MECHANISM:
CHEMISORPTION OR PHYSICOCHEMICAL ADSORPTION
SKELETAL IMAGING
Mechanism: 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. Hydroxyapatite is simply tricalcium
phosphate
Radiopharmaceutical: Tc-99m MDP or
Tc-99m HDP or Tc-99m PYP. Particle size: none- compounds are soluble
In adults, ~45-50% of the injected dose
localizes in bone; the remainder is excreted through the kidneys,
probably by GFR. Imaging typically begun 3 hr post injection. In
children, skeletal uptake is usually 75-80% of the injected dose.
IMAGING OF ACUTE
MYOCARDIAL INFARCTIONS
Mechanism: chemisorption. When
myocardial cells become necrotic following an acute myocardial
infarction, there is an influx of calcium ions which are chemically
converted to hydroxyapatite crystals. 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).
2. Radiopharmaceutical: 99mTc-PYP.
Particle size: none- compound is soluble.
3. Images taken approximately 2 hr post
injection. Optimal imaging time post-infarct is 1-3 days;
after 6 days an infarct is considered "old" and rate of false negative
studies increases significantly.
IMAGING OF ACTIVE
THROMBOSIS
Mechanism: chemisorption. In the
presence of active thrombosis, Tc-Acutect (currently unavailable)
deposited on the thrombus surface. This permitted external detection of
the thrombus using either counting or imaging techniques.
