Abstract: A method of designing a pulse sequence for parallel-transmission MRI includes a) for each one of a plurality of subjects, estimating a linear adjustment transformation (L), converting amplitude maps of RF fields generated by respective transmit channels of a MRI apparatus into respective standardized maps; and b) determining RF waveforms (P) minimizing a discrepancy between subject-specific distributions of flip-angles of nuclear spin and a target distribution, averaged over said subjects, the subject-specific distributions corresponding to the flip-angle distributions achieved by applying a superposition of RF fields, each having a temporal profile described by one of said RF waveforms and a spatial amplitude distribution described by a respective standardized map determined for the subject. A method and an apparatus for performing parallel-transmission MRI using such a pulse sequence are provided.

Abstract: A method of designing a pulse sequence for parallel-transmission MRI includes a) for each one of a plurality of subjects, estimating a linear adjustment transformation (L), converting amplitude maps of RF fields generated by respective transmit channels of a MRI apparatus into respective standardized maps; and b) determining RF waveforms (P) minimizing a discrepancy between subject-specific distributions of flip-angles of nuclear spin and a target distribution, averaged over said subjects, the subject-specific distributions corresponding to the flip-angle distributions achieved by applying a superposition of RF fields, each having a temporal profile described by one of said RF waveforms and a spatial amplitude distribution described by a respective standardized map determined for the subject. A method and an apparatus for performing parallel-transmission MRI using such a pulse sequence are provided.

Abstract: A computer-implemented method of building a database of pulse sequences for parallel-transmission magnetic resonance imaging, includes a) for each of a plurality of subjects, determining an optimal sequence for the subject; b) for each subject, computing the values of the or of a different cost or merit function obtained by playing the optimal sequences for all the subjects; c) aggregating the subjects into a plurality of clusters using a clustering algorithm taking the values, or functions thereof, as metrics; d) for each cluster, determining an averaged optimal sequence for the cluster; e) receiving, as input, a set of features characterizing an imaging subject, comprising at least a morphological feature of the subject; f) associating the subject to one pulse sequence of the database based on the set of features using the computer-implemented classifier algorithm; and g) performing magnetic resonance imaging using the pulse sequence.

Abstract: Systems and methods for designing RF pulses using a technique that directly controls temperature rise via a compression model that is based on virtual observation points (“VOPs”) are provided. Thermal pre-simulations are first carried out for a given RF exposure time, coil, and subject model in order to obtain complex temperature matrices, after which the compression scheme follows. As one example, the thermal model employed can be Pennes' bio-heat equation. Focusing design constraints on the temperature rise instead of the absolute temperature allows for uncertain parameters to be dropped from the thermal model, making it more robust and less prone to errors. In some embodiments, the algorithm used for RF pulse design is the active-set (“A-S”) method.

Abstract: A computer-implemented method of building a database of pulse sequences for parallel-transmission magnetic resonance imaging, includes a) for each of a plurality of subjects, determining an optimal sequence for the subject; b) for each subject, computing the values of the or of a different cost or merit function obtained by playing the optimal sequences for all the subjects; c) aggregating the subjects into a plurality of clusters using a clustering algorithm taking the values, or functions thereof, as metrics; d) for each cluster, determining an averaged optimal sequence for the cluster; e) receiving, as input, a set of features characterizing an imaging subject, comprising at least a morphological feature of the subject; f) associating the subject to one pulse sequence of the database based on the set of features using the computer-implemented classifier algorithm; and g) performing magnetic resonance imaging using the pulse sequence.

Abstract: A method of designing a pulse sequence for parallel-transmission magnetic resonance imaging comprises: a) acquiring, for each member of a cohort, inhomogeneity maps of radio-frequency fields generated within the member; b) computing, for each member of the cohort, a spatial distribution of flip angles of nuclear spins obtained using the pulse sequences, and c) computing a single cost or merit function representative of a difference between the spatial distributions of flip angles and a target distribution, and iteratively adjusting design parameters of the pulse sequences to optimize the cost or merit function; the steps b) and c) being carried out iteratively using a computer. A method of performing parallel-transmission magnetic resonance imaging on a subject using a pulse sequence designed by such a method is provided.

