Arterial spin labeling (ASL) is capable of noninvasively measuring blood flow by magnetically tagging the protons in arterial blood, which has been conventionally achieved using instantaneous (PASL) or continuous (CASL) RF pulses. As an intermediate method, pseudocontinuous ASL (pCASL) utilizes a train of discrete RF pulses to mimic continuous tagging that is often unavailable on imagers due to the requirement of continuous RF transmit capabilities. In the present study, we implemented two versions of pCASL (balanced and unbalanced gradient waveforms in tag and control scans) for both transmit/receive coils and array receivers. Experimental data show a 50% +/- 4% increase of signal-to-noise ratio (SNR) compared with PASL and a higher tagging efficiency than amplitude-modulated (AM) CASL (80% vs. 68%). Computer simulations predict an optimal tagging efficiency of 85% for flow velocities from 10 to 60 cm/s. It is theoretically and experimentally demonstrated that the tagging efficiency of pCASL is dependent upon the resonance offset and flip angle of the RF pulse train. We conclude that pCASL has the potential of combining the merits of PASL, including less hardware demand and higher tagging efficiency, and CASL, which includes a longer tagging bolus and thus higher SNR. These improvements provide a better balance between tagging efficiency and SNR.

Introduction: Standard morphological analyses of trabecular architecture lack explicit segmentations of individual trabecular plates and rods. In this study, a complete volumetric decomposition technique was developed to segment trabecular bone microstructure into individual plates and rods. Contributions of trabecular typeassociated morphological parameters to the anisotropic elastic moduli of trabecular bone were studied. Materials and Methods: Seventy-one human trabecular bone samples from the femoral neck (FN), tibia, and vertebral body (VB) were imaged using CT or serial milling. Complete volumetric decomposition was applied to segment trabecular bone microstructure into individual plates and rods. The orientation of each individual trabecula was determined, and the axial bone volume fractions (aBV/TV), axially aligned bone volume fraction along each orthotropic axis, were correlated with the elastic moduli. The microstructural type-associated morphological parameters were derived and compared with standard morphological parameters. Their contributions to the anisotropic elastic moduli, calculated by finite element analysis (FEA), were evaluated and compared. Results: The distribution of trabecular orientation suggested that longitudinal plates and transverse rods dominate at all three anatomic sites. aBV/TV along each axis, in general, showed a better correlation with the axial elastic modulus (r 2 ‫ס‬ 0.95∼0.99) compared with BV/TV (r 2 ‫ס‬ 0.93∼0.94). The plate-associated morphological parameters generally showed higher correlations with the corresponding standard morphological parameters than the rod-associated parameters. Multiple linear regression models of six elastic moduli with individual trabeculae segmentation (ITS)-based morphological parameters (adjusted r 2 ‫ס‬ 0.95∼0.98) performed equally well as those with standard morphological parameters (adjusted r 2 ‫ס‬ 0.94∼0.97) but revealed specific contributions from individual trabecular plates or rods. Conclusions: The ITS-based morphological analyses provide a better characterization of the morphology and trabecular orientation of trabecular bone. The axial loading of trabecular bone is mainly sustained by the axially aligned trabecular bone volume. Results suggest that trabecular plates dominate the overall elastic properties of trabecular bone.

A new MR imaging-based method for quantifying BW noninvasively has been demonstrated.

Magnetic resonance imaging has previously demonstrated its potential for indirectly mapping myelin density, either by relaxometric detection of myelin water or magnetization transfer. Here, we investigated whether myelin can be detected and possibly quantified directly. We identified the spectrum of myelin in the spinal cord in situ as well as in myelin lipids extracted via a sucrose gradient method, and investigated its spectral properties. Highresolution solution NMR spectroscopy showed the extract composition to be in agreement with myelin's known chemical make-up. The 400-MHz 1 H spectrum of the myelin extract, at 20°C (room temperature) and 37°C, consists of a narrow water resonance superimposed on a broad envelope shifted ∼3.5 ppm upfield, suggestive of long-chain methylene protons. Superimposed on this signal are narrow components resulting from functional groups matching the chemical shifts of the constituents making up myelin lipids. The spectrum could be modeled as a sum of super-Lorentzians with a T 2 * distribution covering a wide range of values (0.008-26 ms). Overall, there was a high degree of similarity between the spectral properties of extracted myelin lipids and those found in neural tissue. The normalized difference spectrum had the hallmarks of membrane proteins, not present in the myelin extract. Using 3D radially ramp-sampled proton MRI, with a combination of adiabatic inversion and echo subtraction, the feasibility of direct myelin imaging in situ is demonstrated. Last, the integrated signal from myelin suspensions is shown, both spectroscopically and by imaging, to scale with concentration, suggesting the potential for quantitative determination of myelin density. myelin in situ | myelin NMR spectrum | super-Lorentzian fitting | ultrashort echo time

