Patent Publication Number: US-9417298-B2

Title: Local SAR reduction in multi-slice pTx via SAR-hopping between excitations

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/644,839, filed on May 9, 2012, and entitled “Local SAR reduction in multi-slice pTx via SAR-hopping between excitations.” 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under EB006847, EB007942, and EB015896 awarded by the National Institutes of Health. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The field of the invention is systems and method for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for designing parallel transmission (“pTx”) radio frequency (“RF”) pulses for use in multislice MRI applications. 
     Parallel transmission RF pulse design methods have been proposed that allow uniform excitation of spins across the field of view and excitation of specific patterns “burnt” into the patient. Electromagnetic (“EM”) simulations of such pulses have shown that the interference of electric fields created by every channel deposit a large amount of energy, as measured by the specific absorption rate (“SAR” expressed in W/kg), at focal locations in the body. When not controlling local SAR explicitly in the pTx pulse design process (i.e. when controlling global SAR or pulse power), local SAR in pTx is often found to be 5 to 10 times greater than in single channel excitations. Another difficulty is that, in contrast to single channel excitation, the ratio of local to global SAR is not constant and depends on the pTx pulse being played. Therefore, both global and local SAR have to be monitored explicitly in pTx experiments. 
     Explicit control of local SAR in pTx is difficult because, in theory, it requires monitoring SAR at hundreds of thousands of locations in the body during the pulse design process and the actual scan. A pulse design algorithm that explicitly incorporates local SAR, global SAR, and peak and average power constraints has been proposed by D. O. Brunner and K. P. Pruessmann in “Optimal design of multiple channel RF pulses under strict power and SAR constraints,”  Magnetic Resonance in Medicine,  2010; 63(5): 1280-1291; however, this study only showed a proof of concept by controlling SAR at a few locations in the body. 
     J. Lee, et al., proposed an approach that controls local SAR at a limited number of “virtual observation points” (“VOPs”), as described in “Local SAR in parallel transmission pulse design,”  Magnetic Resonance in Medicine,  2012; 67(6):1566-78. In this method, fast computational control of local SAR in the entire body is achieved by using a reduced number of SAR constraints. This approach is not optimal, however, because it is based on an approximation of local SAR as a linear combination of the SAR values associated to all VOPs. In other words, it is possible to find pulses that have a better local SAR versus excitation fidelity than those found by this algorithm. Another limitation of this method is that it is not capable of incorporating other constraints than local SAR (e.g. global SAR and/or pulse power). 
     Another class of approaches for reducing local SAR at constant excitation error includes using distinct RF pulses instead of only one RF pulse, as is done conventionally. Such RF pulses can be designed so that they have cancelling SAR hotspot locations and, therefore, an overall six minutes average local SAR that is significantly lower than the local SAR of individual pulses. This is motivated by the fact that the FDA regulates only the six minutes average of local and global SAR, not instantaneous SAR. This type of local SAR reduction technique can be referred to as “SAR hopping” because it allows SAR hotspots to “hop” from pulse to pulse. A limitation of the previous approaches that make use of SAR hopping is that they do not control local SAR explicitly in the entire body, but only at a few locations. Pulses designed using this technique are, therefore, not optimal. A common problem with these approaches is that they use different pulses to excite a single slice of k-space. Since different pulses cannot have exactly the same excitation profiles, this results in ghosting and inaccurate image contrast. 
     It would therefore be desirable to provide a system and method for designing pTx RF pulses for multislice imaging applications that overcome the drawbacks of currently available methods, including suboptimal design performance and the inducement of ghosting artifacts. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the aforementioned drawbacks by providing a system and method for designing pTx RF pulses for use in multislice MRI applications that minimize average local SAR across the pulses using a constraint that enforces SAR hopping between excitations. 
     It is an aspect of the invention to provide a method for designing parallel transmission (pTx) radio frequency (RF) pulses for use in multislice magnetic resonance imaging (MRI). The method includes determining a target magnetization to be achieved in a plurality of slice locations in a subject and selecting a set of compressed specific absorption rate (SAR) matrix points at which SAR can be evaluated. By way of example, the compressed SAR matrix points can be virtual observation points (VOPs). A plurality of RF pulses for achieving the target magnetization are then designed by determining a set of RF waveforms that minimize an average local SAR of the plurality of RF pulses evaluated at the compressed SAR matrix points. The RF waveforms can also minimize a global SAR of the plurality of RF pulses evaluated at the compressed SAR matrix points, the peak power of the RF pulses on each of a plurality of transmit channels, and the average power of the RF pulses on the plurality of transmit channels. 
