BACKGROUND

The present disclosure relates to techniques for reducing image artifacts in magnetic resonance imaging.

SUMMARY

In one aspect, some implementations provide a method for reducing eddy current artifacts of a magnetic resonance imaging (MRI) system that has a main magnet that generates a substantially uniform magnetic field to image a subject therein. The method includes the operations of: energizing a gradient sub-system of the MRI system such that a gradient corresponding to perturbations to the substantially uniform magnetic field are generated, the gradient having a temporal strength that includes a ramp-up phase; measuring the temporal strength of the gradient during the ramp-up phase; and fitting the measured temporal strength during the ramp-up phase simultaneously against one or more temporal model functions characterized. The measured temporal strength is fitted by at least a first set of time constants and a second set of time constants. The first set of time constant and the second set of time constants are derived and then incorporated into a pre-emphasized gradient waveform which, when used to energize the gradient sub-system for imaging the subject placed in the main magnet, generates a corrected gradient in which distortions caused by eddy currents during the ramp-up phase are substantially removed.

Implementations may include one or more of the following features. The operation of measuring the temporal strength of the gradient can include: recording radio frequency (RF) signals from a field camera having more than one probes placed in the substantially uniform magnetic field; and measuring cumulative phases of the recorded RF signals that reflect the perturbations to the substantially uniform magnetic field at where the more than one probes were placed.

In some implementations, measuring the temporal strength of the gradient further includes: determining the temporal strength of the gradient by taking a time derivative of the measured cumulative phase.

In some implementations, fitting the measured temporal strength during the ramp-up phase includes simultaneously fitting against a first temporal model function characterized by the first set of time constants as well as a second temporal model function characterized by the first set of time constants and the second set of time constants.

In some implementations, fitting the measured temporal strength during the ramp-up phase comprises applying a fit only during the ramp-up phase.

In some implementations, the corrected gradient is generated such that radio-frequency (RF) signals for reconstructing an MRI image of the subject are acquired from a k-space trajectory that is substantially identical to the ideal gradient waveform.

In some implementations, the method further includes: accessing data encoding a computational model for one or more components of the gradient sub-system, wherein the computational model specifies ideal gradient amplitudes for each of the one or more components when the gradient corresponding to the perturbations to the substantially uniform magnetic field is generated; determining measured gradient amplitudes of the one or more components after energizing the gradient sub-system; comparing, for each of the one or more components of the gradient sub-system, the measured gradient amplitude and the ideal amplitude specified by the computational model; and determining the set of time constants and the second set of time constants based on the comparison of the measured gradient amplitude and the ideal gradient amplitude for each of the one or more components of the gradient sub-system.

In some implementations, the gradient runs along at least one spatial axis within the main magnetic field.

In some implementations, the method for reducing eddy current artifacts of the MRI system are performed by a computing system including one or more computers, and one or more storage devices storing instructions that, when executed by the one or more computers, cause the one or more computers to perform operations of the method.

In another aspect, some implementations provide a method for reducing eddy current artifacts of a magnetic resonance imaging (MRI) system that has a main magnet that generates a substantially uniform magnetic field to image a subject therein. The method includes the operations of: accessing data encoding a gradient waveform that would otherwise be used on a gradient sub-system of the MRI system to generate a gradient that corresponds to perturbations to the substantially uniform magnetic field; accessing data encoding one or more temporal model functions indicative of a first rising portion immediately followed by a second decaying portion during a ramp-up phase of the gradient; pre-emphasizing the gradient waveform to correct deviations characterized by the first rising portion and the subsequent second decaying portion; and driving the gradient sub-system using the pre-emphasized gradient waveform such that distortions to the gradient caused by eddy currents during the ramp-up phase are substantially removed.

Implementations may include one or more of the following features. The method can further include the operation of receiving and sampling radio-frequency (RF) signals from the subject placed in the main magnetic field when the gradient sub-system is being driven using the pre-emphasized gradient waveform as if the gradient sub-system is being driven using the gradient waveform.

In some implementations, pre-emphasizing the gradient waveform includes the operations of taking a time derivative of the gradient waveform; and convolving the one or more temporal model functions with the time derivative of the gradient waveform.

In some implementations, pre-emphasizing the gradient waveform includes the operation of subtracting, from the gradient waveform, results of convolving the one or more temporal model functions with the time derivative of the gradient waveform.

In some implementations, pre-emphasizing the gradient waveform includes the operation of adding, from the gradient waveform, results of convolving the one or more temporal model functions with the time derivative of the gradient waveform.

In some implementations, the gradient corresponds to perturbation to the main magnetic field along at least one spatial axis.

In some implementations, the gradient waveform represents at least one of a gradient echo acquisition, a spin echo acquisition, an echo-planar imaging (EPI) acquisition, or a spiral acquisition.

