Abstract:
In an MR imaging system, a method is provided for operating an MR gradient coil to provide a pulse sequence which comprises first and second sub-sequences. During the first sub-sequence, the coil is operated to produce a pair of diffusion-weighted imaging pulses, each having a gradient amplitude and a slew-rate. During the second sub-sequence the coil is operated to produce a train of echo-planar imaging pulses, each having a gradient amplitude which is selectively less than the amplitude of the diffusion-weighted pulses, and a slew-rate which is selectively greater than the slew-rate of the diffusion-weighted pulses. Thus, the invention is directed to a gradient system which can provide optimal performance for both diffusion-weighted imaging and echo-planar imaging, while using only a single coil for a given gradient axis. At the same time, the system enables gradient performance parameters to be selected so as to ensure that constraints imposed by the Reilly curve will not be exceeded, and to thereby avoid peripheral nerve stimulation.

Description:
BACKGROUND OF THE INVENTION 
     The invention disclosed and claimed herein is generally directed to a system for driving an MR gradient coil, so that the coil is operated in two or more different modes to produce pulses of selectively different gradient amplitudes and slew-rates during a single MR pulse sequence. More particularly, the invention pertains to a system wherein the gradient coil is operated in a given mode to produce gradient pulses having any gradient amplitude and any slew-rate up to a maximum gradient amplitude and maximum slew-rate, respectively, which correspond to the given mode. A preferred application of the gradient system of the invention is diffussion-weighted echo-planar imaging. 
     Persons of skill in the art have now recognized that an MR imaging sequence which combines diffusion-weighted (DW) imaging and echo-planar imaging (EPI) can serve as an effective clinical tool for the early diagnosis of acute stroke. As is well known, a DW imaging sequence is sensitive to particle motion resulting from diffusion, and comprises two successive gradient pulses of comparatively long time duration. Diffusion sensitivity is characterized by a parameter referred to as b-value which depends quadratically on DW gradient pulse amplitude. Because of the long time duration of the DW pulses, slew-rate, which is a measure of the increment of gradient amplitude in a unit time, is not of primary importance. However, it has been determined that use of a maximum gradient amplitude for the DW gradient waveform results in a shorter echo time (TE), for a given b-value, and therefore provides a higher signal-to-noise ratio (SNR). In one wholebody MR scanner, for example, TE can be reduced from about 99 msec to 72 msec when the amplitude of the DW gradient waveform is increased from 22 mT/m to 40 mT/m, for a b-value of 1100 sec/mm 2 . Such reduction in TE results in a considerable increase in SNR for brain tissue. Moreover, reduced TE decreases repetition time, and therefore increases volume coverage for the same imaging time. Alternatively, higher gradient amplitude allows a higher b-value with greater attendant diffusion sensitivity for a given TE. Using a higher gradient amplitude also makes DW trace imaging more feasible. 
     In EPI, a series of bipolar trapezoidal gradient pulses is used for data acquisition. It is very desirable to provide a high slew-rate, for successive pulses of the series, to reduce the spacing therebetween. In the readout of a combined DW-EPI sequence, a reduced spacing results in smaller off-resonance effects, such as image distortion and blurring. Moreover, it is anticipated that a substantial increase in slew-rate, over the currently used slew-rate of 120 T/m/sec, could provide these benefits in single shot EPI, which is the imaging method of choice for DW imaging. 
     From the above, it would appear that significant increases in both gradient amplitude and slew-rate would be very desirable in a combined DW-EPI sequence. However, simultaneously increasing both parameters could cause peripheral nerve stimulation and be in violation of FDA regulations. As is well known by those of skill in the art, the Reilly curve defines the limits of gradient amplitude and slew rate which are likely to result in nerve stimulation. If the values selected for the amplitude and slew-rate of a particular gradient pulse collectively exceed limits established by the Reilly curve, undesirable peripheral nerve stimulation could occur. Also, systems currently available in the prior art to provide two slew-rates generally require two different coils for each gradient axis. It would be very desirable, both for simplicity and to reduce the cost of gradient amplifiers, to provide a system which could operate a single gradient coil to produce pulses of different combinations of maximum gradient amplitudes and slew-rates. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a new gradient system, which is disposed to operate a single gradient coil in multiple modes. The system is capable of operating the coil to provide gradient pulses having multiple combinations of maximum gradient amplitudes and maximum slew rates. That is, each mode of operation has a corresponding maximum gradient amplitude and maximum slew-rate. When operated in a given mode, the gradient system can produce pulses of any gradient amplitude, up to the corresponding maximum amplitude, and of any slew-rate, up to the corresponding maximum slew-rate. Accordingly, the gradient system can provide optimal gradient performance for both diffusion-weighted imaging and echo-planar imaging. At the same time, performance parameters are employed which ensure that limits established by the Reilly curve, including a selected safety factor, are not exceeded. In one useful embodiment, the invention comprises a dual slew-rate gradient system, which is capable of operating the same gradient coil to selectively produce pulses of two different maximum slew-rates, and of two different maximum amplitudes. 
