PatentDe  


Dokumentenidentifikation EP1231477 29.11.2007
EP-Veröffentlichungsnummer 0001231477
Titel Bilderzeugung durch magnetische Resonanz
Anmelder Philips Medical Systems MR Technologies Finland Oy, Vantaa, FI
Erfinder Kinanen, Ilmari, 02210 Espoo, FI;
Perko, Panu Oskari, 02760 Espoo, FI
Vertreter derzeit kein Vertreter bestellt
DE-Aktenzeichen 60222952
Vertragsstaaten AT, BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LI, LU, MC, NL, PT, SE, TR
Sprache des Dokument EN
EP-Anmeldetag 12.02.2002
EP-Aktenzeichen 022509590
EP-Offenlegungsdatum 14.08.2002
EP date of grant 17.10.2007
Veröffentlichungstag im Patentblatt 29.11.2007
IPC-Hauptklasse G01R 33/389(2006.01)A, F, I, 20051017, B, H, EP

Beschreibung[en]

The present invention relates to magnetic resonance imaging. It finds particular application in conjunction with open MRI systems with a C-shaped flux return path and will be described with particular reference thereto. It will be appreciated, however, that the present invention is useful in conjunction with other open systems, such as systems with H-shaped flux return paths, four poster arrangements, no ferrous flux return path, and the like, and is not limited to the aforementioned application.

In magnetic resonance imaging, a uniform main magnetic field typically denoted Bo is created through an examination region in which a subject to be examined is disposed. The resonance frequency in the field is determined by the field strength and the gyromagnetic ratio of dipoles to be resonated. With open magnetic resonance systems, the main magnetic field is typically vertical, perpendicular to the subject between upper and lower poles. A series of radio frequency (RF) pulses at frequencies coordinated to the resonance frequency are applied to two RF coils, one adjacent each pole, to excite and manipulate magnetic resonance. Gradient magnetic fields are conventionally produced by gradient coils to alter the resonance frequency in a preselected relationship with spatial position. The gradient coils are typically mounted between the RF coils and the poles. The magnetic resonance signals are detected with the two RF coils or localized coils and processed to generate two or three dimensional image representations of a portion of the subject in the examination region.

After pulses are applied by the RF coils, the excited dipoles resonate, that is, they decay back to the state of lowest energy. This is done at a characteristic frequency called the Larmor frequency. The Larmor frequency is a function of the total field strength, i.e., the sum of the main magnetic field and the superimposed gradient field. Thus, when the field strength varies, so does the Larmor frequency. When the field strength varies only with the applied gradients, the accurate calibration of field strength to spatial position within the examination region results in accurate images. However, when the field strength varies due to other, uncalibrated causes, ghosting and other artifacts in the final images result. When the field strength oscillates, the position of anatomical structures oscillate in the resultant image causing ghosting artifacts.

In open magnet systems, the poles are a set distance apart. This distance, along with the current strength in the superconducting magnet and other factors determine the strength of the main magnetic field in the examination region. If this distance changes, the Bo field strength changes causing problems with imaging as discussed previously.

In an open system having a 0.5 m opening, and a 0.23 T main field strength, a change of 1 ppm (part per million) in the aperture causes a 1 ppm change in the field strength, subsequently changing the Larmor frequency. Thus, a 0.5 µm variation in the aperture varies the Larmor frequency by approximately 10 Hz. This is enough of a variance to cause ghosting in the final images. In higher field magnets, the frequency shift is significantly worse.

The aperture of an open system may change from any number of reasons. In a typical C-magnet system as described previously, an acceleration of only one thousandth of earth gravity produces a 1 ppm change in the aperture. Reasons such as people walking in the examination room or adjacent rooms, slamming doors, trucks in the street, and seismic activity can cause variations of this order of magnitude and higher. The acoustic reverberations of gradient and RF activity also cause vibration in the distance between the poles.

