Resident Physics Lectures Christensen Chapter 8 year 1

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 spot coming from focal �blocks all secondary photons � photons not coming from focal spot Focal Spot “Good” photon Patient “Bad” photon XGrid Image Receptor

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

Grid Ratio �Ratio of interspace height to width 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

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 Focal range � distance from convergence line to grid plane �Focal range � working distance range � � width depends on grid ratio smaller ratio has greater range Focal distance

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

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

Grid Disadvantages �Increased patient dose � 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 �uniform loss of radiation over entire film �uniformly light radiograph �no recognizable characteristic (dangerous) dangerous �also occurs when grid tilted

Lateral Decentering �Significant problem in portable radiography �Exact centering not possible

Distance Decentering � 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 �cleanup / scatter rejection �Disadvantage �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