METHOD TO REDUCE ARTEFACTS IN THE MAGNETIC RESONANCE IMAGE DUE TO SPURIOUS MAGNETIC SIGNALS

Abstract: In a magnetic resonance imaging method first a preparatory excitation of spins is applied and subsequently a temporary magnetic compensation gradient is applied in a direction opposite a phase-encoding direction. Thereafter an RF-excitation pulse is generated and a phase-encoding gradient is applied in the phase-encode direction. Finally magnetic resonance signals are received during an acquisition interval. Artefacts in the magnetic resonance image due to magnetisation spuriously generated by the preparatory excitation are avoided.

Description: METHOD TO REDUCE ARTEFACTS IN THE MAGNETIC RESONANCE IMAGE DUE TO SPURIOUS MAGNETIC SIGNALS The invention relates to a magnetic resonance imaging method. 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 static essentially spatially uniform magnetic field. This static magnetic field to a certain extent aligns magnetic spins in the object along 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 static magnetic field. In order to generate magnetic resonance spin echo signals a 180° RF refocusing pulse is applied. The magnetic resonance spin echo signals are received by a receiver while a temporary read-gradient is superimposed on the static magnetic field. The read-gradient is directed along a read-direction. The temporary read-gradient frequency-encodes the magnetic resonance spin echo signals along the read-direction. By way of phase-encoding gradients superimposed on the static magnetic field phase-encoding of the magnetic resonance spin echo signals along the phase-encoding direction is carried out. The phase-encoding direction is 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 avoiding so-called out-of-slice artefacts in the magnetic resonance image. These out-of-slice artefacts are mainly caused by imperfections of the 180° RF refocusing pulse. Such imperfections give rise to a spurious transverse magnetisation from which disturbing magnetic resonance signals are generated. 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. Further, 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 changes also 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 off-set. 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. An object of the invention is to provide a magnetic resonance imaging method which is substantially insensitive to disturbances due to spuriously generated magnetisation, such as the so-called Εemland-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 opposite a phase-encoding direction, - subsequently an RF-excitation pulse is generated, - a phase-encoding gradient is applied in the phase-encode 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 magnetisation, in the object under examination. These 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, it is avoided that these spuriously excited magnetisations cause serious image corruptions, such as the Εemland-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 magnetisation improves the diagnostic quality of the magnetic resonance image in that small details in the object which is imaged are faithfully rendered visible in the magnetic resonance image. Notably, according to the invention it is avoided that in the magnetic resonance image spurious details occur which are not related to the anatomy of the patient who is examined. It is to be noted that the magnetic resonance image forms a technical feature which is of assistance to a physician, e.g. 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 which are defined in the depending Claims. 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 does hardly affect the diagnostic quality of the image. As an alternative, the stripe-formed artefact may be shifted to the edge of the image in 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 is according to the invention not phase-encoded and thus 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. In particular, the magnetic resonance imaging method of the invention is 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 a preparatory excitation and subsequent dephasing of spins in a selected region. By way of a preparatory RF-excitation pulse and a preparatory selection gradient, magnetic spins in the selected region are excited. In particular, this selected region is a region of the object where motion is expected to occur during the 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 to a large extent dephased, the magnetic spins in the REST-slab hardly emit magnetic resonance signals. However, it has appeared that in some practical situations, the dephasing of the magnetic spins in the portion selected by the preparatory excitation and the preparatory selection gradient, i.e. the magnetic spins in the REST-slab, is not complete so that inadvertently magnetic resonance signals are generated from the REST-slab. For example, spurious residual in-phase magnetisation may be generated in the REST-slab owing to imperfections in the spatial uniformity of the static magnetic field. This residual in-phase magnetisation in the REST-slab gives rise to the so-called Εemland'- artefact if no steps are taken. Another example of a preparatory excitation which, if no steps are taken, may cause artefacts in the magnetic resonance image is the so-called SPLR pre-pulse (SPectral Inversion Recovery) in which for example spins and fat or water are selectively excited. Further, the