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 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 expensive PBL requirement has been dropped for manufacturers PBL does not insure tube & bucky are aligned! Cassette

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

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

Scatter Reduction Less scatter = improved image quality quantity of scatter depends on field size primary beam intensity independent of field size

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