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Resident Physics Lectures

Resident Physics Lectures. Christensen, Chapter 8 (year 1) Grids. George David, MS, FAAPM, FACR Associate Professor Department of Radiology Medical College of Georgia. Purpose. Focal Spot. Directional filter for photons Ideal grid passes all primary photons

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Resident Physics Lectures

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  1. Resident Physics Lectures • Christensen, Chapter 8 (year 1) Grids George David, MS, FAAPM, FACR Associate Professor Department of Radiology Medical College of Georgia

  2. Purpose Focal Spot • 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 “Good” photon Patient “Bad” photon X Grid Image Receptor

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

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

  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. Grid Structure

  7. Grid Styles • Parallel • Focused

  8. 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

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

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

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

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

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

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

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

  16. Lateral Decentering • Significant problem in portable radiography • Exact centering not possible

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

  18. 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

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

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

  21. 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

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

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