Hee Kwon Song

MRI measurement of brain iron in patients with restless legs syndrome

Brain iron insufficiency in the restless legs syndrome (RLS) has been suggested by a prior CSF study. Using a special MRI measurement (R2′), the authors assessed regional brain iron concentrations in 10 subjects (five with RLS, five controls). R2′ was significantly decreased in the substantia nigra, and somewhat less significantly in the putamen, both in proportion to RLS severity. The results show the potential utility of this MRI measurement, and also indicate that brain iron insufficiency may occur in patients with RLS in some brain regions.

Digital topological analysis of in vivo magnetic resonance microimages of trabecular bone reveals structural implications of osteoporosis.

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 (μ‐MRI) of the radius obtained at 137 × 137 × 350 μm3 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.

Arterial transit time imaging with flow encoding arterial spin tagging (FEAST)

Arterial spin labeling (ASL) perfusion imaging provides direct and absolute measurement of cerebral blood flow (CBF). Arterial transit time is a related physiological parameter reflecting the duration for the labeled spins to reach the brain region of interest. Most of the existing ASL approaches to assess arterial transit time rely on multiple measurements at various postlabeling delay times, and thus are vulnerable to motion artifact as well as computational error. We describe the use of flow encoding arterial spin tagging (FEAST) technique to measure tissue transit time, which can be derived from the ratio between the ASL signals measured with and without appropriate bipolar gradients. In the present study, we provided a theoretical framework and carried out an experimental validation during steady‐state imaging. The global mean tissue transit time was ∼1100 and 1400 ms for two conditions of bipolar gradients with specific encoding velocity (Venc) of 29 and 8 mm/sec, respectively. The mean tissue transit time measured within cerebral vascular territories was shortest in the deep middle cerebral artery (MCA) territory. Application of the FEAST technique in two patients with cerebrovascular disease demonstrated prolonged tissue transit times in the affected vascular territories which were consistent with results from other MR imaging modalities. Magn Reson Med 50:599–607, 2003.

Cortical Bone Water: In Vivo Quantification with Ultrashort Echo-Time MR Imaging

PURPOSE To develop and evaluate a method based on ultrashort echo-time radial magnetic resonance (MR) imaging to quantify bone water (BW) concentration as a new metric of bone quality in human cortical bone in vivo. MATERIALS AND METHODS Human subject studies were institutional review board approved and HIPAA compliant; informed consent was obtained. Cortical BW concentration was determined with custom-designed MR imaging sequences at 3.0 T and was validated in sheep and human cortical bone by using exchange of native water with deuterium oxide (D(2)O). The submillisecond T2* of BW requires correction for relaxation losses during the radiofrequency pulse. BW was measured at the tibial midshaft in healthy pre- and postmenopausal women (mean age, 34.6 and 69.4 years, respectively; n = 5 in each group) and in patients receiving maintenance hemodialysis (mean age, 51.8 years; n = 6) and was compared with bone mineral density (BMD) at the same site at peripheral quantitative computed tomography, as well as with BMD of the lumbar spine and hip at dual x-ray absorptiometry. Data were analyzed by using the Pearson correlation coefficient and two-sided t tests as appropriate. RESULTS Excellent agreement was obtained ex vivo between the water displaced by using D(2)O exchange and water measured with respect to a reference sample (r(2) = 0.99, P < .001). In vivo, BW in the postmenopausal group was greater by 65% (28.7% +/- 1.3 [standard deviation] vs 17.4% +/- 2.2, P < .001) than in the premenopausal group, and patients with renal osteodystrophy had higher BW (41.4% +/- 9.6) than the premenopausal group by 135% (P < .001) and the postmenopausal group by 43% (P = .02). BMD showed an opposite behavior, with much smaller group differences. Because the majority of BW is in the pore system of cortical bone, this parameter provides a surrogate measure for cortical porosity. CONCLUSION A new MR imaging-based method for quantifying BW noninvasively has been demonstrated.

Topological analysis of trabecular bone MR images

Recently, imaging techniques have become available which permit nondestructive analysis of the three-dimensional (3-D) architecture of trabecular bone (TB), which forms a network of interconnected plates and rods. Most osteoporotic fractures occur at locations rich in TB, which has spurred the search for architectural parameters as determinants of bone strength. Here, the authors present a new approach to quantitative characterization of the 3-D microarchitecture of TB, based on digital topology. The method classifies each voxel of the 3-D structure based on the connectivity information of neighboring voxels. Following conversion of the 3-D digital image to a skeletonized surface representation containing only one-dimensional (1-D) and two-dimensional (2-D) structures, each voxel is classified as a curve, surface, or junction. The method has been validated by means of synthesized images and has subsequently been applied to TB images from the human wrist. The topological parameters were found to predict Youngs modulus (YM) for uniaxial loading, specifically, the surface-to-curve ratio was found to be the single strongest predictor of YM (r/sup 2/=0.69). Finally, the method has been applied to TB images from a group of patients showing very large variations in topological parameters that parallel much smaller changes in bone volume fraction (BVF).

