Abstract:
The present invention, in one form, provides compensation for the effects of an oscillatory B0 eddy current B e (t) in an NMR apparatus. In an NMR apparatus having a transmitter generating a spin excitation signal and a receiver detecting an NMR signal, applied gradient signals are analyzed to estimate a resulting oscillatory B0 eddy current B e (t). The frequency of either the transmitter or the receiver of the NMR apparatus, or both, is shifted in frequency by an amount proportional to B e (t) to achieve compensation. The applied gradient signals are digitized and filtered using a recursive filter that is based upon an oscillatory model of the oscillatory B0 eddy current B e (t)

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to methods and apparatus for correction of distortion of signals in a nuclear magnetic resonance (NMR) system, and more particularly to methods and apparatus for correction of distortion in such NMR systems caused by switching of gradient magnetic fields when such switching results in oscillatory B0 magnetic fields. 
     In at least one known NMR imaging device, nuclear spins are subjected to magnetic fields and excited by a spin excitation signal from a radio frequency transmitter. The magnetic field is uniform and homogeneous. The frequency of the spin excitation signal is such that a resonant matching occurs to a natural Larmor precession frequency for those magnetic spins to be excited. The excited nuclear spins precess about a direction of the homogeneous magnetic field vector at an angle that depends upon the strength and duration of the spin excitation field. If the homogeneous magnetic field varies with time, the precession frequency will also vary. 
     In addition, at least one known NMR and imaging device utilizes gradient magnetic fields for volume selective spectroscopy or imaging. Gradient magnetic fields are applied to encode volume regions of a sample and thereby allow for position sensitive measurements of the nuclear magnetic resonance signal. However, when the gradient magnetic fields are switched on and off, inductive coupling produces current flow in conductive elements of the device. These currents produce undesirable time-dependent magnetic fields that adversely affect signal measurement. An undesirable eddy current field component, i.e., a uniform B0 component, is one result of necessary gradient field changes. Uncompensated B0 eddy currents can lead to image quality problems such as ghosting or to degraded MR (magnetic resonance) spectroscopy performance. 
     In known systems, only exponentially decaying gradient and B0 eddy current errors have been recognized. However, the introduction of shielded magnets with shortened axial extent has resulted in drastically higher static magnetic fields near the edges of the gradient coil. These magnetic fields, in turn, produce higher forces and significant oscillatory eddy currents. Oscillatory B0 eddy currents cause unwanted side lobes in MR spectroscopy and artifacts in MR imaging. 
     It would be desirable to provide apparatus and methods for correcting for oscillatory B0 eddy currents in the presence of changes in the gradient field. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one exemplary embodiment of the present invention, an NMR apparatus having a transmitter generating a spin excitation signal and a receiver detecting an NMR signal analyzes gradient signals to estimate a resulting oscillatory B0 eddy current B e (t). A frequency of either the transmitter or the receiver of the NMR apparatus, or both, is shifted in frequency by an amount proportional to B e (t) to compensate for the oscillatory B0 eddy current. The applied gradient signals are digitized and filtered using a recursive filter derived from an oscillatory model of the eddy current B e (t). The recursive filter has a complex-valued output, and the eddy current B e (t) estimate is the real part of the complex-valued output of the recursive filter. 
     The above described embodiment and others that are described herein effectively compensate for oscillatory B0 eddy currents to provide enhanced NMR image quality and MR spectroscopy performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph illustrating an observed B0 behavior in response to an application of a gradient magnetic field. 
     FIG. 2 is a graph showing a measured oscillatory B0 eddy current in a NMR system. 
     FIG. 3 is a block diagram of an embodiment of a single channel recursive filter for estimating a correction frequency in accordance with the invention. 
     FIG. 4 is a graph showing measured B0 eddy currents in the same NMR system from which FIG. 2 was obtained, but in which frequency shift compensation in accordance with the invention was applied. 
     FIG. 5 is a block diagram of an embodiment of a three-channel recursive filter for estimating a correction frequency in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention, in one embodiment, is a filter that is utilized in an NMR system (not shown) to provide real-time compensation of B0 error for arbitrary gradient inputs. In this embodiment, the NMR system includes a radio frequency transmitter (not shown) that excites nuclear spins subjected to a uniform and homogeneous magnetic field with a radio frequency electromagnetic wave of a selected frequency that resonantly matches the natural Larmor precession frequency of those magnetic spins to be excited. The NMR system also includes a receiver to detect signals resulting from the excited spins. The NMR system also includes one or more coils that provide gradient magnetic fields that are selectively generated by current flowing through the coils. The filter analyzes the currents flowing through these coils to generate an estimate of an oscillatory B0 eddy current, which is, in turn, used to adjust a frequency of the transmitter and/or the receiver. This adjustment compensates for the B0 error, as shown in FIG. 1, which results from rapidly switching on a gradient, that would otherwise be present in the NMR system. 
     The following derivation results in an efficient filter for providing a correction for oscillatory B0 eddy currents B e (t). The modeled eddy currents are written as:                  B   e          (   t   )       =       -            G        (   t   )              t         *     h        (   t   )                 (   1   )                                
     where G(t) is a gradient signal displayed on any of the three axes x, y, or z, and where * denotes convolution, and where the impulse response h(t) is written as:                h        (   t   )       =       u        (   t   )              ∑     k   =   0       N   -   1              α   k                 -   t     /     τ   k              cos        (       2      π                   f   k        t     +     θ   k       )                     (   2   )                                
     with u(t) equal to the unit step function written as:                u        (   t   )       =     {           0   ,     t   &lt;   0                 1   ,     t   ≥   0.                       (   3   )                                
     The receive frequency is shifted by an amount γB e (t), where γ is the gyromagnetic ratio. To perform the shift in real time with a digital transceiver  36 ,  38  as shown in FIG. 5, determination of B e (t) in eq. (1) is made as efficient as possible. As a result of the receive frequency shifted at discrete intervals Δt, if Δt&gt;&gt;(1/f) smallest  and Δt&gt;&gt;τ smallest , where (1/f) smallest  and τ smallest  each represent the smallest values of 1/f k  and τ k , respectively, for all k, then eq. (1) is evaluated with sufficient accuracy using the z-transform. 
     In one embodiment, it will suffice to consider the case N=1. Generalizations for larger N follow by superposition. In this embodiment, the real part of the function u(t)αe −βt   jθ is h(t), where β=1/τ−j2πf. B e (t) is the real part of a complex quantity Y(t)=−(dG(t)/dt)*((u(t)αe −βt e jθ ). 
     The unit step response for Y step (t) is determined by setting G(t)=u (t) so that: 
     
