Abstract: A computer is programmed to acquire calibration data from a calibration scan, the calibration data configured to characterize high order eddy current (HOEC) generated magnetic field error of an imaging system. The computer is also programmed to process the calibration data to generate a plurality of basis coefficients and a plurality of time constants and to calculate a plurality of basis correction coefficients based on the plurality of basis coefficients, the plurality of time constants, and gradient waveforms in a given pulse sequence. The computer is further programmed to execute a diffusion-weighted imaging scan that comprises application of a DW-EPI pulse sequence to acquire MR data from an imaging subject and reconstruction of an image based on the acquired MR data. The computer is also programmed to apply HOEC-generated magnetic field error correction during application of the DW-EPI pulse sequence configured to reduce HOEC-induced distortion in the reconstructed image.

Abstract: A computer is programmed to acquire calibration data from a calibration scan, the calibration data configured to characterize high order eddy current (HOEC) generated magnetic field error of an imaging system. The computer is also programmed to process the calibration data to generate a plurality of basis coefficients and a plurality of time constants and to calculate a plurality of basis correction coefficients based on the plurality of basis coefficients, the plurality of time constants, and gradient waveforms in a given pulse sequence. The computer is further programmed to execute a diffusion-weighted imaging scan that comprises application of a DW-EPI pulse sequence to acquire MR data from an imaging subject and reconstruction of an image based on the acquired MR data. The computer is also programmed to apply HOEC-generated magnetic field error correction during image reconstruction configured to reduce HOEC-induced distortion in the reconstructed image.

Abstract: A computer is programmed to acquire calibration data from a calibration scan, the calibration data configured to characterize high order eddy current (HOEC) generated magnetic field error of an imaging system. The computer is also programmed to process the calibration data to generate a plurality of basis coefficients and a plurality of time constants and to calculate a plurality of basis correction coefficients based on the plurality of basis coefficients, the plurality of time constants, and gradient waveforms in a given pulse sequence. The computer is further programmed to execute a diffusion-weighted imaging scan that comprises application of a DW-EPI pulse sequence to acquire MR data from an imaging subject and reconstruction of an image based on the acquired MR data. The computer is also programmed to apply HOEC-generated magnetic field error correction during application of the DW-EPI pulse sequence configured to reduce HOEC-induced distortion in the reconstructed image.

Abstract: A method of calibrating an imaging sequence includes the application of a pre-scan pulse sequence to acquire MR signals from a region-of-interest to be imaged with an imaging pulse sequence. The pre-scan pulse sequence is interrupted to acquire pre-scan data in a low bandwidth acquisition window. A frequency spectrum is generated from the pre-scan data and displayed to interactively allow a user to establish scan parameters for the imaging pulse sequence.

Abstract: A method and apparatus for reducing vibration-related artifacts in diffusion weighted imaging determines a vibrational frequency of an MR system and modifies scan parameters such that vibrational frequencies induced on the MR system are inconsistent with the vibrational frequency of the MR system. The method and apparatus improves image quality of MR images acquired using diffusion weighted imaging techniques with the MR system. As such, modification and/or redesign of the MR system to reduce vibrational frequency interaction is reduced.

Abstract: A method of pre-scan data acquisition includes the application of a pre-scan pulse sequence to acquire MR signals from a region-of-interest to be imaged with an imaging pulse sequence. The pre-scan pulse sequence applies a pre-scan readout gradient pulse and a pre-scan readout gradient rewinder pulse. MR signals are acquired from a region of interest during application of the pre-scan readout gradient pulse and after application of the pre-scan readout gradient rewinder pulse.

Abstract: A method and apparatus for reducing vibration-related artifacts in diffusion weighted imaging determines a vibrational frequency of an MR system and modifies scan parameters such that vibrational frequencies induced on the MR system are inconsistent with the vibrational frequency of the MR system. The method and apparatus improves image quality of MR images acquired using diffusion weighted imaging techniques with the MR system. As such, modification and/or redesign of the MR system to reduce vibrational frequency interaction is reduced.

Abstract: A method and apparatus for reducing vibration-related artifacts in diffusion weighted imaging determines a vibrational frequency of an MR system and modifies scan parameters such that vibrational frequencies induced on the MR system are inconsistent with the vibrational frequency of the MR system. The method and apparatus improves image quality of MR images acquired using diffusion weighted imaging techniques with the MR system. As such, modification and/or redesign of the MR system to reduce vibrational frequency interaction is reduced.

Abstract: A method for processing digital images includes acquiring a phase difference image which includes one or more phase wraps, creating a modulated phase difference image from the phase difference image, comparing the modulated phase difference image to the phase difference image to locate areas in the phase difference image to be unwrapped, and unwrapping the phase difference image based on the areas located in the comparing step. The unwrapping step is performed by replacing wrapped pixels in the phase difference image with pixels in the modulated phase difference image plus an integer multiple of &pgr;. The integer multiplier is computed by comparing overlapping pixels in the image segments. This has a smoothing effect causing the wrapped pixels in the phase difference image to become unwrapped. As a result of this pixel replacement process, an image of improved quality and information content is produced compared with conventional methods.

Abstract: A method for processing digital images includes acquiring a phase difference image which includes one or more phase wraps, creating a modulated phase difference image from the phase difference image, comparing the modulated phase difference image to the phase difference image to locate areas in the phase difference image to be unwrapped, and unwrapping the phase difference image based on the areas located in the comparing step. The unwrapping step is performed by replacing wrapped pixels in the phase difference image with pixels in the modulated phase difference image plus an integer multiple of &pgr;. The integer multiplier is computed by comparing overlapping pixels in the image segments. This has a smoothing effect causing the wrapped pixels in the phase difference image to become unwrapped. As a result of this pixel replacement process, an image of improved quality and information content is produced compared with conventional methods.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: The ghost artifacts appearing in images produced by an MRI system are measured. An image of a reference phantom is acquired and the ghost artifacts are measured by cross correlating a ghost correlation matrix with the image. The ghost correlation matrix is produced from the image and the cross correlation is computed as the ghost correlation matrix is moved across the image along either the phase encoding or frequency encoding axes.

Abstract: 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.

Abstract: 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.

Abstract: Signal fall-off in axial EPI images as well as its variations is corrected by compensating the EPI pulse sequence with gradient pulses that serve to balance the phase dispersion caused by Maxwell terms. Four embodiments are described which employ the slice-selection gradient to compensate the EPI pulse sequence and a fifth embodiment employs the readout gradient.

Abstract: A method for correcting Maxwell field induced distortion, ghosting, and blurring artifacts in non-axially oriented EPI images is disclosed. In one embodiment phase corrections are calculated and used to offset Maxwell term errors during the image reconstruction process, and in another embodiment corrections are made after the image is reconstructed.

Abstract: An eddy current correction method is provided for use in an MR system disposed to generate difflusion-weighted echo planar images by employing a bipolar diffusion-weighting gradient as well as the nominal components in an echo planar imaging pulse sequence. The correction method comprises the steps of deriving eddy current parameters (i.e., amplitude and time constant) associated with each eddy-current-induced magnetic field component caused by the diffusion-weighting gradient, and generating a set of correction terms, each of the correction terms being a function of the parameters. The method includes the further steps of modifyg a set of ideal echo planar imaging gradients and the receiver phase and frequency to respectively offset the eddy current induced magnetic field gradient and the spatially independent B.sub.0 -magnetic field. The modification occurs in two stages.