Projection Geometry

Question 1

What are the 2 views and problems

Answer 1

• Plain view and Projection view
• 2D image of 3D objects
• Problems
  
  • Unsharpness
  • Magnification
  • Distortion
  • All 3 are always present

Question 2

Why are the 3 issues always present

Answer 2

• The focal spot is an area, never a point
• The X-rays diverge from their source
• Images are 2D Representations of 3D structures

Question 3

Spatial Resolution

Definition

Measure by

Answer 3

• The degree/ability of how well the image can display 2 objects that are close together
• Measured in line pairs per mm (lp/mm)
• Higher spatial resolution is desirable

Question 4

Spatial Resolution

Line Pair

Answer 4

• Line pair
  
  • 1 radiopaque (lead) line and radiolucent space
  • (The black line and space, which is of equal width=Line Pair)
• Lead (radiopaque) line has same thickness as radiolucent space

Question 5

Spatial Resolution

Resolving Power

high or low

Dependent

Answer 5

• The resolving power is measured as the highest number of line pairs per millimeter that can be distinguished on a radiograph
• We hant HIGH spatial resolution
• The spatial resolution is dependent on the type of receptor (fil/digital) used and geometric factors

Question 6

Sharpness

Definition

What leads to unsharpness

Answer 6

• The degree of how well the image can display the boundary (outline) between two differing radiodensitiies
• Sharp boundaries are desirable
• Unsharpness at the boundary of an object is due to penumbra and parallax

Question 7

Penumbra

Definition

Answer 7

• The space of partial illumination between the perfect shadow and full light
• Penumbra is where the unsharpness is
  
  • Cant tell the edge from the boundaries of shadow (blue and red lines)

Question 8

Penumbra Reduction

Answer 8

• Use as small an effective focal spot as pratical -O.4mm
• Increase the distance between the focal spot and the object
• Minimize the distance between the object and image receptor

Question 9

Parallax

Answer 9

• Apparent change in position or size of a subject when viewed from different perspectives
• Only happens in Dental film- double emulsion
• Film on front and back to capture image twice and reduce dosage to patient
• With the 2 outlines the image can be blurry

Question 10

Movement Types 3

Answer 10

• Just like a camera, create blurry lines
  
  • Source movement (tube head movement)
  • Object movement (paient movement)
  • Receptor movement (patient movement of sensor/ film in mouth)

Question 11

Distortion

2 types

Answer 11

• Distortion is present in all radiographs
• Image size distortion
  
  • Due to divergent paths of photons in x ray beam
  • Leads to equal magnification (positive or negative) of the objects
• Image shape distortion
  
  • Due to the alignment of the photon beam in respect with the object and the receptor
  • Unequal magnification indiffernt parts of the same object

Question 12

Image Size Distortion

2 ways to prevent

Answer 12

• Increase in source to object decreases size distortion
  
  • Move the source back and you will get a truer size, the closer the more magnified
• Decrease in object to receptor distance decreases size distortion

Question 13

Image Shape Distortion

2 forms

Answer 13

• Foreshortening
  
  • Object is not parallel to beam and receptor
  • Causing the portion of tooth farther from receptor to be magnified
• Elongatoin
  
  • When the receptor is not perpendicular to source and object, the whole thing is magnified

Question 14

2 General ways to improve image distortion

Answer 14

Have object parallel to receptor

Beam perpendicular to long axis of tooth and receptor

Question 15

Localization of Objects

uses what 3 techniques

Answer 15

• Make 2 images perpendicular to one another
• Buccal Object Rule using shift shot
• Cone beam computed tomography 3D

Question 16

SLOB Rule

Answer 16

• Same Lingual
  
  • Move tube mesially
  • Dot moves mesially
• Opposite Buccal
  
  • Tube moved mesially
  • Dot moved distally