Abstract: A method of designing a refocusing pulse or pulse train for Magnetic Resonance Imaging comprises the steps of: a) determining a phase-free performance criterion representative of a proximity between a rotation of nuclear spins induced by the pulse and a target operator, summed or averaged over one or more voxels of an imaging region of interest; and b) adjusting a plurality of control parameters of the pulse to maximize the phase-free performance criterion; wherein each target operator is chosen so the phase-free performance criterion takes a maximum value when the nuclear spins within all voxels undergo a rotation of a same angle ? around a rotation axis lying in a plane perpendicular to a magnetization field B0, called a transverse plane, with an arbitrary orientation; wherein the angle ? is different from M? radians, with integer M, preferably with ?<? radians and even preferably with ??0.9·? radians.

Abstract: A method of designing a pulse sequence for parallel-transmission magnetic resonance imaging comprises: a) acquiring, for each member of a cohort, inhomogeneity maps of radio-frequency fields generated within the member; b) computing, for each member of the cohort, a spatial distribution of flip angles of nuclear spins obtained using the pulse sequences, and c) computing a single cost or merit function representative of a difference between the spatial distributions of flip angles and a target distribution, and iteratively adjusting design parameters of the pulse sequences to optimize the cost or merit function; the steps b) and c) being carried out iteratively using a computer. A method of performing parallel-transmission magnetic resonance imaging on a subject using a pulse sequence designed by such a method is provided.

Abstract: Systems and methods for designing RF pulses using a technique that directly controls temperature rise via a compression model that is based on virtual observation points (“VOPs”) are provided. Thermal pre-simulations are first carried out for a given RF exposure time, coil, and subject model in order to obtain complex temperature matrices, after which the compression scheme follows. As one example, the thermal model employed can be Pennes' bio-heat equation. Focusing design constraints on the temperature rise instead of the absolute temperature allows for uncertain parameters to be dropped from the thermal model, making it more robust and less prone to errors. In some embodiments, the algorithm used for RF pulse design is the active-set (“A-S”) method.

Abstract: A method of designing a refocusing pulse or pulse train for Magnetic Resonance Imaging comprises the steps of: a) determining a phase-free performance criterion representative of a proximity between a rotation of nuclear spins induced by the pulse and a target operator, summed or averaged over one or more voxels of an imaging region of interest; and b) adjusting a plurality of control parameters of the pulse to maximize the phase-free performance criterion; wherein each target operator is chosen so the phase-free performance criterion takes a maximum value when the nuclear spins within all voxels undergo a rotation of a same angle ? around a rotation axis lying in a plane perpendicular to a magnetization field B0, called a transverse plane, with an arbitrary orientation; wherein the angle ? is different from M? radians, with integer M, preferably with ?<? radians and even preferably with ??0.9·? radians.

Abstract: A method of performing nuclear magnetic resonance imaging of a body comprising at least two populations of nuclei characterized by different spin resonance frequencies, the method comprising the steps of: (a) immerging said body (B) in a static magnetic field (B0) for aligning nuclear spins along a magnetization axis; (b) exposing it to a transverse radio-frequency pulsed field (B1) for flipping said nuclear spins, said radio-frequency pulsed field comprising a train of elementary pulses, each having a constant frequency and amplitude, and a continuous phase; (c) detecting a signal emitted by nuclear spins excited by said radio-frequency pulsed field; characterized in that it also comprises, prior to performing steps (a)-(c), computing a set of optimal parameters (N, ?i, ?i, ?i) of said train of elementary pulses for minimizing the differences between the actual values of the spin-flip angles (FAj) of nuclei belonging to each of said populations and predetermined target values thereof; said predetermined targ

