In vitro measurement of phosphorus in small bone biopsies using 14 MeV neutron activation analysis

In studying diseases of bone such as osteoporosis and renal osteodystrophy it is important to have an index of change in bone mineral content. For this purpose we describe a method for measuring the phosphorus content of 100--150 mg bone biopsies. Femoral and iliac crest biopsies from 6 cadavers were each paired with a standard bone of similar size and irradiated with 14 MeV neutrons for 10 minutes. A special rotor ensured the same activating flux across both sample and standard bone which were both wrapped in aluminium foil. The 28Al generated from bone phosphorus (31P(n,alpha)28Al) and the 27Mg generated from the aluminium foils (27Al(n,p)27Mg), which undergo beta-decay have useful gamma rays at 1779 keV and 844 keV respectively. These gamma rays were counted using a 110 cm3 Ge(Li) detector and a rotating frame permitting four 30 s counts of sample and standard alternately (interval for rotation 10 seconds) in identical geometries. Using this system the phosphorus content of the biopsies could be determined with a coefficient of variation of 4.6% using the standard bone as reference and 6% using aluminium foil as reference. In dense bone the P/dry weight ratio was 30--50% greater than in spongy bone. The clinical value of the method is discussed.