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1. Medical Radiation Sciences > 8. Beam Geometry > Flashcards

8. Beam Geometry Flashcards

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USMLE® Step 1 flashcards
1
Q

What is magnification caused by?

A

The diverging beam, the outer part of the beam makes the object appear larger

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2
Q

What is the magnification factor?

A

How much larger the object appears in the image

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3
Q

What are two ways to reduce magnification?

A
  1. Increase SID
  2. Decrease OID (has a greater effect)
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4
Q

What is distortion affected by?

A
  1. SID
  2. OID
  3. Object position
  4. Thickness of object
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5
Q

What are the 2 types of shape distortion?

A
  1. Foreshortening
  2. Elongation
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6
Q

When does elongation happen?

A

When object is parallel to IR but central ray is angled and when object is parallel to CR but IR is angled

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7
Q

When does foreshortening happen?

A

When object is angled but IR is flat

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8
Q

What are two ways that we can use angles to our advantage?

A
  1. Bisecting angle method
  2. Superimposition
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9
Q

How does the bisecting angle method reduce distortion?

A

The angle of the CR to the IR is 1/2 angle of object to the CR

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10
Q

What is focal spot blur caused by?

A

multiple point sources on the larger fs

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11
Q

What is another word for geometric unsharpness?

A

penumbra

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12
Q

What is the inverse square law caused by?

A

The diverging beam: as you get farther from the source, more and more of the diverging beam misses the object

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13
Q

What are the pros for the LARGE focal spot?

A
  1. More area to be hit by x ray beam
  2. Less pitting of target surface
  3. Higher ratings
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14
Q

What are the cons for the LARGE focal spot?

A
  1. More point sources
  2. More blur
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15
Q

What are the pros for the SMALL focal spot?

A
  1. Sharper image
  2. Less blurring
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16
Q

What are the cons for the SMALL focal spot?

A
  1. Lower ratings
  2. More easily damaged
17
Q

What is the line focus principle?

A

Uses angles to achieve large actual focal spot and effective small focal spot

18
Q

What is the effective focal spot?

A

-Focal spot as seen from the POV of the IR
-Square shape
-Point sources are closer together which creates better spatial resolution

19
Q

What does it mean if the anode angle from the vertical is 45 degrees?

A

Effective size is equal to the incident electron beam size

20
Q

What does it mean if the anode angle from the vertical is less than 45 degrees?

A

The effective size is smaller than incident electron beam size

21
Q

What is the usual anode angle from the vertical

A

12-17 degrees

22
Q

What is the anode heel affect?

A

Caused by the angled target, some photons travel through the target causing attenuation. It happens with steeper angles

23
Q

What are the benefits of the anode heel affect?

A
  1. Can act like a filter (Align anode to thinner portion of the body)
  2. Used for T spine and abdomen
24
Q

How can you reduce the anode heel effect?

A

Longer SID and smaller field sizes

25
Q

What is the biangular anode?

A

Anodes that have two different angles and you can choose either one depending on if you need a sharper image image or a larger field size

26
Q

If you have a larger angle of the anode, what will happen to the actual and effective focal spot?

A

Same actual fs but larger effective fs

27
Q

If you have a larger angle of the anode, what will happen to the anode heel affect?

A

Less anode heel affect

28
Q

If you have a larger, shallower angle of the anode, what will happen to the blur and spatial resolution?

A

Increased focal spot blur and decreased SR.

29
Q

If you have a smaller steeper angle, what will happen to the actual and effective fs?

A

Same actual fs and smaller effective fs

30
Q

If you have a smaller steeper angle, what will happen to the heel affect?

A

More heel affect

31
Q

If you have a smaller steeper angle, what will happen to the fs blur and SR?

A

Decrease in fs blur and better SR

1. Medical Radiation Sciences (18 decks)

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