Abstract: Image artifacts produced by residual magnetization in elements of the gradient field amplifier system are reduced by driving the residual magnetization to a preselected value at the end of each imaging pulse sequence. Reset gradient pulses are produced by the gradient field amplifier system after each signal is acquired to drive the residual magnetization to a selected value. A number of different reset gradient waveforms may be used, and they may be produced in a number of different ways.

Abstract: A technique is disclosed for calibrating time delays between RF and gradient frequency pulses in a magnetic resonance imaging system. The calibration procedure includes the formation of calibration images of a phantom positioned in the gradient field system. Calibration images are processed and compared to one another to determine deviation between locations in the gradient field system and the impact of radio frequency-to-gradient waveform time delays on the deviations. Optimal time delays are identified which minimize the deviations between the calibration images. Multiple axes of the system may be calibrated through the use of symmetrical phantoms and similar pulse sequences of each axis. A spectral-spatial pulse sequence is employed bearing the calibration routine.

Abstract: The present invention, in one form, provides compensation for the effects of an oscillatory B0 eddy current Be(t) in an NMR apparatus. In an NMR apparatus having a transmitter generating a spin excitation signal and a receiver detecting an NMR signal, applied gradient signals are analyzed to estimate a resulting oscillatory B0 eddy current Be(t). The frequency of either the transmitter or the receiver of the NMR apparatus, or both, is shifted in frequency by an amount proportional to Be(t) to achieve compensation.

Abstract: Two incomplete k-space data sets are acquired with an MRI system using two surface coils. The data from the two surface coils is combined in a Sense process to reduce aliasing artifacts in the reconstructed image. The incomplete k-space data is filled out using a modified Homodyne reconstruction method which preserves phase information for the Sense process.

Abstract: An MRI system automatically performs a calibration procedure to calculate optimal compensation parameter values for all three gradient pre-emphasis filters. A single fixture is employed to measure the errors caused by gradient pulses produced by each of the three gradient systems. The measured errors are used to calculate a gradient error function and the optimal compensation parameter values are calculated for each pre-emphasis filter by finding the minimum in the gradient error function.

Abstract: In an MR imaging system, a method is provided for operating an MR gradient coil to provide a pulse sequence which comprises first and second sub-sequences. During the first sub-sequence, the coil is operated to produce a pair of diffusion-weighted imaging pulses, each having a gradient amplitude and a slew-rate. During the second sub-sequence the coil is operated to produce a train of echo-planar imaging pulses, each having a gradient amplitude which is selectively less than the amplitude of the diffusion-weighted pulses, and a slew-rate which is selectively greater than the slew-rate of the diffusion-weighted pulses. Thus, the invention is directed to a gradient system which can provide optimal performance for both diffusion-weighted imaging and echo-planar imaging, while using only a single coil for a given gradient axis.

Abstract: A fast and computationally efficient system and method is disclosed for detecting and tracking the position of a reference structure in the body using a linear phase shift to minimize motion artifacts in magnetic resonance imaging. In one application, the system and method is used to determine the relative position of the diaphragm in the body in order to synchronize data acquisition to the same relative position with respect to the abdominal and thoracic organs to minimize respiratory motion artifacts. The system and method uses the time domain linear phase shift of the reference structure data in order to determine its spatial positional displacement as a function of the respiratory cycle. The signal from a two-dimensional rectangular or cylindrical column is first Fourier transformed to the image domain, apodized or bandwidth-limited, converted to real, positive values by taking the magnitude of the profile, and then transformed back to the image domain.

Abstract: An MRI system acquires NMR signals and digitizes them at a fixed sample rate. A lower, prescribed sample rate is obtained by transforming the acquired NMR signal using a weighting function calculated from scan parameters. The transformation includes multiplying by the weighting function and convolving the NMR signal with the complex conjugate of the weighting function.

Abstract: An MRI system performs a calibration procedure to calculate optimal compensation parameter values for a pre-emphasis filter that alters the shape of gradient waveforms. The gradient field errors are measured using a series of pulse sequences in which different compensation parameter values are sampled. The measured errors are used to calculate a gradient error function and the optimal compensation parameter values are calculated by finding the minimum in the gradient error function.

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: Artifacts in NMR images produced by Maxwell terms during non-rectilinear scans, such as spiral scans, are reduced or eliminated. Phase corrections for in-plane and through-plane blurring are used to offset Maxwell terms errors.