     It is another aspect of the invention to provide an MRI system that includes a magnet system, a plurality of gradient coils, an RF system, and a computer system. The magnet system is configured to generate a polarizing magnetic field about at least a portion of a subject arranged in the MRI system; the plurality of gradient coils are configured to apply at least one magnetic gradient field to the polarizing magnetic field; and the RF system includes at least one RF coil configured to apply an RF field to the subject and to receive magnetic resonance signals therefrom. The computer system is programmed to determine a target magnetization to be achieved in a plurality of slice locations in the subject; select a set of compressed SAR matrix points at which SAR can be evaluated; determine a set of RF waveforms that minimize an average local SAR of a plurality of RF pulses evaluated at the compressed SAR matrix points using a constrained optimization; and direct the RF system to produce the plurality of RF pulses using the determined set of RF waveforms, such that the determined target magnetization is generated in the plurality of slice locations in the subject. 
     The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart setting forth the steps of an example of a method for designing parallel transmission RF pulses for multislice MRI applications; 
         FIG. 2  is a block diagram of an example of an MRI system that can implement the present invention; and 
         FIG. 3  is a block diagram of an example of a parallel transmit and receive RF system that can form a part of the MRI system of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Described here are a system and method for designing radio frequency (“RF”) pulses for parallel transmission (“pTx”) applications, and particularly pTx applications in multislice magnetic resonance imaging (“MRI”). The present invention implements the concept of “SAR hopping” by framing the concept between slice-selective excitations as a constrained optimization problem that attempts designing multiple pulses simultaneously subject to an overall local SAR constraint. That is, the present invention does not “force” the algorithm to shift SAR hotspots from pulse to pulse. Instead, the algorithm automatically finds the set of RF waveforms that yield the best excitation profiles for all pulses (slices) while ensuring that the local SAR of the average of all pulses is below the regulatory limit imposed by the FDA. This approach designs all pulses simultaneously while constraining local SAR, global SAR, and peak and average power explicitly and, therefore, yields pulses that achieve optimal tradeoffs between local and global SAR, pulse power, and excitation fidelity. Ghosting artifacts and inaccurate image contrast are avoided by designing pulses that excite different slices as opposed to different k-space lines. In this approach, a given slice is excited using the same pulse. 
     The present invention includes two different pTx RF pulse design modules that allow for the reduction of local SAR at constant excitation error. The first constrains local SAR explicitly in the pulse design process. This constraint is achieved by compressing the SAR matrices at every location in the body to a much smaller set of compressed SAR matrix points, which may be virtual observation points (“VOPs”). Pulses are then designed that minimize excitation error while ensuring that the local SAR, global SAR, and peak and average power on each channel are below limits set by the user. This approach does not use regularization parameters that indirectly control the tradeoff between SAR, power and excitation fidelity (soft constraints). Instead it enforces SAR and power limits explicitly (hard constraints). This feature of the present invention simplifies the task of the operator and improves image quality by automatically finding optimal tradeoffs between local SAR, global SAR, peak and average pulse power, and excitation fidelity. 
     The second module simultaneously designs multiple pulses that have cancelling SAR hotspot locations. This design constraint may be referred to as “SAR hopping” because it allows SAR hotspots to “hop” from pulse to pulse. Excitation strategies that allow the reduction of the six minute average local SAR based on the design of multiple pulses with cancelling SAR hotspots locations have been proposed before. These methods, however, are limited by sub-optimality and image ghosting. The present invention overcomes these limitations. First, the “SAR hopping” concept is framed as an optimization problem over the RF waveforms of all pulses simultaneously while explicitly constraining the local SAR of the average of all pulses. In other words, multiple RF waveforms are designed such that they minimize the overall mean square error of all pulses while guaranteeing that the six minutes average local SAR of these pulses is below the regulatory limit. 
     Thus, the present invention explicitly controls local SAR, global SAR, peak power, and average power on every transmit channel simultaneously. Local SAR is also controlled in the entire body by compressing the SAR matrices at every location into a dramatically smaller set of compressed SAR matrix points. Unlike previous techniques, the present invention is mathematically optimal in that it yields pulses that achieve the best possible tradeoff between local and global SAR, peak and average power, and excitation fidelity. The present invention does not depend on regularization parameters that indirectly control tradeoffs between SAR, power, and fidelity; instead, it explicitly enforces the global and local SAR as well as power limits. This simplifies the task of the operator and could potentially improve image quality by automatically finding optimal tradeoffs between local SAR, global SAR, peak and average power as well as excitation fidelity. 
     When using several transmit channels to excite the MR signal, SAR at location, r, is computed from the knowledge of the electric fields E 1 (r), . . . , E C (r) created by the C transmit channels; the RF waveforms played on each of these channels; the conductivity, σ(r); and density ρ(r), according to the following: 
     
