Pulmonary activity on labeled leukocyte images: physiologic, pathologic, and imaging correlation

Accurate interpretation of labeled leukocyte images requires knowledge of pulmonary labeled leukocyte uptake: its prevalence and patterns and its correlation with technical, physiologic, and pathologic conditions as well as with other imaging findings. Images obtained shortly after injection of labeled cells are characterized by diffuse pulmonary activity, which decreases over time, until about 4 hours after injection when it becomes indistinguishable from background activity, remaining constant thereafter. Focal pulmonary uptake that is segmental or lobar in appearance is most often associated with bacterial pneumonia. Focal pulmonary uptake that is not segmental or lobar results from technical problems during labeling or reinfusion and is not usually associated with infection. Diffuse pulmonary uptake on images obtained more than 4 hours after reinjection of labeled cells is associated with a variety of pathologic conditions, some of the more common being opportunistic infection, radiation pneumonitis, pulmonary drug toxicity, adult respiratory distress syndrome, and sepsis. However, this pattern is almost never seen in bacterial pneumonia. When pulmonary uptake patterns are analyzed and correlated with the clinical situation, labeled leukocyte scintigraphy can provide useful information about pulmonary disease.