Resident Physics Lectures

1
Resident Physics Lectures

• Christensen, Chapter 7
• X-Ray Beam Restrictors

George David Associate Professor Department of
Radiology Medical College of Georgia
2
Restrictors Types

• Aperture Diaphragms
• Cones and Cylinders
• Collimators

3
Aperture Diaphragms

• Lead sheet with hole in center
• simple
• large penumbra (shadow)
• Lead close to focal spot

Focal Spot
Penumbra
Film
4
Cones Cylinders
Focal Spot
Focal Spot
Film
Film
With cone angle as above, this functions as an
aperture
Reduced Penumbra
5
Collimators

• Advantages
• Provides adjustable rectangular field
• fluoro may also have circular field
• light beam indicates x-ray field
• Configuration
• two sets of shutters
• longitudinal
• transverse
• sets may actually have two pairs of shutters
  operating together

One set two pairs
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Collimators

• Generally include filter light

7
Collimator Features

• Numerical field size indicators
• Field size is SID dependent

8 10 12 14 17
40
8 10 12 14 17
8
Collimator Features

• Field center indicator (crosshairs)
• plastic sheet attached to bottom of collimator
• two black lines drawn on it
• light projects field center on patient

Shutters
Crosshairs
Bottom of Collimator
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Positive Beam Limitation(PBL or Automatic
Collimation)

• senses film size, orientation, SID
• source - image distance
• limits x-ray field to size of film
• expensive
• PBL requirement has been dropped for
  manufacturers
• PBL does not insure tube bucky are aligned!

Cassette
10
Collimator Checks

• X-Ray / Light Field alignment
• Alignment of x-ray field to bucky
• Use tubestand centering devices
• lateral detent
• longitudinal alignment light
• Field Size Indicator accuracy
• PBL accuracy

11
Collimation Patient Dose Reduction

• 10 X 10 cm field has 1/4 the area of a 20 X 20 cm
  field
• small change in field size has large effect on
• volume of irradiated tissue
• scatter

12
Scatter Reduction

• Less scatter improved image quality
• quantity of scatter depends on field size
• primary beam intensity independent of field size

13
Scatter Reduction

• as field size increases, so does fraction of
  scatter photons reaching film
• scatter peaks at 30cm X 30cm field size
• smaller field size may require higher technique
• compensates for loss of density from scatter
  photon