2REVIEW:MATTER OR TUBE INTERACTION? BremsstrahlingCharacteristicCoherentComptonExit/remnantInfraredPhotoelectric
3Scatter Which interaction causes scatter? What factors increase scatter?If low kVp is used, what type of matter interaction increases?
4REDUCING 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?
5Beam restricting devices Aperture diaphragms“hole”Cones/CylindersCollimatorsTwo sets of adjustable lead shutters (pg 144)PBL
6REVIEW INHERENT FILTRATION? ADDED FILTRATION? BY LAW, MINIMUM AMOUNT OF FILTRATION?LONG SCALE CONTRAST AND SCATTER- IS THERE A RELATIONSHIP?
7GRID1913 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.
8WHEN 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
9GRID 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.
10GRID PATTERNS Linear Cross or cross hatched Parallel or non-focused gridFocused gridFocal distance and convergent pointRecommended SIDWafer grid/grid cassette/grid cap
11STATIONARY VS RECIPROCATING GRIDS Potter Bucky diaphragm
12Grid 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