Dynamic MRI with projection reconstruction and KWIC processing for simultaneous high spatial and temporal resolution

A method for dynamic imaging in MRI is presented that enables the acquisition of a series of images with both high temporal and high spatial resolution. The technique, which is based on the projection reconstruction (PR) imaging scheme, utilizes distinct data acquisition and reconstruction strategies to achieve this simultaneous capability. First, during acquisition, data are collected in multiple undersampled passes, with the view angles interleaved in such a way that those of subsequent passes bisect the views of earlier ones. During reconstruction, these views are weighted according to a previously described k‐space weighted image contrast (KWIC) technique that enables the manipulation of image contrast by selective filtering. Unlike conventional undersampled PR methods, the proposed dynamic KWIC technique does not suffer from low image SNR or image degradation due to streaking artifacts. The effectiveness of dynamic KWIC is demonstrated in both simulations and in vivo, high‐resolution, contrast‐enhanced imaging of breast lesions. Magn Reson Med 52:815–824, 2004.

k -Space weighted image contrast (KWIC) for contrast manipulation in projection reconstruction MRI

A novel technique for manipulating contrast in projection reconstruction MRI is described. The method takes advantage of the fact that the central region of k‐space is oversampled, allowing one to choose different filters to enhance or reduce the amount that each view contributes to the central region, which dominates image contrast. The technique is implemented into a fast spin‐echo (FSE) sequence, and it is shown that multiple T2‐weighted images can be reconstructed from a single image data set. These images are shown to be nearly identical to those acquired with the Cartesian‐sampled FSE sequence at different effective echo times. Further, it is demonstrated that T2 maps can be generated from a single image data set. This technique also has the potential to be useful in dynamic contrast enhancement studies, capable of yielding a series of images at a significantly higher effective temporal resolution than what is currently possible with other methods, without sacrificing spatial resolution. Magn Reson Med 44:825–832, 2000.

Multislice double inversion pulse sequence for efficient black‐blood MRI

Over the last several years there has been a rapidly growing interest in high‐resolution MRI of the vascular wall to assess the extent of atherosclerotic lesions. Vessels of particular clinical relevance are the carotid and coronary arteries. Currently, the preferred imaging sequence for these studies is a “black‐blood” technique based on the double‐inversion scheme to null the blood signal. A critical drawback of the black‐blood technique, however, has been its single‐slice nature, as there is only one point in time during the recovery of the blood magnetization from inversion at which the signal is completely nulled. Consequently, the total scan time can become prohibitively long, particularly when an imaging protocol includes several series of these datasets. In this work, a multiple‐slice double‐inversion technique is described that can reduce the scan time by a factor of two or more. It is demonstrated in vivo with examples from carotid and coronary arteries that one can acquire multiple slices with sufficient nulling of blood, following a single set of inversion pulses. Magn Reson Med 47:616–620, 2002.

Intratendinous strain fields of the intact supraspinatus tendon: the effect of glenohumeral joint position and tendon region.

Rotator cuff tears are a common shoulder pathology and are hypothesized to relate to excessive tissue deformation. Few data exist, however, describing deformation of the rotator cuff as an intact, functional unit. Our purpose was to determine regional variations of intratendinous rotator cuff strain over a range of clinically relevant joint positions. A novel, MRI‐based technique was utilized to quantify intratendinous strains in cadaveric shoulder specimens at 15°, 30°, 45°, and 60° of glenohumeral abduction in the scapular plane. The strain data were grouped into superior, middle, and inferior locations across the region where most rotator cuff tears occur clinically. A repeated measures ANOVA assessed the effects of joint position and tendon region on intratendinous strain. Few differences in intratendinous strain existed across tendon regions, but joint position had a pronounced effect. Specifically, intratendinous strain increased with increasing joint angle, and the 60° strain was significantly greater than the 15° strain across all tendon regions. These data suggest that joint position plays a larger role in rotator cuff mechanics than previously believed. Future studies will utilize this technique for quantifying intratendinous strain to assess the effects of partial‐thickness rotator cuff tears.

In vivo MR micro imaging with conventional radiofrequency coils cooled to 77°K

Cryogenically cooled conventional surface coils are shown to provide significant signal‐to‐noise ratio (SNR) gains for MR micro imaging of tissue structure in vivo. Measurements are described which employ a simple, all‐polyvinyl chloride (PVC) vacuum dewar capable of maintaining a bath of liquid nitrogen around the coil, within 5 mm of the tissue to be imaged. Images acquired in vivo at 64 MHz with a 2‐cm diameter copper coil cooled to 77 K demonstrated a gain in SNR of approximately 2.7 ± 0.3 relative to those obtained with the same coil at room temperature under otherwise identical conditions. This increase is consistent with the reduction in coil resistance and the minor contribution to overall resistance from the imaging object. The performance of the coil is illustrated with images from the human finger and rabbit eye and potential applications are discussed. Magn Reson Med 43:163–169, 2000.