preparatory excitation may be an MTC (Magnetisation Transfer Contrast) pre- pulse. Preferably, the time-integral of the gradient fields in the phase-encoding direction over the interval starting after the preparatory excitation until a point in time within the acquisition interval equals a predetermined net gradient. The predetermined net gradient has the same value for all phase-encoding steps, in other words in the (reciprocal) k-space the predetermined net gradient is independent of k y , where k y is the component of the wavevector corresponding to the phase-encoding direction. For example, the predetermined net gradient equals zero. Usually, the sequence starting with the preparatory sequence until the acquisition of the magnetic resonance signals is repeated. In practice such a repetition is carried out very often in order to acquire sufficient magnetic resonance signals for reconstructing the magnetic resonance image with a high diagnostic quality. For example the sequence is repeated 128 or 256 times and the magnetic resonance signal is sampled for 128 or 256 (frequency) samples in the read direction. Owing to said time-integral of the gradients in the phase-encode direction being equal in all repetitions of the sequence, the spuriously generated magnetisation remains non phase-encoded over the repetitions of the sequence. Hence, according to the invention it is made possible to move the artefacts due to spuriously generated magnetisations which are reduced to stripes to the edge of the image by the same phase-shift for all repetitions of the sequence. Thus, reduction of the artefacts into small stripes at the edge of the magnetic resonance image is achieved in a simple way. The invention further pertains to a magnetic resonance imaging system. The magnetic resonance imaging system according to the invention as defined in Claim 5 is capable of performing the magnetic resonance imaging method of the invention which achieves a substantial reduction of artefacts due to spuriously generated magnetisation e.g. due to preparatory excitations. It is noted that the functions of the magnetic resonance imaging system are preferably carried out by a suitably programmed computer. The invention also relates to a computer-programme for performing the magnetic resonance imaging method of the invention. The computer-programme according to the invention as defined in Claim 6. Advantageously, the magnetic resonance imaging system includes a computer into which the computer-programme according to the invention is loaded. These and other aspects of the invention will be elucidated with reference to the embodiments described hereinafter and with reference to the accompanying drawing; therein: Figure 1 is a diagrammatic representation of a magnetic resonance system according to the invention and Figure 2 is a graphic representation of a sequence of excitations and temporary gradients of the magnetic resonance imaging method of the invention. Figure 1 is a diagrammatic representation of a magnetic resonance system according to the invention. The magnetic resonance imaging system includes a main magnet 10 which generates a stationary magnetic field in an examination space 11 in which a part of the patient 30 to be examined is arranged. The stationary magnetic field generated by the main magnet in the examination space is to a large extent spatially uniform. The magnetic resonance imaging system also includes gradient coils 12. The gradient coils 12 generate spatial magnetic field gradients that are superimposed on the stationary magnetic field. Furthermore, the magnetic resonance imaging system includes a transmitter coil 13 and a receiver coil 13. Often, the same coil or antenna is employed alternately as the transmitter coil and the receiver coil. This transmitter and receiver coil 13 is coupled to an electronic transmitter and receiver circuit 15. The patient 30 to be examined is arranged on a patient table 14 and moved into the bore of the main magnet in such an manner that the part of the body to be examined is situated approximately at the centre of the bore. Using the gradient coils a selection gradient is applied to select e.g. a thin slice in the examination space. Subsequently, the transmitter coil emits an RF electromagnetic excitation pulse whereby the spins in the body of the patient and in the selected slice are excited. The excited spins relax to their equilibrium states and emit RF magnetic resonance signals which are received by the receiver coil. The RF magnetic resonance signals are for example generated as gradient echo signals or as spin echo signals or they may also be free-induction decay signals. A reconstruction unit 25 derives an image signal (IS) representing an image of e.g. a cross-section of the body of the patient to be examined from the magnetic resonance signals (RFS). The gradient coils and the transmitter coil are also used to carry out the preparatory excitation such as the REST-pulse. After the excitation of the spins in the selected slice, the gradient coils 12 apply a phase-encoding gradient field, for a predetermined period of time; this phase-encoding gradient field renders the phase of the precession of the magnetisation due to the excited spins location-dependent in a phase-encoding direction in the selected slice. By way of the read gradient field also applied in a read-direction after the phase-encoding gradients gradient echo signals are generated. The