Previously, dampeners, such as rubber pads under the pole and flux return path, have been used to dampen environmental vibration forces. For example, the United States patent US6774633B2 discusses a vertically-aligned open MRI magnet system that includes first and second (i.e., top and bottom) assemblies each having a longitudinally-extending and vertically-aligned axis, a superconductive main coil, and a vacuum enclosure enclosing the main coil. At least one support beam has a first end attached to the first assembly and has a second end attached to the second assembly. A vibration isolation system supports the magnet system, for example, a rubber mat or rubber blocks placed between the magnet system and the floor (Col. 5, Ln. 19-22). Such dampeners were effective for eliminating higher frequency components of vibrations, but lower frequency vibrations in the range of 5-20 Hz were less attenuated. A further disadvantage of using soft material to isolate vibrations is that the magnet is not supported in a firm position and it may shift from the original intended position.

Another method used is active vibration cancellation. These systems are massive and expensive. Typically large mechanical drivers are mounted under the pole and flux return path assembly. Environmental vibration is sensed and converted into counteracting physical movement. In this way, the actuators strive to create equal and opposite cancelling vibrations. In spite of the expense, the ability of these systems to cancel vibrational movement is limited. For example, the United States patent US 6169404 B1 discusses a vibration cancellation system for an MR imager, wherein a superconducting C-shaped magnet includes a vibration sensor at a remote end of separated pole pieces to provide a vibration signal that is phase-inverted and converted to an opposing force by a magnetostrictive actuator connected across the support member between the opposite ends of the pole pieces, to cancel vibrations which would otherwise adversely affect the homogeneity of the magnetic field in an imaging region of the magnet.

In accordance with one aspect of the present invention, a magnetic resonance apparatus is given an imaging region which is defined between upper and lower poles through which a main magnetic field is generated. A gradient coil assembly superimposes magnetic field gradients on the main magnetic field. A radio frequency coil assembly excites magnetic resonance in selected dipoles of a subject disposed in the imaging region. A reconstruction processor reconstructs received magnetic resonance signals into image representations. A force transducer is placed under the lower pole assembly to measure vibrations in the magnetic resonance apparatus.

According to another aspect of the present invention, a method of magnetic resonance imaging is provided. A main magnetic field is induced through an examination region between a pair of pole assemblies. A subject is in the examination region. Magnetic resonance is excited, spatially encoded and received from selected dipoles within the subject. The signals are processed into a human readable form. Vibrations that alter the distance between pole assemblies are measured.

One advantage of the present invention is that it reduces imaging artifacts.

Another advantage of the present invention is that it provides images with sharp contrast.

Another advantage of the present invention is that it provides a more uniform and stable main magnetic field.

Another advantage is that it offers improved stability to an MR system.

Ways of carrying out the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which:

  • FIGURE 1 is a diagrammatic illustration of a magnetic resonance imaging system in accordance with the present invention.

With reference to FIGURE 1, in an open MRI system, an imaging region 10 is defined between an upper pole assembly 12 and a lower pole assembly 14. A pair of magnetic flux sources are disposed adjacent to upper and lower pole pieces 16,18 generating a temporally constant, main magnetic field Bo through the imaging region 10. It is to be appreciated that the open MRI apparatus may have a variety of pole pieces or, in some instances, no pole pieces at all. The magnets for generating the main magnetic field can be positioned at other locations. A ferrous flux return path 20 is provided between the pole assemblies remote from the imaging region 10. The flux return path in the preferred embodiment is a C-shaped path that supports the upper pole assembly. Alternately, the flux return path 20 could be an H-shaped arrangement, a four-poster arrangement, embedded in the walls, or the like. The use of a pair of pole pieces with no defined flux path, just the ambient surroundings through which flux returns, is also contemplated.

For imaging, magnetic field gradient coils 22, 24 are disposed on opposite sides of the imaging region 10 adjacent the pole pieces 16, 18. In the preferred embodiment, the gradient coils are planar coil constructions which are connected by gradient amplifiers 26 to a gradient magnetic field controller 28. The gradient magnetic field controller 28 causes current pulses which are applied to the gradient coils 22, 24 such that gradient magnetic fields are superimposed on the temporally constant and uniform field Bo across the imaging region 10. The gradients of the fields aligned with the main field are typically oriented along a longitudinal or y-axis, a vertical or z-axis and a transverse or x-axis.

