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

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

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

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

Collimator Features Numerical field size indicators Field size dependent on source-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 aligned Cassette