Abstract: A computer-implemented method to correct motion and interpolation effects for functional magnetic resonance imaging (fMRI) analysis is provided. The method estimates the motion on every voxel of the data and removes those effects to leave a residual signal that can be analyzed to high accuracy. The estimation of the motion includes solving a normal matrix equation based on the local translational motion of each voxel of the head, and a regularization parameter that depends on the local spatial structure of the head. Methods to regularize a matrix from the normal equation using the regularization parameter are also provided. A rolling filter implementation for real-time processing and motion correction is provided.

Abstract: A computer-implemented method to correct motion and interpolation effects for functional magnetic resonance imaging (fMRI) analysis is provided. The method estimates the motion on every voxel of the data and removes those effects to leave a residual signal that can be analyzed to high accuracy. The estimation of the motion includes solving a normal matrix equation based on the local translational motion of each voxel of the head, and a regularization parameter that depends on the local spatial structure of the head. Methods to regularize a matrix from the normal equation using the regularization parameter are also provided. A rolling filter implementation for real-time processing and motion correction is provided.

Abstract: Disclosed is a method for regularized high-order shimming of a magnetic field in an MRI system. In one application a spiral pulse sequence is used to acquire field maps, and based on the user selection a shimming region, a least squares calculation of shim currents is performed that minimizes the root mean square (RMS) value of the B0 in homogeneity-over the volume of interest. The singular value decomposition (SVD) is used to enable regularized methods for solving the least squares problem. The regularization allows arbitrary regions to be shimmed without the divergence of shim currents.

Abstract: MRI is used to monitor the time behavior or an organ of interest. Images of such organ may change in time due to physiological motion, and/or due to contrast-agent accumulation. Dynamic applications generally involve acquiring data in a k-t space, which contains both temporal and spatial information. In some dynamic applications, the t axis of the k-t space is not densely filled with information. A method is introduced which can transfer information from the k axes to the t axis, allowing a denser, smaller k-t space to be acquired, and leading to significant reductions in the acquisition time of the temporal frames. Results are presented for cardiac imaging and functional MRI (fMRI). In the case of cardiac imaging, the present method is shown to reduce the data requirement by nearly a factor two. In the case of fMRI, reductions by as much as a factor six can be obtained. The behavior of the method is assessed by comparing the results to data obtained in a conventional way.

Abstract: A method is presented for correcting Maxwell term error artifacts produced by an NMR system during the production of either a phase contrast angiogram or a complex difference angiogram. Phase corrections are made to the reconstructed phase image to eliminate the artifacts. Correction coefficients calculated from the flow encoding magnetic gradient waveforms of the phase contrast pulse sequence are used in a polynomial to calculate a set of phase error corrections. These corrections are then used to adjust the phase at each pixel of the angiogram image.

Abstract: A phase unwrapping method is provided comprising a post-processing MR operation for an array of phase-related MR data elements. The method includes the steps of defining a Region of Interest, of comparing each data element with the first and second data elements which are adjacent thereto in the array, in order to generate respectively corresponding first and second gradient values. A subset of the data elements is constructed, wherein a data element is assigned to the subset only if its corresponding first and second gradient values are less than a selected threshold value VENC/2. The data element subset is employed to generate a fitted phase function having a value at the position of each data element in the array, and the value of the fitted function thereof, for a given data element, is subtracted from the measured value thereof.

Abstract: Disclosed is a method to correct the geometric distortion caused by field inhomogeneity in MR images such as images of patients wearing MR-compatible stereotaxic frames. A previous distortion correction method derives patient-dependent error maps by computing the phase-difference of 3D images acquired at different T.sub.E 's. The time difference (.DELTA.T.sub.E =4.9 ms at 1.5 T) is chosen such that the water and fat signals are in phase. However, .DELTA.T.sub.E is long enough to permit phase wraps in the difference images for frequency offsets greater than 205 Hz. Phase unwrapping techniques resolve these only for connected structures; therefore the phase difference for fiducial rods may be off by multiples of 2 .pi. relative to the head. This uncertainty is removed by using an additional single 2D phase-difference image with .DELTA.T.sub.E =1 ms (during which time no phase-wraps are typically expected) to determine the correct multiple of 2 .pi. for each rod.

Abstract: An in-vivo correction method for the non-linear, shape dependent spatial distortion in MR images due to magnetic field inhomogeneity including inhomogeneity due to susceptibility variations is disclosed. Geometric distortion at the air/tissue and tissue/bone interfaces before and after the correction is quantified using a phantom. The results are also compared to the "distortion-free" CT images of the same phantom. Magnetic susceptibility of cortical cattle bone was measured using a SQUID magnetometer and found to be -8.86 ppm which is quite similar to that of tissue (-9 ppm). The distortion at the bone/tissue boundary was negligible while that at the air/tissue boundary created displacements of about 2.0 mm with a1.5T main magnetic field and a 3.13 mT/m gradient field, a significant value if MR images are used to localize targets with the high accuracy expected for stereotaxic surgery. The correction method reduces the errors to at least the same level of accuracy as CT.

