Francis Moiny - Academia.edu (original) (raw)
Papers by Francis Moiny
Renewable Energy, 2013
ABSTRACT
Magma: Magnetic Resonance Materials in Physics, Biology, and Medicine, 1997
Monte-Carlo computer simulations have proven to be very powerful tools for the analysis of the ma... more Monte-Carlo computer simulations have proven to be very powerful tools for the analysis of the magnetization decay induced by susceptibility gradients, as well for contrast agent characterization, as for the BOLD effect allowing fMRI. A recent vasculature model containing capillaries and venules uses homogeneous magnetized cylinders as models for the vessels. This modeling is questioned by comparing results obtained from simulation results based on two different models, one using homogeneous cylinders and another taking into account the existence of red blood cells, treated as homogeneous magnetized spheres. The results show the nonequivalence of both models, with the modeling by cylinders systematically overestimating the transverse relaxation rates, and the difference increasing with the adopted value of the diffusion coefficient. The discrepancy is attributed to the dominating role, regarding relaxation, of the local magnetic field in the immediate vicinity of the capillaries, which results in the suggestion of elaborating a "mixed modeling": the analytical expressions corresponding to the homogeneous cylinder model could be used except when the spin packets are wandering in the immediate vicinity of the capillaries, where accounting for the existence of individual red blood cells (whose motion may be neglected) seems unavoidable.
Magnetic Resonance in Medicine, 1995
When red blood cells are deoxygenated, hemoglobin, which is then transformed into deoxyhemoglobin... more When red blood cells are deoxygenated, hemoglobin, which is then transformed into deoxyhemoglobin or methemoglobin, becomes paramagnetic. The transverse nuclear magnetic relaxation rate of water protons is considerably enhanced by this chemical transformation. A general agreement exists about the origin of the phenomenon-local field inhomogeneities induced by paramagnetic centers randomly distributed within the cell-but the localization of the region that dominates the relaxation is unclear. We addressed this problem with a computer simulation devoted to the determination of transverse magnetic relaxation of water protons in the presence of superparamagnetic MRI contrast agent candidates. The simulation confirms an earlier experimental result that shares equitably the responsibility for the observed relaxation between intracellular and extracellular water.
Magnetic Resonance in Medicine, 2001
Chemical exchange (CE) theory is compared with two theories of T 2 -shortening caused by microsco... more Chemical exchange (CE) theory is compared with two theories of T 2 -shortening caused by microscopic magnetic centers: inner-and outer-sphere relaxation theory (long-echo limit) and mean gradient diffusion theory (short-echo limit). The CE equation is shown to be identical to these theories in the respective limits and appropriate parameter relationships are derived for spherical particles. The theories are then compared with computer simulations of spherical particles and with a recent general theory, with good agreement in the asymptotic regions. The CE model also reproduces the essential relaxation characteristics in the intermediate range. Finally, good agreement of a CE model with simulations for magnetized cylinders is also demonstrated. The discussion is limited to weakly magnetized particles such that the maximum phase shift during an echo interval is less than one radian, permitting the use of the Luz-Meiboom CE equation. Magn Reson Med 45:1014 -
Magnetic Resonance in Medicine, 2002
Computer simulations of water transverse relaxation induced by superparamagnetic particles are sh... more Computer simulations of water transverse relaxation induced by superparamagnetic particles are shown to disagree with the available theories, covering the slow diffusion domain. Understanding these new simulations, not in the slow diffusion domain, thus requires a new theoretical approach. A "partial refocusing model" is introduced for this purpose; it is based on a spatial division between an inner region where the gradients are too strong for the refocusing pulses to be efficient and an outer region where they are efficient. This model agrees with published simulations of relaxation induced by magnetic dipoles approximated as points. The validity domains of the various models are also compared.
Magnetic Resonance in Medicine, 1991
Computer simulations and experimental approach have been used to characterize the properties of p... more Computer simulations and experimental approach have been used to characterize the properties of particulate MRI contrast agents with special attention paid to the influence of particle size. Up to approximately 50 nm, an increase of the particle diameter leads to a strong enhancement of the transverse magnetization decay rate. For larger grains or aggregates, the decay rate measured without spin-echo formation reaches a plateau. When observed through a spin-echo sequence, the transverse magnetization decay rate becomes slower on increasing the particle size or on shortening the echo time. For these large particles, multiexponential decay rates are observed. Definition and measurement of relaxivity in such systems is discussed.
