Geometric Characteristics of Imaging

All radiographic projections are plagued by three problems:

• Unsharpness (penumbra)
• Magnification
• Distortion

X-rays originate from a small plane (focal spot) as numerous point sources. Each point source forms its own image. Edges of images will not be at the same point on the film or sensor, hence images are always unsharp (blurred).
X-rays travel in diverging lines, which projects edges of objects farther away from each other. This results in magnification.
Structures have 3 dimensions, but are projected onto a 2 dimensional film/sensor. Different parts of the structure are varying distances, hence magnification is different at each distance distorting image.

Unsharpness (Penumbra)

Unsharpness is minimized by:

• Long source-object distance --longer PID
• Short object-flim distance --place film closer to tooth surface
• Small focal spot --done by manufacturer
• Limiting patient motion

Magnification

Equal Enlargement of all parts of the image.
Magnification is reduced by:

• Long source-object distance --longer PID
  
• Short object-flim distance --place film closer to tooth surface
  
• Small focal spot

Distortion

Unequal enlargement of parts of the image. Normal crown to root ratio is 1:2. Distortion appears across all teeth. If patient has naturally long roots then it will appear in only a few teeth.
Reduced by:

• Parallel alignment of teeth and film --film parallel to long axis of tooth
• Perpendicular alignment of beam and teeth/ x-ray is at right angle to film

Summary: Accurate Image Formation

• Long source-object distance --longer PID
  
• Short object-film distance --place film closer to tooth surface
  
• Small focal spot
• Perpendicular alignment of beam and teeth/ x-ray is at right angle to film