Measuring the global cerebral metabolic rate of oxygen (CMRO(2)) is a valuable tool for assessing brain vitality and function. Measurement of blood oxygen saturation (HbO(2)) and flow in the major cerebral outflow and inflow vessels can provide a global estimate of CMRO(2). We demonstrate a rapid noninvasive method for quantifying CMRO(2) by simultaneously measuring venous oxygen saturation in the superior sagittal sinus with magnetic resonance susceptometry-based oximetry, a technique that exploits the intrinsic susceptibility of deoxygenated hemoglobin, and the average blood inflow rate with phase-contrast magnetic resonance imaging. The average venous HbO(2), cerebral blood flow, and global CMRO(2) values in eight healthy, normal study subjects were 64%+/-4%, 45.2+/-3.2 mL per 100 g per minute, and 127+/-7 micromol per 100 g per minute, respectively. These values are in good agreement with those reported in literature. The technique described is noninvasive, robust, and reproducible for in vivo applications, making it ideal for use in clinical settings for assessing the pathologies associated with dysregulation of cerebral metabolism. In addition, the short acquisition time (approximately 30 seconds) makes the technique suitable for studying the temporal variations in CMRO(2) in response to physiologic challenges.

Osteoporosis is a disease characterized by bone volume loss and architectural deterioration. The majority of work aimed at evaluating the structural implications of the disease has been performed based on stereologic analysis of histomorphometric sections. Only recently noninvasive imaging methods have emerged that provide sufficient resolution to resolve individual trabeculae. In this article, we apply digital topological analysis (DTA) to magnetic resonance microimages (-MRI) of the radius obtained at 137 ؋ 137 ؋ 350 m 3 voxel size in a cohort of 79 women of widely varying bone mineral density (BMD) and vertebral deformity status. DTA is a new method that allows unambiguous determination of the three-dimensional (3D) topology of each voxel in a trabecular bone network. The analysis involves generation of a bone volume fraction map, which is subjected to subvoxel processing to alleviate partial volume blurring, followed by thresholding and skeletonization. The skeletonized images contain only surfaces, profiles, curves, and their mutual junctions as the remnants of trabecular plates and rods after skeletonization. DTA parameters were compared with integral BMD in the lumbar spine and femur as well as MR-derived bone volume fraction (BV/TV). Vertebral deformities were determined based on sagittal MRIs of the spine with a semiautomatic method and the number of deformities counted after threshold setting. DTA structural indices were found the strongest discriminators of subjects with deformities from those without deformities. Subjects with deformities (n ‫؍‬ 29) had lower topological surface (SURF) density (p < 0.0005) and surface-to-curve ratio (SCR; a measure of the ratio of platelike to rodlike trabeculae; p < 0.0005) than those without. Profile interior (PI) density, a measure of intact trabecular rods, was also lower in the deformity group (p < 0.0001). These data provide the first in vivo evidence for the structural implications inherent in postmenopausal osteoporosis accompanying bone loss, that is, the conversion of trabecular plates to rods and disruption of rods due to repeated osteoclastic resorption. (J Bone Miner Res 2001;16:1520 -1531)

Osteoporosis is a multifactorial disorder of bone mineral homeostasis affecting the elderly. It is a major public health issue with significant socioeconomic consequences. Recent findings suggest that bone loss-the key manifestation of the disease-is accompanied by architectural deterioration, both affecting the bone's mechanical competence and susceptibility to fracture. This article reviews the potential of quantitative micro MRI (-MRI), including a discussion of the technical requirements for image acquisition, processing, and analysis for assessing the architectural implications of osteoporosis and as a means to monitor the response to treatment. With current technology, the resolution achievable in clinically acceptable scan times and necessary signal-to-noise ratio (SNR) is comparable to trabecular thickness. This limited spatial resolution regime demands processing and analysis algorithms designed to operate under such limiting conditions. It is shown that three different classes of structural parameters can be distinguished, characterizing scale, topology, and orientation. There is considerable evidence that osteoporotic bone loss affects all three classes but that topological changes, resulting from conversion of trabecular plates to rods, with the latter's eventual disconnection, are particularly prominent. Clinical applications discussed can be divided into those dealing with assessment of osteoporotic fracture risk as opposed to the study of the effect of disease progression and regression in response to treatment. Current data suggest that noninvasive assessment of cortical and trabecular bone (TB) architecture by -MRI may provide new surrogate endpoints to assess the efficacy of intervention in osteoporosis treatment and prevention.

Following the publication of this article, the author's noted a misprint in equation (4). The equation should read: %HbO 2 ¼ 1 À 2jDfj gDw do B o DTEðcos 2 y À 1=3ÞHct Â100