Grids are placed between the patient and the x-ray film to reduce the scattered radiation reaching the detector (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 ⁷. The strips can be oriented either linear or crossed in their longitudinal axis. As scattered radiation is increased in "thicker" patients and at larger field sizes, grids are useful in such scenarios to improve image contrast.

Characteristics

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) ⁷. A 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 30-80 strips (or grid lines) per cm. Low frequency is 40 to 50 strips per cm, medium frequency is 50 to 60 strips per cm, and greater than 60 strips per cm is high frequency ⁷.

Primary transmission is the percentage of primary radiation transmitted through the grid. Ideally, a grid should have 100% primary beam transmission while blocking all the scatter radiation. The primary transmission can be calculated by measuring the ratio of intensity of radiation with a grid over the intensity of radiation without a grid as shown below ⁸:

T_p=I₀/I'₀ x 100

where

T_p = transmitted primary radiation

I₀ = intensity with grid

I'₀ = intensity without grid

Primary transmission can also be estimated by the formula below, assuming that the geometric relationship between the anode and the grid is accurate, there is no grid cut-off, and primary radiation is not absorbed in the grid interspaces. Thus, percentage surface area of the interspaces will corresponds to the percentage of primary beam transmission, as shown in the formula below ⁸:

T_p=D/ (D+d) x 100

where

D = interspace width

d = thickness of the strip

Measured primary transmission is always smaller than the estimated primary transmission due to assumptions given above ⁸.

The Bucky factor (also known as grid factor) ^7,8 is the ratio of incident radiation falling the grid to the transmitted radiation. Although Bucky factor is similar to the primary transmission as described above; there is one major difference: Bucky factor measures both primary and secondary (scattered) radiation transmission, thus making it more practical to determine the additional amount of patient's exposure needed when switching from non-grid to grid technique in order to maintain the same image quality. The formula for Bucky/grid factor is ⁸:

Bucky factor = incident radiation / transmitted radiation

Grid factor = exposure necessary with grid / exposure necessary without grid

If Bucky factor is two, the exposure factors also needs to be increased by two to mainatain the same image quality. The value for Bucky factor range from 3 to 5 ⁷. The higher the grid ratio, the higher the Bucky factor. Meanwhile, the higher the energy of the radiation beam, the higher the grid ratio or Bucky factor needed to effectively filter out scattered radiation ⁸.

The contrast improvement factor (K) is the ratio between the contrast with a grid and without a grid. The value of K typically range from 2 to 4 ⁷. The formula is ⁸:

K = contrast with grid / contrast without grid

The contrast improvement factor depends upon grid ratio and lead content of the stripe, which are important to filter out the scattered radiation and improve contrast. To compare the different types of grid, other factors that can affect contrast should be controlled, e.g. kVp, field size, thickness of phantom. According to International Commission on Radiologic Units and Measurements, 100 kVp, large field size, and a phantom of 20-cm thick should be used to compare different types of grid ⁸.

Grid selectivity index measures the ability of a grid to transmit primary radiation while filtering out the scattered radiation, as given by the formula below:

Grid selectivity = fraction of primary radiation transmitted / fraction of scattered radiation transmitted

Fraction of primary radiation transmitted can be estimated by the formula of D/ (D+d) as described above. Meanwhile, the scattered radiation is measured according to kV and grid ratio. Usual values for grid selectivity index is from 6 to 12 ⁷.

Types

virtual grid: no actual grid is used; latest innovation for scatter reduction by digitally reconstructing a radiograph
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 ratios 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

History and etymology

Anti-scatter grids were developed by Gustav Bucky (1880-1963), a German-American radiologist who patented a stationary grid in 1913. Not long after his original invention, Bucky introduced a moving grid to overcome the problems inherent with a static device.

Two American radiologists independently of Bucky also came up with the idea of a moving grid. These were Eugene W Caldwell (1870-1918) ⁶, a radiologist and qualified engineer, who received a patent for an automated timing device to move the grid ⁴; and Hollis E Potter (1880-1964) ⁵, who was the first to present the development at a scientific event, namely the winter meeting of the Central Section of the American Roentgen Ray Society (ARRS) in February 1915 ⁴.