Abstract:
Vibration of the magnet assembly in a MRI system perturbs the magnetic field and adversely affects the images produced by the system. The perturbation of the magnetic field can be sensed by an apparatus having a source which emits a first signal that is reflected off the magnet assembly. A receiver detects a second signal formed by the reflected portion of the first signal. The first and second signals are processed to produce an output signal representing movement of the electromagnet and thus variation of the magnetic field. That output signal can be used by to components of the magnetic resonance imaging system to compensate for the effects that the vibration has on the images.

Description:
BACKGROUND OF THE INVENTION 
     The field of the invention is magnetic resonance imaging, and in particular the invention relates to sensing variation of the B 0  polarizing magnetic field resulting from vibration of the imaging apparatus. 
     Any nucleus, which possesses a magnetic moment, attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency, known as the Larmor frequency, which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant γ of the nucleus). Nuclei which exhibit this phenomena are referred to herein as “spins”. 
     When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M z  is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. However, if the substance or tissue is subjected to a magnetic field (excitation field B 1 ) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M t , which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B 1  is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomena is exploited. 
     When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. 
     To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G x , G y and G z ) which have the same direction as the polarizing field B 0 , but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified. 
     MRI is particularly useful as a medical diagnostic tool. However, the ability to create detailed images which clearly depict anatomical features of the patient, depends upon an extremely stable polarizing magnetic field B 0 . Mechanical vibration of the MRI system perturbs the polarizing magnetic field, thereby producing artifacts in the resultant magnetic resonance images. If the real-time displacement of the magnetic components could be accurately measured, then mathematical models could be used to estimate and correct the magnetic field variation due to that displacement. For whole body magnets, displacements on the order of a micron generate magnetic field changes of approximately one part per million. Thus a motion sensor for artifact correction must be capable of detecting submicron displacements. 
     The obvious approach to measuring the vibration would be to sense the variation of the polarizing magnetic field. However, the imaging system produces other magnetic fields, which vary at radio frequencies and thus can adversely affect the ability to sense changes in the polarizing magnetic field. 
     SUMMARY OF THE INVENTION 
     The present invention provides technique for detecting minute vibration of a magnet of a magnetic resonance imaging system which results in perturbation of the magnetic field. That technique generates a first signal having a predefined frequency X which is directed toward part of the magnetic resonance imaging system which is physically connected to the magnet. The first signal may be selected from among the microwave, ultrasound, sound and light spectra, for example. The signal also may originate from a resonant radio frequency coil whose properties change with displacement. 
     A portion of the first signal gets reflected by that part and then is received as a second signal. The first and second signals are processed to produce a output signal which represents movement of the electromagnet. That output signal can be employed in compensating for effects that the vibration has on the magnetic field and ultimately in images produced by the magnetic resonance imaging system. 
     In the preferred embodiment, the first and second signals are mixed together to produce an resultant signal that then is low-pass filtered. The base-band signal produced by the filtering than is applied to a quadrature detector which produces the output signal representing movement of the electromagnet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an MRI system which incorporates the present invention; and 
     FIG. 2 is a block diagram depicting the present apparatus for measuring displacement of the magnet assembly in that MRI system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, there is shown the major components of a preferred magnetic resonance imaging (MRI) system  100  which incorporates the present invention. The operation of the MRI system  100  is controlled from an operator console  101  which includes a keyboard and control panel  102  and a display  104 . The console  101  communicates through a link  116  with a separate computer system  107  that enables an operator to control the production and display of images on the screen  104 . The computer system  107  includes a number of modules which communicate with each other through a backplane. These include an image processor module  106 , a CPU module  108  and a memory module  113 , known in the art as a frame buffer for storing image data arrays. The computer system  107  is linked to a disk storage  111  and a tape drive  112  for storage of image data and programs, and it communicates with a separate system control  122  through a high speed serial link  115 . 
     The system control  122  includes a set of modules connected together by a backplane. These include a CPU module  119  and a pulse generator module  121  which connects to the operator console  101  through a serial link  125 . It is through this link  125  that the system control  122  receives commands from the operator which indicate the scan sequence that is to be performed. The pulse generator module  121  operates the system components to carry out the desired scan sequence. It produces data which indicates the timing, strength and shape of the RF pulses which are to be produced, and the timing of and length of the data acquisition window. The pulse generator module  121  connects to a set of gradient amplifiers  127 , to indicate the timing and shape of the gradient pulses to be produced during the scan. The pulse generator module  121  connects to a scan room interface circuit  133  which receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  133  that a patient positioning system  134  receives commands to move the patient to the desired position for the scan. 
     The gradient waveforms produced by the pulse generator module  121  are applied to a gradient amplifier system  127  comprised of G x , G y  and G z  amplifiers. Each gradient amplifier excites a corresponding gradient coil in an assembly generally designated  139  to produce the magnetic field gradients used for position encoding acquired signals. The gradient coil assembly  139  forms part of a magnet assembly  141  which includes a polarizing magnet  140  for the B 0  field and a whole-body RF coil  152 . A transceiver module  150  in the system control  122  produces pulses which are amplified by an RF amplifier  151  and coupled to the RF coil  152  by a transmit/receive switch  154 . The resulting signals radiated by the excited nuclei in the patient may be sensed by the same RF coil  152  and coupled through the transmit/receive switch  154  to a preamplifier  153 . The amplified NMR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  150 . The transmit/receive switch  154  is controlled by a signal from the pulse generator module  121  to electrically connect the RF amplifier  151  to the coil  152  during the transmit mode and to connect the preamplifier  153  during the receive mode. The transmit/receive switch  154  also enables a separate RF coil (for example, a head coil or surface coil) to be used in either the transmit or receive mode. 
     The NMR signals picked up by the RF coil  152  are digitized by the transceiver module  150  and transferred to a memory module  160  in the system control  122 . When the scan is completed and an entire array of data has been acquired in the memory module  160 , an array processor  161  operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  115  to the computer system  107  where it is stored in the disk memory  111 . In response to commands received from the operator console  101 , this image data may be archived on the tape drive  112 , or it may be further processed by the image processor  106  and conveyed to the operator console  101  and presented on the display  104 . 
     For a more detailed description of the transceiver  150 , reference is made to U.S. Pat. Nos. 4,952,877 and 4,922,736 which are incorporated herein by reference. 
     Referring still to FIG. 1, MRI system  100  also includes a displacement detector  170  which provides signals to the system control  122  which indicate vibration of the magnet assembly  141 . The displacement detector uses interferometric motion sensing using a beam of radiation in the microwave, ultrasound, sound or light spectra. The present invention will be described in the context of an amplitude modulated microwave interferometer with the understanding that other wavelengths of radiation may be used and that one of ordinary skill in the art would appreciate from the description herein how to implement the present inventive concept utilizing other wavelengths. 
     With reference to FIG. 2, the displacement detector  170  includes a local oscillator  172  which produces a first signal S 1 (t) at the microwave frequency. The output signal of the local oscillator  172  is denoted by the expression: 
     