       
         Y step (t)−αe −βt+jθ u(t)  (4) 
       
     
     If t is sampled at intervals of Δt, the z-trans form of equation (4) is written as:                    Y   ~     step          (   z   )       =       -   α                              j                 θ            (     z     z   -            -   β                   Δ                 t           )       .               (   5   )                                
     Dividing the step response {tilde over (Y)} step (z) by the z-transform of the unit step ũ(z)=z|(z−1) produces the z-transform of the transfer function {tilde over (h)}(z):                  h   ~          (   z   )       =       -   α                            j                 θ            (       z   -   1       z   -            -   β                   Δ                 t           )                 (   6   )                                
     The general response for any gradient signal {tilde over (G)}(z) is then determined by:                  Y   ~          (   z   )       =       -       G   ~          (   z   )            α                              -   j                   θ            (       z   -   1       z   -            -   βΔ                   t           )       .               (   7   )                                
     Determining an inverse z-transform of eq. (7) generates Y at discrete times t n =nΔt in accordance with: 
     
       
         Y(t n )=e −βΔt Y(t n−1 )−αe jθ ΔG(t n ),  (8) 
       
     
     where ΔG(t n )=G(t n )−G(t n−1 ) with B e (t n )=Re[Y(t n )]. 
     Eq. (8) provides the mathematical basis for a recursive digital filter, i.e., an infinite impulse response or IIR filter, for determining B e (t n ). The number of operations required for this filter is much smaller than for evaluating eq. (1) by direct convolution. To use this filter for compensation of oscillatory B0 eddy currents, it is necessary to estimate filter parameters f k , τ k , α k , and θ k  for each k from 0 to N− 1 , where N is a number selected or determined to provide an adequate number of terms to sufficiently approximate the observed oscillatory behavior. In one embodiment, a value N= 1  was determined to adequately correct for oscillatory B0 behavior. 
     In one embodiment, to determine the characteristics of the filter for a specific imaging system, the imaging system is adjusted to optimize the compensation for non-oscillatory eddy currents. B0 eddy currents are then measured using a known driving signal, and a fitting routine is utilized to obtain estimates for the for the parameters of h(t) in eq. (2). In one example using a known NMR imaging system, B0 eddy currents were measured by using a sinusoidal driving signal over a range of frequencies. A quasi-resonance was observed at around  32  Hz. FIG. 2 shows the measured B0 eddy current in the known NMR imaging system resulting from twenty complete cycles of a 31.25 Hz sinusoidal exciting signal with amplitude 2 Gauss/cm (G/cm) on the x-axis. The vertical axis of the plot is normalized by the maximum amplitude of the exciting gradient. 
     Estimates for the parameters of h(t) in eq. (2) were determined for this system, with a simplex fitting routine using the assumption N=1. The estimates obtained were f=31.91 Hz, τ0.36 sec., α=0.000036 cm, and θ=0.16 radians. Eq. (8) for this system can therefore be evaluated in sufficient time to allow an update time of Δt=64 μsec. Since Δt clearly satisfies Δt&gt;&gt;1/f and Δt&gt;&gt;τ for the data of FIG. 2, the z-transform analysis and eq. (8) are good approximations. 
     For eddy currents with sufficient long period and decay constant compared to the filter update interval, the frequency shift can be determined using a recursive filter to minimize computation time with a digital transceiver. In one embodiment and as shown in FIG. 3, a digital recursive filter  10  estimates B e (t n ) from an applied gradient G(t n ). The signal G(t n ) is a digitized representation of a gradient current applied to a magnetic coil (not shown), the representation being produced by an analog-to-digital converter (also not shown). Initially, the digitized G(t n ) is applied to a delay  12 . The delayed signal G(t n−1 ) is then supplied to multiplier  14 , where it is multiplied, for example, by −1. The result of the