At what body-part thickness is the use of a grid recommended?

• A. 5 cm
• B. 8 cm
• C. 15 cm
• D. 20 cm

Answer: (B)

Grids are used to boost image contrast by absorbing scattered photons that are produced as X-rays pass through the patient. Scatter increases with body-part thickness, so once the part reaches a thickness where scatter would noticeably degrade contrast, a grid becomes helpful. About 8 cm is the point at which the amount of scatter becomes significant enough to justify using a grid—the grid reduces that scatter, improving the image, even though some primary photons are lost and exposure may need to be increased to maintain density. For thinner parts (like a few centimeters), scatter is low and a grid isn’t usually necessary; for thicker parts (15 cm, 20 cm), the grid becomes even more important.