MCNP Study of Multi-Hole Collimation Grids for Biological Neutron Radiography

Waterman, Shane W.

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MCNP Study of Multi-Hole Collimation Grids for Biological Neutron Radiography

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Biological Neutron Imaging has been studied recently as a promising imaging modality to discriminate between similar types of human tissue by exploiting the elastic scatter of neutrons by water. Yet the neutron’s high rate of elastic scatter in tissue causes image blur which reduces contrast. Therefore, because neutron elastic scatter is the primary attenuation mechanism the need arises for the addition of a collimator in the imaging setup to prevent scattered neutrons from contributing to image formation. We performed a computer simulation study using the MCNP6 radiation transport code to develop a multi-hole SiO2 collimator model doped with a neutron absorbing isotope to absorb scattered neutrons and allow unscattered neutrons to form a radiograph. We used this model to determine the impact of collimation ratio and dopant isotope concentration. The isotopes used are Boron-10 and Gadolinium-157 in weight percentages varying from 5%-20% and collimation ratios ranging from 1 to 200. A 3cm thick water phantom is used as a human tissue model for the purposes of this investigation. The Michelson Contrast and Modulation Transfer Function are calculated using the simulation data to quantify the impact of the collimator on the output image quality. It is shown that using a doped collimator provides significant image quality improvement in both contrast and spatial resolution. Using a 15% Gd-157 doped collimator with an L/D as low as 25 can result in 50% contrast and a spatial resolution of approximately the detector pixel size with a factor of 40 decrease in neutron flux.

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