Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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A dual-echo pulse sequence for simultaneous acquisition of MR angiography and venography (MRAV) is developed. Data acquisition of the second echo for susceptibility-weighted imaging-based MR venography is added to the conventional three-dimensional (3D) time-of-flight (TOF) MRA pulse sequence. Using this dual-echo acquisition approach, the venography data can be acquired without increasing the repetition time, and, therefore, the scan duration of routine TOF MRA scans is maintained. ⋯ The effect of spatial resolution on vein-to-background contrast is also demonstrated. Venous contrast is improved in high-resolution (0.52 x 0.52 x 1.6 mm(3)) images compared to that in standard-resolution (0.78 x 0.78 x 1.6 mm(3)) images. This MRAV technique enables the acquisition of MR venography without the need of an extra scan or injection of contrast agent in routine clinical brain exams at 3T.
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Most k-space-based parallel imaging reconstruction techniques, such as Generalized Autocalibrating Partially Parallel Acquisitions (GRAPPA), necessitate the acquisition of regularly sampled Cartesian k-space data to reconstruct a nonaliased image efficiently. However, non-Cartesian sampling schemes offer some inherent advantages to the user due to their better coverage of the center of k-space and faster acquisition times. On the other hand, these sampling schemes have the disadvantage that the points acquired generally do not lie on a grid and have complex k-space sampling patterns. ⋯ Because the undersampled non-Cartesian data cannot be reconstructed using a single GRAPPA kernel, several Cartesian patterns are selected for the reconstruction. This flexibility in terms of both the appearance and number of patterns allows this pseudo-Cartesian GRAPPA to be used with undersampled data sets acquired with any non-Cartesian trajectory. The successful implementation of the reconstruction algorithm using several different trajectories, including radial, rosette, spiral, one-dimensional non-Cartesian, and zig-zag trajectories, is demonstrated.
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Although arterial spin labeling (ASL) MRI has been successfully applied to measure gray matter (GM) perfusion in vivo, accurate detection of white matter (WM) perfusion has proven difficult. Reported literature values are not consistent with each other or with perfusion measured with other modalities. In this work, the cause of these inconsistencies is investigated. ⋯ Furthermore, simulations show that even at a spatial resolution of 7 microl voxel size, contamination by the GM signals can exceed 40% of the actual WM signal. From 10-min long flow-sensitive alternating inversion recovery ASL (FAIR-ASL) measurements at 3T in normal subjects (N=7), using highly sensitive detectors, it is shown that single-voxel (7 mul) deep WM perfusion values have an signal-to-noise ratio (SNR) less than 1. The poor sensitivity and heterogeneous transit time limit the applicability of ASL for measurement of perfusion in WM.
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Comparative Study
Rapid in vivo musculoskeletal MR with parallel imaging at 7T.
The purpose of this work was to implement autocalibrating GRAPPA-based parallel imaging (PI) for in vivo high-resolution (HR) MRI of cartilage and trabecular bone micro-architecture at 7T and to evaluate its performance based on comparison of MR-derived morphology metrics between accelerated and conventional images and comparison of geometry factor measures between 3T and 7T. Using an eight channel coil array for trabecular MRI at the ankle, a higher maximum feasible acceleration (R) = 6 and lower geometry factor values than that at 3T were observed. ⋯ Robust quantification of bone and cartilage metrics could be derived from accelerated ankle and knee acquisitions. PI can enhance the clinical feasibility of in vivo bone and cartilage HR-MRI for osteoporosis and osteoarthritis at 7T.
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Our study followed the changes in thalamic nuclei metabolism, hindlimb sensitivity to thermal stimulation, and locomotor function after spinal cord injury (SCI). MR spectroscopy (MRS) was used to examine the thalamic nuclei of rats 1 day before and 1, 3, 6, and 15 days after SCI or sham surgery. All animals were tested before MRS measurements for motor performance and thermal sensitivity. ⋯ The data also showed an increase in inositol (Ins), glutamate, and creatine (Cr) concentrations on the third day postinjury; the Ins concentration remained elevated on the sixth day. In sham-operated animals an increase in NAA concentration was observed on the sixth and fifteenth days after surgery and an increase in Cr concentration on the third day. A positive correlation between Ins concentration and hindlimb sensitivity in both SCI and sham-operated animals suggests changes in glial activity, while changes in NAA levels may indicate the response of thalamic neuronal cells to injury.