Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-06-15
2001-05-08
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000, C324S307000
Reexamination Certificate
active
06229309
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an MR method with a sequence which is repeated several times and during which the nuclear magnetization in an examination zone is excited in the presence of a uniform, steady magnetic field 2, after which an MR signal is received from the examination zone and phase errors occur due to concomitant gradients in the time interval between the excitation of the nuclear magnetization and the reception of the MR signal. The invention also relates to a device for carrying out the method.
2. Description of the Invention
From WO 98 02 757 it is known that MR methods involve so-called concomitant gradients; concomitant gradients are to be understood to mean the gradients of the magnetic fields which extend perpendicularly to the direction of the steady magnetic field. As a consequence of Maxwell's field equations they inevitably occur together with the desired gradients of the magnetic field components extending in the direction of the steady field. Such concomitant gradients may cause image artifacts when slices are examined whose planes do not extend perpendicularly to the direction of the uniform, steady magnetic field (coronal, sagittal, or oblique slices) or when the slices are situated outside the iso-center of an MRI apparatus.
The influence exerted on the image quality by the concomitant gradients is dependent on their magnitude in relation to the steady magnetic field. In the case of strong steady magnetic fields (0.5 Tesla or more), the effect of the concomitant gradients on the image quality generally will hardly be noticeable. However, in the case of lower strengths of the steady magnetic field, for example 15 mT as occurring in the so-called Overhauser imaging methods, their negative effect on the image quality is very pronounced.
In order to compensate the phase errors caused by the concomitant gradients and affecting the image quality, the known MR apparatus utilizes five additional coils whereby correction magnetic fields are generated. These additional coils are controlled by means of pulses which must be adapted to the relevant MR pulse sequence. This solution requires a comparatively large amount of additional means.
Another solution, which is known from Proc. SMRM, London 1985, 1037-103 8, proposes to split the prephasing pulse for the read-out gradients and the phase encoding pulse into two parts wherebetween a 180° RF pulse acts on the examination zone. The phase errors are then zero at the instant at which the read-out gradient is started. It is a drawback of this method that it is necessary to use a 180° RF pulse in each sequence.
SUMMARY OF THE INVENTION
It is an object of the present invention to reduce the effect of the concomitant gradients on the image quality while using few additional means and without using additional 180° RF pulses. In a method of the kind set forth this object is achieved in that the temporal variation of the magnetic fields acting in the examination zone during the time interval between the excitation of the nuclear magnetization and the reception of the MR signals is such that the spatial distribution of the phase error upon reception of the MR signal is at least substantially the same for all sequences (in this context and hereinafter a sequence is to be understood to mean the single excitation of the nuclear magnetization and the subsequent reception of an MR signal).
The invention is based on the consideration that the MR image formed from the MR signals exhibits blurring when the strength of the magnetic field, and hence the phase error, in the examination zone varies from one sequence to another in the time interval between the excitation of the nuclear magnetization and the reception of the MR signals in the individual sequences, said variation being due to the spatial encoding required for imaging. When a temporal variation is imposed on the overall magnetic field acting on the examination zone during the time interval such that the distribution of the phase error is at least approximately equally large for all sequences, the MR image reconstructed from the MR signals then received will no longer contain blurring.
It is true that the phase error can again vary in time due to the concomitant gradients acting subsequent to said time interval, i.e. during the reception of the MR signal, but this variation is the same for all MR signals. This causes merely distortions in the MR image. Whereas the blurring cannot be eliminated at a later stage (i.e. during or after the reconstruction), if necessary the distortion of the MR image can be corrected at a later stage.
A preferred version concerns an approach which can be simply implemented in practice. Preferred versions of this approach are characterized by a bipolar pulse of the gradient magnetic field. Use is made of the fact that the phase encoding is linearly dependent on the time integral over the phase encoding gradients, whereas the phase error is dependent on the time integral of the square of the gradient. The temporal variation of the gradient can thus be configured in such a way that on the one hand the necessary phase encoding is achieved and on the other hand the phase error that is time integral of the square of the gradient, is equally large for all sequences.
A bipolar pulse of this gradient as used in the preferred versions, is to be understood to mean a gradient whose polarity changes from a positive to a negative value (during the time interval). Bipolar pulses of gradients are known per se, for example from DE-PS 40 04 185. According to the known method, however, the bipolar gradient pulses are used for flux compensation. The disturbing effects of the concomitant gradients cannot be corrected by means of the known pulse sequence, inter alia because they have the same temporal variation in all sequences.
In a more preferred embodiment, magnetic field gradients are applied in a phase encoding direction in the time between exciting nuclear magnetization and receiving generated MR signals, wherein the applied gradient comprises an additional bipolar pulse of the gradient magnetic field generated before and/or after a phase-encoding gradient pulse, the time integral over the bipolar pulse being zero, and the time integral over the square of the sum of the phase-encoding gradient pulse and the bipolar pulse being the same for all sequences.
In another more preferred embodiment, magnetic field gradient are similarly applied in phase encoding direction in the time between exciting nuclear magnetization and receiving generated MR signals, wherein the applied gradient is a single bipolar pulse which is modified from one sequence to another and, which has a temporal variation chosen to be such that the time integral over the bipolar pulse of the gradient magnetic field corresponds to the phase encoding required for the relevant sequence, and the time integral over the square of the bipolar pulse of the gradient magnetic field is equally large for all sequences.
It is advantageous if the phase encoding gradient and the readout gradient do not overlap during the time interval.
This invention also is directed to an MR apparatus including a main field magnet, gradient coil system, an RF coil system, a receiver, and a control unit for controlling these elements to perform the methods according to the invention.
REFERENCES:
patent: 4794337 (1988-12-01), Twieg
patent: 5034692 (1991-07-01), Laub et al.
patent: 5929638 (1999-07-01), Aldefeld et al.
patent: 6043656 (2000-03-01), Ma et al.
patent: 6087831 (2000-07-01), Bornert et al.
patent: 4004185 (1990-11-01), None
patent: WO9802757 (1998-01-01), None
“Phase Errors in NMR Images” by D.G. Norris (Univeristy of Aberdeen), in SMRM 1985, pp. 1037-1038.
Fetzner Tiffany A.
Oda Christine
U.S. Philips Corporation
Vodopia John F.
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