       
         S 1 (t)=A cos(ωt)   (1) 
       
     
     where A is the signal amplitude and ω is the angular frequency of the signal. That output is applied to the input of a dual directional coupler  174  and specifically to a power splitter  176  which divides the input signal into two portions. One portion of the first signal is fed to a first output port  179  of the dual directional coupler  174 . The remaining portion of the first signal from the local oscillator  172  passes through a directional coupler to a second port  180  of the dual directional coupler  174 . 
     The second port  180  is connected to a microwave antenna  182  from which the signal radiates through the air to the magnet assembly  141 . The antenna  182  is supported independently from the magnet assembly  141  so that vibration of the latter component does not affect the antenna. The instantaneous distance between the microwave antenna  182  and magnet assembly  141  is given by the expression d+Δ(t), where d represents the nominal distance and Δ(t) denotes the change in distance due to mechanical vibration of the magnet assembly. The nominal distance d is significantly greater than the maximum value of the vibration component Δ(t). The incident first signal from the antenna  182  travels at a velocity c and has a wavelength λ. 
     Some of the first signal which impinges upon a reflector  183  on the magnet assembly  141  is reflected back to the antenna  182 . The time, τ, required for the signal to leave the antenna, strike the magnet assembly and return to the antenna is given by the equation: 
     
       
         τ=2(d+Δ(t))/c   (2) 
       
     
     The reflection produces a second signal S 2 (t) which has a different amplitude B and phase as given by the expression: 
     
       
         S 2 (t)+B cos(ω(t+τ))   (3) 
       
     
     The reflected second signal received by antenna  182  is fed back into the directional coupler  178  which channels this signal to a third output port  184  of the dual directional coupler  174 . Output ports  179  and  184  are connected to inputs of a signal mixer  186  which multiplies the first and second signals S 1 (t) and S 2 (t) together. The product of the mixing is applied to a low-pass filter  138  and the produced baseband intermediate signal, I(t), at the output of the filter  188  may be expressed as:                      I        (   t   )       =       (     AB   /   2     )          cos        (     ω                 t     )                     =       (     AB   /   2     )          cos        (     2          ω        (     d   +     Δ        (   t   )         )       /   c       )                     =       (     AB   /   2     )          cos        (     4          π        (     d   +     Δ        (   t   )         )       /   λ       )                       (   4   )                                
     This intermediate signal forms the output of the displacement detector  170 . 
     The intermediate signal, I(t), from t he displacement detector  170  is applied to a quadrature receiver  190  which is part of the system control  122  in FIG.  1 . The quadrature receiver  190  may be used with either local oscillator signal or the received signal delayed by 90° before mixing in a second mixer. If 2ωd/c+4πd/λ&lt;&lt;1, then the baseband signal may be expressed as:                      Q        (   t   )       =                  (     AB   /   2     )          sin        (     ω                 t     )                     =                  (     AB   /   2     )          sin        (     2          ω        (     d   +     Δ        (   t   )         )       /   c       )                     ≈                AB                     ω        (     d   +     Δ        (   t   )         )       /   c                   =                2      π                     AB        (     d   +     Δ        (   t   )         )       /   λ                     (   5   )                                
     With a quadrature receiver  190 , it is possible to obtain a baseband signal which depends only upon two ω d/t without any amplitude factors. The vibration signal, S b , may be expressed as:                      S   b     =     atan   (       I        (   t   )       /     Q        (   t   )                         =     2          ω        (     d   +     Δ        (   t   )         )       /   c         )               =     4          π        (     d   +     Δ        (   t   )         )       /   λ                     (   6   )                                
     Thus, sub-wavelength variations and distance to the magnet assembly can be determined with high precision. 
     As noted previously, the vibration signal can be utilized in a mathematical models to estimate the magnetic field variation and that estimate than can be employed to correct for the effects due to the vibration. 
     Several modifications can be to the system described above. For example, a frequency-modulated (FM) configuration may be used as well as the amplitude modulated (AM) version described. Single-channel systems may be acceptable for some applications. It is possible to place a separate receiving antenna/transducer at the location of the magnet assembly, thereby boosting the received signal amplitude. Similarly, a reflecting device such as a corner reflector may be placed on the target to increase the received signal at the antenna. 
     Therefore, although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.