9/10: Projection Geometry

Question 1

What are the geometric characteristics?

Answer 1

Image sharpness
Image magnification
Image shape distortion

Question 2

This is the fuzzy, unsharp margin of radiographic image

Answer 2

Unsharpness

Question 3

What are examples of unsharpness margins?

Answer 3

Penumbra
Edge gradient

Question 4

What are the 3 rules to maximize sharpness?

Answer 4

1. Radiation source should be be as small as possible
2. Source-to-object distance should be as long as possible
3. Object-to-receptor distance should be as short as possible

Question 5

This is the shadow of the image we receive

Answer 5

Umbra

Question 6

This is the unsharp number. Area around the margins of the object

Answer 6

Penumbra

Question 7

The closer the image, therefore the larger the focal spot, the

Answer 7

Larger the penumbra

Question 8

The further the image, therefore the smaller the focal spot, the

Answer 8

Smaller the penumbra

Question 9

How can you control source-to-object distance?

Answer 9

Length of cone → larger = less divergence → more sharpness

Question 10

This is the equal enlargement of radiographic image, compared to the actual size of the object

Answer 10

Magnification

Question 11

What are the 2 rules to minimize magnification?

Answer 11

1. Source-to-object distance should be as long as possible
2. Object-to-receptor distance should be as short as possible

Question 12

When you magnify, the image shows

Answer 12

True shape of an object

Question 13

This is the “unequal enlargement”. Variation from true shape of object, unequal magnification of part of object. Improper alignment of receptor, object, beam

Answer 13

Distortion

Question 14

What are 2 rules to minimize shape distortion?

Answer 14

1. Object and receptor should be parallel
2. Beam should be perpendicular to both object and receptor

Question 15

What are the types of distortion?

Answer 15

Foreshortening
Elongation

Question 16

This is when the tooth is not parallel to receptor, and beam directed perpendicular to receptor

Answer 16

Foreshortening

Question 17

This when tooth is not parallel to receptor, and beam directed perpendicular to tooth

Answer 17

Elongation

Question 18

What are the 5 rules for accurate image formation?

Answer 18

1. Focal spot as small as possible
2. Source-object distance as long as possible
3. Object-receptor distance as short possible
4. Object parallel to receptor
5. Beam perpendicular to object and receptor

Question 19

When doing PAs, what should you use for the paralleling technique?

Answer 19

Long cone

Question 20

When doing PAs, what should you use for the bisecting angle technique?

Answer 20

Long or short cone

Question 21

why do you want the receptor paralell to the tooth

Answer 21

increaes S-R distance
increases O-R distance

Question 22

Of the PA techniques, what is the preferred method?

Answer 22

Paralleling technique

Question 23

If the paralleling technique cannot be used, what technique should be used?

Answer 23

Bisecting angle technique

Question 24

The bisecting angle technique is based on what?

Answer 24

Rule of isometry

Question 25

What is the rule of isometry?

Answer 25

2 triangles have 2 equal angles and a common side, then the 2 triangles are equal

Question 26

If you have incorrect horizontal angulation, what do you get

Answer 26

Overlapped contacts

Question 27

This is periapical and bitewing radiograph showing all teeth

Answer 27

FMX

Question 28

What can you use for radiographic localization?

Answer 28

Right angle method
Tube shift (SLOB)

Question 29

what will INCREASE magnification?

Answer 29

S-R distance is decreased
O-R distance is increased

Question 30

in the bisecting angle technique, the beam is perpendicular to what?

Answer 30

bisecting line
neither the tooth or receptor

Question 31

overlapping of contacts occur due error in what?

Answer 31

horizontal angulation

Question 32

according to the SLOB rule, when you move the tubehead mesially, the object moves mesially, where is the object located?

Answer 32

lingually