General Electric Company

Ge Medical Systems Global Technology Company, LLC

Xiaohong Zhou

Joseph K Maier

Matthew A Bernstein

Steven J Huff

Hammond G Reynolds

Graeme Colin McKinnon

Yiping Du

Jason Andrew Polzin

Jingfei Ma

Frederick H Epstein

Paul Edgar Licato

Lawrence E Ploetz

Gousheng Tan

Yping Du

A method and apparatus for correcting ghosting artifacts that are related to Maxwell fields and/or other perturbation magnetic fields is disclosed. The method and apparatus includes acquiring MR image n-space data and an MR reference scan, each having perturbation field effects therein. After determining phase correction values from the MR reference scan and reconstructing an MR image using the phase correction values, a projection phase error is calculated from the reconstructed MR image and then subtracted from the reference scan, the result of which is used to determine a new set of phase correction values. The new set of phase correction values is applied to the acquired MR image data to reconstruct a new image. The reconstructed new image can then be reused to calculate a new projection phase error, which again is subtracted from the reference scan data and the process is repeated until an image of desired ghost artifact reduction is achieved.

Method and apparatus to reduce the effects of maxwell terms and other perturbation magnetic fields in MR images

A method and apparatus for correcting ghost artifacts that are related to orthogonal perturbation magnetic fields is disclosed. The technique includes acquiring MR data and an MR reference scan, each in the presence of orthogonal perturbation magnetic fields. However, the region of interest for the MR reference scan is limited to a relatively narrow band within the imaging subject. Preferably, the narrow band is selected in the vicinity of the magnet iso-center and parallel to the readout direction. Alternatively, the narrow band can also be selected in the limited region where the orthogonal perturbation fields are either minimal or constant along the phase encoding direction, and parallel to the readout direction. The selection of the relatively narrow band is accomplished by either spatially saturating surrounding regions, or using a two-dimensional spatially selective RF pulse. Phase correction values can then be calculated from a more accurately defined slope and intercept of the phase of the MR reference scan and used to correct the MR data. A reconstructed MR image using this technique has reduced ghost artifacts otherwise caused by the adverse effects of the orthogonal perturbation fields in the phase correction process.

Method and apparatus to reduce perturbation field effects in MR images by restricting the region of interest

A magnetic resonance (MR) imaging system including a method and apparatus for reducing image artifacts caused by magnet vibration is disclosed herein. The system includes a magnetic field vibration quantification and compensation scheme including quantifying a vibration error component included in the magnetic field configured to acquire k-space data corresponding to an MR image, and correcting the k-space data using the vibration error component. The vibration-induced magnetic field perturbation includes a spatially invariant magnetic field, a spatially linear readout magnetic field gradient, a spatially linear phase-encoding magnetic field gradient, and a spatially linear slice-selection magnetic field gradient. The vibration error component can be at least one of a spatially independent phase error of the spatially invariant magnetic field, a k -space displacement error of the spatially linear readout magnetic field gradient, a k -space displacement error of the spatially linear phase-encoding magnetic field gradient, and a slice-selection error of the spatially linear slice-selection magnetic field gradient.

Method and apparatus for reducing image artifacts caused by magnet vibration in an MR imaging system

A prescan process for an MR fast spin echo (FSE) pulse sequence adjusts the phases of the RF excitation pulse and RF refocusing pulses in the FSE pulse sequence to reduce the spatially invariant phase errors (zeroth order). The prescan process also calculates the value of a readout gradient compensation pulse, a phase-encoding gradient compensation pulse and a slice-select gradient compensation pulse to reduce the first order phase shifts along the readout, phase-encoding, and slice-select gradient axes.

Fast spin echo phase correction for MRI system

A magnetic resonance (MR) imaging system including a method and apparatus for reducing image artifacts caused by magnet vibration is disclosed herein. The system includes a magnetic field vibration quantification and compensation scheme including correcting for a vibration error induced in k-space data corresponding to an MR image. The vibration error can be at least one of a spatially independent phase error caused by a spatially invariant magnetic field, a k,-space displacement error caused by a perturbation gradient along the readout direction, a k,-space displacement error caused by a perturbation gradient along the phase-encoding direction, and a slice-selection error caused by a perturbation gradient along the slice-selection direction. The vibration error is used to appropriately modify and/or add to its corresponding magnetic field component, or its equivalent, in the pulse sequence.

Method and apparatus to compensate for image artifacts caused by magnet vibration in an MR imaging system

A technique is provided for generating images on an MRI system in which errors due to gradient pulses are compensated. The errors are identified in advance, such as in a calibration sequence performed on the MRI system. Receiver phase adjustment and logical gradient error values are derived from the identified error values. The calibration sequence may be a modified version of the MRI imaging sequence used to produce the images. The correction values may be based upon corrections at the center of k-space. The technique is particularly useful in compensating for effects of eddy currents in pulse sequences employing high slew rate gradient pulses, such as diffusion weighted echo planar imaging sequences.

