18F-Fluoride PET in the Assessment of Malignant Bone Disease.

In the 1970s, 18F-labeled sodium fluoride (18F-NaF), one of the most ubiquitous positron-emitting radiopharmaceuticals, was briefly used for skeletal scintigraphy before the introduction of 99mTc-labeled diphosphonates with optimal characteristics for conventional g cameras (1,2). 18F-NaF is an analog of the hydroxyl group found in hydroxyapatite bone crystals and therefore an avid bone seeker (3). As with 99mTc-based bone-scanning radiopharmaceuticals that are bound to bone by chemical absorption, fluorine is directly incorporated into the bone matrix, converting hydroxyapatite to fluoroapatite (4). Because the protein-bound proportion is less for 18F-NaF than for 99mTc-medronate, 18FNaF is more rapidly cleared from the plasma and excreted by the kidneys, with first-pass extraction approaching 100% (5). One hour after injection, only 10% of 18F-NaF remains in the plasma (1). Its desirable characteristics of high and rapid bone uptake accompanied by rapid blood clearance result in a high bone-to-background ratio in a short time. High-quality images of the skeleton can be obtained less than 1 h after the intravenous administration of 18F-NaF. Areas of high uptake on scans can result from any process that increases the exposed bone crystal surface or the blood flow.