Projection Geometry

Question 1

What are plain projections?

Answer 1

• Conventional radiograph made w/ stationary x-ray source
• Displays 2D image of body part
• ENTIRE VOLUME of tissue b/n x-ray source & image receptor is = projected onto 2D image

Question 2

Regarding Plain Projections how can you MAXIMIZE value from a radiograph?

Answer 2

1) Clear understanding of normal anatomy

2) Mentally reconstruct a 3D view of anatomic structure of interest

Question 3

What is sharpness?

Answer 3

How well a boundary b/n 2 areas of differing radio density is revealed

Question 4

What is spatial resolution?

Answer 4

How well a radiograph is able to reveal SMALL OBJECTS that are close together

Question 5

What are the 2 distinct yet interdependent features ?

Answer 5

Image Sharpness & Resolution

Note: Desirable to optimize imaging–>BOTH high sharpness & resolution

Question 6

What are 3 ways to optimize image sharpness?

Answer 6

1) Use SMALL effective focal spot
2) INCREASE distance b/n focal spot & object by using a LONG OPEN ended cylinder
3) MINIMIZE distance b/n onject & image receptor

Question 7

Small focal spot = _____________

Answer 7

Small area of unsharpness

Question 8

Large focal spot = _______________

Answer 8

Large area of unsharpness

Question 9

• Cathode shoots to __________

- ___________ are a product of this.

Answer 9

• Anode

- Photons

Question 10

Regarding Image Size Distortion, Increasing distance from focal spot to object does what to Anode?

Answer 10

• Distance from anode and object of interest, you don’t get as much unsharpness in the image.
• When it is closer to anode you get more unsharpness (divergence).

Question 11

Regarding Image Size Distortion, decreasing distance from focal spot to object does what to Anode?

Answer 11

• Divergence, wide zone of unsharpness.

- Try to find optimum distance

Question 12

What is Paralleling Technique?

Answer 12

1) Preferred method for making radiograph
2) Placing image receptor parallel w/ long axis of tooth
3) Minimizes image distortion

Question 13

Describe the Technique for Paralleling

Answer 13

1) Postion receptor TOWARD MIDDLE of oral cavity away from teeth
(however, does not result in some image magnification & loss of sharpness)

2) Overcome limitations:
a) Use relative long aiming cylinder “cone” to INCREASE focal spot -object distance

b) “Cone” directs only most central & parallel rays of beam to teeth & image receptor.
(Decrease image magnification & Increase sharpness

c) Image receptors used to support receptor in pt mouth to align beam

Question 14

What is Foreshortening( Image Shape Distortion)?

Answer 14

• Unequal magnification

- Image receptor is placed PERPENDICULAR to the X-ray beam and the tooth is at an angle.

Question 15

What is Elongation (Image Shape Distortion)?

Answer 15

-Tooth is closer to anode (source to object distance is decreased) & receptor is NOT perpendicular to the beam.

Question 16

What is the ideal placement of receptor and beam?

Answer 16

Receptor parallel to long axis of the tooth and beam lined up perpendicular to receptor

Question 17

What is Object Localization?

Answer 17

1) Deriving 3D info from 2D radiographic image

• EX: determine location of foreign object or impacted (canine) tooth w/ in jaw
• Take a base image and an occlusal image to see where tooth is on palatal

Question 18

What are 2 methods for Object Localization ?

Answer 18

1) 2 images (views) projected @ right angeles (90) to each other
2) Tube shift technique-identifies spatial postion
3) Rationale derives from manner in which relative positions of x-ray image of 2 separate objects change when projection angle at which images were made changes

(AKA buccal object rule or SLOB rule [Same Lingual Opposite Buccal]

Question 19

If object does not move w/ respect to reference object when tube head is moved, object is _________

Answer 19

• In the MIDDLE

Question 20

What is the Eggshell Effect?

Answer 20

1) Pain images- images that project a 3 D volume onto a 2 D receptor- may produce an eggshell effect of CORTICATED structures
2) Radiograph of a hard boiled egg. Note the rim of eggshell is opaque even tho it is uniform in thickness

Question 21

• __________ view of the egg being exposed to an x-ray beam
• Top photon has a __________ path through _______ of the egg & a _________ path through shell of egg than __________ photon.
• As a result, _________ traveling through ___________ of a curved surface are more attenuated than photons traveling @ ________ angles to surface.

Answer 21

• Schematic
• Tangential
• Apex
• Longer
• Photons
• Periphery
• Right

Question 22

• Regarding the Eggshell effect, an _______ lesion on the buccal surface of the mandible on an occlusal view.
• Expanded cortex is more ________ that the region _______ the border as a result of the eggshell effect.

Answer 22

• Expansile
• Opaque
• Inside

Note: More cortical bone on the outer surface