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Resident Physics Lectures

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Resident Physics Lectures

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    1. Resident Physics Lectures Christensen, Chapter 7 X-Ray Beam Restrictors

    2. Restrictors Types Aperture Diaphragms Cones and Cylinders Collimators

    3. Aperture Diaphragms Lead sheet with hole in center simple large penumbra (shadow) Lead close to focal spot

    4. Cones & Cylinders

    5. 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

    6. Collimators Generally include filter & light

    7. Collimator Features Numerical field size indicators Field size is SID dependent

    8. 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

    9. 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!

    10. 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

    11. 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

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

    13. 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

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