Scatter Which interaction causes scatter? What factors increase scatter? If low kVp is used, what type of matter interaction increases?
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?
Beam restricting devices Aperture diaphragms “hole” Cones/Cylinders Collimators Two sets of adjustable lead shutters (pg 144) PBL
REVIEW INHERENT FILTRATION? ADDED FILTRATION? BY LAW, MINIMUM AMOUNT OF FILTRATION? LONG SCALE CONTRAST AND SCATTER- IS THERE A RELATIONSHIP?
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.
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
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.
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
STATIONARY VS RECIPROCATING GRIDS Potter Bucky diaphragm
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
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
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
A typical grid is or has LinearWhy? Focussed Why? Mid ratioWhy? Focal range of 40 or 72 inchesWhy?
AIR GAP (Works like a grid) Increased OID Less scatter reaching IR Why is this limited in its usefulness?