Jerome L. Ackerman

Density of organic matrix of native mineralized bone measured by water- and fat-suppressed proton projection MRI.

Water‐ and fat‐suppressed projection MR imaging (WASPI) utilizes the large difference between the proton T  2* s of the solid organic matrix and the fluid constituents of bone to suppress the fluid signals while preserving solid matrix signals. The solid constituents include collagen and some molecularly immobile water and exhibit very short T  2* . The fluid constituents include mobile water and fat, with long T  2* . In WASPI, chemical shift selective low‐power π/2 pulses excite mobile water and fat magnetization which is subsequently dephased by gradient pulses, while the magnetization of collagen and immobile water remains mostly in the z‐direction. Additional selective π pulses in alternate scans further cancel the residual water and fat magnetization. Following water and fat suppression, the matrix signal is excited by a short hard pulse and the free induction decay acquired in the presence of a gradient in a 3D projection method. WASPI was implemented on a 4.7 T MR imaging system and tested on phantoms and bone specimens, enabling excellent visualization of bone matrix. The bone matrix signal per unit volume of bovine trabecular specimens was measured by this MR technique and compared with that determined by chemical analysis. This method could be used in combination with bone mineral density measurement by solid state 31P projection MRI to determine the degree of bone mineralization. Magn Reson Med 50:59–68, 2003.

Nuclear Magnetic Resonance Spin‐Spin Relaxation of the Crystals of Bone, Dental Enamel, and Synthetic Hydroxyapatites

Studies of the apatitic crystals of bone and enamel by a variety of spectroscopic techniques have established clearly that their chemical composition, short‐range order, and physical chemical reactivity are distinctly different from those of pure hydroxyapatite. Moreover, these characteristics change with aging and maturation of the bone and enamel crystals. Phosphorus‐31 solid state nuclear magnetic resonance (NMR) spin‐spin relaxation studies were carried out on bovine bone and dental enamel crystals of different ages and the data were compared with those obtained from pure and carbonated hydroxyapatites. By measuring the31P Hahn spin echo amplitude as a function of echo time, Van Vleck second moments (expansion coefficients describing the homonuclear dipolar line shape) were obtained and analyzed in terms of the number density of phosphorus nuclei.31P magnetization prepared by a 90° pulse or by proton‐phosphorus cross‐polarization (CP) yielded different second moments and experienced different degrees of proton spin‐spin coupling, suggesting that these two preparation methods sample different regions, possibly the interior and the surface, respectively, of bone mineral crystals. Distinct differences were found between the biological apatites and the synthetic hydroxyapatites and as a function of the age and maturity of the biological apatites. The data provide evidence that a significant fraction of the protonated phosphates (HPO4−2) are located on the surfaces of the biological crystals, and the concentration of unprotonated phosphates (PO4−3) within the apatitic lattice is elevated with respect to the surface. The total concentration of the surface HPO4−2 groups is higher in the younger, less mature biological crystals.

Phosphate Ions in Bone: Identification of a Calcium–Organic Phosphate Complex by 31P Solid-State NMR Spectroscopy at Early Stages of Mineralization

Previous 31P cross-polarization and differential cross-polarization magic angle spinning (CP/MAS and DCP/MAS) solid-state NMR spectroscopy studies of native bone and of the isolated crystals of the calcified matrix synthesized by osteoblasts in cell culture identified and characterized the major PO4−3 phosphate components of the mineral phase. The isotropic and anisotropic chemical shift parameters of the minor HPO4−2 component in bone mineral and in mineral deposited in osteoblast cell cultures were found to differ significantly from those of brushite, octacalcium phosphate, and other synthetic calcium phosphates. However, because of in vivo and in vitro evidence that phosphoproteins may play a significant role in the nucleation of the solid mineral phase of calcium phosphate in bone and other vertebrate calcified tissues, the focus of the current solid-state 31P NMR experiments was to detect the possible presence of and characterize the phosphoryl groups of phosphoproteins in bone at the very earliest stages of bone mineralization, as well as the possible presence of calcium-phosphoprotein complexes. The present study demonstrates that by far the major phosphate components identified by solid-state 31P NMR in the very earliest stages of mineralization are protein phosphoryl groups which are not complexed with calcium. However, very small amounts of calcium-complexed protein phosphoryl groups as well as even smaller, trace amounts of apatite crystals were also present at the earliest phases of mineralization. These data support the hypothesis that phosphoproteins complexed with calcium play a significant role in the initiation of bone calcification.

