Resident Physics Lectures Christensen, Chapter 8 (year 1) Grids George David, MS, FAAPM, FACR 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 Image Receptor

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

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

Grid Structure

Grid Styles Parallel Focused

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 Reality: lead strips block some primary Interspace Lead

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

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

Grid Disadvantages Increased patient dose Positioning critical 3-6 times Positioning critical poor positioning results in grid cutoff

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 also occurs when grid tilted uniform loss of radiation over entire film uniformly light radiograph no recognizable characteristic (dangerous) also occurs when grid tilted

Lateral Decentering Significant problem in portable radiography Exact centering not possible

Distance Decentering Grid too far from or too close to focal spot Far focus-grid decentering Near focus-grid decentering Grid too far from or too close to focal spot cutoff at periphery dark center

Combined lateral & focus-grid distance decentering Easy to recognize Uneven exposure Image light on one side, dark on the other Dangerous Can mimic clinical conditions

Moving Grids Grids move ~1- 3 inches during exposure Motion blurs out lead strip shadows

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

Air Gap Alternative to grid use By-product of magnification radiography Very effective in removing scatter originating closest to image receptor Much attenuation of scatter in the body Air gap decreases capture angle

Mammo Cellular Grid Similar to nuclear medicine collimator Used by some mammography vendors