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Resident Physics Lectures Christensen, Chapter 8Grids George David Associate Professor Department of Radiology Medical College of Georgia.

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Presentation on theme: "Resident Physics Lectures Christensen, Chapter 8Grids George David Associate Professor Department of Radiology Medical College of Georgia."— Presentation transcript:

1 Resident Physics Lectures Christensen, Chapter 8Grids George David Associate Professor Department of Radiology Medical College of Georgia

2 Purpose Directional filter for photons Ideal grid passes all primary photons photons coming from focal spot blocks all secondary photons photons not coming from focal spot Film Patient “Good” photon “Bad” photon Grid X Focal Spot

3 Grid Construction Lead ~.05“ thick upright strips (foil) Interspace material between lead strips maintains lead orientation materials fiber aluminum wood Lead Interspace

4 Grid Ratio Ratio of interspace height to width h w Grid ratio = h / w Lead Interspace

5 Grid Ratio Expressed as X:1 Typical values 8:1 to 12:1 for general work 3:1 to 5:1 for mammography Grid function generally improves with higher ratios h w Grid ratio = h / w

6 Lines per Inch # lead strips per inch grid width Typical: Lines per inch = W + w w = thickness of interspace (mm) W = thickness of lead strips (mm) w W

7 Grid Structure

8 Grid Patterns Orientation of lead strips as seen from above Types Linear Linear Cross hatched Cross hatched 2 stacked linear grids ratio is sum of ratios of two linear grids very sensitive to positioning & tilting Rare; only found in specials

9 Grid Styles Parallel Focused

10 Parallel Grid lead strips parallel useful only for small field sizes large source to image distances

11 Focused Grid Slightly angled lead strips convergence line Strip lines converge to a point in space called convergence line Focal distance distance from convergence line to grid plane Focal range working distance range width depends on grid ratio smaller ratio has greater range Focal range Focal distance

12 Ideal Grid passes all primary radiation Reality: lead strips block some primary Lead Interspace

13 Ideal Grid block all scattered radiation Reality: lead strips permit some scatter to get through to film Lead Interspace

14 Primary Transmission Fraction of a scatter-free beam passed by grid Ideally 100% (never achieved) Lead Interspace

15 Primary Transmission Typical values: % Theoretic calculation: (fraction of grid that is interspace) Tp (%)= 100 X W / (W+w) where W = Interspace thickness w = lead strip thickness actual transmission < theoretical primary attenuated by interspace material focusing imperfections w W W+w

16 Bucky Factor Radiation incident on grid transmitted radiation indicates actual increase in exposure because of grid’s presence due to attenuation of both primary & secondary radiation

17 Bucky Factor Measures fraction of radiation absorbed by grid high ratio grids have higher bucky factors

18 Bucky Factor Higher bucky factor means higher x-ray technique higher patient dose typically 3-6

19 More Lines / inch at Same Ratio Means Less Lead Content & Less Contrast Improvement thinner lead & same ratio less lead (less thickness, same height) Same interspace dimensions same contrast improvement for 133 line 10:1 and 80 line 8:1 grids h d Grid ratio = h / d

20 Grid Disadvantages Increased patient dose Positioning critical poor positioning results in grid cutoff loss of primary radiation because images of lead strips projecte wider

21 Grid Cutoff focused grids used upside down lateral decentering (or angulation) focus- grid distance decentering combined lateral & focus-grid distance decentering

22 Upside Down Focused Grid Dark exposed band in center Severe peripheral cutoff

23 Lateral Decentering uniform loss of radiation over entire film uniformly light radiograph dangerous no recognizable characteristic (dangerous)

24 Lateral Decentering also occurs when grid tilted both result in uniform loss of intensity no other clinical clues may be mistaken for technique problems Can be compensated for by increasing patient exposure

25 Lateral Decentering Significant problem in portable radiography Compensate by over-exposing patient exact centering not possible minimizing lateral decentering low ratio grids long focal distances

26 Distance Decentering Grid too close or too far from focal spot Darker center All parallel grids have some degree of distance decentering Focused to infinity

27 Far focus-grid decentering Near focus-grid decentering cutoff at periphery dark center cutoff proportional to grid ratio decentering distance

28 Minimizing Distance Decentering Cutoff low grid ratio small fields

29 Combined lateral and focus-grid distance decentering Easy to recognize uneven exposure film light on one side, dark on the other

30 Moving Grids Motion starts with second trigger Grids move ~1- 3 inches must be fast enough not to see grid lines for short exposures Motion blurs out lead strip shadows

31 Moving Grid Disadvantages $$$ Vibration Potential May limit minimum exposure time Increases patient dose lateral decentering from motion up to 20% loss of primary evenly distributes radiation on film stationary grid makes interspace gaps darker for same amount of radiation

32 Grid Tradeoff Advantage cleanup / scatter rejection Disadvantage increased patient dose increased exposure time increase tube loading positioning & centering more critical $$$

33 Grid Selection use low ratios for low kVp, high ratios for high kVp book recommends 8:1 below 90 kVp 12:1 above 90 kVp

34 Air Gap Techniques Principle radiation scatters uniformly decrease in scatter (most scatter misses film) air gap decreases angle of capture; increases angle of escape Negligible attenuation in air gap Angles of escape

35 Air Gap air gap very effective in removing scatter originating closest to film much of scatter nearest tube doesn’t reach film Much attenuation of scatter in the body Air gap decreases capture angle

36 Air Gap Applications Magnification Radiography including mammography geometry causes air gap Grid not used with air gap

37 Mammo Cellular Grid


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