System design for in vivo neutron activation analysis measurements of manganese in the human brain: based on Monte Carlo modeling M.L. Arnold*, F.E. McNeill, W.V. Prestwich, D.R. Chettle Department of Physics and Astronomy, Medical Physics & Radiation Sciences Unit, McMaster University, 1280 Main St. West, Hamilton, ON, Canada L8S 4M1 Abstract Manganese is an essential nutrient required by the human body, but conversely, over exposure to the element may cause central nervous system damage. The technique of in vivo neutron activation analysis, using the McMaster KN-accelerator, is being investigated as a possible method of noninvasively determining manganese concentrations within the human body. Since the brain is the primary target of damage from exposure it would be the ideal site for measurements. Thus, Monte Carlo simulations have been undertaken to de®ne the optimum experimental parameters for such a measurement, examining the use of possible moderator, re¯ector and collimator materials. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Neutron activation analysis; Manganese; In vivo; Brain; Monte Carlo 1. Introduction The toxic eects of manganese (Mn) on the central nervous system have been documented for over 150 years, however as of yet there does not exist a good biomarker which can be used as an indicator of ex- posure and the possibility of developing adverse eects (Greger, 1999). To date, one of the best biomarkers is T 1 weighted MRI, which shows an increase in signal intensity in the basal ganglia region of the brain for exposed nonhuman primates (reviewed by Pal et al., 1999), suggesting an increase in Mn in this region, but unfortunately providing no quantitative information. Recently, there have been studies dealing with the optimization of systems for the technique of in vivo neutron activation analysis (IVNAA) (Ma et al., 1999; Dilmanian et al., 1998; McNeill and Chettle, 1998). IVNAA can be used to quantify the amount of various elements within the body, and the feasibility of this technique for quantitatively measuring Mn within the basal ganglia is currently being investigated. The McMaster KN-accelerator would be used to provide neutrons, by means of the 7 Li(p,n) 7 Be reaction (threshold of 1.88 MeV), and Mn activation would occur via the 55 Mn(n,g ) 56 Mn reaction (s=13 barn). Following the irradiation process, the 847 keV gamma rays emitted when 56 Mn decays (half-life=2.58 h, branching ratio=100%) would be counted outside of the body using NaI detectors. At present, the ir- radiation process has been modeled using Monte Carlo simulations to determine the optimum experimental parameters which should be used, and future work will deal with modeling of the entire two step process to Applied Radiation and Isotopes 53 (2000) 651±656 0969-8043/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S0969-8043(00)00199-8 www.elsevier.com/locate/apradiso * Corresponding author. Fax: +1-905-546-1252. E-mail address: arnoldml@mcmail.CIS.mcmaster.ca (M.L. Arnold).