Bone Scan

Excerpt

Skeletal scintigraphy, bone scintigraphy, or the most commonly used term, "bone scan," constitutes a versatile and valuable nuclear medicine modality. The examination is typically performed using the radiotracer technetium-99m (Tc-99m) complexed to a diphosphonate, either methylene diphosphonate (MDP) to form Tc-99m MDP or hydroxydiphosphonate (HDP) to form Tc-99m HDP. Tc-99m is cost-effective and exhibits favorable imaging characteristics, including high spatial resolution, an optimal photopeak at 140 keV for γ-camera detection, and a relatively short physical half-life of 6 hours, which allows adequate image acquisition while minimizing radiation exposure.

Tc-99m phosphonates were first introduced into clinical practice in 1971 by Subramanian et al, with several subsequent formulations developed until the establishment of Tc-99m MDP in 1975, which remains the predominant radiotracer in skeletal scintigraphy. Single-photon emission computed tomography combined with computed tomography (SPECT/CT) may be employed to enhance anatomic localization. The precise mechanism by which Tc 99m MDP or Tc 99m HDP localizes to bone involves the binding of bisphosphonate analogs to crystalline hydroxyapatite in the extracellular mineral phase of bone via chemisorption. Tc 99m phosphonates accumulate in bone in proportion to osteoblastic activity and, to a lesser extent, blood flow, which governs tracer delivery.

Consequently, any process that increases osteoblastic activity is associated with elevated radiotracer uptake, rendering the study highly sensitive but relatively nonspecific. The specificity of skeletal scintigraphy depends on integration of clinical history, correlation with complementary imaging modalities, and meticulous assessment of radiotracer uptake patterns, including mono-ostotic versus polyostotic, axial versus appendicular, periarticular versus metaphyseal or diaphyseal, and focal versus fusiform or linear distribution.

Fluorine-18 sodium fluoride (F18-NaF) positron emission tomography (PET) has recently experienced renewed use for metabolic bone imaging, driven by temporary shortages of Tc-99m and the widespread adoption of PET/computed tomography (PET/CT) technology. F18-NaF, a calcium analog, was first introduced in 1963, and its 511 keV photons were detectable with general-purpose rectilinear scanners or early positron detectors. The short physical half-life of fluorine-18 (110 minutes) and dependence on cyclotron production limited widespread availability.

The subsequent introduction of Tc-99m generators, phosphate radiotracers, and γ cameras contributed to F18-NaF’s relative obscurity for approximately 4 decades. Modern F18-NaF PET/CT offers several advantages over Tc-99 m phosphonate bone scans, including superior spatial resolution, an enhanced target-to-background ratio, and increased sensitivity. Limitations include higher cost, slightly elevated radiation exposure, and a potentially higher false-positive rate resulting from increased uptake at sites of degenerative changes.