2 REVIEW:MATTER OR TUBE INTERACTION? BremsstrahlingCharacteristicCoherentComptonExit/remnantInfraredPhotoelectric
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/CylindersCollimatorsTwo 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 GRID1913 Dr.Gustave Bucky.1/4 inch thick and range from 8 x10 to 17 x17Lead strips separated by interspacesCenter 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 GRID FREQUENCY MADE FROM? Higher the Frequency, the thinner the interspace materialGRID RATIORATIO OF HEIGHT OF LEAD STRIPS TO THE DISTANCE BETWEEN THEM h/DHIGHER THE RATIO, THE THE FREQUENCYWATCH 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 gridFocused gridFocal distance and convergent pointRecommended SIDWafer 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