     Generally, the invention can be embodied as a method for operating an MR gradient coil during a specified imaging sequence. Such method comprises the steps of driving the coil in a first mode, during a first sub-sequence, to produce a number of first gradient pulses in which the gradient amplitudes are equal to or lower than a first maximum gradient amplitude and the slew-rates are equal to or lower than a first maximum slew-rate, and driving the coil in a second mode, during a second sub-sequence, to produce a number of second gradient pulses, in which the gradient amplitudes and slew-rates are equal to or lower than a second maximum gradient amplitude and second maximum slew-rate. In general, the operating modes can be in any order. In a preferred embodiment, the coil is operated during the first sub-sequence to produce a set of diffusion-weighted imaging pulses, and is operated during the second sub-sequence to produce a train of EPI pulses. 
     In a further embodiment, the invention is directed to a gradient amplifier or other apparatus for driving a single MR gradient coil in multiple modes of operation. The amplifier is operable in a first mode disposed to drive the coil to produce a number of first gradient pulses, each having an amplitude and slew-rate which do not exceed a first maximum gradient amplitude and a first maximum slew-rate, respectively, and operable in a second mode to drive the coil to produce a number of second gradient pulses, each having an amplitude and a slew-rate which do not exceed a second maximum gradient amplitude and a second maximum slew-rate, respectively. The second maximum gradient amplitude is selectively less than the first maximum gradient amplitude, and the second maximum slew-rate is selectively greater than the first maximum slew-rate. The apparatus further comprises a switch disposed to switch the gradient amplifier from one of the modes to the other, during a single brief time period. In a preferred embodiment, the amplifier is operable in the first mode to drive the gradient coil to produce a set of diffusion-weighted imaging pulses, and is operable in the second mode to dive the coil to produce a train of echo planar imaging pulses. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing basic components of an MR system for use in practicing embodiments of the invention. 
     FIG. 2 is a diagram showing a pulse sequence comprising a combination of DW and EPI gradient pulses, which may be produced in accordance with an embodiment of the invention. 
     FIG. 3 shows a Reilly curve comprising a plot of gradient amplitude versus slew-rate which is likely to result in peripheral nerve stimulation in a preferred embodiment. 
     FIG. 4A is a graph showing gradient amplitude versus slew-rate for a first operational mode of a gradient system operated in accordance with the invention. 
     FIG. 4B is a graph showing gradient amplitude versus slew-rate for a second operational mode of a gradient system operated in accordance with the invention. 
     FIGS. 5A and 5B are schematic views respectively showing a gradient amplifier for an embodiment of the invention in two modes of operation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there are shown the basic components of an MR system or scanner  10  which may be operated in accordance with the invention described herein. System  10  includes an RF transmit coil  12 , as well as a cylindrical magnet  14  for generating a main or static magnetic field B o  in the bore thereof. RF coil  12  is operated to transmit RF excitation signals into a patient or other subject of imaging  16  residing in the magnet bore, in order to produce MR signals. System  10  further includes gradient coils  18 ,  20  and  22  for generating G x , G y  and G z  magnetic field gradients relative to orthogonal X-, Y- and Z-reference axes, respectively. FIG. 1 shows each of the gradient coils  18 ,  20  and  22  respectively driven by gradient amplifiers  24 ,  26  and  28 , and RF coil  12  driven by transmit amplifier  30 . FIG. 1 further shows an RF coil  32 , which is operated in association with a receive amplifier  34  to acquire MR signals from subject  16 . In some arrangements, coil  32  and coil  12  comprise the same RF coil, which is operated in alternate modes during the imaging sequence. System  10  is further provided with a pulse sequence control  36 , which is operated to control the RF and gradient amplifiers, and to thereby generate pulse sequences to produce and acquire sets of MR signals. System  10  also includes system control and data processing electronics  38 , for operating respective components of system  10  in accordance with the invention. The construction, functions, and interrelationships of components of MR system  10  are well known and described in the prior art, such as in U.S. Pat. No. 5,673,969, issued Sep. 30, 1997 to Zhou et al. 
     Referring to FIG. 2, there is shown a gradient pulse sequence, which comprises two sub-sequences occurring during time periods t 1  and t 2 , respectively. The gradient pulses of the two sub-sequences may be produced by operating one of the gradient amplifiers of MR system  10 , such as G x  amplifier  24 , to selectively drive its corresponding gradient coil  18 . 
     FIG. 2 further shows the sub-sequence of time period ti comprising a pair of DW imaging pulses  40   a  and  40   b,  each having an amplitude G DW . As stated above, it is very desirable to increase the amplitude of the DW pulses, to reduce TE for a given b-value and to thereby enhance SNR. 
     Referring further to FIG. 2, the sub-sequence of time period t 2  is shown to comprise a train of bipolar EPI gradient pulses, such as pulses  42   a  and  42   b.  Positive EPI pulses have a gradient amplitude G EPI , and negative EPI pulses have a gradient amplitude −G EPI . As stated above, increasing the slew-rate in a train of EPI gradient pulses, to decrease the spacing therebetween reduces off-resonance and eddy current effects such as image distortion and blurring, thereby improving single shot EPI images. Reducing eddy-current related distortion is particularly important in DW imaging because such distortion is a serious obstacle for combining DW images with different b-values, and in DW trace imaging. As previously stated, the sequence of FIG. 2 may provide a valuable tool for the early diagnosis of acute stroke. 
     As described above, the Reilly curve pertains to peripheral nerve stimulation in patients of MR imaging. Such effect can result from excessively rapid change in a gradient magnetic field, and is related to both gradient amplitude and slew-rate. As is well known, the derivative dB/dt provides a measure of the peripheral nerve stimulation threshold, where B(t) represents magnetic field strength as a function of time. More specifically, the dB/dt threshold of peripheral nerve stimulation caused by a ramping gradient and corresponding to the Reilly curve, represented herein as dB/dt Reilly, can be expressed as follows: 
     
