Spin echo | EPFL Graph Search

In magnetic resonance, a spin echo or Hahn echo is the refocusing of spin magnetisation by a pulse of resonant electromagnetic radiation. Modern nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) make use of this effect. The NMR signal observed following an initial excitation pulse decays with time due to both spin relaxation and any inhomogeneous effects which cause spins in the sample to precess at different rates. The first of these, relaxation, leads to an irreversible loss of magnetisation. But the inhomogeneous dephasing can be removed by applying a 180° inversion pulse that inverts the magnetisation vectors. Examples of inhomogeneous effects include a magnetic field gradient and a distribution of chemical shifts. If the inversion pulse is applied after a period t of dephasing, the inhomogeneous evolution will rephase to form an echo at time 2t. In simple cases, the intensity of the echo relative to the initial signal is given by e–2t/T2 where T2 is the tim

Comparison of 2D simultaneous multi-slice and 3D GRASE readout schemes for pseudo-continuous arterial spin labeling of cerebral perfusion at 3 T

Tom Hilbert, Tobias Kober, Bénédicte Marie Maréchal

Objective In this perfusion magnetic resonance imaging study, the performances of different pseudo-continuous arterial spin labeling (PCASL) sequences were compared: two-dimensional (2D) single-shot readout with simultaneous multislice (SMS), 2D single-shot echo-planar imaging (EPI) and multishot three-dimensional (3D) gradient and spin echo (GRASE) sequences combined with a background-suppression (BS) module. Materials and methods Whole-brain PCASL images were acquired from seven healthy volunteers. The performance of each protocol was evaluated by extracting regional cerebral blood flow (rCBF) measures using an inline morphometric segmentation prototype. Image data postprocessing and subsequent statistical analyses enabled comparisons at the regional and sub-regional levels. Results The main findings were as follows: (i) Mean global CBF obtained across methods was were highly correlated, and these correlations were significantly higher among the same readout sequences. (ii) Temporal signal-to-noise ratio and gray-matter-to-white-matter CBF ratio were found to be equivalent for all 2D variants but lower than those of 3D-GRASE. Discussion Our study demonstrates that the accelerated SMS readout can provide increased acquisition efficiency and/or a higher temporal resolution than conventional 2D and 3D readout sequences. Among all of the methods, 3D-GRASE showed the lowest variability in CBF measurements and thus highest robustness against noise.

SPRINGER2020

Quantitative Microstructural Imaging for Clinical Use

Gian Franco Piredda

Today, Magnetic Resonance Imaging (MRI) is a well-established medical imaging modality thanks to its excellent soft tissue contrast. Conventional MR image contrasts can be weighted towards one or more physical properties that discriminate different tissues, which represents one of the main advantages of MRI compared to other imaging techniques. However, conventional contrasts always depend on the combination of different tissue properties, imaging parameters and employed hardware. Quantitative MRI (qMRI) techniques allow moving from such relative contrast information to a single, absolute measure of one or more separate tissue properties.The measurement of water proton relaxation rates represents one major approach for quantitatively characterizing the human brain tissue microstructure with MRI. MR relaxation mechanisms depend on complex interactions among protons and between protons and their surrounding lattice. Hence, important determining factors of the physical parameters measured with qMRI are, for example, the concentration of water, macromolecules (for instance abundant in myelin) and paramagnetic atoms (e.g., iron). The exhibited sensitivity and specificity of qMRI acquisitions towards the microstructural properties of brain tissues drove an incremental investigation around qMRI metrics as biomarkers for alterations caused by disease. However, the practical use of qMRI in clinical settings is still hindered by difficulties in delivering precise quantification in an adequate resolution and within acquisition times comparable to conventional imaging. Moreover, to exploit the full potential of quantitative maps, normative values of physical parameters in healthy tissues are required, enabling the comparison of tissue properties from a single patient to normal values, potentially improving diagnosis and follow-up assessments.With a focus on brain relaxometry and myelin water imaging, this thesis aims at bringing qMRI closer to clinical routine by tackling challenges ranging from the image acquisition to its practical application. More specifically, a validated T2 relaxometry sequence for myelin imaging - the multi-echo gradient and spin echo sequence - has been implemented and subsequently accelerated by combining it with parallel imaging to achieve whole brain coverage in a clinical compatible acquisition time. The direct estimation of microstructural features from relaxometry data has been also investigated using a fast protocol for T1 and T2 mapping and a data-driven approach that takes advantage of recent advances in the machine learning domain. Finally, the clinical value of qMRI in conjunction with normative atlases of physical properties has been explored to detect personalized changes in tissue parameters that reflect the underlying pathology.

EPFL2021

Quantitative Microstructural Imaging for Clinical Use

Gian Franco Piredda

EPFL2021