For exciting magnetic resonance in selected nuclei, an upper radio frequency coil 30 and a lower radio frequency coil 32 are disposed between the gradient coils 22, 24 adjacent the imaging region 10. The coils 30,32 generate narrow spectrum RF frequency magnetic fields in a band around a selected resonance frequency, typically denoted B1, within the imaging region. The coils 30, 32 are connected to one or more RF transmitters 34 that transmits pulses designated by an RF pulse controller 36. RF screens are disposed between the RF coils 30, 32 and the gradient coils 22, 24 to minimize the generation of RF eddy currents in the gradient coils 24, 26. The RF coils 30, 32 transmit B1 magnetic field pulses into the imaging region.

A sequence controller 40 accesses a sequence memory 42 to withdraw one or more RF and gradient pulse imaging sequences, which are implemented by the gradient controller 28, and the RF pulse controller 36 in a coordinated relationship. Typically, the sequence controller 40 causes the pulse controller and the RF transmitter to transmit pulses into the imaging region commensurate with the application. That is, different sequences are designed to illustrate different features of the subject.

In applications in which the radio frequency coils 30, 32 operate in both transmit and receive modes, magnetic resonance signals are picked up by the radio frequency coils 30, 32. The resonance signals are demodulated by one or more receivers 50, preferably digital receivers. The digitized signals are processed by a reconstruction processor 52 into volumetric or other image representations which are stored in a volumetric image memory 54. A video processor 56, under operator control, withdraws selected image data from the volume memory and formats it into appropriate data for display on a human readable display 58, such as a video monitor, active matrix monitor, liquid crystal display, or the like.

In order to sense environmental disturbances that result in the vibration of the floor or any other support structure of the magnet assembly, a force transducer 60 is disposed underneath the lower pole assembly 14 between the lower pole assembly and the floor in the preferred embodiment. Preferred sensors have accurate, readily anticipated responses to frequencies in the 2-70 Hz range. The force transducer is preferably a strain gauge type transducer, and has approximately the same compressibility of hard rubber shoes previously used to dampen vibrations, which compressibility dampens vibrations in the 20-70 Hz range. Alternately, piezoelectric discs can be used. The force transducer produces an output voltage waveform at least in the 5-20 Hz frequency range indicative of any vertical compressions experienced by the pole assemblies. The output waveform is processed by a vibration analyzer 62 which determines the corresponding changes in the distance between the upper pole assembly 12 and the lower pole assembly 14 due to the vibration, the attractions in the main Bo field due to the changes, and ultimately the corrections to compensate. Preferably, the signal may be processed by a digital signal processor to have a high degree of control intelligence before being fed to the vibration analyzer 62. In the preferred embodiment, higher frequency vibrations are dampened by the transducer and do not affect the stability of the system. Frequencies below 2 Hz are filtered by high pass filters because their effect on the magnet is not significant. The effect of the voltage waveform components in the range of 5-20 Hz on the Bo field is measured during initial calibration and appropriate corrections are calculated.

In one preferred embodiment, the analysis of the vibration waveform is used to correct the Bo field strength. As the MR assembly is vibrated up and down, the massive upper pole assembly 12 has such inertia that the interpole spacing between pole pieces 16, 18 expands and contracts. This variance causes the strength of the main field to vary. As the pole pieces come closer together, the field strengthens. Conversely, as they move apart, the field weakens. Given the vibration waveform from the transducer 60, a variance of the distance with time is found by measurement during design and set up. Mathematically it is relatively simple to take an instantaneous &Dgr;d of the gap between pole pieces and convert it into a &Dgr;Bo. Look up tables are also contemplated. At least one and preferably a pair of shim coils 70, 72 controlled by a shim coil control 74 produces a magnetic field to counteract the calculated change in the magnetic field due to vibrations. For example, an activity waveform of the shim coil is inverted and scaled in comparison to the vibration waveform. The resultant effect of both the vibrations and shim coil counteract, resulting in a temporally constant main field.

In a second preferred embodiment, the vibration waveform is used to adjust the spatial encoding gradient pulses. The resonant frequency is a function of the field strength. In one common mode, the RF pulse has a frequency which excites a whole slice or slab at the location where the sum of a slice select gradient and the Bo field has a preselected strength. Subsequent gradient fields vary the main field to shift the frequencies of the resonating dipoles to preselected frequencies at each spatial increment. However, when vibrations vary the main Bo field, the spatial location at which expected resonant frequencies occur are shifted. The receiver 50 correlates frequency of received signals with spatial position in the read gradient dimension. Thus, if dipoles are resonating at a shifted frequency under the read gradient, the reconstruction processor 52 assigns a shifted spatial position in that read direction. Thus, if the Bo field oscillates or varies with time, a ghosted, blurred image results.