Abstract: A method of separating fat and water proton NMR images in the presence of local magnetic field inhomogeneities uses multiple images with phase shift between the water and fat signals. The phase shifts need not be multiples of .pi. and are selected so that at least two images are not "fat opposed" and so that images are not symmetric in phase shift around a "fat aligned" image. A switch function is developed from the argument of the combined images to unambiguously identify the predominant chemical species of each voxel. Wrap arounds in the argument resulting from the limited range of trigonometric functions are treated first by subtracting a low order polynomial, then detecting and removing wrap around discontinuities.

Abstract: NMR image data acquired from a human subject is corrected for view-to-view motion caused by respiration. As each view is acquired the position of the subject is also measured to indicate the distance of the anterior abdominal wall from a reference position. The acquired NMR image data is corrected using the associated measured distances and a simplified model of the motion.

Abstract: Determination of the shimming fields for a polarizing NMR magnet is performed by acquiring two NMR images with evolution times differing by .tau.. A pixel by pixel division of the images yields a third image whose phase is proportional to magnetic field inhomogeneity. Several such inhomogeneity images are acquired within a cylindrical volume and the data of the images is fit according to a set of polynomials approximating the fields of the magnet shim coils. The coefficients of this fitting procedure are used to set the current level in the corresponding shim coils. The interaction between correction fields is accommodated by an empirically derived correction matrix. This procedure may be repeated to provide more accurate shimming.

Abstract: An image data set acquired by an NMR system using an echo-planar pulse sequence is corrected for systemic errors. Errors are measured by acquiring a reference data set using the same echo-planar pulse sequence without phase encoding, and the measured errors are employed to correct phase errors in the image data set.

Abstract: A method of separating fat and water proton NMR images in the presence of local magnetic field inhomogeneities uses three images acquired with .pi., O, and -.pi. phase shift between the water and fat signals. A switch function is developed from the argument of the combined images to unambiguously identify the predominant chemical species of each pixel. Wrap arounds in the argument resulting from the limited range of trigonometric functions are treated first by subtracting a low order polynomial, fit to the phase image, to accentuate and remove wrap around discontinuities, and second by using trend analysis to detect and remove discontinuities caused by the wrap arounds.

Abstract: In an NMR system a method for measuring the motion of spins moving in an unknown direction includes four separate measurement cycles. The first cycle establishes a reference phase and each of the subsequent three measurement cycles includes motion encoding field gradients for measuring motion along respective pairs of Cartesian coordinates. All four measurements are combined to calculate each of the motion components along x, y and z coordinates.

Abstract: An NMR system for producing images carries out a procedure in which field maps are produced that indicate inhomogeneities in the transmitted RF excitation field and inhomogeneities in the RF receive field. Intensity NMR data is acquired using a set of different RF excitation field strengths and separate transmit and receive arrays are produced by fitting a characteristic curve to the data. A variety of images which display RF inhomogeneities may be produced from these two arrays.

Abstract: NMR data indicative of the motion of the anterior abdominal wall of a subject is acquired just prior to each image data pulse sequence in an NMR scan. The acquired motion NMR data is processed on-line to produce a signal indicative of the displacement of the abdominal wall from a reference position. The displacement values may be employed to produce a signal indicative of a respiration phase that can be used in connection with motion artifact reduction techniques.

Abstract: Displacement values which indicate respiration phase are acquired along with each view of an NMR scan. Two or more such NMR scans are conducted and the deviation of each displacement value from a smooth reference curve is employed to measure the integrity of its associated NMR image data. The NMR image data from the separate scans are combined to reduce random noise and motion artifacts, and the combination is accomplished by weighting the data in accordance with its measured integrity.

Abstract: In-vivo shimming is performed by acquiring two NMR images with evolution times differing by ##EQU1## where .omega..sub.1, .omega..sub.2, are the Larmor frequencies of the dominant proton species of the imaged object. A pixel by pixel division of the image yields a third image whose phase is proportional to magnetic field inhomogeneity. The phase of the third image set has periodic discontinuities as a result of the periodicity of the arctangent function used to extract the phase from the image. These discontinuities are removed by taking the derivative of the phase and integrating the product of that derivative and a weighting function where the weighting function has zero weight at the points of discontinuity. Off-center quadratic terms in the inhomogeneity are removed prior to fitting the linear shimming gradients.

Abstract: A method is provided for characterizing the spurious, time dependent magnetic field gradient response and for compensating therefor. In accordance with the method, two small sample objects are placed at two positions with respect to the system isocenter where the gradient to be compensated has different values. A magnetic field gradient pulse is applied to the sample, followed by the application after a variable time of a 90.degree. radio frequency pulse. The phase of the resulting free induction decay (FID) signal is monitored as a function of total time since the end of the gradient pulse. These data are related to the impulse response of the gradient and are employed to construct compensation filters for both the gradient coils and the polarizing field shim coil.

Abstract: In an NMR imaging system spins in a set of slices are simultaneously excited by a composited RF excitation pulse in each NMR pulse sequence of a scan. The composite RF excitation pulse contains a set of terms, each of which excites a separate slice and each of which is modulated by a k.sub.y -dependent phase factor. The k.sub.y -dependent phase factor causes the iamge of each slice to be offset in the resulting reconstruction so that each slice image can be separated from the others.