multiplication is supplied to adder  16  where it is added to G(t n ) to generate ΔG(t n ). The result of this addition is supplied to multiplier  18 , where it is multiplied by a quantity −αe jθ , parameters α and θ having been estimated in advance. A resulting signal is added by adder  20  to a signal that is a function of an output Y(t n ) of adder  20  delayed by delay  22  and multiplied by e −βΔt  at multiplier  23 , where β is determined in advance by estimation, and Δt is a discrete sampling time. Function  24  takes the real part of Y(t n ) to produce output B e (t n ) It will be recognized by those skilled in the art that filter  10  in FIG. 3 can readily be implemented in special purpose hardware or as software or firmware that is executed by a processor (not shown) in an NMR system. It will also be recognized by those skilled in the art that various modifications of filter  10  are possible, including modification that operate directly on G(t n ) without generating an intermediate result ΔG(t n ). 
     A digital recursive filter having the parameters fitted to the oscillatory B0 eddy current shown in FIG. 2 was implemented. This filter was used to shift the imaging system receive frequency of the known NMR system while a B0 eddy current was excited using the same driving signal as for FIG.  2 . As shown by the results illustrated in FIG. 4, oscillatory B0 eddy currents were effectively corrected by shifting the receive frequency by an amount predicted from the exciting gradient signals and eddy current impulse response. 
     In one embodiment and as shown in FIG. 5, a correction circuit  25  is employed to compensate for oscillatory B0 eddy currents resulting from a set of gradient signals G x (t), G y (t), G z (t). The G x (t), G y (t), G z (t) signals, in one embodiment, are signals such as those generated by a set of G x , G y , and G z  pulse control modules, or amplifiers  26  in one known NMR system. All signals represented in FIG. 5 are digitized. In this embodiment, each of a set of samples G x (t n ), G y (t n ), G z (t n ) of gradient signals G x (t), G y (t), G z (t) is input to a filter  10  of a type as shown in FIG.  3 . Although illustrated as identical filters, in one embodiment, each filter  10  employs different internal parameters α, β, and θ if necessary. In one embodiment, appropriate parameters are obtained for each filter  10  from observed B0 eddy currents resulting from separate tests in which each gradient coil is separately excited. A simplex fitting routine is used to determine the parameters of h(t) in eq. (4), as described above. This technique provides estimates of parameters α, β, and θ in each filter  10  so that outputs B e,x (t n ), B e,y (t n ), B e,z (t n ) are added by adder  28  to produce an output B e (t n ) representing a composite effect of signals G x (t), G y (t), G z (t) on a uniform, homogeneous magnetic field. Output B e (t n ) is applied to a multiplier  30 , where it is multiplied by γ to obtain an instantaneous frequency correction γB e (t n ) This correction may be applied to a variable frequency oscillator (VFOs)  32  that is coupled to NMR RF transmitter  36  and receiver  38 . The correction provides a frequency control for transmitter  36  and receiver  38 . An excitation signal  52  is applied to excite a substance exposed to a magnetic field generated by a magnet  50 , and a received signal  54  is processed by a receiver  38 . 
     It will be recognized that some or all of the digital filtering and processing of signals in FIG. 5 including that performed by correction circuit  25  may be performed by special purpose signal processing hardware. However, such processing may also be performed by a processor executing software or firmware instructions. Furthermore, the frequency correction signals of this invention may be applied to frequency synthesized VFOs, including those employing direct digital synthesis. 
     From the preceding description of various embodiments of the present invention, it is evident that the invention provides efficient correction for oscillatory B0 eddy currents in NMR applications. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. Accordingly the spirit and scope of the invention are to be limited only by the terms of the appended claims and their equivalents.