Magma (New York, N.Y.)
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Magma (New York, N.Y.) · Apr 2014
ReviewQuantification in magnetic resonance spectroscopy based on semi-parametric approaches.
Magnetic resonance spectroscopy (MRS) is a value-added modality to magnetic resonance imaging (MRI) that is often used in diagnosis, treatment and progression monitoring, as well as in non-destructive, non-invasive studies of disease states in humans and model systems in animals. The availability of high magnetic field strengths and use of hyperpolarized nuclei, combined with the possibility of acquiring spectra at very short echo-time, have dramatically increased the potential of MRS. For the last two decades, a challenge has been to quantify short echo-time proton spectra that exhibit many metabolites, and to estimate their concentrations. ⋯ The semi-parametric approaches, QUEST, AQSES, TARQUIN, LCModel and SiToolsFITT, are reviewed and discussed according to handling of macromolecule signal and unknown decay of the metabolite signal (lineshape). Estimation of noise-related errors on model parameters and compromise used in real-world applications are detailed, with emphasis on the bias-variance trade-off. Applications of the semi-parametric methods QUEST and AQSES to quantification of MRS, HRMAS and MRSI data are also provided.
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Magma (New York, N.Y.) · Aug 2013
ReviewThe role of tissue microstructure and water exchange in biophysical modelling of diffusion in white matter.
Biophysical models that describe the outcome of white matter diffusion MRI experiments have various degrees of complexity. While the simplest models assume equal-sized and parallel axons, more elaborate ones may include distributions of axon diameters and axonal orientation dispersions. These microstructural features can be inferred from diffusion-weighted signal attenuation curves by solving an inverse problem, validated in several Monte Carlo simulation studies. ⋯ The purpose of this review is to provide an update on the current understanding of how various properties of the tissue microstructure and the rate of water exchange between microenvironments are reflected in diffusion MRI measurements. We focus on the use of biophysical models for extracting tissue-specific parameters from data obtained with single PGSE sequences on clinical MRI scanners, but results obtained with animal MRI scanners are also considered. While modelling of white matter is the central theme, experiments on model systems that highlight important aspects of the biophysical models are also reviewed.
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Magma (New York, N.Y.) · Jun 2012
Highly accelerated PSF-mapping for EPI distortion correction with improved fidelity.
This study presents an improved point-spread-function (PSF) mapping-based distortion correction method and accelerated PSF acquisition for distortion correction in EPI without loss of quality or reliability compared to full encoding. ⋯ The proposed EPI-PSF-based distortion correction improves correction of EPI and accelerates PSF reference data acquisition and computation.
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Magma (New York, N.Y.) · Jun 2012
Local SAR management by RF shimming: a simulation study with multiple human body models.
Parallel transmission facilitates a relatively direct control of the RF transmit field. This is usually applied to improve the RF field homogeneity but might also allow a reduction of the specific absorption rate (SAR) to increase freedom in sequence design for high-field MRI. However, predicting the local SAR is challenging as it depends not only on the multi-channel drive but also on the individual patient. ⋯ Parallel transmission can become an important tool to control and manage the local SAR in the human body. The practical use of local SAR constraints is feasible with consistent results for a variety of body models.