Renewable Energy, 2013
ABSTRACT
Magma: Magnetic Resonance Materials in Physics, Biology, and Medicine, 1997
Monte-Carlo computer simulations have proven to be very powerful tools for the analysis of the ma... more Monte-Carlo computer simulations have proven to be very powerful tools for the analysis of the magnetization decay induced by susceptibility gradients, as well for contrast agent characterization, as for the BOLD effect allowing fMRI. A recent vasculature model containing capillaries and venules uses homogeneous magnetized cylinders as models for the vessels. This modeling is questioned by comparing results obtained from simulation results based on two different models, one using homogeneous cylinders and another taking into account the existence of red blood cells, treated as homogeneous magnetized spheres. The results show the nonequivalence of both models, with the modeling by cylinders systematically overestimating the transverse relaxation rates, and the difference increasing with the adopted value of the diffusion coefficient. The discrepancy is attributed to the dominating role, regarding relaxation, of the local magnetic field in the immediate vicinity of the capillaries, which results in the suggestion of elaborating a "mixed modeling": the analytical expressions corresponding to the homogeneous cylinder model could be used except when the spin packets are wandering in the immediate vicinity of the capillaries, where accounting for the existence of individual red blood cells (whose motion may be neglected) seems unavoidable.
Magnetic Resonance in Medicine, 1995
When red blood cells are deoxygenated, hemoglobin, which is then transformed into deoxyhemoglobin... more When red blood cells are deoxygenated, hemoglobin, which is then transformed into deoxyhemoglobin or methemoglobin, becomes paramagnetic. The transverse nuclear magnetic relaxation rate of water protons is considerably enhanced by this chemical transformation. A general agreement exists about the origin of the phenomenon-local field inhomogeneities induced by paramagnetic centers randomly distributed within the cell-but the localization of the region that dominates the relaxation is unclear. We addressed this problem with a computer simulation devoted to the determination of transverse magnetic relaxation of water protons in the presence of superparamagnetic MRI contrast agent candidates. The simulation confirms an earlier experimental result that shares equitably the responsibility for the observed relaxation between intracellular and extracellular water.
Magnetic Resonance in Medicine, 2001
Chemical exchange (CE) theory is compared with two theories of T 2 -shortening caused by microsco... more Chemical exchange (CE) theory is compared with two theories of T 2 -shortening caused by microscopic magnetic centers: inner-and outer-sphere relaxation theory (long-echo limit) and mean gradient diffusion theory (short-echo limit). The CE equation is shown to be identical to these theories in the respective limits and appropriate parameter relationships are derived for spherical particles. The theories are then compared with computer simulations of spherical particles and with a recent general theory, with good agreement in the asymptotic regions. The CE model also reproduces the essential relaxation characteristics in the intermediate range. Finally, good agreement of a CE model with simulations for magnetized cylinders is also demonstrated. The discussion is limited to weakly magnetized particles such that the maximum phase shift during an echo interval is less than one radian, permitting the use of the Luz-Meiboom CE equation. Magn Reson Med 45:1014 -
Magnetic Resonance in Medicine, 2002
Computer simulations of water transverse relaxation induced by superparamagnetic particles are sh... more Computer simulations of water transverse relaxation induced by superparamagnetic particles are shown to disagree with the available theories, covering the slow diffusion domain. Understanding these new simulations, not in the slow diffusion domain, thus requires a new theoretical approach. A "partial refocusing model" is introduced for this purpose; it is based on a spatial division between an inner region where the gradients are too strong for the refocusing pulses to be efficient and an outer region where they are efficient. This model agrees with published simulations of relaxation induced by magnetic dipoles approximated as points. The validity domains of the various models are also compared.
Magnetic Resonance in Medicine, 1991
Computer simulations and experimental approach have been used to characterize the properties of p... more Computer simulations and experimental approach have been used to characterize the properties of particulate MRI contrast agents with special attention paid to the influence of particle size. Up to approximately 50 nm, an increase of the particle diameter leads to a strong enhancement of the transverse magnetization decay rate. For larger grains or aggregates, the decay rate measured without spin-echo formation reaches a plateau. When observed through a spin-echo sequence, the transverse magnetization decay rate becomes slower on increasing the particle size or on shortening the echo time. For these large particles, multiexponential decay rates are observed. Definition and measurement of relaxivity in such systems is discussed.