Perfluorinated Organic Liquids and Emulsions as Biocompatible NMR Imaging Agents for 19F and Dissolved Oxygen

Emulsions of fluorocarbons are finding considerable use in physiology for intravascular oxygen transport. Their wide clinical application as blood substitutes, anti-shock, and anti-ischemic agents seems imminent. Whole body NMR imaging is rapidly gaining clinical application and may one day almost completely supplant X-ray imaging. All of the 19F compounds used in biocompatible fluorocarbon emulsions give 19F signals identical to those in the corresponding neat liquid. In concentrations of 10% w/v they are readily imaged. The paramagnetic oxygen molecule reduces T1 in such a way as to make possible whole body imaging of oxygen. T1 typically decreases from 1-4 to 0.3-0.5 seconds and is an inverse linear function of oxygen tension. Spin-lattice relaxation times versus oxygen tensions from 0 to 600 torr have been obtained for F-decalin, F-tributylamine, and F-44E. The usefulness of these 19F effects in clinical NMR imaging depends upon the sensitivity of the method and the tolerable dose. The 19F signal may find use in monitoring 19F compounds as vapors or gases dissolved in plasma or in perfluorocarbons in neat liquid or particle form.

EVALUATION OF BONE MINERAL DENSITY USING THREE-DIMENSIONAL SOLID STATE PHOSPHORUS-31 NMR PROJECTION IMAGING

Abstract. A solid state magnetic resonance imaging technique is used to measure true three-dimensional mineral density of synthetic hydroxyapatite phantoms and specimens of bone ex vivo. The phosphorus-31 free induction decay at 2.0 T magnetic field strength is sampled following application of a short, hard radiofrequency excitation pulse in the presence of a fixed amplitude magnetic field gradient. Multiple gradient directions covering the unit sphere are used in an efficient spherical polar to Cartesian interpolation and Fourier transform projection reconstruction scheme to image the three-dimensional distribution of phosphorus within the specimen. Using 3–6 Gauss/cm magnetic field gradients, a spatial resolution of 0.2 cm over a field of view of 10 cm is achieved in an imaging time of 20–35 minutes. Comparison of solid state magnetic resonance imaging with dual energy X-ray absorptiometry (DXA), gravimetric analysis, and chemical analysis of calcium and phosphorus demonstrates good quantitative accuracy. Direct measurement of bone mineral by solid state magnetic resonance opens up the possibility of imaging variations in mineral composition as well as density. Advantages of the solid state magnetic resonance technique include avoidance of ionizing radiation; direct measurement of a constituent of the mineral without reliance on assumptions about, or models of, tissue composition; the absence of shielding, beam hardening, or multiple scattering artifacts; and its three-dimensional character. Disadvantages include longer measurement times and lower spatial resolution than DXA and computed tomography, and the inability to scan large areas of the body in a single measurement, although spatial resolution is sufficient to resolve cortical from trabecular bone for the purpose of measuring bone mineral density.

Methods for detecting and imaging a temperature of an object by nuclear magnetic resonance

A novel and improved method to detect indirectly a temperature of an object employing nuclear magnetic resonance techniques (NMR). The method involves obtaining an NMR spectrum to determine chemical shift, relaxation times, spin-spin couplings or quadrupole couplings for an element of a compound having at least one conformational isomer wherein the compound is influenced by a temperature of the object. Uniquely, the present invention may detect temperature in the body of an animal. Further, the present invention discloses a novel method to determine and monitor thermal physiological states in an animal as well as determine and monitor thermal states in an object. Because of the unique and advantageous non-invasive, non-destructive and non-ionizing properties, the present invention may be employed in an object or animal continuously. The present invention also provides for the thermal imaging, or NMR thermography, by one-, two-, or three-dimensional reconstruction techniques from chemical shift, relaxation times, (e.g., T1 or T2,) spin-spin couplings and quadrupole couplings for an element of a compound having at least one conformational isomer in an object or animal influenced by a temperature in the object or animal. The methods disclosed are applicable to inanimate and animate solids.

