1 / 22

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

shannonwilson
Télécharger la présentation

Resident Physics Lectures

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related