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x-ray scatter

SCATTER. The Issue: 50-90% of x-rays reaching receptor are scatter, which degrades image contrastFactors Affecting ScatterScatter increases with amount of tissue exposedScatter increases with kVp. Scatter and X-ray Field Size. Scatter Escape Angle:. scatter is emitted in all directionsonly scatter reaching image within area of interest (field size) degrades contrast. The smaller the field, the larger the escape angle.

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x-ray scatter

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    1. X-RAY SCATTER

    2. SCATTER The Issue: 50-90% of x-rays reaching receptor are scatter, which degrades image contrast Factors Affecting Scatter Scatter increases with amount of tissue exposed Scatter increases with kVp

    3. Scatter and X-ray Field Size

    4. Scatter Escape Angle: scatter is emitted in all directions only scatter reaching image within area of interest (field size) degrades contrast. The smaller the field, the larger the escape angle

    5. FACTORS AFFECTING SCATTER Scatter increase with amount of tissue exposed Field size: irradiated volume and escape angle Thickness: irradiated volume Levels off at some thickness/size (scatter range) Scatter increases with kVp Higher fraction of Compton (source of scatter) Scatter in more penetrating: more reaches film Scatter is more biased forward (toward film)

    6. CONTROLLING SCATTER Minimize Scatter Production Lower kVp (but usually fixed by penetrability) Reduce thickness (eg, compression in mammo) Minimize X-ray field size: Collimation Prevent Scatter From Reaching Image Grids Air gap

    7. COLLIMATION

    8. Functions of X-ray Beam Restrictors Minimize Patient Radiation Dose: Irradiates only necessary tissue “Square” Function: Proportional to beam area e.g.: reducing 10x10” field (100 in2) to 8x8” field (64 in2) reduces irradiated tissue by 36% Minimize Scatter: Improves image contrast Scatter reduced in ~proportion to beam area

    9. Types of Restrictors: Apertures, Cones

    10. Apertures and Cones Beam defined by fixed “hole” (aperture) Penumbra depends on distance of defining aperture from focal spot (ie-top or bottom) Used if small number of beam sizes/shapes Examples: Dental, Mammography

    11. X-ray Beam Collimators

    13. Functions ofX-ray Beam Collimators:

    14. Collimator Requirements Continuous range of size (down to 2x2” or smaller) and shape (rectangular) by pairs of independent blades Light field indication (must indicate x-ray edges to <2% os SID) Penumbra reduced by upper/lower shutters

    15. Collimator Congruence Testing

    16. SCATTER AND GRIDS

    17. GRID TERMINOLOGY Types of Grids Linear grid: Strips parallel to table (and tube) Crossed grid: superimposed perpendicular grids Focused grid: grid lines “converge” to focal line Parallel grid : focused at infinity Grid Numbers: Grid Ratio: predicts scatter-removal ability Grid Frequency: grids lines per inch

    18. X-RayScatterRemoval Grids

    19. Bucky’s Original Grid

    20. Linear Grid

    21. Crossed Grid (Superimposed Linear

    22. Focused Grid

    23. Parallel Grid (Focused at Infinity)

    24. Grid Ratio and Scatter Clean-up

    25. Grid Ratio and Scatter Clean-up-con’t

    26. Grid Ratio Selection Trade-off: primary absorption and scatter removal High scatter exams (high kV, thick part): 10:1up to 16:1 Otherwise: 8:1

    27. Grid Performance Grid Primary Transmission Bucky Factor (Tube Load, Patient Dose) Contrast Improvement Factor (“real” test) Lead Content (assuming “good” grid design) Grid Cutoff

    28. Grid Performance: Bucky Factor The Bucky Factor is the ratio of incident radiation intensity reaching the grid to the transmitted radiation intensity passing through the grid. This is the factor by which mAs --- and thus patient dose --- must be increased when using the grid.

    29. Grid Performance: Bucky Factor

    30. Contrast Improvement Factor The Contrast Improvement Factor (K) is the ratio of contrast with the grid to contrast without the grid. Since the purpose for using the grid is to improve contrast (scatter removal is just a means to that end), many consider this the “real” test of a grid

    31. Contrast Improvement Factor

    32. Grid Performance: Lead Content

    33. Grid Cutoff Upside Down Lateral Decentering Distance Decentering Combined lateral+distance decentering Angulation

    34. Grid Cutoff Upside Down Lateral Shift

    35. Lateral Decentering Grid Cutoff (40” Focal Distance)

    36. Grid Cutoff: Distance

    37. Distance Decentering Grid Cutoff (40” Focal Distance)

    38. Grid Cutoff: Combined/Angulation

    39. Reciprocating Grid: Bucky Factor

    40. Scatter Removal: Air Gap

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