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SCATTER AND GRIDS

SCATTER AND GRIDS. Chapter 5, Fauber Supplement-Chapter 11, Bushong. REVIEW:MATTER OR TUBE INTERACTION?. Bremsstrahling Characteristic Coherent Compton Exit/remnant Infrared Photoelectric. Scatter. Which interaction causes scatter? What factors increase scatter?

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SCATTER AND GRIDS

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  1. SCATTER AND GRIDS Chapter 5, Fauber Supplement-Chapter 11, Bushong

  2. REVIEW:MATTER OR TUBE INTERACTION? • Bremsstrahling • Characteristic • Coherent • Compton • Exit/remnant • Infrared • Photoelectric

  3. Scatter • Which interaction causes scatter? • What factors increase scatter? • If low kVp is used, what type of matter interaction increases?

  4. REDUCING SCATTER • Beam restriction • Collimation – as collimation increases what happens to patient’s dose? • Collimation – as collimation increases what happens to the production of scatter? • Collimation- as collimation increases what happens to the scale of contrast- shorter or longer?

  5. Beam restricting devices • Aperture diaphragms • “hole” • Cones/Cylinders • Collimators • Two sets of adjustable lead shutters (pg 144) • PBL

  6. REVIEW • INHERENT FILTRATION? • ADDED FILTRATION? • BY LAW, MINIMUM AMOUNT OF FILTRATION? • LONG SCALE CONTRAST AND SCATTER- IS THERE A RELATIONSHIP?

  7. GRID • 1913 Dr.Gustave Bucky. • 1/4 inch thick and range from 8 x10 to 17 x17 • Lead strips separated by interspaces • Center line directs the radiographer as to which way the lines are running.

  8. WHEN TO USE A GRID • Pt thickness • kVp chosen (some argument here!!!) • What exactly does a grid do? • (see pg 124) • Absorbs scatter while allowing primary radiation to pass through

  9. GRID CONSTRUCTION • INTERSPACE MATERIAL • MADE FROM? • GRID FREQUENCY • Higher the Frequency, the thinner the interspace material • GRID RATIO • RATIO OF HEIGHT OF LEAD STRIPS TO THE DISTANCE BETWEEN THEM h/D • HIGHER THE RATIO, THE ---------THE FREQUENCY • WATCH OUT FOR TESTING TRICKS- THE THICKNESS OF THE LEAD IS NOT USED TO CALCULATE GRID RATIO.

  10. GRID PATTERNS • Linear • Cross or cross hatched • Parallel or non-focused grid • Focused grid • Focal distance and convergent point • Recommended SID • Wafer grid/grid cassette/grid cap

  11. STATIONARY VS RECIPROCATING GRIDS Potter Bucky diaphragm

  12. Grid Conversion • See page 130, table 5-2 • Add 10:1 Bucky Factor/GCF = 5 • Let’s discuss an easier way to increasing grid ratio and missing factor

  13. GRID BOO BOOS • Grid cut-off –FOUR TYPES • Upside down focused grid boo boo • Off level grid boo boo • Off center grid boo boo • Off focus grid boo boo

  14. Grid cut-off -4 TYPES Upside down focused grid boo boo Off level grid boo boo Off center grid boo boo Off focus grid boo boo Loss of density, edges Over all reduction of density Over all reduction of density Loss of density, periphery GRID BOO BOOS

  15. A typical grid is or has • Linear Why? • Focussed Why? • Mid ratio Why? • Focal range of 40 or 72 inches Why?

  16. AIR GAP (Works like a grid) • Increased OID • Less scatter reaching IR • Why is this limited in its usefulness?

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