8. Beam Geometry

Question 1

What is magnification caused by?

Answer 1

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

Question 2

What is the magnification factor?

Answer 2

How much larger the object appears in the image

Question 3

What are two ways to reduce magnification?

Answer 3

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

Question 4

What is distortion affected by?

Answer 4

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

Question 5

What are the 2 types of shape distortion?

Answer 5

1. Foreshortening
2. Elongation

Question 6

When does elongation happen?

Answer 6

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

Question 7

When does foreshortening happen?

Answer 7

When object is angled but IR is flat

Question 8

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

Answer 8

1. Bisecting angle method
2. Superimposition

Question 9

How does the bisecting angle method reduce distortion?

Answer 9

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

Question 10

What is focal spot blur caused by?

Answer 10

multiple point sources on the larger fs

Question 11

What is another word for geometric unsharpness?

Answer 11

penumbra

Question 12

What is the inverse square law caused by?

Answer 12

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

Question 13

What are the pros for the LARGE focal spot?

Answer 13

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

Question 14

What are the cons for the LARGE focal spot?

Answer 14

1. More point sources
2. More blur

Question 15

What are the pros for the SMALL focal spot?

Answer 15

1. Sharper image
2. Less blurring

Question 16

What are the cons for the SMALL focal spot?

Answer 16

1. Lower ratings
2. More easily damaged

Question 17

What is the line focus principle?

Answer 17

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

Question 18

What is the effective focal spot?

Answer 18

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

Question 19

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

Answer 19

Effective size is equal to the incident electron beam size

Question 20

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

Answer 20

The effective size is smaller than incident electron beam size

Question 21

What is the usual anode angle from the vertical

Answer 21

12-17 degrees

Question 22

What is the anode heel affect?

Answer 22

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

Question 23

What are the benefits of the anode heel affect?

Answer 23

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

Question 24

How can you reduce the anode heel effect?

Answer 24

Longer SID and smaller field sizes

Question 25

What is the biangular anode?

Answer 25

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

Question 26

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

Answer 26

Same actual fs but larger effective fs

Question 27

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

Answer 27

Less anode heel affect

Question 28

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

Answer 28

Increased focal spot blur and decreased SR.

Question 29

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

Answer 29

Same actual fs and smaller effective fs

Question 30

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

Answer 30

More heel affect

Question 31

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

Answer 31

Decrease in fs blur and better SR