Normal Skeletal Uptake

Mild to moderate FDG uptake can be normally seen within red marrow, most commonly affecting the vertebral bodies, sternum, pelvis, and proximal long bones (it is not unusual to see fairly prominent uptake within the proximal half of the femurs). This red marrow uptake can be quite heterogeneous at times, yet still normal.

Diffuse Skeletal Uptake:

Intense and diffuse reactive marrow uptake is frequently seen as the result of chemotherapy and/or colony stimulating factors (discussed in detail, here).

If rather diffuse and intense uptake is noted without a history of recent chemotherapy or colony stimulating factors, be suspicious of infiltrating disease such as lymphoma. 

Focal Skeletal Uptake:

Focal skeletal uptake warrants extremely close assessment of the co-registered CT images to exclude malignancy. Other etiologies of focal skeletal uptake include fracture, arthropathy, infection and several benign bone lesions.