The effectiveness of a grid in absorbing SR is determined by the grid ratio. The grid ratio is that of lead strip depth to radiolucent spaces width. For instance, a grid with 0.25-mm translucent spaces and 2-mm deep lead strips would have a grid ratio of 2 0.25, or B. Such a grid would be called an 8:1 grid, a common ratio. The efficiency of grids increases with increased grid ratio. This means that an 8:1 grid is more efficient than a 5:1 grid. With the lower grid ratios, less fog is eliminated, but more latitude is allowed in positioning and in SID selection.
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