149 research outputs found
New bryophyte taxon records for tropical countries 2
Norris & T. Kop. Sabah, Mt. Kinabalu, Mary Strong Clemens. 10741, 15.11.1915 (L) as âCampylopus metzlerioides Broth. nom. nud.â The species was known before (mostly as Atractylocarpus comosus Dix.) from Sumatra, Celebes, New Guinea, Bhutan and Nepal [JPF]
Aggregated motion estimation for real-time MRI reconstruction
Real-time magnetic resonance imaging (MRI) methods generally shorten the
measuring time by acquiring less data than needed according to the sampling
theorem. In order to obtain a proper image from such undersampled data, the
reconstruction is commonly defined as the solution of an inverse problem, which
is regularized by a priori assumptions about the object. While practical
realizations have hitherto been surprisingly successful, strong assumptions
about the continuity of image features may affect the temporal fidelity of the
estimated images. Here we propose a novel approach for the reconstruction of
serial real-time MRI data which integrates the deformations between nearby
frames into the data consistency term. The method is not required to be affine
or rigid and does not need additional measurements. Moreover, it handles
multi-channel MRI data by simultaneously determining the image and its coil
sensitivity profiles in a nonlinear formulation which also adapts to
non-Cartesian (e.g., radial) sampling schemes. Experimental results of a motion
phantom with controlled speed and in vivo measurements of rapid tongue
movements demonstrate image improvements in preserving temporal fidelity and
removing residual artifacts.Comment: This is a preliminary technical report. A polished version is
published by Magnetic Resonance in Medicine. Magnetic Resonance in Medicine
201
Fast T2 Mapping with Improved Accuracy Using Undersampled Spin-echo MRI and Model-based Reconstructions with a Generating Function
A model-based reconstruction technique for accelerated T2 mapping with
improved accuracy is proposed using undersampled Cartesian spin-echo MRI data.
The technique employs an advanced signal model for T2 relaxation that accounts
for contributions from indirect echoes in a train of multiple spin echoes. An
iterative solution of the nonlinear inverse reconstruction problem directly
estimates spin-density and T2 maps from undersampled raw data. The algorithm is
validated for simulated data as well as phantom and human brain MRI at 3 T. The
performance of the advanced model is compared to conventional pixel-based
fitting of echo-time images from fully sampled data. The proposed method yields
more accurate T2 values than the mono-exponential model and allows for
undersampling factors of at least 6. Although limitations are observed for very
long T2 relaxation times, respective reconstruction problems may be overcome by
a gradient dampening approach. The analytical gradient of the utilized cost
function is included as Appendix.Comment: 10 pages, 7 figure
Joint T1 and T2 Mapping with Tiny Dictionaries and Subspace-Constrained Reconstruction
Purpose: To develop a method that adaptively generates tiny dictionaries for
joint T1-T2 mapping.
Theory: This work breaks the bond between dictionary size and representation
accuracy (i) by approximating the Bloch-response manifold by piece-wise linear
functions and (ii) by adaptively refining the sampling grid depending on the
locally-linear approximation error.
Methods: Data acquisition was accomplished with use of an 2D radially sampled
Inversion-Recovery Hybrid-State Free Precession sequence. Adaptive dictionaries
are generated with different error tolerances and compared to a heuristically
designed dictionary. Based on simulation results, tiny dictionaries were used
for T1-T2 mapping in phantom and in vivo studies. Reconstruction and parameter
mapping were performed entirely in subspace.
Results: All experiments demonstrated excellent agreement between the
proposed mapping technique and template matching using heuristic dictionaries.
Conclusion: Adaptive dictionaries in combination with manifold projection
allow to reduce the necessary dictionary sizes by one to two orders of
magnitude
Reconstruction and Dissection of the Entire Human Visual Pathway Using Diffusion Tensor MRI
The human visual system comprises elongated fiber pathways that represent a serious challenge for diffusion tensor imaging (DTI) and fiber tractography: while tracking of frontal fiber bundles may be compromised by the nearby presence of air-filled cavities, nerves, and eye muscles, the anatomic courses of the three main fiber bundles of the optic radiation are subject to pronounced inter-subject variability. Here, tractography of the entire visual pathway was achieved in six healthy subjects at high spatial accuracy, that is, at 1.8âmm isotropic spatial resolution, without susceptibility-induced distortions, and in direct correspondence to anatomic MRI structures. Using a newly developed diffusion-weighted single-shot STEAM MRI sequence, we were able to track the thin optic nerve including the nasal optic nerve fibers, which cross the optic chiasm, and to dissect the optic radiation into the anterior ventral bundle (Meyer's loop), the central bundle, and the dorsal bundle. Apart from scientific applications these results in single subjects promise advances in the planning of neurosurgical procedures to avoid unnecessary damage to the visual fiber system
Suppression of MRI Truncation Artifacts Using Total Variation Constrained Data Extrapolation
The finite sampling of k-space in MRI causes spurious image artifacts, known as Gibbs ringing, which result from signal truncation at the border of k-space. The effect is especially visible for acquisitions at low resolution and commonly reduced by filtering at the expense of image blurring. The present work demonstrates that the simple assumption of a piecewise-constant object can be exploited to extrapolate the data in k-space beyond the measured part. The method allows for a significant reduction of truncation artifacts without compromising resolution. The assumption translates into a total variation minimization problem, which can be solved with a nonlinear optimization algorithm. In the presence of substantial noise, a modified approach offers edge-preserving denoising by allowing for slight deviations from the measured data in addition to supplementing data. The effectiveness of these methods is demonstrated with simulations as well as experimental data for a phantom and human brain in vivo
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