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Anti-scatter grid is a device for limiting the amount of radiation scatter created in a radiographic exposure reaching the detector.[1][2]

The grid is constructed of a series of alternating parallel strips of lead and a radiolucent substance such as a plastic, carbon fibre, aluminium, even paper. The grid is placed between the patient and the detector during the exposure. Primary beam radiation passes through the radiolucent strips as it travels roughly parallel to them, but scattered radiation which has, almost by definition, deviated from the parallel beam, cannot easily pass through the grid as it encounters the lead strips at an angle, and is attenuated, or lost, from the beam.

Grids are used particularly in examinations where a large quantity of scatter is created, i.e., those involving a large volume of tissue being irradiated and those requiring low energy i.e. voltage. The scatter would otherwise degrade the image by reducing the contrast and resolution. Use of a grid, however, requires a greater radiation exposure to the patient as a good deal of primary beam is also attenuated by the lead slats, and for this reason grids are not used for all examinations.

The single most important parameter that influences the performance of an anti-scatter grid, is the grid ratio.[3] The grid ratio is the ratio of the height to the width of the interspaces (not the grid bars) in the grid. Grid ratios of 8:1, 10:1, and 12:1 are most common on radiography. A 5:1 grid is most common for mammography.[3] The grid is essentially a one-dimensional Collimator and increasing the grid ratio increases the degree of collimation. Higher grid ratios provide better scatter cleanup, but they also result in greater radiation doses to the patient.[3]
References

R. Highnam; J.M. Brady (6 December 2012). Mammographic Image Analysis. Springer Science & Business Media. p. 58. ISBN 978-94-011-4613-5.
Robin J. Wilks (1987). Principles of Radiological Physics. Churchill Livingstone. ISBN 978-0-443-03780-1.
Jerrold T. Bushberg (2002). The Essential Physics of Medical Imaging. Lippincott Williams & Wilkins. p. 169. ISBN 978-0-683-30118-2.

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