Interaction potentials of molecular clusters with applications to collision rate calculations

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Physical limits to human brain B0 shimming, engineering implications thereof

Objective: As the MRI main magnetic field rises for improved Signalto-Noise Ratio, susceptibility-induced B0-inhomogeneity increases proportionally, aggravating related artifacts. Considering only susceptibility disparities between air and biological tissue, we explore the topological conditions for which perfect shimming could be performed in a Region of Interest (ROI) such as the human brain or part thereof.Materials and Methods: After theoretical considerations for perfect shimming, spherical harmonic (SH) shimming simulations of veryhigh degree are performed, based on a 100-subject database of 1.7-mmresolved brain fieldmaps acquired at 3 T . In addition to the whole brain, shimmed ROIs include slabs targeting the prefrontal cortex, both or single temporal lobes, or spheres in the frontal brain above the nasal sinus.Results and Discussion: We show “perfect” shimming is possible only if the ROI can be contained in a sphere that does not enclose sources of magnetic field inhomogeneity, which are gathered at the air-tissue interface. We establish a 12 Hz inhomogeneity hard shim limit at 7 T for whole brain shimming, that can only be attained at shimming degree higher than 90. On the other hand, under limited power and SH degree resources, 3D regionspecific shimming is shown to greatly improve homogeneity in critical zones such as the prefrontal cortex and around ear canals

A Fieldmap-Driven Few-Channel Shim Coil Design for MRI of the Human Brain

International audienceWe exploit the inter-subject similarity of inhomogeneous static magnetic field patterns arising in the human brain under MRI examination to design a small set of shim coils providing performance equivalent to numerous coils based on high-order Spherical Harmonics corrections. A hundred brain B 0-maps were first collected at 3 T. Ideal subject-specific electric current density stream functions are then computed with low power constraints, on a cylindrical surface. This step is repeated over tens of brain maps so that a Principal Component Analysis can be applied to the stream functions; the main components result in the small set of coils. Both 50-subject hold-out and 10-fold cross-validation are employed to evaluate consistency of the proposed system performance over a posteriori subjects. Simulations show that only 3 cylindrical coils manage to capture the principal magnetic field profiles in the human brain, thus providing a better static field inhomogeneity mitigation than that obtained from 16 unlimited-power high-order Spherical Harmonics coils, with inhomogeneity greatly reduced in the pre-frontal cortex compared to 2 nd-order shimmed baseline field acquisitions. The approach provides a very reduced channel count system for mitigating complex B 0-inhomogeneity patterns. Thus, a compact, cost-effective system could be conceived and driven by relatively low-budget electronics. The method should therefore have a strong impact in both Ultra-High and portable low-field MRI/MRS. Moreover, this technique can be applied to the design of shim coils addressing anatomies other than the brain. Keywords: B 0 inhomogeneity, human brain shimming, MRI, shim coil design, stream functions, Ultra-High-Field MRI, whole brain shimming

Physical Limits to Human Brain B0 Shimming with Spherical Harmonics, Engineering Implications Thereof

International audienceObjective: As the MRI main magnetic field rises for improved Signalto-Noise Ratio, susceptibility-induced B0-inhomogeneity increases proportionally, aggravating related artifacts. Considering only susceptibility disparities between air and biological tissue, we explore the topological conditions for which perfect shimming could be performed in a Region of Interest (ROI) such as the human brain or part thereof.Materials and Methods: After theoretical considerations for perfect shimming, spherical harmonic (SH) shimming simulations of very high degree are performed, based on a 100-subject database of 1.7-mmresolved brain fieldmaps acquired at 3 T. In addition to the whole brain, shimmed ROIs include slabs targeting the prefrontal cortex, both or single temporal lobes, or spheres in the frontal brain above the nasal sinus. Results and Discussion: We show "perfect" SH shimming is possible only if the ROI can be contained in a sphere that does not enclose sources of magnetic field inhomogeneity, which are gathered at the air-tissue interface. We establish a 13Hz inhomogeneity hard shim limit at 7 T for whole brain SH shimming, that can only be attained at shimming degree higher than 90. On the other hand, under limited power and SH degree resources, 3D regionspecific shimming is shown to greatly improve homogeneity in critical zones such as the prefrontal cortex and around ear canals

Artifacts and pitfalls in diffusion MRI.

International audienceAlthough over the last 20 years diffusion MRI has become an established technique with a great impact on health care and neurosciences, like any other MRI technique it remains subject to artifacts and pitfalls. In addition to common MRI artifacts, there are specific problems that one may encounter when using MRI scanner gradient hardware for diffusion MRI, especially in terms of eddy currents and sensitivity to motion. In this article we review those artifacts and pitfalls on a qualitative basis, and introduce possible strategies that have been developed to mitigate or overcome them

Two-spoke placement optimization under explicit specific absorption rate and power constraints in parallel transmission at ultra-high field

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Accelerating Parallel Transmit Array B1 Mapping in High Field MRI With Slice Undersampling and Interpolation by Kriging

International audienceTransmit arrays have been developed to mitigate the RF field inhomogeneity commonly observed in high field magnetic resonance imaging (MRI), typically above 3T. To this end, the knowledge of the RF complex-valued B1 transmit-sensitivities of each independent radiating element has become essential. This paper details a method to speed up a currently available B1-calibration method. The principle relies on slice undersampling, slice and channel interleaving and kriging, an interpolation method developed in geostatistics and applicable in many domains. It has been demonstrated that, under certain conditions, kriging gives the best estimator of a field in a region of interest. The resulting accelerated sequence allows mapping a complete set of eight volumetric field maps of the human head in about 1 min. For validation, the accuracy of kriging is first evaluated against a well-known interpolation technique based on Fourier transform as well as to a B1-maps interpolation method presented in the literature. This analysis is carried out on simulated and decimated experimental B1 maps. Finally, the accelerated sequence is compared to the standard sequence on a phantom and a volunteer. The new sequence provides B1 maps three times faster with a loss of accuracy limited potentially to about 5%

On Variant Strategies To Solve The Magnitude Least Squares Optimization Problem In Parallel Transmission Pulse Design And Under Strict SAR And Power Constraints

International audienceParallel transmission has been a very promising candidate technology to mitigate the inevitable radio-frequency field inhomogeneity in magnetic resonance imaging (MRI) at ultra-high field (UHF). For the first few years, pulse design utilizing this technique was expressed as a least squares problem with crude power regularizations aimed at controlling the specific absorption rate (SAR), hence the patient safety. This approach being suboptimal for many applications sensitive mostly to the magnitude of the spin excitation, and not its phase, the magnitude least squares (MLS) problem then was first formulated in 2007. Despite its importance and the availability of other powerful numerical optimization methods, this problem yet has been faced exclusively by the pulse designer with the so-called variable exchange method. In this paper, we investigate other strategies and incorporate directly the strict SAR and hardware constraints. Different schemes such as sequential quadratic programming (SQP), interior point (I-P) methods, semi-definite programming (SDP) and magnitude squared least squares (MSLS) relaxations are studied both in the small and large tip angle regimes with real data sets obtained in-vivo on a human brain at 7 Tesla. Convergence and robustness of the different approaches are analyzed, and recommendations to tackle this specific problem are finally given. Small tip angle and inversion pulses are returned in a few seconds and in under a minute respectively while respecting the constraints, allowing the use of the proposed approach in routine