Experimental results on deuterium NMR in the solid state by magic angle sample spinning

Abstract Magic angle sample spinning has been applied to remove the first order quadrupole broadening in NMR spectra of deuterium in the solid state. The free induction decay of the rotating sample consists of a series of spatially induced echoes which were sampled in synchronism with the rotation to produce an isotropic decay. A minute adjustment of the axis of rotation with respect to the laboratory field direction was necessary since the spectra were narrowed by about three orders of magnitude. The resulting spectra exhibit resolved, isotropic, chemically shifted lines and indicate the feasibility of high resolution deuterium NMR in solids.

Water- and fat-suppressed proton projection MRI (WASPI) of rat femur bone.

Investigators often study rats by μCT to investigate the pathogenesis and treatment of skeletal disorders in humans. However, μCT measurements provide information only on bone mineral content and not the solid matrix. CT scans are often carried out on cancellous bone, which contains a significant volume of marrow cells, stroma, water, and fat, and thus the apparent bone mineral density (BMD) does not reflect the mineral density within the matrix, where the mineral crystals are localized. Water‐ and fat‐suppressed solid‐state proton projection imaging (WASPI) was utilized in this study to image the solid matrix content (collagen, tightly bound water, and other immobile molecules) of rat femur specimens, and meet the challenges of small sample size and demanding submillimeter resolution. A method is introduced to recover the central region of k‐space, which is always lost in the receiver dead time when free induction decays (FIDs) are acquired. With this approach, points near the k‐space origin are sampled under a small number of radial projections at reduced gradient strength. The typical scan time for the current WASPI experiments was 2 hr. Proton solid‐matrix images of rat femurs with 0.4‐mm resolution and 12‐mm field of view (FOV) were obtained. This method provides a noninvasive means of studying bone matrix in small animals. Magn Reson Med 57:554–567, 2007.

Nuclear magnetic resonance microscopy of atheroma in human coronary arteries.

The purpose of this study was to use direct nuclear magnetic resonance (NMR) microscopy to quantitate and image accumulations of atheroma lipids in human coronary arteries and to validate the results by comparison with histologic preparations. NMR microscopy was performed on a superconducting experimental NMR imaging system operating at 2 Tesla with a probe designed for short echo time (TE), strong B 1 field strength, and small samples. Data acquisition used multiple-offset chemical encoding with offsets based on the thermotropic spectral signature of atheroma lipids within the human arterial vessel wall. Three separate channels of image data yielded color axis display of atheroma within the vessel walls. Atheroma location by histology was identified by rarefaction of stroma, as the lipids are extracted in the process of embedding in paraffin. Perimeters, areas, and a shape index (perimeter2:4πarea) of lumen, atheroma, and outer wall were determined and compared for NMR vs histology. There was no significant difference in the measurements with the exception of luminal shape indices, which were uniformly larger by histology, attributable to flattening of the vessels during histologic preparation. NMR measurement of atheroma content of coronary artery walls agreed well with histology (r = 0.996). NMR microscopy with color axis display proved able to quantitate and image atheroma in coronary arteries, obviating the distortions and lipid removal associated with fixation, embedding, and sectioning for histology.

Structure, Composition, and Maturation of Newly Deposited Calcium-Phosphate Crystals in Chicken Osteoblast Cell Cultures

Characterization of the very early calcium phosphate (CaP) crystals deposited in bone or in osteoblast cell cultures has been hampered by the overwhelming presence of organic matrix components and cells that obscure spectral analyses. We have overcome this problem using isolated protein‐free crystals and have obtained new data including31P nuclear magnetic resonance (NMR) spectra for the first time from mineral crystals deposited during osteoblast calcification in culture. Crystals were isolated from cultures at two time points: (a) at first calcium accumulation (day 8–10) and (b) after 60 days of culture, to assess maturational changes. The analyses show that the chemical composition overall and short range order of the early and mature crystals are characteristic of the apatite crystals found in young embryonic chick bone in vivo. No mineral phase other than apatite was detected by any of the methods used.31P NMR spectroscopy identified the HPO4 groups as those present in bone apatite. Similar to bone apatites, no OH groups were detected by Fourier transform infrared (FTIR) spectroscopy. The temporal maturational changes in composition and structure of the mineral phase were difficult to assess because of the continuous deposition of crystals throughout culturing. The pathway of the maturational changes observed were similar to those occurring in chick bone in vivo and synthetic apatite crystals in vitro although to a much smaller extent.