Multinuclear solid-state three-dimensional MRI of bone and synthetic calcium phosphates

Abstract

Multinuclear three-dimensional solid-state MRI of bone, tooth, and synthetic calcium phosphates is demonstrated in vitro and in vivo with a projection reconstruction technique based on acquisition of free induction decays in the presence of fixed amplitude magnetic field gradients. Phosphorus-31 solid-state MRI provides direct images of the calcium phosphate constituents of bone substance and is a quantitative measurement of the true volumetric bone mineral density of the bone. Proton solid-state MRI shows the density of bone matrix including its organic constituents, which consist principally of collagen. These solid-state MRI methods promise to yield a biological picture of bone richer in information concerning the bone composition and short range-crystalline order than the f luid-state images provided by conventional proton MRI or the density images produced by radiologic imaging techniques. Three-dimensional solidstate projection reconstruction MRI should be readily adaptable to both human clinical use and nonmedical applications for a variety of solids in materials science.The solid substance (1) of cancellous and compact bone, consisting of solid phases of calcium phosphate mineral and organic matrix, is most often characterized in vivo by noninvasive methods including ionizing radiation, among which are emission tomography based on radiolabeled pharmaceuticals, x-ray computed tomography, and dual energy x-ray absorptiometry. One of the few nonradiative methods is conventional (i.e., fluid state) proton NMR imaging (or MRI), which has been used recently to characterize the total bone substance (mineral plus matrix) in a specific region of cancellous bone tissue indirectly by imaging the marrow space. In effect, a negative image of the bone substance is produced. If the image has sufficiently high spatial resolution such that the marrow spaces between the bony trabeculae are resolved (which is difficult to achieve for humans in vivo), the microarchitectural details of the trabecular network may be geometrically analyzed in various ways (2), and the volume fraction of bone substance (which is related to the bone mineral density) may be determined. If the image has insufficient spatial resolution, signal dephasing caused by the microscopic distribution of magnetic susceptibility discontinuities between marrow and bone substance can be used to derive information (e.g., textural information) about the trabecular network (3, 4), which may or may not be related to the mass of bone.However, none of the noninvasive characterization methods is capable of yielding direct information concerning the masses of bone mineral and matrix independently and are, therefore, incapable of yielding the extent of mineralization (the ratio of actual mineral density to full potential mineralization). Most importantly, they provide no information about the chemical composition or chemical structure of the mineral.Bone mineral is a nonstoichiometric calcium-deficient apatite, whose crystal structure is basically that of the sy...

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