       
         
           
             
               
                 
                   
                     
                       SAR 
                       ⁡ 
                       
                         ( 
                         r 
                         ) 
                       
                     
                     = 
                     
                       
                         
                           σ 
                           ⁡ 
                           
                             ( 
                             r 
                             ) 
                           
                         
                         
                           2 
                           ⁢ 
                           
                             ρ 
                             ⁡ 
                             
                               ( 
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                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   
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                                   ⁡ 
                                   
                                     ( 
                                     
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     which can be approximated as 
     
       
         
           
             
               
                 
                   
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                     ⁡ 
                     
                       ( 
                       r 
                       ) 
                     
                   
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                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           
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                             ⁡ 
                             
                               ( 
                               
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                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
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                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
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                               ) 
                             
                           
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                           Q 
                           ⁡ 
                           
                             ( 
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                             rf 
                             ⁡ 
                             
                               ( 
                               
                                 i 
                                 ⁢ 
                                 
                                     
                                 
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                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
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                   ( 
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     In Eqn. (2), rf(iΔt) is the vertical concatenation of RF values played on all channels at time iΔt; { . . . } H  indicates the Hermitian transpose operator; T is the pulse length; N T  is the number of RF samples; and Q(r) is the correlation matrix of electric fields created by all channels at the location, r. SAR averaging over a given volume (e.g., a ten gram or one gram volume) as prescribed by the FDA can be done at the level of the correlation matrices, Q(r) by summing the original matrices contained in the averaging volume. Eqn. (2) is thus valid even when performing SAR averaging. The material properties and other constants are incorporated in the definition of the correlation matrix, Q(r) to simplify notations. 
     In theory, explicit control for local SAR in the design of MRI pulses should be performed by controlling SAR at every position, r, of the body model used to compute the electric fields. Assuming an average tissue density of 1000 kg/m 3  body, 1 g (10 g) of tissue corresponds to a volume 10 −6  m 3  (10 −5  m 3 ). For the averaging process to be reasonably accurate, at least 50 Q-matrices would need to be summed, which means that the linear resolution of the body model would need to be at least 2.7 mm (5.8 mm). At these resolutions, typical body models contain hundreds of thousands of voxels. Controlling SAR at so many locations would make the pulse design process extremely slow and, therefore, not applicable in the clinic. To solve this problem, the method of the present invention controls SAR in the entire body using a compression of the original SAR matrices, SAR (r) to reduce the number of evaluation points. By way of example, one method for compressing the SAR matrices, SAR(r) can include forming a significantly smaller set of virtual observation points (“VOPs”). The compression of the SAR matrices, SAR(r), can be carried out as described by G. Eichfelder and M. Gebhardt in “Local specific absorption rate control for parallel transmission by virtual observation points,”  Magnetic Resonance in Medicine,  2011; 66(5): 1468-1476. The VOP compression scheme allows for the reduction of the number of SAR matrices by a factor of 300 or more, while ensuring that the local SAR estimation error associated with the compression process is an overestimation. The safety margin afforded by this overestimation is a beneficial feature of the algorithm. Another example of how the SAR matrices, SAR(r), can be compressed is to use the compression method described by A. Sbrizzi, et al., in “Fast design of local N-gram specific absorption rate-optimized radiofrequency pulses for parallel transmit systems,”  Magnetic Resonance in Medicine,  2012; 67(3):824-834. Unlike the VOP method, this compression technique does not guarantee that the SAR error is an overestimation. Although the succeeding description is provided with respect to using VOPs, it will be appreciated by those skilled in the art that the VOPs can be readily exchanged with other compressed samplings of the SAR matrices, SAR(r). 
     Other popular model reduction methods, like truncated SVD, could be used to reduce the number of SAR matrices, but these methods typically result in local SAR estimation errors that are sometime positive and sometime negative. That is, in some cases these methods underestimate local SAR, which is potentially harmful to the patient. The local SAR overestimation error associated with the VOP compression scheme is bounded by a user-defined maximum allowed error that indirectly controls the number of VOPs. A tight control of local SAR thus requires more VOPs than a loose one. 
     As described above, local SAR averaged over six minutes can be reduced by designing multiple pulses that have their SAR hotspots at different locations. Such pulses, played during a six minute time interval, have a lower overall local SAR than if a single pulse was used during the same period. In order to avoid in-plane ghosting, these pulses can be used to excite different slices. This idea is implemented as an optimization problem in which multiple pulses are designed simultaneously and the local SAR of the average of all pulses is constrained explicitly using compressed SAR matrix points, such as VOPs, 
     