precession frequencies of the excited magnetisation in the selected slice are rendered location-dependent in the read-direction. Using inter alia Fourier analysis of the magnetic resonance signals, an image, being the magnetic resonance image, of a cross- section in the selected slice of the patient can thus be reconstructed. The gradient coils 12 are energised by means of a power supply 21 whereto the various gradient coils 12 are coupled. The power supply 21 is controlled by a front-end controller 20. The function of the front-end controller is carried out, for example, by a suitably programmed computer or processor. The front-end controller 20 also tunes the transmitter and receiver circuit 15 .Furthermore, the transmitter and receiver circuit 15 is coupled to a modulator 22. The modulator 22 and the transmitter and receiver circuit 15 activate the transmitter coil 13 so as to transmit the desired (RF) excitation pulses. The receiver coil is coupled to a pre-amplifier 23. The pre-amplifier 23 amplifiers the magnetic resonance signal (RFS) and the amplified signal is applied to a demodulator 24. The demodulator demodulates the amplified signal. The demodulated signal from the demodulator contains the actual information concerning the local spin densities in the selected slice. Using Fourier transformation and inverse Radon transformation, the reconstruction unit 25 derives the image signal (IS), which represents image information of the selected slice. The image of the selected slice is displayed on a monitor 26; to this end the image signal (IS) is applied to the monitor. The image signal (IS) is also stored in a buffer unit 27, while awaiting further processing or printing as a hard-copy. Figure 2 is a graphic representation of a sequence of excitations and temporary gradients of the magnetic resonance imaging method of the invention. Figure 2 schematically indicates the excitation pulses and temporary gradients in various (phase-encode and read) directions on a single time axis. First, the preparatory excitation PE is generated which is for example accompanied by dephasing magnetic gradient fields (not shown). The RF excitation RFE is emitted later than the preparatory excitations. After the preparatory pulse PE, and notably before the RF excitation RFE the compensation magnetic gradient CG is applied in the phase-encode direction. After the RF excitation RFE a phase-encoding gradient PEG is — applied which performs a phase-encoding of the magnetisation generated by the RF excitation RFE. Any inadvertently generated spurious magnetisation due to the preparatory excitation is not phase-encoded due to the combined actions of the compensation gradient CG and the phase-encoding gradient PEG. The magnetic resonance signals RFS are acquired during the acquisition interval AQ while the read-gradient RG is applied. The example shown is a sequence of the FFE-type (fast field echo). Only one gradient echo is shown in Figure 2, but multiple read-gradients may be successively applied so as to generate multiple gradient echoes. It is noted that the invention may also advantageously applied in magnetic resonance imaging methods which make use of (multiple) spin-echo sequences The read-gradient RG is implemented as a bipolar gradient-pair so as to avoid dephasing of the spins due to the read gradient field.

Claims: CLAIMS: 1. A magnetic resonance imaging method wherein - a preparatory excitation of spins is applied, - subsequently a temporary magnetic compensation gradient is applied in a direction opposite a phase-encoding direction, - subsequently an RF-excitation pulse is generated, - a phase-encoding gradient is applied in the phase-encode direction and - magnetic resonance signals are received during an acquisition interval. 2. A magnetic resonance imaging method as claimed in Claim 1, wherein - the preparatory excitation includes that spins in a selected region are excited and dephased. 3. A magnetic resonance imaging method as claimed in Claim 1 wherein - the time-integral of the temporary magnetic compensation gradient and the phase-encoding gradient over the time interval from the preparatory excitation until a point of time in the acquisition interval essentially equals a predetermined net integrated gradient. 4. A magnetic resonance imaging method as claimed in Claim 3, wherein - the predetermined net integrated gradient is equal to zero. 5. A magnetic resonance imaging system arranged to - apply a preparatory excitation of spins, - subsequently apply a temporary magnetic compensation gradient in a direction opposite a phase-encoding direction, - subsequently generate an RF-excitation pulse, - apply a phase-encoding gradient and - receive magnetic resonance signals during an acquisition interval. 6. A computer-programme including instructions to - apply a preparatory excitation of spins, - subsequently apply a temporary magnetic compensation gradient in a direction opposite a phase-encoding direction, - subsequently generate an RF-excitation pulse, - apply a phase-encoding gradient and - receive magnetic resonance signals during an acquisition interval.

Inventors: PEEREN GERARDUS N, PRINS WILLEM M, PEEREN, GERARDUS, N., PRINS, WILLEM, M.
Applicants: KONINKL PHILIPS ELECTRONICS NV, PEEREN GERARDUS N, PHILIPS SVENSKA AB, PRINS WILLEM M, KONINKLIJKE PHILIPS ELECTRONICS N.V., PEEREN, GERARDUS, N., PHILIPS AB, PRINS, WILLEM, M.
Application year: 1999
Main classification: G01R33
IPC codes: A61B5, G01R33