Skeletal, Infection, and Inflammation Imaging
Bone scan patterns and infection specificity
Tc-99m MDP adsorbs to the hydroxyapatite crystal surface, so uptake reflects osteoblastic activity and regional blood flow rather than tumor type. Three-phase imaging (flow, blood pool, delayed) separates hyperemia from bone turnover: cellulitis produces increased flow and blood pool without focal delayed uptake, whereas osteomyelitis adds focal delayed uptake. A superscan - diffuse skeletal uptake with faint kidneys - suggests widespread metastases or metabolic bone disease. The flare phenomenon is increased uptake after effective therapy, not progression. Purely lytic or highly aggressive lesions may be photopenic.
Specificity is the weak point. Labeled leukocyte imaging (In-111 oxine or Tc-99m HMPAO) adds specificity for infection; it is sensitive for prosthetic hip and knee infection but less so for spinal infection, where FDG PET and MRI are preferred. Gallium-67 remains useful for sarcoidosis, discitis, and some chronic infections. FDG PET is excellent for vasculitis, sarcoidosis, fever of unknown origin, and spinal infection, but is nonspecific in the early postoperative period.
Traps: calling a photopenic defect aggressive without corroboration; relying on labeled white cells for the axial skeleton; ignoring that marrow replacement causes cold defects; and misreading a Charcot joint as osteomyelitis in a diabetic foot.
Worked example: a three-phase bone scan shows increased flow and blood pool across the whole foot but a focal delayed hot spot at the first metatarsal head - the diffuse hyperemia is cellulitis, while the focal delayed uptake indicates osteomyelitis.