Grids in Radiography

Grids in Radiography

• A grid is a device placed under the table between the patient and the IR to absorb scatter radiation.
• Grids are constructed of tiny lead strips held in place by a radiolucent interspacing material.
• The effectiveness of a grid is determined by the grid ratio, which ranges from 5:1 to 16:1.
• The grid ratio determines how much variation in the direction of the incoming photon is allowed without the photon being absorbed by the grid.
• Grids with the same ratio may have many strips close together or fewer strips farther apart, which is called the grid frequency.
• Grid frequencies range from 60 to 196 lines/inch.
• Grids for general-purpose use are called focused grids because the lead strips are aligned in the direction of the diverging primary x-ray beam.
• The lead strips of a focused grid are precisely aligned with the x-ray beam at a specific source-image receptor distance (SID), which is called the grid radius.
• The range of distances within which the grid will not absorb an undue amount of useful radiation is referred to as the focal range of the grid.
• The SID used with a grid should always be within the grid’s focal range.
• The most commonly used SIDs are 40, 48, and 72 inches, and each usually requires a different grid with a suitable focal range.
• Grids with extra-long focal ranges are now available, some of which have a focal range of 40 to 72 inches.