Plain Film

1
Resident Physics Lectures

• Plain Film
• Tomography

George David Associate Professor Department of
Radiology Medical College of Georgia
2
Body Section Tomography

• Not CT
• Body Section Imaging
• Goal
• keep plane of interest in focus
• blur all other plans
• enhances contrast
• Popularity decreasing because of
• CT
• MRI

3
Tomography History

• Predates CT by decades
• Used to be popular for inner ear studies
• Still used at MCG for IVPs

4
Tomography Blurring

• Blurring accomplished by synchronous movement of
  tube film
• tube bucky physically connected by rod
• rod pivots around fulcrum
• tube moves one direction
• film moves in other direction

5
Conventional Tomography Blurring

• Image produced on film
• Objects above or below fulcrum plane change
  position on film thus blur

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Tomography Blurring

• the further from the fulcrum an object is, the
  more it blurs!

7
Tomography Blurring

• objects shaped oriented in direction of motion
• elongate
• do not blur

8
Tomographic Blurring

• Blurring improved by use of complex motions
• circular
• tri-spiral
• elliptical

Hypocycloidal
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Complex Blurring

• The more complex the blurring motion
• the sharper the tomographic image
• better blurring
• the more expensive the machine

10
Tomo Patient Doses

• Tomo can be high dose procedure
• several rads not unusual
• one image per cut with film
• cuts at many levels routinely employed to find
  cut of interest
• each cut exposes entire field
• Can do multiple images per cut with digital
  receptor

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Thickness of Cut

• Same as thickness of region in focus
• Determined by angle tube moves
• Larger angles yield thinner cuts
• more motion more blurring
• Smaller angles yield thicker cuts
• 0o (stationary) yields conventional film

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Thickness of Cut
Smaller Angle Thicker Cut
Larger Angle Thinner Cut
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Cut Thickness

• Approximate cut thickness for linear tomo

Tomo Angle Thickness of Cut (mm)
(degrees) ----------------------------------------
--------------- 0 Infinite (non-tomo)
2 31 4 16 6 11 10 6 20 3 35 2 50
1
14
Grids Tomo

• Linear grid used
• Grid lines parallel with table for linear tomo
• Grid must change orientation (rotate) in complex
  motion tomo
• eliminates grid cutoff