RESUMO
PURPOSE: In children, red marrow is present in the majority of bones, including the shafts of long bones. Shielding can reduce exposure to radiosensitive tissues adjacent to the collimated field of view. The goal of this study was to determine the shield placement for adjacent bone marrow (bone marrow that is not in the primary beam) that results in the greatest reduction in exposure during knee imaging. METHOD: Radiation exposure was measured with both an ion chamber and a cassette in 2 locations using a knee phantom. Location 1 was directly superior to the collimated field of view facing the primary beam on the anterior surface of the phantom. Location 2 was directly superior to the collimated field of view on the posterior surface of the phantom. Sources of exposure outside collimation include tube leakage, off-focus radiation, scatter radiation, and remnant primary radiation. RESULTS: Using an independent 2-tailed t test, it was determined that shielding in the first location resulted in a significant reduction in exposure outside of collimation. DISCUSSION: This study reinforced the finding that collimation does not entirely eliminate exposure outside the field of view. Shielding should be used in conjunction with collimation to reduce exposure as much as possible. Shielding on the surface of the patient facing the primary beam results in the greatest reduction in exposure outside of collimation. CONCLUSION: The importance of shielding as much pediatric long bone, and therefore, red marrow, as possible cannot be overstated. Shielding on the surface of the patient facing the primary beam provides the best decrease in exposure outside of collimation.