In vivo measurement of accumulated bone fluorides by nuclear magnetic resonance

We have developed a localized noninvasive nuclear magnetic resonance (NMR) method for determining the accumulated bone fluoride content in human index fingers. Using a 27 MHz "split-ring" resonator probe, we measure the total fluoride (F) content within a 6.5 cm length of the distal end of the index finger from a calibrated determination of the intensity of the observed 19F free induction decay (FID) signal. Since fluoride impurities accumulate almost exclusively in bone mineral, the 19F resonance signal is broadened by rigid lattice magnetic dipole-dipole interactions, causing the FID signal to have a relatively short lifetime of approximately 75 microseconds. This short signal lifetime, plus the weakness of the signal strength prevents conventional magnetic resonance imaging equipment from observing the 19F in bone mineral. Nevertheless, we have achieved an in vivo sensitivity of +/- 0.5 mg fluoride in an observation period of 30 min. Assuming an index finger has between 0.25 and 0.5 g of bone calcium, this sensitivity is just sufficient to detect fluorides in the index finger of most adults whose bone fluoride concentration is greater than 2.0 mg fluoride/g calcium (0.8 mg/g ash weight). We are particularly interested in using this new NMR technique to monitor the rate of bone fluoride accumulation in osteoporotic patients receiving therapeutic levels of fluoride in their diets.