# Resident Physics Lectures

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

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 Focal Spot “Good” photon Patient “Bad” photon X Grid Film

Grid Construction Lead Interspace ~ .05“ thick upright strips (foil)

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

Grid Ratio Expressed as X:1 h Grid ratio = h / w 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

Lines per Inch # lead strips per inch grid width Typical: 103 25.4
W + w w W w = thickness of interspace (mm) W = thickness of lead strips (mm)

Grid Structure

Grid Patterns Orientation of lead strips as seen from above Types
Linear 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

Grid Styles Parallel Focused

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

Focused Grid Slightly angled lead strips
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

Ideal Grid passes all primary radiation

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

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

Primary Transmission Typical values: 55 - 75%
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

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

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

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

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

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

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

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

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

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

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

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

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

Minimizing Distance Decentering Cutoff
low grid ratio small fields

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

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

\$\$\$ 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

increased patient dose increased exposure time increase tube loading positioning & centering more critical \$\$\$

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

Air Gap Techniques Principle Negligible attenuation in air gap
Angles of escape 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

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

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

Mammo Cellular Grid