Voxel size effects in three-dimensional nuclear magnetic resonance microscopy performed for trabecular bone dosimetry

An important problem in internal dosimetry is the assessment of energy depo sition by beta particles within trabecular regions of the skeleton. Recent dosimetry methods for trabecular bone are based on Monte Carlo particle tra nsport simulations within three-dimensional (3D) images of real human bone samples. Nuclear magnetic resonance (NMR) microscopy is a 3D imaging techni que of choice due to the large signal differential between bone tissue and the water-tilled marrow cavities. Image voxel sizes currently used in NMR m icroscopy are between 50 mum and 100 mum, but the images are time consuming to acquire and can only be performed at present for in vitro samples. It i s therefore important to evaluate what resolution is best suitable in order to properly characterize the trabecular microstructure, to adequately pred ict the tissue dosimetry, and to minimize imaging time. In this. work, a ma thematical model of trabecular bone, composed of a distribution of spherica l marrow cavities, was constructed. The mathematical model was subsequently voxelized with different voxel sizes (16 mum to 1000 mum) to simulate 3D N MR images. For each image, voxels are assigned to either hone or marrow acc ording to their enclosed marrow fraction. Next, the images are coupled to t he EGS4 electron transport code and absorbed fractions to bone and marrow a re calculated for a marrow source of monoenergetic electrons. Radionuclide S values are also determined far the voxelized images with results compared to data calculated for the pure mathematical sample. The comparison shows that for higher energy electrons (>400 keV), good convergence of the result s is seen even within images of poor resolution. Above 400 keV, a voxel res olution as large as 300 mum results in dosimetry errors below 5%. For low-e nergy electrons and high-resolution images, the self-dose to marrow is also determined to within 5% accuracy. Nevertheless, increased voxelization of the image overestimates the surface area of the bone-marrow interface leadi ng to errors in the cross-dose to bone as high as 25% for some low-energy b eta emitters. (C) 2000 American Association of Physicists in Medicine. [S00 94-2405(00)00211-X].