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 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

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

Collimator Features Numerical field size indicators Field size is dependent on source to image distance SID Focal spot to image receptor 8 10 12 14 17 40” 8 10 12 14 17

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 are aligned! Cassette