1. 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 Configuration
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

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

7. Collimator Features Numerical field size indicators
Field size is SID dependent
40”

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

9. 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