       
         
           
             
               
                 
                   
                     
                       min 
                       x 
                     
                     ⁢ 
                     
                       
                          
                         
                           Ax 
                           - 
                           b 
                         
                          
                       
                       2 
                       2 
                     
                   
                   ; 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     where x and b are the concatenation of the RF waveforms and the target magnetizations, respectively, of all pulses being designed simultaneously. Similarly, the system matrix, A, is the block diagonal of the system matrices corresponding to individual pulses. These individual system matrices can be computed from the small flip angle magnetic resonance signal equation as follows,
 
[ A   p ] i,(jc)   =iγm   0 ( r   i ) S   c ( r   i ) e   −2πik(jΔt)·r     i   ,  (4);
 
     where A p  is the system matrix of the p th  pulse; γ is the proton gyromagnetic ratio; m 0 (r i ) is the steady state magnetization at location r i ; S c (r i ) is the B1+ transmit pulse profile of the c th  transmit channel at location r i ; and k(t) is the transmit k-space trajectory. 
     The optimization problem of Eqn. (3) is constrained by the following constraints: 
     
       
         
           
             
               
                 
                   
                     
                       
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                       SAR 
                       local 
                     
                   
                   ; 
                 
               
               
                 
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                   5 
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                   ( 
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                   ( 
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     These constraints can be described as follows: Eqn. (5) constrains local SAR at every compressed SAR matrix point; Eqn. (6) constrains global SAR; Eqn. (7) constrains peak power for each channel; and Eqn. (8) constrains average power on every channel. As described above, the local SAR of the average of all pulses is being constrained in the present invention, and not the local SAR of each pulse individually. In doing so, the SAR hopping concept is enforced as a constraint in the pulse design process. The many-pulses SAR kernel, S j , is the block diagonal matrix, 
     
       
         
           
             
               
                 
                   
                     
                       S 
                       j 
                     
                     = 
                     
                       
                         1 
                         P 
                       
                       ⁡ 
                       
                         [ 
                         
                           
                             
                               
                                 VOP 
                                 j 
                               
                             
                             
                               0 
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               ⋱ 
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               0 
                             
                             
                               
