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Ch18 The Grid
Principles of Radiographic Imagine- Carlton
Question | Answer |
---|---|
The Grid | Absorbs scatter, improves contrast |
X-Ray Beam - as it passes through the body three things happen: | Pass through unaffected or un-attenuated, Absorbed by the body photoelectric effect, interact and change direction (Comptons) - produces scatter (patient is source of scatter) |
Effect of scatter on image contrast | overall graying/fog, undercuts true image, DOES NOT produce image details, densities added in a negative sense |
Scatter Contribution to patient dose and poor image quality | major contributor, once produced, how can it be reduced to not degrade image quality? -GRIDS |
Function of the grid | IMPROVE CONTRAST |
Function of the grid | Higher kVp - grid efficiency goes DOWN |
How does the grid work? | Grid removes scatter by absorption (lead strips) improve contrast image, ideal all exit or remnant beam should pass through grid... |
Exit X-Ray Beam | Scattered photons- of no diagnostic value, add to overall density-graying effect (fog), decreases contrast = low contrast poor contrast resolution, the % of comptons increases |
The amount of scatter radiation INCREASES when: | Increase in patient thickness, Increase in field size, decrease in the atomic number of the tissue, such as soft tissue (electrons are most loosely bound) |
Grid Use | Thicker, larger body parts, procedures that require high kVp techniques, absorbs scatter radiation (cleans up image) |
Grid Rules | Thickness exceeds 10 cm, kVp exceeds 60 kvp |
1913 Gustav Peter Bucky | Original idea, crude design, thin lead strips |
1920 Hollis Potter | Lead strips running in one direction, thinner lead strips |
Potter-Bucky Diaphragm | Grid moves during exposure |