Spinks T J, Goode A W, Ranicar A S, Steere E
Phys Med Biol. 1984 Apr;29(4):385-94. doi: 10.1088/0031-9155/29/4/008.
Measurement of total body nitrogen is assuming increasing importance in the nutritional evaluation of seriously ill patients. Nitrogen has been previously measured either by counting (i) the annihilation radiation from 13N immediately after neutron irradiation with 14 MeV neutrons or (ii) the 'prompt' gamma rays from thermal neutron capture by 14N during irradiation with 14 MeV neutrons or with those produced by isotopic sources or a cyclotron. The present work describes studies into the feasibility of measuring 13N produced by irradiation with a neutron beam from the MRC Cyclotron. A complication of this method is that 13N is also produced in a reaction with 16O. Direct measurement of oxygen by use of the reactions 16O(n, p)16N or 16O(n, 2n)15O enables this interference to be estimated. The former reaction is possible with both 14 meV and cyclotron-produced neutrons but the 7.1 s half-life of 16N requires detectors to be placed in or very close to the irradiation site. In our particular circumstances this is not possible but the more energetic cyclotron neutron spectrum allows the production of 15O which has a half-life of 2.05 min and can be measured in a remote whole-body counter. A disadvantage with the cyclotron beam, in comparison with 14 MeV neutrons, is that a higher dose is required for similar accuracy. A reproducibility of about 4% is obtained with a dose equivalent of 0.01 Sv.
全身氮含量的测量在重症患者的营养评估中变得越来越重要。以前测量氮的方法有两种：一是在被14兆电子伏中子辐照后，立即计数13N的湮没辐射；二是在被14兆电子伏中子辐照期间，计数14N俘获热中子产生的“瞬发”伽马射线，或者计数由同位素源或回旋加速器产生的中子所产生的“瞬发”伽马射线。本研究描述了利用医学研究委员会（MRC）回旋加速器产生的中子束辐照产生的13N进行测量的可行性研究。该方法的一个复杂之处在于，13N也会在与16O的反应中产生。通过利用16O(n, p)16N或16O(n, 2n)15O反应直接测量氧，可以估算这种干扰。前一种反应对于14兆电子伏中子和回旋加速器产生的中子都可行，但16N的7.1秒半衰期要求探测器放置在辐照部位内部或非常靠近辐照部位的地方。在我们的具体情况下，这是不可能的，但能量更高的回旋加速器中子能谱可以产生半衰期为2.05分钟的15O，并且可以在远程全身计数器中进行测量。与14兆电子伏中子相比，回旋加速器束的一个缺点是，为了达到相似的精度需要更高的剂量。当剂量当量为0.01希沃特时，可获得约4%的重现性。
