Grids are placed between the patient and the X-ray film to reduce the scattered radiation (produced mainly by compton effect) and thus improve image contrast.
They are made of parallel strips of lead with an interspace having an aluminum or organic spacer. The strips can be oriented either linear or crossed in their longitudinal axis. As the scatter radiation is increased in "thicker" patients and at larger field sizes, grids are useful in such scenarios to improve image contrast.
The working ability of a grid is described by the grid ratio, which is the ratio of height of the lead strips to the distance between two strips (the interspace). The higher the grid ratio, the better the image contrast but at a cost of increased patient dose. Grid ratio of 8:1 is generally used for 70-90 kVp technique and 12:1 is used for >90 kVp technique.
- focussed grids (most grids): strips are slightly angled so that they focus in space
- parallel grid: used for short fields or long distances
- moving grids (also known as Potter-Bucky grids): eliminates the fine grid lines that may appear on the image when focussed or parallel grids are used
Grids are commonly used in radiography, with grid ratio available in even numbers, such as 4:1, 6:1, 8:1, 10:1 or 12:1.
Generally used where the anatomy is >10 cm:
- spine (except lateral cervical)
- contrast studies
- barium studies (including lateral cervical)
- breast (mammography): uses 4:1 grid ratio
- x-ray production
- x-ray tubes
- tube rating
- interaction with matter
- beam collimators
- air gap technique
- intensifying screen
- x-ray film
- image intensifier
- digital radiography
- digital image
- x-ray artifacts
- radiation units
- radiation safety
- radiation detectors