An AP chest radiograph was obtained with 40 inches SID, 14x17 field size, 2.5 mm Al filtration, and a 12:1 grid. Which change would increase receptor exposure?

An AP chest radiograph was obtained with 40 inches SID, 14x17 field size, 2.5 mm Al filtration, and a 12:1 grid. Which change would increase receptor exposure?

• A. Increased SID
• B. Decreased grid ratio
• C. Decreased collimation
• D. Increased filtration

Answer: (B)

The key idea is how grids control the amount of radiation that actually reaches the image receptor. A grid is used to reduce scatter and improve contrast, but it also attenuates the primary beam. The amount of primary radiation the grid absorbs increases with grid ratio—the higher the ratio, the more primary photons are blocked. So, by decreasing the grid ratio, you reduce the grid’s absorption of the primary beam, allowing more photons to reach the receptor and increasing receptor exposure. This happens even if the other factors (SID, filtration, field size) stay the same, which is why lowering the grid ratio is the most direct way to boost receptor exposure in this scenario. In contrast, increasing the source-to-image distance lowers exposure due to the inverse-square law, additional filtration decreases exposure by removing more photons, and reducing collimation (increasing field size) would also raise exposure, but the grid ratio change provides the clearest, most predictable method to increase the exposure reaching the receptor in this setup.

The key idea is how grids control the amount of radiation that actually reaches the image receptor. A grid is used to reduce scatter and improve contrast, but it also attenuates the primary beam. The amount of primary radiation the grid absorbs increases with grid ratio—the higher the ratio, the more primary photons are blocked. So, by decreasing the grid ratio, you reduce the grid’s absorption of the primary beam, allowing more photons to reach the receptor and increasing receptor exposure. This happens even if the other factors (SID, filtration, field size) stay the same, which is why lowering the grid ratio is the most direct way to boost receptor exposure in this scenario.

In contrast, increasing the source-to-image distance lowers exposure due to the inverse-square law, additional filtration decreases exposure by removing more photons, and reducing collimation (increasing field size) would also raise exposure, but the grid ratio change provides the clearest, most predictable method to increase the exposure reaching the receptor in this setup.