Quantification of Sodium in Bone and Soft Tissue with In Vivo Neutron Activation Analysis
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Excess sodium (Na) intake is directly related to hypertension and an increased risk of developing many chronic diseases, but there is currently no method to directly quantify Na retained in the body. Because of this, the locations of Na storage and its exchange mechanisms are not well known. This information is critical for understanding the impact of increased Na intake in modern diets. In order to non-invasively quantify Na in bone and soft tissue, a compact deuterium-deuterium (DD) neutron generator-based in vivo neutron activation analysis (IVNAA) system was developed. MCNP was used to design a custom irradiation assembly to maximize Na activation in hand bone while minimizing dose. In order to test the system, live pigs were used. Two 100% efficient high purity germanium (HPGe) detectors collected Na-24 counts over 24 hours post irradiation. From the pig studies, a two-compartment model of exchange was developed to quantify Na in bone and in soft tissue. The right legs of four live pigs, two on a low Na diet and two on a high Na diet, both for 14 days, were irradiated inside the customized irradiation cave for 10 minutes (45 mSv dose to the leg) and then measured with the HPGe detectors. The spectra were analyzed to obtain the net Na counts at different time points. Analysis shows exponential decrease of Na in the leg during the first one hour of measurement, while the change was minimal at the second hour, and the counts were stabilized at the second and third 2 hour measurements, taken 7 and 21 hours post irradiation. Bone Na and soft tissue Na concentrations were calculated using calibration lines created with bone and soft tissue equivalent Na phantoms as well as the parameters obtained from the two-compartment model. The results show that the difference in bone and soft tissue Na between the pigs on high vs low Na diets was significant. With these results, we conclude that DD neutron generator-based IVNAA can be used to accurately quantify Na in bone and soft tissue in vivo and the system is a potential valuable tool for nutrition studies.