The Experts below are selected from a list of 297 Experts worldwide ranked by ideXlab platform
Guido Nolte - One of the best experts on this subject based on the ideXlab platform.
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the magnetic lead field theorem in the quasi static approximation and its use for magnetoencephalography forward calculation in realistic Volume conductors
Physics in Medicine and Biology, 2003Co-Authors: Guido NolteAbstract:The equation for the magnetic lead field for a given magnetoencephalography (MEG) channel is well known for arbitrary frequencies omega but is not directly applicable to MEG in the quasi-static approximation. In this paper we derive an equation for omega = 0 starting from the very definition of the lead field instead of using Helmholtz's reciprocity theorems. The results are (a) the transpose of the conductivity times the lead field is divergence-free, and (b) the lead field differs from the one in any other Volume conductor by a gradient of a scalar function. Consequently, for a piecewise homogeneous and isotropic Volume conductor, the lead field is always tangential at the outermost surface. Based on this theoretical result, we formulated a simple and fast method for the MEG forward calculation for one shell of arbitrary shape: we correct the corresponding lead field for a Spherical Volume conductor by a superposition of basis functions, gradients of harmonic functions constructed here from Spherical harmonics, with coefficients fitted to the boundary conditions. The algorithm was tested for a prolate spheroid of realistic shape for which the analytical solution is known. For high order in the expansion, we found the solutions to be essentially exact and for reasonable accuracies much fewer multiplications are needed than in typical implementations of the boundary element methods. The generalization to more shells is straightforward.
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the magnetic lead field theorem in the quasi static approximation and its use for magnetoencephalography forward calculation in realistic Volume conductors
Physics in Medicine and Biology, 2003Co-Authors: Guido NolteAbstract:The equation for the magnetic lead field for a given magnetoencephalography (MEG) channel is well known for arbitrary frequencies ω but is not directly applicable to MEG in the quasi-static approximation. In this paper we derive an equation for ω = 0 starting from the very definition of the lead field instead of using Helmholtz's reciprocity theorems. The results are (a) the transpose of the conductivity times the lead field is divergence-free, and (b) the lead field differs from the one in any other Volume conductor by a gradient of a scalar function. Consequently, for a piecewise homogeneous and isotropic Volume conductor, the lead field is always tangential at the outermost surface. Based on this theoretical result, we formulated a simple and fast method for the MEG forward calculation for one shell of arbitrary shape: we correct the corresponding lead field for a Spherical Volume conductor by a superposition of basis functions, gradients of harmonic functions constructed here from Spherical harmonics, with coefficients fitted to the boundary conditions. The algorithm was tested for a prolate spheroid of realistic shape for which the analytical solution is known. For high order in the expansion, we found the solutions to be essentially exact and for reasonable accuracies much fewer multiplications are needed than in typical implementations of the boundary element methods. The generalization to more shells is straightforward.
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current multipole expansion to estimate lateral extent of neuronal activity a theoretical analysis
IEEE Transactions on Biomedical Engineering, 2000Co-Authors: Guido Nolte, G CurioAbstract:High-resolution magnetoencephalography (MEG) allows for a detailed description of focal neuronal current sources going far beyond the dipole approximation which merely indicates the center and magnitude of neuronal activity. Higher order multipole coefficients can be related to other bulk properties, like spatial extent or curvature. The possibility and limitations of measuring spatial extent by interpreting reconstructed multipole coefficients was tested under realistic noise conditions and for model misspecifications; for this analysis the primary cortical response ("N20") to electric median nerve stimulation was modeled by a one dimensional source distribution. The forward calculation was done analytically up to octapolar order for a Spherical Volume conductor. The multipole expansion is shown to estimate the lateral source extent with negligible bias; this estimate is to first-order stable against additional source features, like gyral curvature or spatial extent in a second direction (gyral depth, neuronal length). For a dipole moment of 20 nAm a lateral extent of 2 cm can be detected for a realistic noise level with large but experimentally still reasonable effort. Approximating a realistic head model by a sphere results in errors larger than the extent to be estimated; accordingly, studies on human cortical evoked responses will require multipole fitting in realistic head models.
