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SCATTER AND GRIDS Chapter 6, Fauber Supplement-Chapter 14, Bushong.

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Presentation on theme: "SCATTER AND GRIDS Chapter 6, Fauber Supplement-Chapter 14, Bushong."— Presentation transcript:

1 SCATTER AND GRIDS Chapter 6, Fauber Supplement-Chapter 14, 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 147) 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 154, table 6-2  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 BOO BOOS  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  Loss of density, periphery

15 A typical grid is or has  LinearWhy?  Focussed Why?  Mid ratioWhy?  Focal range of 40 or 72 inchesWhy?

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


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