Grids

Grids are placed between the patient and the x-ray film to reduce the scattered radiation (produced mainly by the Compton effect) and thus improve image contrast.

They are made of parallel strips of high attenuating material such as lead with an interspace filled with low attenuating material such as carbon fiber 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 the height of the lead strips (h) to the distance between two strips, i.e. the interspace (D). Grid ratio of 8:1 is generally used for 70-90 kVp technique and 12:1 is used for >90 kVp technique. The strip line density (number of strips per cm) is 1/(D+d), where d is the thickness of the strip. This is typically 20-60 strips per cm.

Types

• focused grids (most grids): strips are slightly angled so that they focus in space so must be used at specified focal distances
• parallel grid: used for short fields or long distances
• moving grids (also known as Potter-Bucky or reciprocating grids): eliminates the fine grid lines that may appear on the image when focused or parallel grids are used; cannot be used for portable films

Uses

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:

• abdomen
• skull
• spine (except lateral cervical)
• contrast studies
  • IVU
  • RGU
  • MCU
  • barium studies (including lateral cervical)
• breast (mammography): uses 4:1 grid ratio

Characteristics

The Bucky Factor is the ratio of radiation on the grid to the transmitted radiation. It indicates the increase in patient dose due to the use of a grid. It is typically two to six. 

The contrast improvement factor is the ratio between the contrast with a grid and without a grid. it is typically two. Image contrast can be improved by increasing the grid ratio via increasing the height of the lead strips or reducing the interspace. However, this leads to increased x-ray tube loading and radiation exposure to the patient.