Image B is obtained using Indium-111 (In-111) radiolabeled white blood cells.  In-111 radiolabeled white blood cells injected into the body accumulate normally in the spleen, liver, and bone marrow and pathologically in areas of active inflammation/infection.

Image A is a technetium-99m (Tc-99m) sulfur colloid study.   Tc-99m sulfur colloid is taken up by reticuloendothelial cells.   Since the bone marrow contains reticuloendothelial cells, imaging with Tc-99m sulfur colloid gives an image showing the distribution of bone marrow within the body.  The bone marrow distribution within the body is often asymmetric, especially after bone surgery such as a joint replacement. 

Tc-99m sulfur colloid images are often obtained in conjunction with In-111 white blood cell images in order to distinguish between normal uptake of In-111 white blood cells within bone marrow versus abnormal uptake in areas of inflammation/infection.  Areas of uptake on the In-111 white blood cell images that are not present on the Tc-99m sulfur colloid images are presumed to represent areas of active inflammation/infection.