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 method is described for increasing the temporal resolution of MR fluoroscopy procedures. A central portion of the image requiring a higher temporal resolution is updated more frequently using less-than-complete newly acquired NMR data. A running average of the signals produced by peripheral structures is used to form the periphery of the image and to remove artifact-producing data from the less-than-complete NMR data.

Abstract: Several algorithms are presented for determining the trajectory of a particle in motion given cine phase contrast (PC) velocity field data. The PC velocity field is usually measured in a fixed frame of reference (spatial coordinates) and must be transformed to the velocity in material coordinates, i.e., the velocity experienced by individual points in the medium. The material velocity field can then be integrated to yield the trajectory of a particular point. Therefore, to determine the trajectory of a particular point, the given spatial velocity field must be transformed to the material velocity field. While there exists a mathematical relationship that maps velocity fields in spatial coordinates to material coordinates, this mapping can be applied only if particle trajectories are known. Although there appears to be a dilemma here, through an optimization technique, we have devised an iterative procedure to determine the optimal trajectory in the sense to be defined.

Abstract: Temporal resolution in dynamic magnetic resonance imaging is obtained by establishing a representation of static material in the field of view by averaging all signal acquisitions for a full field of view, and Fourier transforming the averaged data to yield an averaged image. The known dynamic portion of the image is set to zero to yield an image restricted to the static portion of the image. The averaged static portion is then inverse transformed to generate a synthetic MR raw data set that is an estimate of the data that would be collected if the object consisted only of the static material. The synthetic data are subtracted from the MR data for each view to eliminate the data for static material. The resulting data then represents the dynamic material which can be individually reconstructed.

Abstract: A closed-form integration method is derived and analyzed for computing motion trajectories from velocity field data, in particular as measured by phase contrast cine (PC) MR imaging. By modeling periodic motion as composed of Fourier harmonics and integrating the material velocity of the tracked point in the frequency domain, this method gives an unbiased trajectory estimate in the presence of white measurement noise and eddy current effects. When applied to practical data, the method can incorporate compensation for the frequency response of the data acquisition method, offering a further improvement on the tracking accuracy. In simulation and phantom studies, the estimated trajectories were in excellent agreement with the true trajectories. Encouraging results have also been obtained on data from human beings.

Abstract: The present invention, in one form, is an apparatus for performing image reconstruction using data obtained by a four beam helical scan. In reconstructing an image, projection data arrays are generated. Such projection data is then weighted by weighting factors to generate a weighted projection data array. The weighted projection data array is filtered and back projected to generate an image data array. The image data arrays for the beams are then summed to generate a slice image data array.

Abstract: A CT apparatus for scanning compact structures associated with a larger body uses radiation source producing a reduced field-of-view to simplify construction and reduce exposure of the larger body. Truncation artifacts in the reconstructed image caused by volume elements in the larger body imaged by the radiation beam only for projections at some angles, are reduced by acquiring two projections at two different energies and combining those projections to compensate for the attenuation of the radiation by the volume elements of the larger body.

Abstract: An MRI system includes a noise filter which receives each acquired NMR signal during a scan and detects short-duration noise pulses by sensing the signal level in a band of frequencies outside the imaging bandwidth. A blanking circuit suppresses the NMR signal during the time interval each noise pulse is detected to remove the noise pulse prior to image reconstruction.

Abstract: A CT apparatus for scanning compact structures associated with a larger body uses radiation source producing a reduced field-of-view to simplify construction and reduce exposure of the larger body. Truncation artifacts in the reconstructed image caused by volume elements in the larger body imaged by the radiation beam only for projections at some angles, are reduced by acquiring two projections at two different energies and combining those projections to compensate for the attenuation of the radiation by the volume elements of the larger body.

Abstract: An x-ray computed tomography system for helically scanning a patient translates the patient as projections of the patient at various beam angles are obtained. The projection data are weighted prior to reconstruction to account for multiply measured projection data and the deviation of the cone beam from a parallel beam approximation.

Abstract: A 3rd generation CT system produces gain calibration factors (.gamma.) for each channel of a detector array while a scan is being performed. Redundant views are acquired during the scan but the x-ray intensity values in the second view are shifted to the adjacent detector channel by focal spot wobbling the x-ray source. Gain calibration factors (.gamma.) are calculated using the ratio (.beta.) of x-ray intensity values measured at adjacent detector channels.

