics (1 decks)

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