Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-03-01
2002-07-23
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000
Reexamination Certificate
active
06424152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a magnetic resonance imaging method.
2. Description of Related Art
A magnetic resonance imaging method is disclosed in the European patent application EP 0 526 983.
In order to generate a magnetic resonance image of an object to be examined, the object is placed in a steady, essentially spatially uniform magnetic field. This steady magnetic field to a certain extent aligns magnetic spins in the object in the direction of the magnetic field. In the known magnetic resonance imaging method a 90° RF excitation pulse is applied to excite magnetic spins in a selected part of an object to be examined, such as a patient to be medically examined. The part of the object in which the magnetic spins are excited is selected by way of a temporary selection gradient which is superimposed on the steady magnetic field. A 180° RF refocusing pulse is applied in order to generate magnetic resonance spin echo signals. The magnetic resonance spin echo signals are received by a receiver while a temporary read gradient is superimposed on the steady magnetic field. The read gradient is directed in a read direction. The temporary read gradient frequency-encodes the magnetic resonance spin echo signals in the read direction. Phase-encoding of the magnetic resonance spin echo signals in the phase-encoding direction is carried out by way of phase-encoding gradients superimposed on the steady magnetic field. The phase-encoding direction extends at right angles to the read direction. The phase-encoding gradient and the read gradient effect spatial encoding of the magnetic resonance spin echo signals. The magnetic resonance image is reconstructed from these spatially encoded magnetic resonance spin echo signals.
The known method concerns the avoidance of so-called out-of-slice artefacts in the magnetic resonance image. Such out-of-slice artefacts are mainly caused by imperfections of the 180° RF refocusing pulse. Such imperfections give rise to a spurious transverse magnetisation which leads to disturbing magnetic resonance signals. According to the known method the phase-encoding gradient is applied before the 180° RF refocusing pulse. Consequently, the disturbing magnetic resonance signals due to the imperfections of the 180° RF refocusing pulse are not phase-encoded so that in the magnetic resonance image the out-of-slice artefacts are collapsed into a single column or row of the magnetic resonance image. Although the known method is quite effective in avoiding the out-of-phase artefacts, the known method appears to be restricted to the reduction of corruptions of the image that are due to imperfections of the 180° RF refocusing pulse.
Furthermore, in the known method a predetermined phase-offset is applied to the 90° RF excitation pulse so that the phase of the excited magnetic spins is offset by the predetermined phase-offset. The receiver also changes the phase of the received magnetic resonance spin echo signal by the predetermined phase-offset. The phase of the 180° RF refocusing pulse is not offset. Consequently, the receiver offsets the phase of the spurious magnetic resonance signals due to imperfections of the 180° RF refocusing pulse by the predetermined phase-offset. The predetermined phase-offset of the magnetic resonance spin echo signals due to the 90° RF excitation is cancelled by the predetermined phase-offset of the receiver. Hence, the artefact that is collapsed into a single line is shifted to an edge of the image where it is the least annoying.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.
SUMMARY OF THE INVENTION
An object of the invention is to provide a magnetic resonance imaging method which is substantially insusceptible to disturbances due to spuriously generated magnetisation, such as the so-called ‘Eemland-artefact’.
This object is achieved by the magnetic resonance imaging method according to the invention wherein
a preparatory excitation of spins is applied,
subsequently a temporary magnetic compensation gradient is applied in a direction which opposes a phase-encoding direction,
subsequently an RF excitation pulse is generated,
a phase-encoding gradient is applied in the phase-encoding direction, and
magnetic resonance signals are received during an acquisition interval.
The temporary magnetic compensation gradient causes a pre-compensating phase-encoding of any spuriously excited spins, i.e. spuriously generated magnetization, in the object under examination. Such spuriously excited spins are, for example, generated in the preparatory excitation. The combined effect of the pre-compensating phase-encoding and the phase-encoding that is applied after the RF-excitation is that the spuriously excited spins have no phase-encoding after the phase-encoding gradient. Hence, these spuriously excited spins cannot give rise to phase-encoded magnetic resonance signals. Consequently, these spuriously excited magnetizations are prevented from causing serious image corruptions such as the ‘Eemland-artefact’. As the magnetic resonance signals due to such spuriously excited spins are effectively not phase-encoded, these magnetic resonance signals give rise to only small stripes in the magnetic resonance image that is reconstructed from the magnetic resonance signals. The reduction of image corruptions due to spuriously generated magnetization enhances the diagnostic quality of the magnetic resonance image in that small details in the object being imaged are faithfully rendered visible in the magnetic resonance image. According to the invention it is notably avoided that the magnetic resonance image contains spurious details which are not related to the anatomy of the patient being examined. It is to be noted that the magnetic resonance image forms a technical feature which is of assistance to a physician, for example an MRI-radiologist, to reach a diagnosis.
These and other aspects of the invention will be elaborated in more detail on the basis of the following embodiments.
Preferably, a predetermined phase-offset is applied to the RF-excitation pulse and a compensating phase-offset is applied to the receiver so as to shift the small stripes to an edge of the image where the corruption of the image by such small stripes hardly affects the diagnostic quality of the image. As an alternative, the stripe-like artefact may be shifted to the edge of the image during the reconstruction of the image from the magnetic resonance signals. Notably the RF-excitation and the receiver as well as the preparatory excitation involve respective predetermined phases. Thus, there is a definite phase-difference between the magnetic resonance signals due to the RF-excitation and spurious excitations due to the preparatory excitation. The spurious excitations cause a disturbance that according to the invention is not phase-encoded and hence is collapsed into a stripe. Owing to the definite phase-difference in the reconstruction of the image from the magnetic resonance signals the stripe can be shifted to the edge of the image.
The magnetic resonance imaging method of the invention is particularly effective in mitigating artefacts due to the application of so-called REST slabs (Regional Saturation Technique). Such REST slabs are often applied to avoid perturbation due to motion of a portion of the object to be examined. REST involves preparatory excitation and subsequent dephasing of spins in a selected region. Magnetic spins in the selected region are excited by way of a preparatory RF-excitation pulse and a preparatory selection gradient. Such a selected region is notably a region of the object where motion is expected to occur during magnetic resonance imaging. Usually this portion is indicated as a REST slab. Subsequently, the excited magnetic spins are dephased, notably by a dephasing gradient field. Because the magnetic spins in the REST slab are dephased to a large exten
Peeren Gerardus N.
Prins Willem M.
Fetzner Tiffany A.
Koninklijke Philips Electronics , N.V.
Vodopia John
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