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perturbative analytical solutions of the electric forward problem for realistic Volume conductors
Journal of Applied Physics, 1999Co-Authors: Guido Nolte, Gabriel CurioAbstract:The magnetic field induced by a current dipole situated in a realistic Volume conductor cannot be computed exactly. Here, we derive approximate analytical solutions based on the fact that in magnetoencephalography the deviation of the Volume conductor (i.e., the head) from a Spherical approximation is small. We present an explicit integral form which allows to calculate the nth order Taylor expansion of the magnetic field with respect to this deviation from the corresponding solution of the electric problem of order n−1. Especially, for a first order solution of the magnetic problem only the well-known electric solution for a Spherical Volume conductor is needed. The evaluation of this integral by a series of Spherical harmonics results in a fast algorithm for the computation of the external magnetic field which is an excellent approximation of the true field for smooth Volume conductor deformations of realistic magnitude. Since the approximation of the magnetic field is exactly curl-free it is equally go...
Johan Nuyts - One of the best experts on this subject based on the ideXlab platform.
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characterization of a preclinical pet insert in a 7 tesla mri scanner beyond nema testing
Physics in Medicine and Biology, 2020Co-Authors: Willy Gsell, Cesar Molinos, C Correcher, Sarah Belderbos, Jens Wouters, Sven Junge, Michael Heidenreich, Greetje Vande Velde, Ahmadreza Rezaei, Johan NuytsAbstract:This study evaluates the performance of the Bruker positron emission tomograph (PET) insert combined with a BioSpec 70/30 USR magnetic resonance imaging (MRI) scanner using the manufacturer acceptance protocol and the NEMA NU 4-2008 for small animal PET. The PET insert is made of 3 rings of 8 monolithic LYSO crystals (50 × 50 × 10 mm3) coupled to silicon photomultipliers (SiPM) arrays, conferring an axial and transaxial FOV of 15 cm and 8 cm. The MRI performance was evaluated with and without the insert for the following radiofrequency noise, magnetic field homogeneity and image quality. For the PET performance, we extended the NEMA protocol featuring system sensitivity, count rates, spatial resolution and image quality to homogeneity and accuracy for quantification using several MRI sequences (RARE, FLASH, EPI and UTE). The PET insert does not show any adverse effect on the MRI performances. The MR field homogeneity is well preserved (Diameter Spherical Volume, for 20 mm of 1.98 ± 4.78 without and -0.96 ± 5.16 Hz with the PET insert). The PET insert has no major effect on the radiofrequency field. The signal-to-noise ratio measurements also do not show major differences. Image ghosting is well within the manufacturer specifications (<2.5%) and no RF noise is visible. Maximum sensitivity of the PET insert is 11.0% at the center of the FOV even with simultaneous acquisition of EPI and RARE. PET MLEM resolution is 0.87 mm (FWHM) at 5 mm off-center of the FOV and 0.97 mm at 25 mm radial offset. The peaks for true/noise equivalent count rates are 410/240 and 628/486 kcps for the rat and mouse phantoms, and are reached at 30.34/22.85 and 27.94/22.58 MBq. PET image quality is minimally altered by the different MRI sequences. The Bruker PET insert shows no adverse effect on the MRI performance and demonstrated a high sensitivity, sub-millimeter resolution and good image quality even during simultaneous MRI acquisition.
Katrina Yezziwoodley - One of the best experts on this subject based on the ideXlab platform.
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computation of circular area and Spherical Volume invariants via boundary integrals
Siam Journal on Imaging Sciences, 2020Co-Authors: Riley Oneill, Pedro Anguloumana, Jeff Calder, Bo Hessburg, Peter J Olver, Chehrzad Shakiban, Katrina YezziwoodleyAbstract:We show how to compute the circular area invariant of planar curves, and the Spherical Volume invariant of surfaces, in terms of line and surface integrals, respectively. We use the divergence theo...