In this second embodiment, the vibration analyzer 62 uses the vibration waveform to variably shift at least the frequency sensitivity of the receiver, as ghosted in FIGURE 1 to hold the total magnetic field constant at each spatial location along the read direction constant. Preferably, a local oscillator/synthesizer 80 generates an RF pulse modulated by the vibration waveform such that the receiver signal is substantially unaffected by the vibrations. This eliminates read direction ghosting.

In a third preferred embodiment, the sensed vibrations or oscillations are correlated to oscillating shifts in the resonance frequency and phase. The reconstruction processor 52 is programmed to alter the phase encoding of the signal corresponding to the vibration induced force changes. Optionally, a correction is made to spatial location as well as corresponding to frequency effects in the read direction. The transmitter and receiver may be adjusted, as necessary to transmit and demodulate in a frequency spectrum corresponding to the shifting resonance frequencies across the examination region.

In an alternate embodiment, multiple force transducers are disposed underneath the MR assembly. Their contributions are mathematically weighted according to their position. This embodiment is useful for non-uniform variations in the main magnetic field. If the distance between the sides of the pole pieces move less than the other sides, non-uniform changes in the Bo field can be sensed and corrected. Optionally, a hinge 90 supports the back of the magnet assembly, while the front of the magnet assembly under the pole is supported by the force transducer 60.

In another alternate embodiment, one or more force transducers adjacent to a firm support or accelerometers are placed on the vertical portion of the flux return path to measure horizontal displacement. The vibration analyzing processor 62 generates analogous corrections to those discussed above for horizontal vibration induced Bo field variations.


