Exam category: Image Production → Image Acquisition & Evaluation
Why it matters: Collimation, grids, compensating filters, and distance are the tools you use to shape the exposure between the tube and the image receptor, and adding a grid or changing distance means recalculating the mAs. The registry tests the grid conversion factors and both distance formulas.
A grid sits between the patient and the image receptor and filters out scatter radiation before it ever reaches the image, which increases radiographic contrast. A compensating filter physically evens out the exposure across a body part too uneven for one setting to cover. And distance obeys the inverse square law — with a mirror-image mAs formula that runs the opposite direction, which is exactly where the errors happen.
A grid absorbs scatter radiation exiting the patient before it reaches the IR, which increases radiographic contrast. It also absorbs some of the useful beam, so adding or changing a grid always requires a mAs adjustment to maintain exposure.
Whether to use a grid at all comes down to two factors together, not either one alone: part thickness and kVp. A part 10 cm (4 inches) or larger, imaged at a kVp above 60, produces enough scatter that a grid is indicated.
Scattered photons carry no anatomical information, so fog can't be corrected once it's recorded — only stopping scatter before the receptor fixes it. A grid's contrast improvement factor (k) (image contrast with the grid ÷ without) is a separate number from the GCF below, typically 1.5–2.5 — in other words, a grid roughly doubles image contrast; a crossed grid cleans up scatter on both axes, so it beats a parallel grid of double its ratio — a 6:1 crossed grid outperforms a 12:1 parallel grid, at the cost of critical centering and higher dose. Taller strips relative to the interspace also narrow the angle a grid accepts, so higher-ratio grids have less positioning latitude than lower-ratio grids.
Grid construction is described two ways, and they are not interchangeable:
What frequency changes is how visible the lead lines are. A high-frequency grid shows less distinct grid lines on the image than a low-frequency grid, because its strips are narrower and more closely spaced. What frequency does not set is scatter cleanup or the mAs penalty — both of those track grid ratio: compare two grids at the same frequency and the higher-ratio one cleans up more scatter and costs more mAs. The two characteristics are related but not locked together — if grid strip width is held constant, a higher frequency forces a thinner interspace and therefore a higher ratio, but that is a conditional relationship, not a guarantee that any high-frequency grid outranks any low-frequency one.
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Two grids share a 10:1 ratio, but one has 60 lines/cm and the other 30 lines/cm. What does the 60 lines/cm grid do differently?
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