Beam Restrictors - Department of Radiology
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Transcript Beam Restrictors - Department of Radiology
Resident Physics Lectures
Christensen, Chapter 7
X-Ray Beam
Restrictors
George David
Associate Professor
Department of Radiology
Medical College of Georgia
Restrictors Types
Aperture
Diaphragms
Cones and Cylinders
Collimators
Aperture Diaphragms
Lead sheet with hole in center
◦ simple
◦ large penumbra (shadow)
Lead close to focal spot
Focal Spot
Penumbra
Film
Cones & Cylinders
Focal Spot
Focal Spot
Film
Film
With cone angle as above, this
functions as an aperture
Reduced Penumbra
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
Collimators
Generally include filter & light
Filter
Mirror
Focal Spot
Lamp
Shutters (only
1 set shown)
Collimator Features
Numerical field size indicators
◦ Field size is SID dependent
8 10 12 14 17
40”
8 10 12 14 17
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
Positive Beam Limitation
(PBL or Automatic Collimation)
senses film size,
orientation, & SID
◦ source - image distance
limits
x-ray field to
size of film
No longer required
PBL does not
insure tube &
bucky are aligned!
Cassette
Collimator Checks
X-Ray / Light Field alignment
Alignment of x-ray field to bucky
Field Size Indicator accuracy
PBL accuracy