We calculate the effects of subvoxel variations in magnetic susceptibility on MR image intensity for spin-echo (SE) and gradient-echo (GE) experiments for a range of microscopic physical parameters. The model used neglects the overlap of gradients from one magnetic inclusion to the next, and so is valid for low volume fractions and weak perturbations of the magnetic field. Transverse relaxation is predicted to deviate significantly from linear exponential decay in both SE and GE at a particle radius of 2.5 microns. Calculated changes in transverse relaxation rates for SE and GE increase linearly with volume fraction of high-susceptibility regions of 5 microns diameter, but increase with about the 3/2 power of volume fraction of regions with 15 micron spacing between centers. This sensitivity to the actual size and spacing of magnetized regions may allow them to be measured on the basis of contrast. without being resolved in images. GE and SE decay rates are approximately twice as sensitive to long cylinders of 5 microns diameter than to spheres of the same size, for diffusion constants of 2.5 micron 2/ms. Calculated changes in transverse decay rates increase with approximately the square of field and susceptibility variation for 5-microns spheres and a diffusion constant of 2.5 microns 2/ms. This exponent is smaller for cylindrical magnetized regions of the same size, and also depends on the diffusion constant. We discuss possible applications of our theoretical results to the analysis of the effects of high-susceptibility contrast agents in brain. Experimental data from the literature are compared with calculated signal changes according to the model. The monotonic dependence of decay rates on the volume of distribution of the contrast agent suggests that cerebral blood volume and flow could be measured using MR contrast.

Proton NMR spectroscopy has proven useful in the detection of cancer in lymph node tissue. However, due to the high fat content of this type of tissue, 2D 1H COSY measurements (requiring acquisition times of 4-5 h or longer) are necessary to obtain the spectral information necessary for diagnosis. T2-filtered proton magic-angle spinning (MAS) NMR spectroscopy provides 1D spectra of lymph nodes in approximately 20 min with sufficient spectral resolution allowing for identification of changes in cellular chemistry due to the presence of malignant cells. MAS data from lymph nodes of five control and six rats with mammary adenocarcinoma (R13762) demonstrated increases in the signal intensity of resonances associated primarily with lactate (delta = 4.12 ppm) P < 0.0004, creatines/lysine (delta = 3.04 ppm) P < 0.0032, and glutamate/ glutamine (delta = 2.36 ppm) P < 0.0002 in metastatic compared with normal lymph nodes. The infiltration of lymph nodes by malignant cells is an important prognostic factor for many cancers. The rapid assessment of node tissue without the introduction of sampling errors (inherent in currently employed histological procedures) would allow postoperative therapy decisions to be made more efficiently.

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...