                                 VOP 
                                 j 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                   ; 
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     where P is the number of pulses being designed. As mentioned earlier, x(t) is the concatenation of the RF values played on every channel and all pulses at time, t. As such, the constraint defined by Eqn. (5) represents the SAR value averaged over all pulses associated with the j th  VOP. When a SAR compression method other than the VOP method is used, the compressed SAR matrix samplings will replace the VOP j  terms in Eqn. (9). 
     The optimization problem in Eqn. (3) subject to the constraints of Eqns. (5)-(8) is convex. Notably, the least squares objective is convex and the constraints define a convex set because all SAR matrices are semi-definite positive. To solve this optimization problem, a primal/dual interior point algorithm, such as the one described by S. P. Boyd and L. Vandenberghe in  Convex Optimization  (Cambridge Univ. Press, 2004) can be used. In spokes pulses, the number of unknowns (e.g., the spokes amplitudes) is typically small (e.g., ≦100) whereas the number of SAR and power constraints is typically large (e.g., ≧1000). Therefore, each Newton iteration of the primal/dual procedure can be solved exactly using a Schur complement, which is a square matrix of size equal to the number of unknowns. Using the Schur complements allows for exact and fast inversion. The convergence of the primal/dual iterations can be tuned by using a Karush-Kuhn-Tucker relaxation parameter that is iteration dependent. For instance, a large relaxation can be used when constraints are active and almost no relaxation can be used when no constraint is active. 
     Referring now to  FIG. 1 , a flowchart setting forth the steps of an example of a method for designing parallel transmission RF pulses for multislice MRI applications is illustrated. The method begins with the selection of a target magnetization, b, to be achieved in two or more slice locations during the multislice imaging application, as indicated at step  102 . A system matrix, A, is then formed, as indicated at step  104 . By way of example, the system matrix, A, can be formed in accordance with a small tip angle magnetic resonance signal model, such as the one in Eqn. (4). This step may thus include providing B1+ sensitivity maps, which can be obtained using any number of methods known in the art. Similarly, this step may include providing transmit k-space trajectories, such as spokes trajectories. 
     Using the target magnetization, b, and the system matrix, A, a set of optimal RF waveforms is determined, as indicated at step  106 . Preferably, these RF waveforms are determined using the optimization problem set forth in Eqn. (3). This optimization is constrained using one or more of the following constraints: a local SAR constraint, as indicated at  108  and described above in Eqn. (5); a global SAR constraint, as indicated at  110  and described above in Eqn. (6); a peak power constraint, as indicated at  112  and described above in Eqn. (7); and an average power constrain, as indicated at  114  and described above in Eqn. (8). It is noted again that the local SAR constraint  108  is a constraint on the average local SAR attributable to all of the simultaneously designed RF pulses, and not the local SAR attributable to each RF pulse in isolation. This constraint thus automatically enforces the SAR hopping concept described above. In addition, it is noted that the local SAR constraint  108  and the global SAR constraint  110  are assessed at compressed SAR matrix points, which may be VOPs, thereby reducing the computational complexity of the RF design process. 
     The method thus proceeds by providing the determined optimal RF waveforms to an MRI system, as indicated at step  116 . The MRI system is then capable of employing these RF waveforms to produce pTx RF pulses to achieve the target magnetization in multiple slice locations in accordance with the desired multislice imaging application. 
     Referring particularly now to  FIG. 2 , an example of a magnetic resonance imaging (“MRI”) system  200  that can implement the present invention is illustrated. The MRI system  200  includes an operator workstation  202 , which will typically include a display  204 ; one or more input devices  206 , such as a keyboard and mouse; and a processor  208 . The processor  208  may include a commercially available programmable machine running a commercially available operating system. The operator workstation  202  provides the operator interface that enables scan prescriptions to be entered into the MRI system  200 . In general, the operator workstation  202  may be coupled to four servers: a pulse sequence server  210 ; a data acquisition server  212 ; a data processing server  214 ; and a data store server  216 . The operator workstation  202  and each server  210 ,  212 ,  214 , and  216  are connected to communicate with each other. For example, the servers  210 ,  212 ,  214 , and  216  may be connected via a communication system  240 , which may include any suitable network connection, whether wired, wireless, or a combination of both. As an example, the communication system  240  may include both proprietary or dedicated networks, as well as open networks, such as the internet. 
     The pulse sequence server  210  functions in response to instructions downloaded from the operator workstation  202  to operate a gradient system  218  and a radiofrequency (“RF”) system  220 . Gradient waveforms necessary to perform the prescribed scan are produced and applied to the gradient system  218 , which excites gradient coils in an assembly  222  to produce the magnetic field gradients G x , G y , and G z  used for position encoding magnetic resonance signals. The gradient coil assembly  222  forms part of a magnet assembly  224  that includes a polarizing magnet  226  and a whole-body RF coil  228 . The whole-body RF coil  228  may include a single-channel transmit coil or a multiple-channel transmit coil array that is capable of implementing the method of the present invention. For instance, the whole-body RF coil  228  may include a two channel transmit coil array. 
     RF waveforms are applied by the RF system  220  to the RF coil  228 , or a separate local coil (not shown in  FIG. 2 ), in order to perform the prescribed magnetic resonance pulse sequence. Responsive magnetic resonance signals detected by the RF coil  228 , or a separate local coil (not shown in  FIG. 2 ), are received by the RF system  220 , where they are amplified, demodulated, filtered, and digitized under direction of commands produced by the pulse sequence server  210 . The RF system  220  includes one or more RF transmitters for producing a wide variety of RF pulses used in MRI pulse sequences. The RF transmitter is responsive to the scan prescription and direction from the pulse sequence server  210  to produce RF pulses of the desired frequency, phase, and pulse amplitude waveform. The generated RF pulses may be applied to the whole-body RF coil  228  or to one or more local coils or coil arrays (not shown in  FIG. 2 ). 
     The RF system  220  also includes one or more RF receiver channels. Each RF receiver channel includes an RF preamplifier that amplifies the magnetic resonance signal received by the coil  228  to which it is connected, and a detector that detects and digitizes the I and Q quadrature components of the received magnetic resonance signal. The magnitude of the received magnetic resonance signal may, therefore, be determined at any sampled point by the square root of the sum of the squares of the I and Q components:
 
