Skeletal dosimetry via NMR microscopy: investigations of sample reproducibility and signal source

Nuclear magnetic resonance microscopy has been used for several years as a means of quantifying the 3D microarchitecture of the cancellous regions of the skeleton. These studies were originally undertaken for the purpose of developing non-invasive techniques for the early detection of osteoporosis and other bone structural changes. Recently, nuclear magnetic resonance microscopy has also been used to acquire this same 3D data for the purpose of both (1) generating chord length data across bone trabeculae and marrow cavities and (2) generating 3D images for direct coupling to Monte Carlo radiation transport codes. In both cases, one is interested in the reproducibility of the dosimetric data obtained from nuclear magnetic resonance microscopy. In the first of two studies, a trabecular bone sample from the femoral head of a 51-y-old male cadaver was subjected to repeated image acquisition, image processing, image coupling, and radiation transport simulations. The resulting absorbed fractions at high electron energies (4 MeV) were shown to vary less than 4% among four different imaging sessions of the same sample. In a separate study, two femoral head samples were imaged under differing conditions of the NMR signal source. In the first case, the samples were imaged with intact marrow. These samples were then subjected to marrow digestion and immersed in Gd-doped water, which then filled the marrow cavities. Energy-dependent absorbed fraction profiles for both the marrow-intact and marrow-free samples showed essentially equivalent results. These studies thus provide encouragement that skeletal dosimetry models of improved patient specificity can be achieved via NMR microscopy in vivo.