U.S. Philips Corporation

Gerrit H Van Yperen

Peter Van Der Meulen

Miha Fuderer

Johan Samuel Van Den Brink

Johannes Petrus Groen

Johannes J Cuppen

Antoon F Mehlkopf

Hans Gerard Jenniskens

Michael Jozef Duijvestijn

Antoon F Mehlkope

The invention relates to a method for imaging diffusion parameters of a body by means of magnetic resonance. The method involves an adjustment pulse sequence and an imaging pulse sequence. The adjustment pulse sequence measures preparation MR signals. The imaging pulse sequence measures position-dependent MR signals. The imaging pulse sequence is adjusted on the basis of information from the preparation MR signals measured during the adjustment pulse sequence.

Magnetic resonance method and device

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

In an MRI device (1) operating according to a spin-echo method, switched gradient magnetic fields are applied in the form of slice selection (231-233), phase encoding (243-243, 243'243') and read gradients (252-253). 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 (7) to be examined and causing artefacts in a reconstructed image. Another source of disturbance may be phase-distortion in the RF amplifier. By modifying a gradient (251, 231') in between the excitation pulse (221) and the first refocusing pulse to (222) in the spin-echo sequence and/or a change in phase of the RF-pulses, the effects of the disturbances can largely be compensated for. The additional gradient size is adjusted by measuring the position in time and the relative phase of spin-echo signals (162, 163) in a preparatory sequence (121-173).

Magnetic resonance imaging method and apparatus employing eddy current

In a magnetic resonance imaging method multiple refocusing RF-pulses (22-26) are applied and spin-echo signals (62-66) obtained following an excitation RF-pulse (21). Imperfections in the applied RF-pulses (21-26) and the switched gradient magnetic fields (31-36, 42-46, 51-56) cause deviations from the desired phases of the nuclear dipole moments which appear as artefacts or ghosts in a resulting image. By a suitable arrangement of the application of the phase encoding values (242-248; 342-348) imposed during the measurements and/or the addition of supplemental measurements, the effects of the phase deviations in an image are significantly reduced, resulting in images in which less artefacts and ghosts are visible. Reduction of artefacts is also obtained by weighting of measured NMR-signals.

Method and apparatus for magnetic resonance imaging

In a magnetic resonance imaging method multiple refocusing RF-pulses (22-26) are applied and spin-echo signals (62-66) obtained following an excitation RF-pulse (21). Sequences are executed twice, in time-reversed order. Both measurements are used in the reconstruction of an image. This results in images in which artefacts and blurring due to T.sub.2 -decay is reduced.

In a spin-echo magnetic resonance sequence a phase encoding gradient magnetic field (39) is applied after the 180.degree. rephasing pulse (32.sub.1). After detection of the spin-echo signal (33) the position dependent phases are compensated for by applying a further gradient magnetic field (39'), identical in size but opposite in sign. The phase difference (.phi..sub.32,1 -.phi..sub.31,1) between the RF-pulses (31.sub.1,32.sub.1) applied within a sequence is constant over the sequences. With no position dependent effects left at the end of a sequence the next sequence can be started immediately following the earlier one. A repetition time TR substantially shorter than the spin-spin relaxation time T.sub.2 is feasible, thereby developing a steady state of the magnetization. A TR of 50 ms or less can be obtained, as well as strong signals for long T.sub.2 substances and good T.sub.2 contrast. RF spoiling by changing phases of RF-pulses in subsequent sequences can destroy the T.sub.2 signal and provide images with pure T.sub.1 contrast.

Method and apparatus for rapid spin echo magnetic resonance imaging

In magnetic resonance imaging various error sources lead to deterioration of the image quality. One class of errors is formed by errors which vary only slowly over the time required to sample a data line but vary substantially over the time required to acquire data for the complete magnetic resonance image. These error sources are, for example external magnetic fields, motion due to respiration, drift in amplifiers or drift phenomena in permanent magnets due to temperature influences. It is proposed to utilize mutually intersecting data lines in the Fourier domain so as to estimate these error sources and to use these estimates to correct the data sets obtained before execution of image reconstruction.

Magnetic resonance imaging method and device for reducing image errors

The known magnetic resonance imaging method of determining the r.f. power or pulse angle of an r.f. electromagnetic 90.degree.-pulse or 180.degree.-pulse has the drawback that the measurement inaccuracy is often high, that the experimentation time is comparatively long and that its use is limited to r.f. electromagnetic 90.degree.-pulses or 180.degree.-pulses only. In order to mitigate these drawbacks, it is proposed to determine the pulse angle of an r.f. electromagnetic pulse having a pulse angle .alpha. from two out of five echo resonance signals, including one stimulated echo resonance signal, generated by means of three r.f. electromagnetic .alpha..degree.-pulses, .alpha. having an arbitrary value (calibration phase), after which any .beta. can be adjusted by adaptation of the applied r.f. power.

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Eindhoven, Netherlands

Gerrit H Van Yperen