Abstract: According to the known method, a reference measurement is performed by measurement of magnetic resonance signals, without application of a magnetic gradient field to introduce phase encoding in the magnetic resonance signals. According to the invention, two measurements are performed with a read-out gradient of opposite polarity at substantially corresponding instants, relative to an instant at which the contributions to the phase error due to frequency deviations are zero. The advantage of the novel method resides in the fact that a higher insensitivity to field inhomogeneities and chemical shifts is achieved.

Abstract: An MR system for interventional procedures, includes an MR device and an invasive device. The MR device is arranged to acquire images of a part of an object. A part of the invasive device can be imaged in an MR image by providing a coil or a conductor loop which has two non-magnetic conductors which are situated at some distance from one another underneath the surface of the invasive device. A reduction of artefacts due to the movement of a patient or the invasive device can be obtained by sampling an MR signal, whereby an auxiliary magnetic field is periodically applied during the time in which the plurality of lines in k-space are scanned.

Abstract: In a magnetic resonance method for imaging a part of a body which is arranged in a steady magnetic field, magnetic resonance signals are measured utilizing an initial read gradient and an alternating read gradient. The method can be used, for example for phase contrast angiography and for flow velocity measurements in a body of a patient. In order to counteract artifacts in the image due to inhomogeneities of the steady magnetic field, the time elapsing between an excitation RF pulse exciting spins in the part of the body and the initial read gradient is varied in successive pulse sequences. However, this causes artifacts in the fluid flow images due to the phase error which itself is caused by the spins flowing in the direction of the gradient field. In order to counteract these artifacts, the first-order moment of the initial read gradient and a first order lobe of the alternating read gradient is kept substantially constant in each pulse sequence.

Abstract: In an EPI or GRASE magnetic resonance method, phase errors caused by the alternation of the read gradient polarity (G.sub.x) are corrected by obtaining a set of reference measurements with both polarities at low values in the phase encoding (k.sub.y) direction. From these reference measurements the phase errors are estimated. Subsequently, the measurements covering the whole of k-space are corrected and a magnetic resonance image obtained in which artefacts due to the alternating polarity of the read gradient are suppressed.

Abstract: In an MRI device operating according to a spin-echo method, switched gradient magnetic fields are applied in the form of slice selection, phase encoding and read gradients. The switching of the gradients causes eddy currents in metal parts of the apparatus. The eddy currents disturb the applied magnetic fields, thereby changing the phases of the precessing nuclear spins of a body to be examined and causing artefacts in a reconstructed image. The disturbing effects of the eddy currents is compensated for by supplementing a sequence of switched gradient magnetic fields with additional gradient fields. These additional fields have zero net effect but induce further eddy currents which compensate for the disturbing effects of the eddy currents generated by the regular switched gradient magnetic fields. The additional gradient fields are chosen such that the total time-integrated strength of a gradient in any interval between two RF-pulses is substantially equal to the desired value.

Abstract: An MR angiography method in which an angiogram is formed from the difference between amplitude and/or phase information contained in first and second complex images produced by different gradient waveforms in respective first and second types of excitation sequences. The gradient waveforms are chosen so that in the angiogram obtained from the first and second complex images, ideally, signals orginating from excited stationary spin nuclei are not present while the signals orginating from moving spin nuclei, due to flow in blood vessels, are present. However, stationary spin nuclei can be somewhat differently excited in the first and second sequence due to the different gradient waveforms used which cause different eddy currents to flow in metal parts of the apparatus.

Abstract: During known MRI measurements, for example spin echo and inversion recovery measurements, phase errors occur in pixels, inter alia due to eddy currents caused by the application or interruption of magnetic field gradients during a measurement. There is proposed an inversion recovery method combined with a method where the magnetization is not inverted in advance in order to determine for each pixel the phase error in an image obtained by means of an inversion recovery method, so that inversion recovery images can be obtained which do not contain phase errors.

Abstract: In known spin echo imaging examination a waiting period is customarily observed between two successive spin echo measurements in order to allow for the magnetic moment to relax in the direction of the uniform magnetic field Bo. In addition to this waiting period, the use of 180.degree. pulses also leads to a comparatively long examination time which increases the risk of image artefacts due to motions of, for example a patient to be examined. This comparatively long examination time is also unfavorable in view of the radiation load. In order to avoid these drawbacks, a method is proposed in which the electromagnetic pulses used are exclusively excitation pulses in the form of .alpha..degree. pulses, where 0<.alpha.<90.

Abstract: The invention relates to the determination of an NMR distribution in which an alternating gradient field is applied while sampling the NMR signal (FID signal, nuclear spin echo signal). The frequency of the alternating gradient field is comparatively low (order of magnitude of 100 Hz) and the field has from a few to some tens of cycles during each measurement period. While an FID signal is being sampled, the image frequency field matrix is scanned using a zig-zag (oscillating) line pattern during each line when data is provided for elements in from a few to some tens of rows in the image frequency matrix. By applying preparation gradient fields, the image frequency matrix can be filled by means of successive zig-zag line patterns which have been shifted with respect to one another and which thus enable a uniform sampling density to be provided in the image frequency space.