       
           dB/dt|   Reilly =α(1+⊕/θτ)  Eqn. (1)  
       
     
     In Equation (1), α=54 T/sec, β=132 msec, τ is the duration of the gradient ramp when the gradient is switched from 0 to an amplitude G, and θτ is the total duration of the gradient ramp. From the above definition of τ and θτ, it is seen that θ=1 when a gradient is switched from 0 to G, and θ=2 when a gradient is switched from −G to G (e.g., for adjacent bipolar pulses). In a simplified model, dB/dt is related to slew-rate SR by: 
     
       
           dB/dt=L·SR   Eqn. (2)  
       
     
     In Equation (2) L is a scaling factor which depends on gradient coil usefully chosen for a preferred gradient coil embodiment to be 0.344 m, and slew-rate is given by the expression SR=G/τ. 
     In a clinical setting, the maximum allowed value of dB/dt (i.e., dB/dt| actual ) must be lower than the peripheral nerve stimulation threshold by a safety factor f s , in order to avoid any possibility of peripheral nerve stimulation, that is: 
     
       
           dB/dt|   actual   ≦f   s   ·dB/dt|   Reilly   Eqn. (3)  
       
     
     As stated above, safety factor f s  usefully is 66% of the Reilly curve. From Equations (1)-(3), the relationship between gradient amplitude and slew-rate becomes: 
     
       
         1+β· SR/θG≧L·SR /( f   s ·α).  Eqn. (4)  
       
     
     Using θ=2, α=54 T/sec, β=132  82  sec, L=0.344 m, and f s =0.66, Equation (4) can be rewritten as follows: 
     
       
         1+66(μsec)·10 3   SR (T/m/sec)/ G (mT/m)≧ SR (T/m/sec)/103.6(T/m/sec)  Eqn. (5)  
       
