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
