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 (partial shadow)
Lead close to focal spot
Focal Spot
Penumbra
Receptor

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

5. Collimators Advantages Two sets of shutters
Adjustable rectangular field
fluoro may have circular field
Light beam indicates x-ray field
Two sets of shutters
longitudinal
transverse
Each set may have 2 pairs of shutters operating together
One set;
two pairs

6. Radiographic Collimators
Generally include filter & light
Lamp
Mirror
Filter
Focal Spot
Shutters (only 1 set shown)

7. Collimator Features Numerical field size indicators
Field size dependent on source-image distance
SID
Focal spot to image receptor
40”

8. Positive Beam Limitation (PBL or Automatic Collimation)
Senses cassette size, orientation, & SID
source - image distance
limits x-ray field to size of receptor
PBL does not insure tube & bucky aligned
Cassette