Chemisorption

Another important mechanism is known as physicochemical adsorption or chemisorption. The phosphate or phosphonate groups on currently used bone agents bind instantaneously, avidly, and essentially irreversibly to the hydroxyapatite structure of bone tissue. In addition, by the same mechanism, they localize in lesions metastatic to bone. Tc-99m MDP, Tc-99m HDP, and Tc-99m PYP all bind to bone tissue by this mechanism. Typically, 40-50% of the injected dose localizes in bone; the remainder is excreted through the kidneys. Since bone uptake is relatively slow, especially in adults, it is common practice to begin imaging 3 hr post injection.

A closely related example is the imaging of acute myocardial infarctions with Tc-99m PYP 3. When myocardial cells become necrotic following an acute myocardial infarction, there is an influx of calcium ions into the cells. The Ca²⁺ ions react with circulating phosphate ions to form Ca₃(PO₄)₂ crystals, known as hydroxyapatite. Tc-99m pyrophosphate binds avidly and irreversibly to these crystals at the periphery of the infarct where some perfusion is maintained (none localizes in the central region of the infarct). Images are routinely taken approximately 2 hr post injection. Optimal imaging time post-infarct is 1-3 days; after 6 days an infarct is considered "old" and the rate of false negative studies increases significantly.