Resident Physics Lectures Christensen Chapter 7 XRay Beam

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 Film With cone angle as above, this functions as an aperture Reduced Penumbra

Collimators �Advantages ◦ 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 Filter Mirror include filter & light Focal Spot Lamp 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