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In vivo neutron activation analysis
IVNAA is a relatively new body composition technique that allows the determination of specific chemical elements in the body. The body is bombarded with fast neutrons of known energy level. The neutrons can be captured by chemical elements (as part of molecules) in the body, resulting in a transition state of higher energy for that element – energy that is finally emitted as gamma rays. For example, capture of neutrons by nitrogen results in the formation of the isotope 15N, which will emit the excess energy as gamma rays:
14N + 1n → 15N* + gamma rays
where 14N is nitrogen with atomic mass 14, 15N is nitrogen with atomic mass 15, and 1n is a neutron.
With IVNAA, many elements in the body can be determined, including calcium, phosphorus, nitro-gen, oxygen, potassium, and chlorine.
The information obtained at the atomic level can be converted to more useful information. For example, from total body nitrogen total body protein can be calculated as 6.25 times the total nitrogen, assuming that body protein consists of 16% nitrogen. The advantage of the method is that the chemical body composition can be determined in vivo and can be compared with other, indirect, techniques. For fundamental studies and for validation of existing techniques in special groups of subjects, for example in different ethnic groups, elderly subjects, obese sub-jects, or in the diseased state, the methodology can be of great importance. The disadvantage of IVNAA is not only the price. The subject is irradiated, with the radiation dose used depending on the number and kind of elements to be determined. It is relatively low for nitrogen (0.26 mSv) but high for calcium (2.5 mSv).
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