Grid ==
a device that, when introduced between the patient and the film cassette (or image intensifier), reduces the amount of scattered radiation that can reach the film cassette (Fig. 1). The grid was invented by Gustave Bucky already in 1913 and is still the most effective device for reduction of scattered radiation. It is usually made of a thin plate, in which very thin strips of high-attenuating material, commonly lead, is placed in a linear, parallel pattern. The spaces between the strips are filled with low-attenuating material. In older grids this material can be paper or aluminium, but in modern grids it is usually made of carbon fibre. The lead strips are always higher than the distance between them. The grid ratio is defined as the ratio of the height of the lead strips to the distance between them, and is, accordingly, always greater than unity (Fig. 2). The higher the grid ratio, the more efficient reduction of scattered radiation can be accomplished. Common grid ratios range from 5 to 15. Generally, higher grid ratios are used for higher kVps and lower ratios for lower kVps.
The number of strips per cm in grids has long been between 30 and 40, but technological advances in the manufacture of grids have made it possible to produce grids with 60-70 lines per cm. In such grids, the strips are not visible to the naked eye, which means that they do not have to be moved during exposure (see below).
The grid pattern refers to the orientation of the strip pattern. In a linear grid, the strips are placed in a parallel pattern, in a crossed grid two linear grids are placed perpendicular to each other.
The grid can be parallel or focused. In a parallel grid, the strips are mounted parallel to each other which means that the grid is "focused" at infinite distance. Such a grid can only be used together with small fields or at very large focal distances and is therefore rarely used. In a focused grid, the strips are angled and focused at a specific distance which means that the grid can only be used at a specific focal distance.
The grid can either be used as a stationary grid or a moving grid. Moving a grid during exposure will blur out the shadows of the lead strips in the image. The moving grid was invented by Hollis Potter in 1920 and the device has since then been called a Potter-Bucky grid.