In some implementations, the gradient waveform leads to an accumulation of phase errors caused by eddy currents, and wherein the accumulation of phase errors is substantially removed by the pre-emphasized gradient waveform.

In another aspect, some implementations provide a magnetic resonance imaging (MRI) system that includes: a housing having a bore in which a subject to be imaged is placed; a main magnet accommodated by said housing and configured to generate a substantially uniform magnet field within the bore; a group of shimming coils placed around said housing and configured to increase a homogeneity of the magnetic field; pulse generating coils to generate and apply radio frequency (RF) pulses in sequence to scan the subject; a gradient sub-system comprising gradient coils to provide perturbations to the substantially uniform magnet field; and a control unit coupled to the main magnet and configured to perform operations.

The operations performed by the control unit include: accessing data encoding a gradient waveform that would otherwise be used on the gradient sub-system to generate a gradient that corresponds to the perturbations to the substantially uniform magnet field; accessing data encoding one or more temporal model functions indicative of a first rising portion followed by a second decaying portion during a ramp-up phase of the gradient; pre-emphasizing the gradient waveform to correct deviations characterized by the first rising portion and the subsequent second decaying portion; and driving the gradient sub-system using the pre-emphasized gradient waveform such that distortions to the gradient caused by eddy currents during the ramp-up phase are substantially removed and MRI signals are acquired from the subject accordingly in response to the applied RF pulses.

Implementations may include one or more of the following features. The control unit is configured to perform the operation of receiving and sampling radio-frequency (RF) signals from the subject placed in the main magnetic field when the gradient sub-system is being driven using the pre-emphasized gradient waveform as if the gradient sub-system is being driven using the gradient waveform.

In some implementations, the control unit is configured to pre-emphasize the gradient waveform by taking a time derivative of the gradient waveform; and convolving the one or more temporal model functions with the time derivative of the gradient waveform.

In some implementations, the control unit is configured to pre-emphasize the gradient waveform by subtracting, from the gradient waveform, results of convolving the temporal model functions with the time derivative of the gradient waveform.

In some implementations, the gradient corresponds to perturbation to the substantially uniform magnetic field along at least one spatial axis.

In some implementations, the gradient waveform represents at least one of a gradient echo acquisition, a spin echo acquisition, an echo-planar imaging (EPI) acquisition, or a spiral acquisition.

In some implementations, the gradient waveform leads to an accumulation of phase errors caused by eddy currents of the gradient sub-system, and wherein the accumulation of phase errors is substantially removed by the pre-emphasized gradient waveform.

In another aspect, some implementations provide a second magnetic resonance imaging (MRI) system including: a housing having a bore in which a subject to be imaged is placed; a main magnet accommodated by said housing and configured to generate a substantially uniform magnet field within the bore; a group of shimming coils placed around said housing and configured to increase a homogeneity of the magnetic field; pulse generating coils to generate and apply radio frequency (RF) pulses in sequence to scan the subject; a gradient sub-system comprising gradient coils to provide perturbations to the substantially uniform magnet field, and a control unit coupled to the main magnet and configured to perform operations.

The operations performed by the control unit include: energizing a gradient sub-system of the MRI system such that a gradient corresponding to perturbations to the substantially uniform magnetic field are generated, the gradient having a temporal strength that includes a ramp-up phase; measuring the temporal strength of the gradient during the ramp-up phase; and exporting data encoding the measured temporal strength during the ramp-up phase. The measured temporal strength is fitted simultaneously against one or more temporal model functions characterized by at least a first set of time constants and a second set of time constants. The first set of time constants and the second set of time constants are derived and then incorporated into a pre-emphasized gradient waveform which, when used to energize the gradient sub-system for imaging the subject placed in the main magnet, generates a corrected gradient in which distortions caused by eddy currents during the ramp-up phase are substantially removed.

Implementations may include one or more of the following features. The control unit is configured to measure the temporal strength of the gradient by recording radio frequency (RF) signals from a field camera having more than one probes placed in the substantially uniform magnetic field. The cumulative phases of the recorded RF signals are measured, which reflect the perturbations to the substantially uniform magnetic field at where the more than one probes were placed.

In some implementations, the cumulative phases of the recorded RF signals are measured by taking a time derivative of the measured cumulative phase to determine the temporal strength of the gradient.

In some implementations, the measured temporal strength is fitted simultaneously against one or more temporal model functions. The measured temporal strength is simultaneously fitted against a first temporal model function characterized by the first set of time constants as well as a second temporal model function characterized by the first set of time constants and the second set of time constants.

In some implementations, the measured temporal strength is fitted simultaneously by a fit only during the ramp-up phase.

In some implementations, the corrected gradient is generated such that radio-frequency (RF) signals for reconstructing an MRI image of the subject are acquired from a k-space trajectory that is substantially identical to when the corrected gradient is uncorrected.

In some implementations, the gradient runs along at least one spatial axis within the main magnetic field.

The details of one or more aspects of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

