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) material between lead strips maintains lead orientation materials fiber aluminum wood Lead Interspace

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

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

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

Grid Cassette Grid built into cassette front Sometimes used for portables formerly used in mammography low grid ratios focused

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

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

Grid Performance Measurements Primary Transmission (Tp) Bucky Factor (B) contrast improvement factor (K)

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

Measuring Primary Transmission small area beam scatterer in beam far from grid virtually no scatter reaches grid measure radiation intensity with & without grid ratio X 100 is Primary Transmission (Tp) Focal Spot Lead Diaphragm Grid Detector

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 Measurement large x-ray field thick phantom ratio of intensity measurement with & without grid Grid Detector

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

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

Contrast Improvement Factor Ratio of contrast with & without grid Scatter reduces appearance of contrast No Scatter Scatter

Contrast Improvement Factor Depends on kVp field size phantom thickness increase in any of above means more scatter less contrast lower contrast improvement factor

Contrast Improvement Factor Better contrast improvement with higher ratio more lead content in grid

Lead Content of Grid Definition weight per unit area grams (Pb) / cm2 of grid

More Lines / inch at Same Ratio Means Less Lead Content & Contrast Improvement thinner lead & same ratio less lead (less thickness, same height) Same interspace dimensions h d Grid ratio = h / d

More Lines / inch at Same Ratio Means Less Lead Content & Contrast Improvement thinner interspace & less height to maintain ratio less lead (less height, same thickness) h d Grid ratio = h / d

Lead Content of Grid more lines / inch for same ratio means less lead content & thus less contrast improvement puts practical limit on lines per inch 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 at correct position but tilted both result in uniform loss of intensity no other clinical clues may be mistaken for technique problems Can be compensated for by over-exposing patient

Lateral Decentering cutoff increases with Higher grid ratio Greater decentering distance smaller focal distances r b L = ----- X 100 fo L = loss of primary radiation (%) r = grid ratio b = lateral decentering distance (inches) fo = focal distance of grid (inches)

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

X Near focus-grid decentering Far focus-grid decentering target below convergent line cutoff more severe than far decentering Far focus-grid decentering target above convergent line X

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

Combined lateral and focus-grid distance decentering Cutoff proportional to grid ratio decentering distance Cutoff inversely proportional to grid focal distance Less cutoff for longer focus grids cutoff greater for near than for far distance decentering

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 for single phase generators grid motion must not synchronize with pulses note error in book, page 111 (omits “not”)

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

Grid Tradeoff Advantage Disadvantage cleanup / scatter rejection 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 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

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 Air Gap Chest Radiography air gap used as alternative to grid SID increased from 6 feet to 10 feet to maintain geometric unsharpness Grid not used with air gap

Air Gap Optimization Air gap more effective for thicker body parts first inch of air gap most effective in contrast improvement image sharpness deteriorates with increasing gap (magnification) compensate with greater SID smaller focal spot

Mammo Cellular Grid