Grids are manufactured to accommodate all standard film sizes from 9×12 to 45×45 mm or 8″x10″ to 18″x18″. Customized grids to specifications are available upon request. Grids for use inside cassettes are 3 mm (1/8″) smaller than the corresponding film size, whereas grids for use outside the cassettes are approximately 20 mm (1″) larger than the corresponding film size. Extra large grids are made from two or more smaller sized grids that can be fitted together up to a maximum size of 49 cm x 49 cm (19″x19″).
The focal distance (f0) is the distance between the X-ray tube focal spot and the film. Each lead strip is aligned with the X-ray tube focal spot in a defined angle.
Depending on type of X-ray examination, different focal distances are required. Standard focal distances are 80, 100, 105, 115, 130, 140, 150 and 180 cm. Other focal distances can be made in a range from 40 cm up to infinity. A parallel grid has = ∞. ( ∞=infinity)
Focal range is the distance interval in which an X-ray tube can be used with a certain grid. With lower ratio grids, the focal range is considerably wider, with a higher ratio, the range is narrower.
Grid ratio is defined as the height of the lead strip compared to the distance between the strips. Generally speaking, a grid with a higher grid ratio is more efficient than that with a low ratio.
A high ratio requires, however, a higher exposure rate, since more radiation is absorbed by the grid. It is advisable to use grids with a higher ratio, when images of heavy objects and exposures at higher kV are made.
Grids are manufactured with the following ratios as standard:
Parallel and Focused N30 r 6, 8, 10
Parallel and Focused N40 r 6, 8, 10, 12
Parallel and Focused N70 r 6, 8, 10, 12, 16
Other ratios can be manufactured on special request.
Parallel grids are normally made on the “prismatic section” principle, i.e. the strip heights taper off towards the edges. This reduces the cut-off of primary radiation at the edges – an essential, when large cassettes and short focus-film distances are used.
Parallel grids are also made, where the strip heights even at the edges are retained. Such grids are classified as grids with ratio r, whereas the grids made on the “prismatic section” principle have the ratio ro.
Parallel grids give satisfactory results, when the required image size is small enough, and the exposure admits a low grid ratio. This can be advantageous particularly with bed side exposures, where the absence of a grid/focal distance allow a greater latitude in positioning the X-ray tube.
For radiography and fluoroscopy of thicker objects, where higher kV and shorter exposure times are necessary, focused grids, which are more effective at higher kV, can be used at short focus-film distances. This applies even to general surveys.
When focused grids are used, the central beam of the X-rays is centered at the focal distance. This in order to get the highest possible grid efficiency. The degree of deviation from the indicated focal distance that is admissible is dependent on the grid ratio.
The higher the grid ratio, the narrower the deviation limits.
Two parallel or focused grids of half the thickness and ratio are assembled into one cross grid.
The scatter absorption of a cross grid is approximately the same as that of a one component grid of the same ratio.
However, the 2-directional absorption of a cross grid absorbs more scatter originating from the interior of the object than with a conventional grid of the same ratio. A cross grid is suitable, where a grid with very high ratio is required.
Cross grids are advantageous, when deviating from a right angle arrangement of the film plane, since the cross grid only has half the ratio of the linear grid but in two directions.
High kV Technique
At kilo voltages above 125 kV, the scatter radiation increases significantly and necessitates the use of the most efficient grid. Primary absorption in the grid is of little importance.
A grid of high ratio and high number of lines to the cm should therefore be selected despite of the difficulty of correct alignment. Optimum quality is achieved by the use of high definition intensifying screens and a relatively long focal-film distance.
In order to minimize geometrical unsharpness, LYSHOLM focused grid: N70, r 12 or r16 are recommended.