Anspruch[de]
Magnetresonanz-Bildgebungsgerät, das Folgendes umfasst: eine obere Polbaugruppe (12) und eine untere Polbaugruppe (14), die zwischen sich eine Bildgebungsregion (10) definieren, durch die ein Hauptmagnetfeld verläuft, wobei das Hauptmagnetfeld im Betrieb Stärkeschwankungen in Reaktion auf Schwankungen des Abstands zwischen den Polbaugruppen (12, 14) erfährt; eine Gradientenspulenbaugruppe (22, 24), um dem Hauptmagnetfeld codierte Magnetfeldgradienten zu überlagern; ein Hochfrequenzmittel (30, 32) zum Anregen von Magnetresonanz in ausgewählten Dipolen eines in der Bildgebungsregion (10) angeordneten Subjekts, wobei die Magnetresonanz mit den Schwankungen der Magnetfeldstärke schwankt; ein Rekonstruktionsprozessormittel (52) zum Rekonstruieren der empfangenen Magnetresonanz zu einer Bilddarstellung des Subjekts in der Bildgebungsregion (10); mindestens ein Schwingungserfassungsmittel (60) in mechanischer Verbindung mit mindestens einer der Polbaugruppen (12, 14) zum Messen von Schwingungen in dem Magnetresonanzgerät, und ein Schwingungsanalysemittel (62) zum Analysieren von Signalen von dem Schwingungserfassungsmittel (60), dadurch gekennzeichnet, dass das Schwingungsanalysemittel konfiguriert ist, , um, basierend auf den gemessenen Schwingungen, eine Magnetfeld-Shimspule und/oder einen Hauptoszillator, der mindestens eine Hochfrequenz steuert, und/oder einen Magnetfeldgradienten zu steuern, um ihrer Auswirkung auf das Magnetresonanzgerät entgegenzuwirken Magnetresonanzgerät nach Anspruch 1 , wobei das Schwingungsanalysemittel (62) die Magnetfeld-Shimspule (70) steuert, um ein Magnetfeld zu erzeugen, das den durch Schwingungen verursachten Magnetfeldveränderungen entgegenwirkt. Magnetresonanzgerät nach Anspruch 1, wobei das Schwingungsanalysemittel (62) Korrektursignale erzeugt, um die Betriebsfrequenz eines Hauptoszillators zu verändern, der mindestens eine Empfängerfrequenz steuert, um Fehlern entgegenzuwirken, die den durch die gemessenen Schwingungen verursachten Hauptmagnetfeldveränderungen zuzurechnen sind. Magnetresonanzgerät nach Anspruch 1, wobei das Schwingungsanalysemittel (62) eine Phasencodierung entsprechend modifiziert, um Effekte von schwingungsinduzierter Feldmodulation bei der Phasencodierung aufzuheben. Magnetresonanzgerät nach einem der Ansprüche 1 bis 4, das weiterhin Folgendes umfasst: einen Rückflusspfad (20); und ein zweites Schwingungserfassungsmittel, das mit dem Rückflusspfad verbunden ist, um horizontale Schwingungskomponenten zu messen. Magnetresonanzgerät nach einem der Ansprüche 1 bis 5, wobei das mindestens eine Schwingungserfassungsmittel (60) einen Kraftaufnehmer vom Dehnungsmessgerättyp umfasst. Magnetresonanzgerät nach einem der Ansprüche 1 bis 6, das weiterhin Folgendes umfasst: ein Gelenkteil (90), das unter einem Teil eines Rückflusspfades angeordnet ist. Magnetresonanzgerät nach einem der Ansprüche 1 bis 7, das weiterhin Folgendes umfasst: ein Mittel zum Dämpfen von Schwingungen über 70 Hz. Magnetresonanzgerät nach einem der Ansprüche 1 bis 8, das weiterhin Folgendes umfasst: ein Gradientenfeldmittel (22, 24, 26, 28), um dem Hauptmagnetfeld Gradientenmagnetfelder zu überlagern. Magnetresonanzgerät nach Anspruch 2, wobei das Schwingungsanalysemittel (62) dafür vorgesehen ist, Schwankungen zu kompensieren durch mindestens entweder: Steuern von Shimspulen (70, 72) zur Erzeugung von schwankenden Shim-Magnetfeldern zwischen den Polbaugruppen (12, 14), die die Hauptmagnetfeldschwankungen aufheben; Anpassen eines Gradientenfeldmittels (22, 24, 26, 28); oder Anpassen des Rekonstruktionsprozessormittels (52). Verfahren zur Magnetresonanzbildgebung, das Folgendes umfasst: Induzieren eines Hauptmagnetfeldes durch eine Untersuchungsregion (10) zwischen einem Paar in einem Abstand voneinander angeordneten Polbaugruppen (12, 14), wobei ein Subjekt in der Untersuchungsregion (10) aufgenommen wurde; Anregen von Magnetresonanz in ausgewählten Dipolen des Subjekts; räumliches Codieren des Hauptmagnetfelds mit Gradientenfeldern; Messen von Schwingungen, die einen Abstand zwischen den Polbaugruppen (12, 14) verändern; Empfangen und Demodulieren von Magnetresonanzsignalen von den in Resonanz schwingenden Dipolen; Verarbeiten der empfangenen Resonanzsignale zu einer von Menschen lesbaren Form, dadurch gekennzeichnet, dass das Verfahren weiterhin das Anpassen des Hauptmagnetfelds und/oder eines Magnetgradientenfelds und/oder eines Hochfrequenzfelds basierend auf den gemessenen Schwingungen umfasst. Verfahren nach Anspruch 11, das weiterhin Folgendes umfasst: Erzeugen eines Schwingungsmodells, das den gemessenen Schwingungen entspricht; und Kompensieren der Schwingungen durch Anpassen einer Frequenz eines Hauptoszillators basierend auf dem Schwingungsmodell. Verfahren nach Anspruch 12, das weiterhin Folgendes