Sodium fluoride is the inorganic compound with the formula NaF. This colorless solid is a source of the fluoride ion in diverse applications. Sodium fluoride is less expensive and less hygroscopic than the related salt potassium fluoride. Fluoride salts are used to enhance the strength of teeth by the formation of fluorapatite, a naturally occurring component of tooth enamel. In the US, sodium fluoride was once used to fluoridate drinking water, but hexafluorosilicic acid or the related sodium salt are more widely used. Toothpaste often contains sodium fluoride to prevent cavities. Alternatively, sodium fluoride is used as a cleaning agent, e.g. as a "laundry sour." A variety of specialty chemical applications exist in synthesis and extractive metallurgy. The fluoride is the reagent for the synthesis of fluorocarbons. Representative substrates include electrophilic chlorides including acyl chlorides, sulfur chlorides, and phosphorus chloride. Like other fluorides, sodium fluoride finds use in desilylation in organic synthesis. In medical imaging, fluorine-18-labelled sodium fluoride is used in positron emission tomography (PET). Relative to conventional bone scintigraphy carried out with gamma cameras or SPECT systems, PET offers more sensitivity and spatial resolution. A disadvantage of PET is that fluorine-18 labelled sodium fluoride is less widely available than conventional technetium-99m-labelled radiopharmaceuticals. Sodium fluoride is used to conserve tissue samples in biochemistry and medicinal testing as fluoride ions stop glycolysis by inhibiting the enzyme enolase. Sodium fluoride is often used together with iodoacetic acid, which inhibits the enzyme aldolase. It also is used in RIPA lysis buffer as phosphatase inhibitor along with Na3VO4.