     
     Referring to FIG. 3, there is shown a Reilly curve  44  generated by plotting gradient amplitude versus slew-rate, for the condition where peripheral nerve stimulation results. More specifically, Reilly curve  44  is a plot of Equation (5). Thus, a gradient pulse characterized by a point on the graph of FIG. 3 which lies in the region  48 , i.e., above Reilly curve  44 , is likely to cause peripheral nerve stimulation in a patient. On the other hand, a gradient pulse characterized by a point lying in the region  50 , i.e., below curve  44  is unlikely to cause peripheral nerve stimulation. 
     Referring further to FIG. 3, there is shown a rectangular area  49 , which represents the range of allowed gradient amplitudes, as well as the range of allowed slew-rates, for a first operating mode of an embodiment of the invention. Such range is bounded by a maximum amplitude of 40 mT/m, and a maximum slew-rate of 125 T/m/sec. Such maximum values intersect curve  44  at a point  45 . There is also shown a rectangular area  51 , which represents the range of allowed gradient amplitudes, as well as the range of allowed slew-rates, for a second operating mode of an embodiment of the invention. Such range is bounded by a maximum amplitude of 22.1 mT/m, and a maximum slew-rate of 150 T/m/sec. Such maximum values intersect curve  44  at a point  47 . 
     Referring again to FIG. 2, if the amplitude G DW  of pulses  40   a  and  40   b  is to be increased, in order to realize the benefits described above in connection with DW imaging, Equation (5) may be employed to determine the maximum allowable slew-rate for the DW pulses  40   a  and  40   b.  For example, one study has suggested that a gradient amplitude of 40 mT/m could provide considerable improvement in image quality and clinical value for the DWI sequence. At the same time, a slew-rate of 77 T/m/sec was found to be sufficient for the DW gradient waveform. From Equation (5), the maximum allowed slew-rate for a peak gradient amplitude of 40 mT/m is found to be 125 T/m/sec. It will be readily apparent that a slew-rate of 77 T/m/sec is well below such allowed maximum. 
     In like manner, another study has shown that an increase of slew-rate above the currently used rate of 120 T/m/sec will be beneficial to single shot EPI. At the same time, it has been found that for the EPI readout waveform, a gradient amplitude of 22 mT/m is found to be sufficient for a commonly used field-of-view. In a preferred embodiment, if slew-rate is selected to be 150 T/m/sec, the maximum allowed gradient amplitude is found from Equation (5) to be 22.1 mT/m. 
     In accordance with the invention, it has been recognized that if a two-mode gradient system is provided to drive a gradient coil, the coil can produce pulses having optimal parameter values for DWI and EPI, respectively. More particularly, a two-mode gradient system provides a reduced gradient slew-rate when the peak gradient amplitude is higher than a specified value, and provides a higher gradient slew-rate when the peak gradient amplitude is lower than the specified value. At the same time, from the relationship given by Equation (5), respective parameter values can be set for both DWI and EPI pulses which are within the constraints described above in connection with the Reilly curve. The performance of a two-mode gradient system, comprising an embodiment of the invention, is depicted in FIGS. 4A and 4B, collectively, for the specific values computed above. FIG. 4A shows a slew-rate of 125 T/m/sec used when the peak gradient amplitude of a pulse is between 22.1 mT/m and 40 mT/m in a preferred embodiment. FIG. 4B shows a slew-rate of 150 T/m/sec used when the peak gradient amplitude of a pulse is below 22.1 mT/m in the same preferred embodiment. 
     Referring to FIGS. 5A and 5B, there is shown a very useful and comparatively simple configuration for implementing an embodiment of the invention. More specifically, there is shown gradient amplifier  24  comprising power modules  52  and  54 , each providing a voltage V and a current I. The two power modules are interconnected in part through conductive paths  56  and  58 , to supply power to gradient coil  18 . Power modules  52  and  54  are also selectively interconnected by means of switch components  60 ,  62  and  64 . 
     FIG. 5A shows switch  60  open and switches  62  and  64  closed. This has the effect of connecting power modules  52  and  54  in parallel. Accordingly, the current supplied to gradient coil  18  is the sum of the current generated by the two power modules, that is, 2I, but the voltage applied to gradient coil  18  is V. The increased current has the effect of driving coil  18  to produce pulses of double the gradient amplitude, but with the same slew-rate compared to the amplitude and slew-rate produced by a single power module. 
     FIG. 5B shows switch  60  closed and switches  62  and  64  open. This has the effect of connecting power modules  52  and  54  in series. Accordingly, the voltage applied to gradient coil  18  is the sum of the voltages generated by the two power supplies, that is, 2V but with current I. The increased voltage has the effect of driving coil  18  to produce pulses of double the slew-rate, but with the same amplitude as produced by a single power module. 
     It will be readily apparent that the switching of the two power modules, between the parallel and series modes, must be fast enough, such as on the order of 1 msec, that they can be switched from one mode to the other between waveforms  40   b  and  42   a  in FIG. 2 of the DW-EPI pulse sequence. This may be achieved, for example, by means of a programmable circuit residing in MR system control  38 , and connected to operate switch components  60 ,  62  and  64 . 
     It is to be emphasized that in a modification of the invention, a gradient system may be provided which is operable in more than two modes to drive a specified gradient coil. For a particular mode, the coil is driven to produce gradient pulses of any selected amplitude and any slew-rate, up to a specified maximum gradient amplitude and maximum slew-rate, respectively, which correspond to the particular operational mode. Also, the order in which different operating modes occur during an imaging sequence is entirely arbitrary. 
     It is to be emphasized further that the gradient system of the invention can be readily employed to produce other pulse sequences, which will readily occur to those of skill in the art, in addition to the diffusion weighted-EPI pulse sequence described above. 
     Obviously, many other modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the disclosed concept, the invention may be practiced otherwise than as has been specifically described.