Abstract: An x-ray computed tomography system for helically scanning a patient translates the patient as projections of the patient at various beam angles are obtained. A cone beam reconstruction method, which accounts for the divergence of the rays of a fan beam of x-rays, is used to reconstruct multiple narrow slices which are combined to provide a larger slice with an improved slice profile. The conical shape of the fan beam causes some voxels to be scanned more than once for a given beam angle and this extra data is weighted and combined with its counterpart data to improve the quality of the image.

Abstract: A long train of spin echoes is produced using a RARE excitation pulse sequence, and during each spin echo an annular segment of a long k-space spiral as determined by read-out magnetic gradients is detected. At the end of the echo train the entire k-space spiral will have been covered. Each of the segments can be a unique annular portion of the k-space spiral. Alternatively, fewer annular segments of the spiral can be provided, with the fewer annular segments rotated in k-space and replayed to cover interleaved paths in k-space. The imaging gradients are refocused at the time of each spin-echo pulse in order to permit the long echo pulse trains of RARE imaging. Each spiral segment is surrounded by gradient lobes that move out from the k-space origin to the beginning of the segment, and move back to the origin from the end of the segment. Advantageously, the magnetic gradient lobes can be produced concurrently with parasitic echo crusher gradients at the beginning and end of each spin-echo pulse.

Abstract: MRI signals detected by a phased array of multicoils are combined to provide phase contrast image signals with improved signal-to-noise ratio. Detected complex image signals are obtained for different magnetic gradients and are combined to obtain phase, magnitude, and magnitude weighted image signals for imaging.

Abstract: Motion of a deformable region within an object is tracked using phase contrast magnetic resonance imaging measurements by defining the region with a plurality of vertices, obtaining magnetic resonance signals representative of at least one velocity component from each of the vertices at a plurality of time frames, and tracking motion of the region based on movement of the plurality of vertices. The region can be two-dimensional such as a polygon and ellipse or three-dimensional such as a polyhedron. The deformation can be homogeneous or non-homogeneous in which case the velocity component is defined as a polynomial of at least second order.

Abstract: A method and apparatus to reduce the noise in three-dimensional phase contrast magnetic resonance velocity measurements exploits the property that blood is incompressible and therefore the velocity field describing its flow must be divergence-free. The divergence-free condition can be incorporated by the projection operation on Hilbert space where the velocity measurements are projected onto the space of divergence-free velocity fields. The reduction of noise is achieved since the projection operation eliminates the noise component that is not divergence-free. Higher quality angiograms are produced due to the noise reduction in the velocity measurement signals.

Abstract: A CT apparatus for scanning compact structures associated with a larger body uses radiation source producing a reduced field-of-view to simplify construction and reduce exposure of the larger body. Truncation artifacts in the reconstructed image caused by volume elements in the larger body imaged by the radiation beam only for projections at some angles, are reduced by acquiring two projections at two different energies and combining those projections to compensate for the attenuation of the radiation by the volume elements of the larger body.

Abstract: Measures of a velocity component during a plurality of time frames of cyclical motion of a region of interest are obtained using contrast cine MRI techniques. Trajectories of the region of interest are calculated in a backward direction and in a forward direction. The trajectories are then combined to define a motion trajectory. The forward and backward trajectories are weighted prior to combining with the forward trajectory being more heavily weighted for early frames and the backward trajectory being more heavily weighted for late frames.

Abstract: Measures of a velocity component during a plurality of time frames of cyclical motion of a region of interest are obtained using contrast cine MRI techniques. A position error is obtained between a known starting location and a calculated cyclical motion and is used for recalculating cyclical motion of the region of interest using the measures of velocity and the measure of error in velocity.

Abstract: A dual energy x-ray system for determining bone density and the like compensates for variable flux density associated with the different absorptions and the production of the high and low energies of x-rays by controlling both the voltage biasing the x-ray tube and the relative dwell times during which the x-ray tube is at those voltages, the dwell time being increased for the lower voltage to compensate for lower flux density. Compensation for variations in flux density improves the resultant signal-to-noise ratio of the measurements taken and thereby provides more accurate measurement of materials with minimized examination time.