Altered biodistribution and incidental findings on gallium and labeled leukocyte/bone marrow scans

Gallium-67 citrate and labeled leukocyte imaging are established procedures for diagnosing inflammation and infection. Knowledge of the normal biodistribution of these tracers, variations, and unusual disease presentations improves the accuracy of their interpretation. During the first 24 hours after injection, the principal excretory pathway of gallium is renal; subsequently, excretion is primarily colonic. By 72 hours, approximately 75% remains in the body, equally distributed among soft tissues, liver, and bone/bone marrow. This normal distribution is subject to considerable variation. Nasopharyngeal and lacrimal gland uptake can be prominent. Breast uptake, generally faint and symmetric, is intense in hyperprolactinemic states such as pregnancy. Colonic uptake is very variable. Normally healing surgical incisions concentrate gallium for variable amounts of time. In patients receiving multiple transfusions renal, bladder, and bone activity are increased; liver and colon uptake are decreased. The contrast agent gadolinium exerts similar effects. At 24 hours after injection, the normal biodistribution of indium labeled leukocytes is limited to liver, spleen, and bone marrow. The normal biodistribution of technetium-labeled leukocytes includes, in addition to the reticuloendothelial system, colon, urinary tract, and occasionally gall bladder. Images obtained shortly after injection of labeled leukocytes show intense pulmonary activity, which decreases over time. Except in cystic fibrosis, segmental or lobar pulmonary activity indicates bacterial pneumonia. Diffuse pulmonary uptake is associated with various conditions but rarely with bacterial pneumonia. Labeled leukocytes do not accumulate in surgical wounds that heal by primary intention. They do accumulate in wounds healing by secondary intention, such as ostomies and skin grafts. Because labeled leukocytes accumulate in the bone marrow, complementary bone marrow imaging helps differentiate marrow activity from infection. Labeled leukocyte imaging is not useful for diagnosing spinal osteomyelitis because 50% or more of cases present as nonspecific decreased activity. This test is not useful for diagnosing septic arthritis because labeled leukocytes accumulate in inflammatory, noninfectious arthritis. Nodal uptake in patients with lower extremity joint prostheses produces incongruent white blood cell/marrow images in the absence of infection. Careful attention to uptake patterns minimizes this problem. Radiation effects on bone marrow activity are dramatic. Acutely, there is intense, diffusely increased activity. As inflammation subsides, and marrow becomes fibrotic, the irradiated area appears as decreased activity.