umfasst: Anpassen des Frequenzcodiergradienten zum Kompensieren der gemessenen Schwingungen basierend auf dem Schwingungsmodell; und Kompensieren der Schwingungen durch Anpassen eines Phasencodiergradienten, um die gemessenen Schwingungen basierend auf dem Vibrationsmodell zu kompensieren. Verfahren nach Anspruch 12 oder 13, das weiterhin Folgendes umfasst: Kompensieren der Schwingungen basierend auf dem Schwingungsmodell durch Anpassen einer Stärke des Hauptmagnetfelds. Verfahren nach einem der Ansprüche 12 bis 14, wobei der Schritt des Kompensierens der Schwingungen Folgendes umfasst: Einführen einer Shimspule (70) zum Erzeugen von Feldern, die den Feldschwankungen, welche den Veränderungen des Abstands zwischen den Polbaugruppen (12,14) aufgrund von Schwingungen zuzuschreiben sind, ungefähr entsprechen und ihnen entgegengesetzt sind. Verfahren nach einem der Ansprüche 11 bis 15, das weiterhin Folgendes umfasst: Dämpfen eines Teils der Schwingungen. Verfahren nach einem der Ansprüche 11 bis 16, das weiterhin Folgendes umfasst: Messen von horizontalen Schwingungen und Dämpfen eines Teil der horizontalen Schwingungen. Verfahren nach einem der Ansprüche 11 bis 17, wobei der Schritt der Schwingungsmessung Folgendes umfasst: Messen von Schwingungen mit Schwingungsfrequenzen von mehr als 2 Hz und weniger als 70 Hz. Verfahren nach einem der Ansprüche 11 bis 18, das weiterhin Folgendes umfasst: Dämpfen von Schwingungen mit Schwingungsfrequenzen über 70 Hz.
Anspruch[en]
A magnetic resonance apparatus comprising: an upper pole assembly (12) and a lower pole assembly (14) defining an imaging region (10) therebetween through which a main magnetic field extends, the main magnetic field undergoing strength fluctuations in response to fluctuations in the distance between the pole assemblies (12, 14) in use; a gradient coil assembly (12, 14) for superimposing encoded magnetic field gradients upon the main magnetic field; a radio frequency means (30, 32) for exciting magnetic resonance in selected dipoles of a subject disposed in the imaging region (10), which magnetic resonance fluctuates with the main field strength fluctuations; a reconstruction processor means (52) for reconstructing received magnetic resonance into an image representation of the subject in the imaging region (10); at least one vibration sensing means (60) in mechanical connection with at least one of the pole assemblies (12, 14) for measuring vibrations in the magnetic resonance apparatus, and a vibration analyzer means (62) for analyzing signals from the vibration sensing means (60), characterized in that the vibration analyzer means is configured to control one or more of a magnetic field shim coil, a main oscillator controlling at least a radio-frequency and a magnetic field gradient, based on the measured vibrations to counteract their effect on the magnetic resonance apparatus. Magnetic resonance apparatus as claimed in claim 1, wherein the vibration analyzer means (62) controls the magnetic field shim coil (70) to generate a magnetic field that counteracts magnetic field variations caused by the vibrations. Magnetic resonance apparatus as claimed in claim 1, wherein the vibration analyzer means (62) generates correction signals to change the operating frequency of a main oscillator controlling at least a receiver frequency to counteract errors attributable to variations in the main magnetic field caused by the measured vibrations. Magnetic resonance apparatus as claimed in claim 1, wherein the vibration analyzer means (62) modifies a phase encoding accordingly to cancel effects of vibration induced field modulation in phase encoding. Magnetic resonance apparatus as claimed in any one of claims 1 to 4, further including: a flux return path (20); and a second vibration sensing means connected with the flux return path for measuring horizontal vibration components. Magnetic resonance apparatus as claimed in any one of claims 1 to 5, wherein the at least one vibration sensing means (60) includes a strain gauge type force transducer. Magnetic resonance apparatus as claimed in any one of claims 1 to 6, further including: a hinge (90) disposed under a portion of a flux return path. Magnetic resonance apparatus as claimed in any one of claims 1 to 7, further including: a means for damping vibrations above 70 Hz. Magnetic resonance apparatus as claimed in any one of claims 1 to 8, further including: a gradient field means (22, 24, 26, 28) for superimposing gradient magnetic fields on the main magnetic field. Magnetic resonance apparatus as claimed in claim 2, wherein the vibration analyzer means (62) is arranged to compensate for the fluctuations by at least one of: controlling shim coils (70, 72) to generate fluctuating shim magnetic fields between the pole assemblies (12,14) which cancel the main magnetic field fluctuations; adjusting a gradient field means (22, 24, 26, 28); and adjusting the reconstruction processor means (52). A method of magnetic resonance imaging comprising: inducing a main magnetic field through an examination region (10) between a pair of spaced pole assemblies (12, 14), a subject being received in the examination region (10); exciting magnetic resonance in selected dipoles of the subject; spatially encoding the main magnetic field with gradient fields; measuring vibrations that alter a spacing between the pole assemblies (12, 14); receiving and demodulating magnetic resonance signals from the resonating dipoles; processing the received resonance signals into a human readable form, characterized in that the method further comprises adjusting one or more of the main magnetic field, a magnetic gradient field and a radio-frequency field based on the measured vibrations. A method as claimed in claim 11, further including: producing a vibration model corresponding to the measured vibrations; and compensating for the vibrations by adjusting a frequency of a main oscillator based on the vibration model. A method as claimed in claim 12, further including: adjusting the frequency encode gradient to compensate for the measured vibrations based on the vibration model; and compensating for the vibrations by adjusting a phase encode gradient to compensate for the measured vibrations based on the vibration model. A method as claimed in claim 12 or claim 13, further including: compensating for the vibrations based on the vibration model by adjusting a strength of the main magnetic field. A method as claimed in any one of claims 12 to 14, wherein the step of compensating for the vibrations includes: inducing a shim coil (70) to produce fields approximately equal and opposite to field fluctuations attributable to the alterations in the spacing between the pole assemblies (12, 14) caused by the vibrations. A method as claimed in any one of claims 11 to 15, further including: damping a portion of the vibrations. A method as claimed in any one of claims 11 to 16, further including: measuring horizontal vibrations and dampening a portion of the horizontal vibrations. A method as claimed in any one of claims 11 to 17, wherein the vibration measuring step includes: measuring vibrations with vibrational frequencies greater than 2 Hz and less than 70 Hz. A method as claimed in any one of claims 11 to 18, further including: damping vibrations with vibrational frequencies above 70 Hz.
Anspruch[fr]
Appareil à résonance magnétique comprenant: un assemblage formant pôle supérieur (12) et un assemblage formant pôle inférieur (14) délimitant entre eux une région d'imagerie (10) à travers laquelle s'étend un champ magnétique principal, le champ magnétique principal subissant des fluctuations d'intensité en réaction à des fluctuations de la distance séparant les assemblages formant pôle (12, 14) en service; un assemblage de bobines de gradients (22, 24) pour superposer des gradients de champ magnétique codés sur le champ magnétique principal; des moyens à radiofréquence (30, 32) pour exciter une résonance magnétique dans des dipôles sélectionnés d'un sujet placé dans la région d'imagerie (10), cette résonance magnétique fluctuant avec les fluctuations de l'intensité du champ magnétique principal; un moyen à processeur de reconstruction (52) pour reconstruire la résonance magnétique reçue en une représentation par une image du sujet dans la région d'imagerie (10); au moins un moyen de détection de vibrations (60) en liaison mécanique avec au moins un des assemblages formant pôle (12, 14) pour mesurer des vibrations dans l'appareil à résonance magnétique, et un moyen analyseur de vibrations (62) pour analyser des signaux provenant du moyen de détection de vibrations (60), caractérisé en ce que le moyen analyseur de vibrations est configuré pour régir un ou plusieurs des éléments suivants: une bobine de correction de champ magnétique, un oscillateur principal commandant au moins une radiofréquence et un gradient de champ magnétique, sur la base des vibrations mesurées, afin de contrecarrer leur effet sur l'appareil à résonance magnétique. Appareil à résonance magnétique suivant la revendication 1, dans lequel le moyen analyseur de vibrations (62) commande la bobine de correction de champ magnétique (70) afin de générer un champ magnétique qui contrecarre les variations de champ magnétique causées par les vibrations. Appareil à résonance magnétique suivant la revendication 1, dans lequel le moyen analyseur de vibrations (62) génère des signaux de correction pour modifier la fréquence de fonctionnement d'un oscillateur principal commandant au moins une fréquence de récepteur afin de contrecarrer des erreurs pouvant être