Abstract: A method of performing nuclear magnetic resonance imaging of a body (B), comprising: immerging said body in a static magnetic field B0 for aligning nuclear spins along a magnetization axis; exposing it to a transverse radio-frequency pulsed field Bi for flipping said nuclear spins by a predetermined angle; and detecting a signal emitted by flipped nuclear spins; the method being characterized in that it comprises the preliminary steps of: (i) determining a statistical distribution of the amplitude of said radio-frequency pulsed field within a volume of said body; and (ii) computing a set of optimal parameters of a composite radio-frequency pulsed field for jointly minimizing the dispersion of the spin flip angles distribution within said volume of the body, due to B1 and possibly B0 inhomogeneities, and the errors between the actual spin flip angles and their predetermined target value, wherein said radio-frequency pulsed field consists of a train of elementary pulses having a constant frequency and amplitude

Abstract: A method of performing nuclear magnetic resonance imaging of a body, comprising: immerging said body in a static magnetic field for aligning nuclear spins along a magnetization axis; exposing said body to a gradient pulse and to a transverse radio-frequency pulse for performing slice-selective excitation of said nuclear spins, thus flipping the nuclear spins of atoms contained within a slice of said body; detecting a signal emitted by excited nuclear spins; and reconstructing a magnetic resonance image of said slice of the body on the basis of the detected signal; the method being characterized in that said radio-frequency pulse is constituted by a train of slice-selective elementary pulses, approximately equivalent to a train of elementary rectangular pulses with constant frequencies which are designed for compensating for inhomogeneity of the radio-frequency field within the body.

Abstract: A method of exciting nuclear spins in a sample, wherein a plurality of transmit coils are driven in parallel to emit respective radio-frequency excitation pulses, the method comprising computing the phases and/or amplitudes of said excitation pulses by solving an optimization problem for minimizing the difference between the excitation distribution within said sample and a target excitation distribution, and being characterized in that: said optimization problem includes a cost function depending on the power emitted by said transmit coils through respective coil-dependent weighting coefficients; and in that the phases and/or amplitudes of the excitation pulses are computed iteratively, each iteration step comprising: solving said optimization problem based on present values of the weighting coefficient, and subsequently updating the value of at least one of said coefficients so as to control in a predetermined way the local specific absorption rate—SAR—distribution within the sample.

Abstract: A method of performing nuclear magnetic resonance imaging of a body comprising at least two populations of nuclei characterized by different spin resonance frequencies, the method comprising the steps of: (a) immerging said body (B) in a static magnetic field (B0) for aligning nuclear spins along a magnetization axis; (b) exposing it to a transverse radio-frequency pulsed field (B1) for flipping said nuclear spins, said radio-frequency pulsed field comprising a train of elementary pulses, each having a constant frequency and amplitude, and a continuous phase; (c) detecting a signal emitted by nuclear spins excited by said radio-frequency pulsed field; characterized in that it also comprises, prior to performing steps (a)-(c), computing a set of optimal parameters (N, ?i, ?i, ?i) of said train of elementary pulses for minimizing the differences between the actual values of the spin-flip angles (FAj) of nuclei belonging to each of said populations and predetermined target values thereof; said predetermined targ

Abstract: A method of performing nuclear magnetic resonance imaging of a body (B), comprising: immerging said body in a static magnetic field B0 for aligning nuclear spins along a magnetization axis; exposing it to a transverse radio-frequency pulsed field Bi for flipping said nuclear spins by a predetermined angle; and detecting a signal emitted by flipped nuclear spins; the method being characterized in that it comprises the preliminary steps of: (i) determining a statistical distribution of the amplitude of said radio-frequency pulsed field within a volume of said body; and (ii) computing a set of optimal parameters of a composite radio-frequency pulsed field for jointly minimizing the dispersion of the spin flip angles distribution within said volume of the body, due to B1 and possibly B0 inhomogeneities, and the errors between the actual spin flip angles and their predetermined target value, wherein said radio-frequency pulsed field consists of a train of elementary pulses having a constant frequency and amplitude