Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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The ability to visualize blood flow in a vessel-selective manner is of importance in a range of cerebrovascular diseases. Conventional X-ray methods are invasive and carry risks to the patient. Recently, a noninvasive dynamic angiographic MRI-based technique has been proposed using vessel-encoded pseudocontinuous arterial spin labeling, yielding vessel-selective angiograms of the four main brain-feeding arteries. ⋯ These maps are also used to generate synthesized images of blood inflow without bias from T(1) decay and radio frequency effects, greatly improving collateral vessel visibility in the patient with Moya-Moya disease. Relative volume flow rates in downstream vessels are also quantified, showing the relative importance of each feeding artery. This framework is likely to be of use in assessing collateral blood flow in patient groups.
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Quantification of blood oxygen saturation on the basis of a measurement of its magnetic susceptibility demands knowledge of the difference in volume susceptibility between fully oxygenated and fully deoxygenated blood (Δχ(do) ). However, two very different values of Δχ(do) are currently in use. In this work we measured Δχ(do) as well as the susceptibility of oxygenated blood relative to water, Δχ(oxy) , by MR susceptometry in samples of freshly drawn human blood oxygenated to various levels, from 6 to 98% as determined by blood gas analysis. Regression analysis yielded 0.273 ± 0.006 and -0.008 ± 0.003 ppm (cgs) respectively, for Δχ(do) and Δχ(oxy) , in excellent agreement with previous work by Spees et al. (Magn Reson Med 2001;45:533-542).
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Single-shot echo-planar imaging has been used widely in diffusion magnetic resonance imaging due to the difficulties in correcting motion-induced phase corruption in multishot data. Readout-segmented EPI has addressed the multishot problem by introducing a two-dimensional nonlinear navigator correction with online reacquisition of uncorrectable data to enable acquisition of high-resolution diffusion data with reduced susceptibility artifact and T*(2) blurring. The primary shortcoming of readout-segmented EPI in its current form is its long acquisition time (longer than similar resolution single-shot echo-planar imaging protocols by approximately the number of readout segments), which limits the number of diffusion directions. ⋯ In this study, the effects of homodyne and projection onto convex sets reconstructions on estimates of the fractional anisotropy, mean diffusivity, and diffusion orientation in fiber tracts and raw T(2)- and trace-weighted signal are compared, along with signal-to-noise ratio results. It is found that projections onto convex sets reconstruction with 3/5 segments in a 2 mm isotropic diffusion tensor image acquisition and 9/13 segments in a 0.9 × 0.9 × 4.0 mm(3) diffusion-weighted image acquisition provide good fidelity relative to the full k-space parameters. This allows application of readout-segmented EPI to tractography studies, and clinical stroke and oncology protocols.
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A new technique is introduced in this study for in vivo measurement of arterial cerebral blood volume by combining arterial spin labeling with a segmented multiphase balanced steady-state free precession (bSSFP) readout sequence. This technique takes advantage of the phenomenon that the longitudinal magnetization of flowing blood is not or only marginally disturbed (besides T(1) relaxation) by the bSSFP ± α pulse train. When the blood water exchanges into tissue, it becomes quickly saturated by the bSSFP pulse train due to 0 velocity and reduced T(1), T(2) relaxation times. ⋯ Both Bloch equation simulation and in vivo experiments were carried out to demonstrate the feasibility for quantifying cerebral blood volume in arteries, arterioles, and capillaries using two variants of the proposed method. Functional MRI of visual cortex stimulation was further performed using multiphase bSSFP-based arterial spin labeling and compared with vascular-space occupancy contrast. The proposed multiphase bSSFP-based arterial spin labeling technique may allow separation of cerebral blood volume of different vascular compartments for functional MRI studies and clinical evaluation of the cerebral vasculature.
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Fast imaging trajectories are used in MRI to speed up the acquisition process, but imperfections in the gradient system create artifacts in the reconstructed images. Artifacts result from the deviation between k-space trajectories achieved on the scanner and their original prescription. ⋯ This is done efficiently, by comparing the Fourier transforms of the input and measured waveforms of a single high-bandwidth test gradient waveform. This new method is tested for spiral, interleaved echo-planar, and three-dimensional cones imaging, demonstrating its ability to reduce reconstructed image artifacts for various k-space trajectories.