Figure 3. [18F]-SODIUM FLUORIDE

Fluoride-18 sodium fluoride (18F-NaF) was the first widely used radiopharmaceutical for skeletal scintigraphy, it was introduced in 1962 and approved by theFDA in 197 for clinical use.
Pharmacokinetics:
18F-NaF has negligible plasma protein binding, rapid blood and renal clearance, and high bone uptake, which allows whole-body imaging as early as 45–60 min after its venous injection.
The rate-limiting step of 18-NaF bone uptake is blood flow.
Around 30% of the injected dose is sequestered within erythrocytes; however, this fact does not affect the bone uptake  because it freely diffuses across membranes, and almost all the radiopharmaceutical delivered  is retained by the bone after a single pass of blood.
18F-NaF is rapidly cleared from plasma, only 10% remains one hour after the injection, depending on the urine flow rate.
18F-NaF uptake and retention depends on the exposed area of the bone’s surface, which is larger in a variety of benign and malignant bone disorders.
The relationship between osteoblastic and osteoclastic activities determine the incorporation of 18F-NaF into the bone matrix, an increase in the osteoblastic activity increases the 18F-NaF uptake.
For bone deposition, 18F-ions need to pass from plasma through the extracellular space into the shell of bound water surrounding each hydroxyapatite crystal, After chemisorption onto hydroxyapatite, 18F rapidly exchanges for an  OH-  ion on the surface of the hydroxyapatite matrix to form fluoroapatite and migrates into the crystalline matrix of the bone, where is retained until the bone gets remodeled.

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