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computation of circular area and Spherical Volume invariants via boundary integrals
arXiv: Numerical Analysis, 2019Co-Authors: Riley Oneill, Pedro Anguloumana, Jeff Calder, Bo Hessburg, Peter J Olver, Chehrzad Shakiban, Katrina YezziwoodleyAbstract:We show how to compute the circular area invariant of planar curves, and the Spherical Volume invariant of surfaces, in terms of line and surface integrals, respectively. We use the Divergence Theorem to express the area and Volume integrals as line and surface integrals, respectively, against particular kernels; our results also extend to higher dimensional hypersurfaces. The resulting surface integrals are computable analytically on a triangulated mesh. This gives a simple computational algorithm for computing the Spherical Volume invariant for triangulated surfaces that does not involve discretizing the ambient space. We discuss potential applications to feature detection on broken bone fragments of interest in anthropology.
R M Bowley - One of the best experts on this subject based on the ideXlab platform.
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electric fields induced in a Spherical Volume conductor by temporally varying magnetic field gradients
Physics in Medicine and Biology, 2002Co-Authors: Martin Bencsik, Richard Bowtell, R M BowleyAbstract:A homogeneous Spherical Volume conductor is used as a model system for the purpose of calculating electric fields induced in the human head by externally applied time-varying magnetic fields. We present results for the case where magnetic field gradient coils, used in magnetic resonance imaging (MRI), form the magnetic field, and we use these data to put limits on the rates of gradient change with time needed to produce nerve stimulation. The electric field is calculated analytically for the case of ideal longitudinal and transverse linear field gradients. We also show results from computer calculations yielding the electric field maps in a sphere when the field gradients are generated by a real MRI gradient coil set. In addition, the effect of shifting the sphere within each gradient coil Volume is investigated. Numerical analysis shows similar results when applied to a model human head.
Willy Gsell - One of the best experts on this subject based on the ideXlab platform.
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characterization of a preclinical pet insert in a 7 tesla mri scanner beyond nema testing
Physics in Medicine and Biology, 2020Co-Authors: Willy Gsell, Cesar Molinos, C Correcher, Sarah Belderbos, Jens Wouters, Sven Junge, Michael Heidenreich, Greetje Vande Velde, Ahmadreza Rezaei, Johan NuytsAbstract:This study evaluates the performance of the Bruker positron emission tomograph (PET) insert combined with a BioSpec 70/30 USR magnetic resonance imaging (MRI) scanner using the manufacturer acceptance protocol and the NEMA NU 4-2008 for small animal PET. The PET insert is made of 3 rings of 8 monolithic LYSO crystals (50 × 50 × 10 mm3) coupled to silicon photomultipliers (SiPM) arrays, conferring an axial and transaxial FOV of 15 cm and 8 cm. The MRI performance was evaluated with and without the insert for the following radiofrequency noise, magnetic field homogeneity and image quality. For the PET performance, we extended the NEMA protocol featuring system sensitivity, count rates, spatial resolution and image quality to homogeneity and accuracy for quantification using several MRI sequences (RARE, FLASH, EPI and UTE). The PET insert does not show any adverse effect on the MRI performances. The MR field homogeneity is well preserved (Diameter Spherical Volume, for 20 mm of 1.98 ± 4.78 without and -0.96 ± 5.16 Hz with the PET insert). The PET insert has no major effect on the radiofrequency field. The signal-to-noise ratio measurements also do not show major differences. Image ghosting is well within the manufacturer specifications (<2.5%) and no RF noise is visible. Maximum sensitivity of the PET insert is 11.0% at the center of the FOV even with simultaneous acquisition of EPI and RARE. PET MLEM resolution is 0.87 mm (FWHM) at 5 mm off-center of the FOV and 0.97 mm at 25 mm radial offset. The peaks for true/noise equivalent count rates are 410/240 and 628/486 kcps for the rat and mouse phantoms, and are reached at 30.34/22.85 and 27.94/22.58 MBq. PET image quality is minimally altered by the different MRI sequences. The Bruker PET insert shows no adverse effect on the MRI performance and demonstrated a high sensitivity, sub-millimeter resolution and good image quality even during simultaneous MRI acquisition.