Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1998-02-27
2001-01-09
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06172502
ABSTRACT:
The invention relates to a method of forming a magnetic resonance image of a part of a body situated in a static magnetic field by:
a. generating a magnetic resonance signal by means of an excitation RF pulse and gradients, in which
b. a measuring set of signal values is obtained by application of a read-out gradient in a first direction with an alternating polarity and further gradients on the magnetic field and the simultaneous measurement of the generated magnetic resonance signal at measuring points along substantially parallel lines in the k-space, and in which a phase correction of the measuring set is determined by measuring a reference set of signal values,
c. determining an image by transformation and correction of phase errors of the measuring set of signal values.
The invention also relates to a magnetic resonance device for performing this method of forming magnetic resonance images of objects.
A method of the kind set forth is known from EP-A-490528. In the cited Application the excitation RF pulse is understood to be an RF pulse for exciting nuclear spins in a part of the body. The gradients are to be understood to mean additional, temporary magnetic gradient fields whose respective directions extend perpendicularly to one another. One of said gradients is the read-out gradient which is oriented in the first direction. Furthermore, in an Echo-Planar-Imaging (EPI) pulse sequence the read-out gradient has an alternating polarity. The part of the read-out gradient between two polarity reversals is referred to as a lobe of the read-out gradient. A lobe of positive polarity is called a positive lobe and a lobe of negative polarity a negative lobe. Furthermore, one of the other said gradients, whose direction extends transversely of the direction of the read-out gradient, is referred to as a phase encoding gradient. This phase encoding gradient is used for phase encoding of the magnetic resonance signal. Furthermore, the k-space in the cited Application designates a spatial frequency domain in which the magnetic resonance signal is measured along a trajectory and the measured values yield the inverse Fourier transform of the image. The trajectory in the k-space is determined by the time integral of the applied gradients over the time interval from the excitation pulse until the measuring instant.
If the time is too short to measure an adequate number of signal values, the generating of magnetic resonance signals can be repeated by means of additional RF pulses and gradients for the measurement of further signal values.
The phase of magnetic resonance signals is influenced by delays in the RF receiver, the system generating the gradient, and time constants upon the switching of the read-out gradient. After Fourier transformation, the phase errors thus caused become manifest as image artefacts. In order to determine phase errors, the known method measures signal values for a reference set by generating a magnetic resonance signal by means of a pulse sequence which is identical, except for the absence of a phase encoding gradient, to the pulse sequence used to generate the magnetic resonance signal for measuring the signal values of the measuring set. Subsequently, a phase correction is derived from the reference set. Upon reconstruction of an image, the transformed signal values of the measuring set for positive and/or negative lobes of the read gradient are corrected by the phase correction derived from the reference set. Finally, an image is derived by further transformation. It is a drawback of this method that the phase corrections determined may have been influenced by local frequency deviations due to field inhomogeneities or chemical shifts, so that they cannot be corrected in the described manner because the position-dependency of the local frequency deviations is not fully known from the one-dimensionally encoded reference set of resonance signals.
It is inter alia an object of the invention to provide a method which counteracts the contribution of local frequency deviations to the phase correction. To this end, the method according to the invention is characterized in that a first and a second series of signal values of the reference set are measured with opposite polarity of the read-out gradient and at substantially corresponding instants, relative to an instant at which phase errors due to local frequency deviations are zero, the phase corrections being derived from the first and the second series of signal values. The information for executing said phase corrections is obtained by using, during measurement of corresponding signal values, read-out gradient lobes of opposite polarity at corresponding instants for the two series of signal values of the reference set, and by determining the phase difference between the corresponding signal values. As a result of this step, the phase error contributions caused by local frequency deviations are equal for corresponding measurements of the two series of signal values, and hence do not contribute to the phase difference. As a result, the determination of the phase error due to delays in the RF receiver and time constants in systems for generating the gradients, necessary for the correction methods to be used, becomes more accurate. The method of the invention offers the advantage that the phase error is insusceptible to local frequency deviations caused by field inhomogeneities and chemical shifts. A further advantage consists in that other methods of counteracting the effects of local frequency deviations on the images derived can be used in combination with said step, for example a method as mentioned in “Interference between echo time shifting and correction scans in multi-shot EPI and GRASE pulse sequences”, by J. P. Mugler, published in Abstracts SMR 1995, page 758. The cited article describes a method in which a time shift of the read-out gradient is used in a GRASE method and a multi-shot EPI method.
A special version of the method according to the invention is characterized in that the read-out gradients exhibit the same variation in time but are of opposite polarity during the measurement of the two series of signal values of the reference set. The read-out gradient for measuring the second series of the reference set can thus be simply derived from the read-out gradient used for measuring the first series of the reference set.
A further version of the method according to the invention is characterized in that, except for a time shift, the read-out gradients are the same as a function of time during the measurement of the two series of signal values of the reference set. This step is taken preferably if, due to the reversal of the polarity of the read-out gradient, phase differences occur in the two series of signal values other than those for which the correction is suitable. This effect occurs if upon switching of the read-out gradient not only the effects to be corrected, having short time constants with respect to a lobe of the read-out gradient, occur but also effects with a longer time constant with respect to the duration of a lobe of the read-out gradient. In the latter case the effects do not offer a suitable estimate as regards the phase differences between the transformed signals for the positive and the negative lobes of the read-out gradient.
A further version of the method of the invention is characterized in that the reference set is measured in at least one two-dimensional sub-space of the k-space. This step is used preferably when the phase errors to be corrected also have a position-dependency in a direction perpendicular to the direction of the read-out gradient. The advantage of this step resides in the fact that the reference set then also contains information concerning said phase errors with a position-dependency in a direction perpendicular to the read-out gradient. This method is known per se from EP-A-644437.
A further version of the invention is characterized in that a refocusing RF pulse is applied subsequent to the excitation RF pulse. In a refocused magnetic resonance signal m
Groen Johannes P.
Van Ensbergen Gerald
Van Muiswinkel Arianne M. C.
Arana Louis
U.S. Philips Corporation
Vodopia John F.
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