U.S. Philips Corporation

Antoon F Mehlkopf

Johannes H Den Boef

Johannes Petrus Groen

Peter Van Der Meulen

Gerrit H Van Yperen

Willem Marten Prins

Cornelis L Ham

Rolf J Lioen

A magnetic resonance apparatus includes a magnet system (1) for generating a steady magnetic field in a measuring space (35), a gradient coil system (3) for generating gradient fields in the measuring space, and a power supply source (7) for the gradient coils (3) which comprises at least one gradient signal generator (9) and a number of gradient amplifiers (11), each of which is connected between an output of the gradient signal generator and at least one of the gradient coils. In order to cancel annoying noise caused by the gradient coils (3), the apparatus also includes a noise cancellation device which includes the following elements: a) a first filter device (39) having a number of inputs which corresponds to the number of gradient amplifiers (11), each of the inputs being connected to an output of the gradient signal generator (9), and also includes a corresponding number of outputs, the first filter device having transfer functions, all of which correspond approximately to the transfer function of one of the chains extending from the gradient signal generator (9), via one of the gradient amplifiers (11), the gradient coil (coils) (3) connected to this gradient amplifier, and air, to a region (40, 42) which is referred to as a region of silence and in which the sound of the gradient coils (3) is to be cancelled; b) an adder circuit (51), having inputs which are connected to outputs of the first filter device (39) and an output (63) which is connected to an input of a sound reproducing device which comprises at least one sound reproducer (57) arranged in or near the region of silence (40); and c) a signal delay device (41) for inducing a predetermined time difference between the signal transfer from the gradient signal generator (9) to the gradient coils (3) on the one hand and the signal transfer from the gradient signal generator to the first filter device (39) on the other hand.

Magnetic resonance apparatus with noise cancellation

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.

Eddy current compensation in magnetic resonance imaging

A magnetic resonance imaging method is known for the reduction of image errors in magnetic resonance images which are caused by quantization errors in a DAC (26) for generating a phase encoding gradient waveform in an MRI measuring sequence (ms1, ms2) for generating magnetic resonance signals (S(t), S.sub.1 (t)). The known method has the drawback that inter alia the effect of differential non-linearity of the DAC (26) on the image errors is not taken into account. The invention proposes an MRI method in which such image errors are also reduced. To achieve this, prior to and/or during application of the measuring sequences (ms1, ms2), deviations of phase encoding gradient areas relative to desired areas are determined and image data in an image matrix derived from the resonance signals are corrected on the basis of said deviations.

Magnetic resonance imaging method and device for reducing image errors

An MRI apparatus is proposed with an optimally adjusted gain of the detection chain, said MRI apparatus being used for generating pulse and gradient sequences which comprise an excitation pulse having a predetermined excitation angle and a phase encoding gradient. In order to obtain a data set wherefrom an MR image is reconstructed, the pulse and gradient sequence is repeated a number of times, for example 128 times, while varying the value of the time integral of the phase encoding gradient, for lower profile numbers there being chosen an excitation angle which is smaller or greater than the predetermined excitation angle in order to prevent overdriving of the detection chain. The detection chain of such an apparatus is optimally used for obtaining a data set for an MR image.

MRI apparatus having an optimally adjusted detection chain and an

An MRI method and device for fast determination of the transverse relaxation time constant T2 utilizes the fact that this time constant introduces asymmetry into all magnetic resonance echo signals (::exp(-t/T2) enabling determination of the time constant T2 from the asymmetry of the echo signals. During the determination of a spin density distribution the value of the time constant T2 can be determined for each pixel. After Fourier transformation of the resonance signal, integration of the imaginary terms is performed (in the coordinate direction of the measuring gradient). The quotient of the current integral and the real term of the associated pixel is an accurate measure of the time constant T2 in the relevant pixel.

MRI method and device for fast determination of the transverse

A magnetic resonance imaging device and method are disclosed in which, during acquisition of measuring data the receiver gain is switched over in dependence of the signal strength, so that in principle every data set of the various measuring cycles has an optimum signal to noise ratio.

Magnetic resonance device with a selectable gain signal amplifier

The invention relates to a magnetic resonance imaging device comprising a transmitter/receiver which, with the exception of a few components is all digital. The transmitter comprises a digital frequency synthesizer, a single sideband modulator and a selective power amplifier, the single sideband modulator being controlled by a phase locked loop oscillator. The receiver comprises a selective pre-amplifier, a frequency mixing stage which is connected either to an output of the phase locked loop oscillator or to an output of the single sideband modulator, and an analog-to-digital converter which samples the output signal of the frequency mixing stage. The demodulation frequency for the receiver is chosen so that, after demodulation, a frequency band is obtained which is situated to one side of 0 Hz.

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Eindhoven, Netherlands

Antoon F Mehlkopf