Abstract:
In an open magnetic resonance system, pole pieces ( 16, 18 ) on opposite sides of an imaging region ( 10 ) are supported by a ferrous flux return path ( 20 ). During normal operation, environmental disturbances that cause low frequency vertical vibrations in the floor cause corresponding fluctuations in the main field of the magnet. A spacing between the pole pieces ( 16, 18 ) expands and contracts with the vibration causing a strength of the main field to fluctuate. A force transducer ( 60 ) under the magnet assembly measures the magnitude of the vibrations. A vibration analyzer ( 62 ) analyzes the vibrations waveform and calculates compensation for the changes in the interpole spacing. The vibration analyzer ( 62 ) controls one or more of a shim coil ( 70 ) which produces a field equal and opposite to the main field fluctuations, a reconstruction processor ( 52 ) to compensate for magnetic field resonance frequency fluctuations attributable to the main field fluctuations, and gradient magnetic fields applied by gradient field coils ( 22, 24 ).

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the diagnostic imaging arts. 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 B 0  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 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 main B 0  field strength oscillates, the position of anatomical structures oscillate in the resultant image causing ghosting and other 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 B 0  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 (1 mg) 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. 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. I this way, the actuators strive to create equal and opposite canceling vibrations. In spite of the expense, the ability of these systems to cancel vibrational movement is limited. 
     The present invention provides a new and improved method and apparatus that overcomes the above referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a magnetic resonance apparatus is given an imaging region 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 a more limited aspect of the present invention, a vibration analyzer amplifies and processes signals to affect an operating frequency of a main oscillator to counteract the vibrations. 
     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. 
     According to another aspect of the present invention, a magnetic resonance apparatus is provided. A substantially constant magnetic field is generated by a magnet between two pole assemblies, the field fluctuating due to an inconsistent distance between pole assemblies. A radio frequency transmitter induces resonance in selected dipoles that resonate at a resonance frequency, the frequency fluctuating as the main field fluctuates. A radio frequency receiver receives and demodulates the emitted resonance signals. A vibration sensor is connected with at least one of the pole assemblies. A vibration analyzing processor analyzes the sensed vibrations and determines compensations. 
     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. 
     Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a magnetic resonance imaging system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 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 B 0  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, imbedded 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 B 0  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 magnetic fields in a band around a selected resonance frequency, typically denoted B 1 , 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 B 1 , 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 B 0  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 B 0  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 B 0  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 Δd of the gap between pole pieces and convert it into a ΔB 0 . 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 B 0  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 B 0  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 B 0  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 FIG. 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 B 0  field can be sensed and corrected. Optionally, a hinge  80  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  68  generates analogous corrections to those discussed above for horizontal vibration induced B 0  field variations. 
     The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.