Method and apparatus for producing diffusion weighted MR images

A technique is disclosed for determining errors in MRI sequences resulting from eddy currents generated by pulsed gradient fields. During a calibration sequence, with no phase-encoding, gradient pulses and readout sequences are applied along physical axes of a scanner and data sets are acquired for each combination. A reference data set is acquired with no gradient pulses applied. The resulting data sets are processed by one dimensional Fourier transformation, and the transformed data is analyzed to determine spatially invariant and linear gradient errors. The phase errors may be averaged for each physical axis. The technique is particularly useful in determining errors in diffusion weighted echo planar imaging sequences.

Method and apparatus for identifying errors in magnetic resonance imaging examinations

A method for acquiring spatially and spectrally selective MR images by means of an MR imaging system includes the step of selecting an SPSP pulse sequence, comprising a succession of RF sub-pulses and an oscillatory gradient magnetic field, which is disposed to select a slice through a subject. The method further includes measuring specified parameters of a perturbation magnetic field associated with the imaging system, and deriving an expression for the perturbation field from respective measured parameters and from the oscillatory gradient magnetic field. A specified ideal frequency modulation function, associated with the SPSP sequence, is disposed to offset the slice to a particular spatially localized region of the subject. The SPSP pulse sequence is modified by adjusting the frequency modulation function in specified corresponding relationship with the expression. The modified SPSP pulse sequence is then applied to the subject to excite a selected spectral species in the spatially localized region, while substantially reducing signal intensity loss resulting from the perturbation magnetic field.

Method for acquiring spatially and spectrally selective MR images

A method is provided for determining errors in MR imaging which result from translational motion of an object. In accordance with the method, an MR point source is rigidly joined to the object in selected spatial relationship, and for movement in unison therewith. An MR system is operated to acquire an overall k-space signal which represents an image of the object and of the point source collectively, the overall k-space signal being contaminated by phase errors which result from the motion. A k-space data set which represents an image of the point source alone, and which remains contaminated by the phase errors, is filtered or separated out from the overall k-space signal. The MR system is operated in selected association with the point source to acquire a reference k-space data set, which represents an image of the point source alone but which is unaffected by the phase errors resulting from the motion. The reference k-space data set is then selectively compared with the filtered k-space data set, to determine the phase errors. Alternatively, the reference k-space data set can be replaced by a plurality of k-space alignment lines in parallel relationship to the phase-encoding axis.

Method employing point source to determine motion induced errors in MR

A method is provided for correcting for effects of translational motion or movement of an object in MR imaging. In accordance with the method, an MR point source is joined to the object for translational movement in unison therewith, the point source being maintained in selectively spaced-apart relationship with the object. The method further comprises acquiring a set of k-space data representing images of the object and point source collectively, and deriving a function representing the square of the magnitude of the k-space data set. An inverse Fourier transform operation is applied to the derived function to generate a correlation function with a number of correlation components, a specified one of the correlation components comprising the cross-correlation of the object and the point source. All of the correlation components are then discarded, except for the specified correlation component, which provides an adjusted MR image representing the object from which effects of the translational motion have been substantially eliminated.

Method for reducing translational motion artifacts in MR imaging

Two methods are disclosed to remove the image artifacts produced by Maxwell terms arising from the imaging gradients in an echo planar imaging pulse sequence. In the first method, the frequency and phase errors caused by the Maxwell terms are calculated on an individual slice basis and subsequently compensated during data acquisition by dynamically adjusting the receiver frequency and phase. In the second method, two linear phase errors, one in the readout direction and the other in the phase-encoding direction, both of which arise from the Maxwell terms, are calculated on an individual-slice basis. These errors are compensated for in the k-space data after data acquisition.

Correction of artifacts caused by maxwell terms in slice offset echo

In a method for providing an MR image of a section taken through an object, wherein the section is acquired in the presence of in-plane translational motion of the object, an MR system is operated to acquire a set of imaging data points from the section, the set of imaging data points being contaminated by phase errors resulting from the motion. The MR system is further operated to acquire a plurality of correction data point sets, each correction data point in one of said sets being acquired along an alignment line or other trajectory and at a location coinciding with the location of one of the imaging data points. The imaging data points respectively coinciding with the correction data points comprise, collectively, a subset of the imaging data points. The phase difference between each correction data point set and its corresponding subset of imaging data points is determined, and then used to remove the phase errors from the set of imaging data points.

Method for correcting motion-induced errors in MR imaging

An MRI system includes a gradient compensation system which appends magnetization reset gradient waveforms to imaging gradient waveforms produced during a scan. The magnetization compensation gradients maintain the residual magnetization in the MRI system at a constant level which reduces image artifacts.

Method for compensating an MRI system for residual magnetization

Fast spin echo pulse sequences are adjusted to reduce, or eliminate image artifacts caused by Maxwell terms arising from the linear imaging gradients. The waveforms of the slice selection, phase encoding and readout gradients are adjusted in shape, size or position to eliminate or reduce the phase error caused by the spatially quadratic Maxwell terms.

Method for reducing Maxwell term artifacts in fast spin echo MR images

A method is disclosed to remove the image artifacts produced by Maxwell terms arising from the imaging gradients in an echo planar imaging pulse sequence. The frequency and phase errors caused by the Maxwell terms are calculated on an individual slice basis. During the subsequent data acquisition these errors are compensated by dynamically adjusting the receiver frequency and phase.

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Pewaukee, WI, United States of America

Xiaohong Zhou