attribuées à des variations dans le champ magnétique principal causées par les vibrations mesurées. Appareil à résonance magnétique suivant la revendication 1, dans lequel le moyen analyseur de vibrations (62) modifie un codage de phase de manière correspondante pour annuler les effets d'une modulation de champ induite par les vibrations dans le codage de phase. Appareil à résonance magnétique suivant l'une quelconque des revendications 1 à 4, comprenant en outre: un trajet de retour de flux (20); et un second moyen de détection de vibrations connecté au trajet de retour de flux pour mesurer les composantes de vibrations horizontales. Appareil à résonance magnétique suivant l'une quelconque des revendications 1 à 5, dans lequel ledit au moins un moyen de détection de vibrations (60) comprend un transducteur de force du type à jauge de contrainte. Appareil à résonance magnétique suivant l'une quelconque des revendications 1 à 6, comprenant en outre: une articulation (90) disposée en dessous d'une partie d'un trajet de retour de flux. Appareil à résonance magnétique suivant l'une quelconque des revendications 1 à 7, comprenant en outre: un moyen pour amortir des vibrations au-dessus de 70 Hz. Appareil à résonance magnétique suivant l'une quelconque des revendications 1 à 8, comprenant en outre: des moyens à champs de gradients (22, 24, 26, 28) destinés à superposer des champs magnétiques de gradients sur le champ magnétique principal. Appareil à résonance magnétique suivant la revendication 2, dans lequel le moyen analyseur de vibrations (62) est destiné à compenser les fluctuations par au moins l'une des actions suivantes: commander les bobines de correction (70, 72) pour générer des champs magnétiques de correction fluctuants entre les assemblages formant pôle (12, 14), qui annulent les fluctuations du champ magnétique principal; régler un moyen à champ de gradient (22, 24, 26, 28); et régler le moyen à processeur de reconstruction (52). Procédé d'imagerie par résonance magnétique comprenant: l'induction d'un champ magnétique principal à travers une région d'examen (10) entre une paire d'assemblages formant pôle espacés (12, 14), un sujet étant reçu dans la région d'examen (10); l'excitation de la résonance magnétique dans des dipôles sélectionnés du sujet; le codage spatial du champ magnétique principal avec des champs de gradients; la mesure de vibrations qui modifient un espacement entre les assemblages formant pôle (12, 14); la réception et la démodulation de signaux de résonance magnétique provenant des dipôles résonnants; le traitement des signaux de résonance reçus en une forme lisible par l'homme, caractérisé en ce que le procédé comprend en outre le réglage d'un ou de plusieurs des champs suivants: le champ magnétique principal, un champ de gradient magnétique et un champ de radiofréquence sur la base des vibrations mesurées. Procédé suivant la revendication 11, comprenant en outre: la production d'un modèle de vibrations correspondant aux vibrations mesurées; et la compensation des vibrations par réglage d'une fréquence d'un oscillateur principal sur la base du modèle de vibrations. Procédé suivant la revendication 12, comprenant en outre: le réglage du gradient de codage de fréquence afin de compenser les vibrations mesurées sur la base du modèle de vibrations; et la compensation des vibrations par réglage d'un gradient de codage de phase afin de compenser les vibrations mesurées sur la base du modèle de vibrations. Procédé suivant la revendication 12 ou 13, comprenant en outre: la compensation des vibrations sur la base du modèle de vibrations par réglage d'une intensité du champ magnétique principal. Procédé suivant l'une quelconque des revendications 12 à 14, dans lequel l'étape de compensation des vibrations comprend: le fait d'induire une bobine de correction (70) à produire des champs approximativement égaux et opposés à des fluctuations de champ pouvant être attribuées aux altérations de l'espacement entre les assemblages formant pôle (12, 14) causées par les vibrations. Procédé suivant l'une quelconque des revendications 11 à 15, comprenant en outre: l'amortissement d'une partie des vibrations. Procédé suivant l'une quelconque des revendications 11 à 16, comprenant en outre: la mesure des vibrations horizontales et l'amortissement d'une partie des vibrations horizontales. Procédé suivant l'une quelconque des revendications 11 à 17, dans lequel l'étape de mesure des vibrations comprend: la mesure de vibrations dont les fréquences vibratoires sont supérieures à 2 Hz et inférieures à 70 Hz. Procédé suivant l'une quelconque des revendications 11 à 18, comprenant en outre; l'amortissement de vibrations dont les fréquences vibratoires sont supérieures à 70 Hz.






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