BACKGROUND

The field of the disclosure relates generally to systems and methods of magnetic resonance imaging (MRI), and more particularly, to systems and methods of calibrating asymmetric gradient concomitant field correction parameters in MRI.

MRI has proven useful in diagnosis of many diseases. MRI provides detailed images of soft tissues, abnormal tissues such as tumors, and other structures, which cannot be readily imaged by other imaging modalities, such as computed tomography (CT). Further, MRI operates without exposing patients to ionizing radiation experienced in modalities such as CT and x-rays.

In a MRI system, a large magnet produces a very strong magnetic field over the patient's body. The magnetic field produced by the large magnet is quite uniform or homogeneous. Gradient coils in the MRI system produce gradients which distorts this uniform magnetic field. In general, the gradients cause the change in field strength of the magnetic field in the patient body from one point to another.

In some MRI systems, high gradient amplitude coils are used to achieve higher image quality or when good spatial resolution is required. High gradient amplitude coils are capable of producing up to 400 mT/m magnetic gradient fields and/or may have coil efficiencies of η=0.13-0.32 mT/m/A. This compares with coil efficiencies of conventional gradient coils of η=0.08-0.10 mT/m/A. Examples of such gradient coils are the Connectome gradients with a maximum gradient amplitude (Gmax) of 300 mT/m, the Compact 3T MRI system with Gmax=80 mT/m, and the MAGNUS (microstructure anatomy gradient for neuroimaging with ultrasfast scanning) gradient system with Gmax=200-400 mT/m, depending on the gradient driver maximum current output. Of these, the Compact 3T and MAGNUS gradient coils are smaller head-sized asymmetric gradient coils. As such, the maximum gradient fields that are generated by high gradient coils are far in excess of conventional gradients coils. This presents formidable systems engineering issues as the gradient coils and the associated gradient field corrections depend on accurate and proper calibration of gradient coil parameters. The gradient field corrections are necessary to compensate for non-linearity in the gradient field strength which gets worse farther from isocenter of the magnetic field.

Therefore, there is a need for an improved magnetic resonance imaging system and method.

BRIEF DESCRIPTION

In accordance with an embodiment of the present technique, a method for correcting concomitant gradient effects in a magnetic resonance imaging (MRI) system is presented. The method includes determining a plurality of first phase difference measurements between two acquisitions using a plurality of first bipolar gradient waveforms applied to a first gradient coil axis or direction. A first gradient coil axis' constant is determined based on the plurality of first phase different measurements. With the determined first gradient coil (axis) constant, compensatory gradient waveforms related to that gradient coil axis are determined. Finally, the compensatory gradient waveforms are applied along with target gradient waveforms to compensate for a concomitant gradient field. The compensatory waveforms may be applied on other gradient axes in addition to the first gradient coil axis.

In accordance with another embodiment of the present technique, a method for correcting concomitant gradient effects in an MRI system is presented. The method includes determining a plurality of first phase difference measurements between two acquisitions using a plurality of first bipolar gradient waveforms applied to a first gradient coil (axis). The method further includes determining a plurality of second phase difference measurements between two acquisitions using a plurality of second bipolar gradient waveforms applied to a second gradient coil (axis). First and second gradient coil constants are determined based on the plurality of first phase difference measurements and the plurality of second phase difference measurements respectively. The method also includes determining compensatory gradient waveforms based on the first and second gradient coil constants and applying the compensatory gradient waveforms with target gradient waveforms to compensate for a concomitant gradient field.

In accordance with yet another embodiment of the present technique, a MRI system is presented. The MRI system includes a magnet configured to generate a polarizing magnetic field about at least a portion of a subject arranged in the MRI system and a gradient coil assembly including a plurality of gradient coils configured to apply at least one gradient field to the polarizing magnetic field. The MRI system further includes a radio frequency (RF) system configured to apply an RF field to the subject and to receive magnetic resonance signals from the subject and a processing system. The processing system is programmed to determine a plurality of phase difference measurements between two acquisitions using a plurality of bipolar gradient waveforms applied to at least one gradient coil of the plurality of gradient coils. The processing system is also programmed to determine at least one gradient coil constant based on the plurality of phase difference measurements and to determine compensatory gradient waveforms based on the at least one gradient coil constant. The processing system is further programmed to apply the compensatory gradient waveforms to the plurality of gradient coils to compensate for a concomitant gradient field.