 M =√{square root over (I 2   +Q   2 )}  (10);
 
     and the phase of the received magnetic resonance signal may also be determined according to the following relationship: 
     
       
         
           
             
               
                 
                   φ 
                   = 
                   
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           Q 
                           I 
                         
                         ) 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     The pulse sequence server  210  also optionally receives patient data from a physiological acquisition controller  230 . By way of example, the physiological acquisition controller  230  may receive signals from a number of different sensors connected to the patient, such as electrocardiograph (“ECG”) signals from electrodes, or respiratory signals from a respiratory bellows or other respiratory monitoring device. Such signals are typically used by the pulse sequence server  210  to synchronize, or “gate,” the performance of the scan with the subject&#39;s heart beat or respiration. 
     The pulse sequence server  210  also connects to a scan room interface circuit  232  that 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  232  that a patient positioning system  234  receives commands to move the patient to desired positions during the scan. 
     The digitized magnetic resonance signal samples produced by the RF system  220  are received by the data acquisition server  212 . The data acquisition server  212  operates in response to instructions downloaded from the operator workstation  202  to receive the real-time magnetic resonance data and provide buffer storage, such that no data is lost by data overrun. In some scans, the data acquisition server  212  does little more than pass the acquired magnetic resonance data to the data processor server  214 . However, in scans that require information derived from acquired magnetic resonance data to control the further performance of the scan, the data acquisition server  212  is programmed to produce such information and convey it to the pulse sequence server  210 . For example, during prescans, magnetic resonance data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server  210 . As another example, navigator signals may be acquired and used to adjust the operating parameters of the RF system  220  or the gradient system  218 , or to control the view order in which k-space is sampled. In still another example, the data acquisition server  212  may also be employed to process magnetic resonance signals used to detect the arrival of a contrast agent in a magnetic resonance angiography (“MRA”) scan. By way of example, the data acquisition server  212  acquires magnetic resonance data and processes it in real-time to produce information that is used to control the scan. 
     The data processing server  214  receives magnetic resonance data from the data acquisition server  212  and processes it in accordance with instructions downloaded from the operator workstation  202 . Such processing may, for example, include one or more of the following: reconstructing two-dimensional or three-dimensional images by performing a Fourier transformation of raw k-space data; performing other image reconstruction algorithms, such as iterative or backprojection reconstruction algorithms; applying filters to raw k-space data or to reconstructed images; generating functional magnetic resonance images; calculating motion or flow images; and so on. 
     Images reconstructed by the data processing server  214  are conveyed back to the operator workstation  202  where they are stored. Real-time images are stored in a data base memory cache (not shown in  FIG. 2 ), from which they may be output to operator display  212  or a display  236  that is located near the magnet assembly  224  for use by attending physicians. Batch mode images or selected real time images are stored in a host database on disc storage  238 . When such images have been reconstructed and transferred to storage, the data processing server  214  notifies the data store server  216  on the operator workstation  202 . The operator workstation  202  may be used by an operator to archive the images, produce films, or send the images via a network to other facilities. 
     The MRI system  200  may also include one or more networked workstations  242 . By way of example, a networked workstation  242  may include a display  244 ; one or more input devices  246 , such as a keyboard and mouse; and a processor  248 . The networked workstation  242  may be located within the same facility as the operator workstation  202 , or in a different facility, such as a different healthcare institution or clinic. 
     The networked workstation  242 , whether within the same facility or in a different facility as the operator workstation  202 , may gain remote access to the data processing server  214  or data store server  216  via the communication system  240 . Accordingly, multiple networked workstations  242  may have access to the data processing server  214  and the data store server  216 . In this manner, magnetic resonance data, reconstructed images, or other data may be exchanged between the data processing server  214  or the data store server  216  and the networked workstations  242 , such that the data or images may be remotely processed by a networked workstation  242 . This data may be exchanged in any suitable format, such as in accordance with the transmission control protocol (“TCP”), the internet protocol (“IP”), or other known or suitable protocols. 