Abstract: A method is provided for rapidly measuring change in T.sub.2 * in a region of an MR imaging subject, the method employing a double gradient echo sequence with spiral readout. In this technique, two gradient echoes are acquired after a single RF excitation. A first readout trajectory spirals out from the center of k-space, i.e., k.sub.r =0, to some maximum radius, and then a second readout trajectory spirals back towards the center of k-space. A gradient echo is formed each time a k-space trajectory passes through the center of k-space. The signal intensity of the two echoes allows the calculation of the T.sub.2 * values on a pixel by pixel basis. The ability to measure changes in T.sub.2 * allows measurement of change in perfusion to be quantified.

Abstract: A prescan for an MRI system (FIG. 1) automatically sets the rf power level to produce an excitation field that nutates spins in the image plane the amount prescribed by the scan. Nutation angle is measured with a pulse sequence comprised of three slice selective excitation pulses that produce a set of frequency encoded NMR echo signals (E.sub.1, S.sub.1, E.sub.2, E.sub.23, E.sub.13). Nutation angle is calculated at locations in the image slice from the Fourier transformation of these signals and the maximum nutation angle is employed to calculate the final rf power level setting for the scan.

Abstract: A CT apparatus for reducing aliasing in reconstructed images uses an x-ray tube with a translatable focal spot to double the spatial sampling rate, over that achieved by a conventional CT machine. Radial resolution artifacts in the image, identified to the "bleeding through" of previous samples from different focal spot positions into the present sample are removed by a convolution process employing the inverse of the detector transfer function. Timing of the data sampling with respect to the changing of the wobble positions is also employed to minimize the bleed through and to improve signal-to-noise ratio.

Abstract: A method of reducing image artifacts in images acquired with fan beam, helical scanning, tomographic imaging systems uses half scans of less than 360.degree. of projection data of an imaged object on each side of the slice plane being imaged. The half scans are weighted with half scan weighting factors to compensate for redundant data and are weighted with helical scanning weighting factors to interpolate the half scans projection data to projection data at the slice plane. The imaged object may be moved one slice thickness for each 360.degree. of scanning so that the half scans are concentrated closer to the slice plane thereby reducing interpolation errors. Alternatively, for a series of slice images, the imaged object may be moved one slice thickness for each half scan to reduce average slice acquisition time.

Abstract: An x-ray CT system has a detector which revolves about the object being imaged to acquire attenuation data from many different angles through a range of at least 180.degree.. The response of the detector has a time lag which as the detector revolves tends to blur the attenuation data. The acquired data is compensated for the resolution degradation by convolving the data with a function that is the inverse of a response function of the detector. An image can be reconstructed from this compensated data. However, this compensation process tends to reintroduce noise that was reduced by the blurring. As a result, a modified version of the detector response function is applied to the compensated data to reduce the noise without degrading the image resolution to an unacceptable level.

Abstract: A method of reducing image helical scanning; artifacts in computed tomography imaging systems divides 360.degree. of projection data into two half scans. Separate weighting functions are applied to the two half scans and they are reconstructed to an image per conventional reconstruction methods. The weighting functions provide effective interpolation and extrapolation of the half scan data to a slice plane centered in the projection data. In one embodiment, the weighting functions are feathered with a cubic function to remove weighting induced image artifacts.

Abstract: A CT apparatus reduces errors in projection data acquired in helical scanning. The imaged object moves concurrently along a translation axis and the x-ray beam is periodically translated with the imaged object so as to subtend a single predetermined volume element during the acquisition of one projection set of data for a first slice. The x-ray beam then returns to its starting position and tracks a second predetermined volume element within a next slice. The x-ray beam may be translated by moving the focal point or a collimator or a combination of both. Helical scans with a pitch requiring sweeping of the x-ray beam beyond the detector limits are accommodated by limiting the sweep to a lessor compliance distance. The angular rate of the sweep is held constant within this compliance distance during the sweep.

Abstract: A method of reducing image artifacts in images acquired with fan beam, helical scanning, tomographic imaging systems rebins the fan beam data into parallel beam projection sets. Data is spliced from first areas of the parallel beam projection data to second areas to create a set of 2.pi. radians of complete projection data. The 2.pi. of projection data is divided into half scans which are interpolated and extrapolated to produce a new projection set at the slice plane with reduced helical offset error. A feathering function is used to blend the discontinuities in the interpolation and extrapolation weights to prevent streak image artifacts.

Abstract: A method of reducing image helical scanning artifacts in fourth generation computed tomography imaging systems, rebins 360.degree. of projection data into two half scans. The data of the half scans is spliced so as to create 360.degree. of contiguous detector vertex projections. A weighting function is applied to the two half scans and they are reconstructed to an image per conventional reconstruction methods. The weighting functions provide effective interpolation and extrapolation of the half scan data to a slice plane centered in the projection data. In one embodiment, the weighting functions are feathered with a cubic function to remove weighting induced image artifacts.