     As shown in  FIG. 2 , the RF system  220  may be connected to the whole-body RF coil  228 , or, as shown in  FIG. 3 , a transmission section of the RF system  220  may connect to one or more transmit channels  302  of an RF coil array  304  and a receiver section of the RF system  220  may connect to one or more receiver channels  306  of the RF coil array  304 . The transmit channels  302  and the receiver channels  306  are connected to the RF coil array  304  by way of one or more transmit/receive (“T/R”) switches  308 . In alternative configurations of the RF system  228  in which the receive coils are a separate collection of coils than the transmit coils, T/R switches  308  are not needed and are not used. Instead, in such a configuration the receive array is “detuned” during transmission so that it does not couple to the transmitter. Likewise, during reception, the transmitter is detuned. In this manner, the transmit and receive paths do not mix. 
     Referring particularly to  FIG. 3 , the RF system  220  includes one or more transmit channels  302  that produce a prescribed RF excitation field. The base, or carrier, frequency of this RF excitation field is produced under control of a frequency synthesizer  310  that receives a set of digital signals from the pulse sequence server  210 . These digital signals indicate the frequency and phase of the RF carrier signal produced at an output  312 . The RF carrier is applied to a modulator and up converter  314  where its amplitude is modulated in response to a signal, R(t), also received from the pulse sequence server  210 . The signal, R(t), defines the envelope of the RF excitation pulse to be produced and is produced by sequentially reading out a series of stored digital values. These stored digital values may be changed to enable any desired RF pulse envelope to be produced. 
     The magnitude of the RF excitation pulse produced at output  316  may be attenuated by an exciter attenuator circuit  318  that receives a digital command from the pulse sequence server  210 . The attenuated RF excitation pulses are then applied to a power amplifier  320  that drives the RF coil array  304 . 
     The MR signal produced by the subject is picked up by the RF coil array  302  and applied to the inputs of the set of receiver channels  306 . A preamplifier  322  in each receiver channel  306  amplifies the signal, which is then attenuated by a receiver attenuator  324  by an amount determined by a digital attenuation signal received from the pulse sequence server  210 . The received signal is at or around the Larmor frequency, and this high frequency signal is down converted in a two step process by a down converter  326 . The down converter  326  first mixes the MR signal with the carrier signal on line  312  and then mixes the resulting difference signal with a reference signal on line  328  that is produced by a reference frequency generator  330 . The down converted MR signal is applied to the input of an analog-to-digital (“A/D”) converter  332  that samples and digitizes the analog signal. As an alternative to down conversion of the high frequency signal, the received analog signal can also be detected directly with an appropriately fast analog-to-digital (“A/D”) converter and/or with appropriate undersampling. The sampled and digitized signal is then applied to a digital detector and signal processor  334  that produces 16-bit in-phase (I) values and 16-bit quadrature (Q) values corresponding to the received signal. The resulting stream of digitized I and Q values of the received signal are output to the data acquisition server  212 . In addition to generating the reference signal on line  328 , the reference frequency generator  330  also generates a sampling signal on line  336  that is applied to the A/D converter  332 . 
     Thus, a method for designing pTx RF pulses that allows generating slice selective pulses capable of creating highly uniform excitation patterns at multiple slice locations while creating negligible SAR deposition in the patient (&lt;10 W/kg @ 100% duty cycle) has been provided. The method includes designing multiple RF pulses simultaneously subject to local SAR constraints for the average of all pulses, thereby enforcing “SAR hopping”. The method is fast enough (e.g., designing a pair of 2-spokes pulses in 23 seconds) to be used in clinic. 
     The method of the present invention improves upon other RF pulse design method by accounting for ghosting artifacts and sub-optimality. Previous “SAR hopping” techniques, such as those described by I. Graesslin, et al., in “Local SAR constrained hotspot reduction by temporal averaging,”  Proceedings of the International Society for Magnetic Resonance in Medicine , Stockholm, Sweden, 4932, produce ghosting artifacts and inaccurate image contrast because distinct pulses are used to acquire a single slice of k-space data. Because it is not possible to design pulses with exactly the same excitation phase profiles, phase variation between the excitation of different k-space lines creates ghosting. Moreover, variations in the magnitude of the excitation profiles of different pulses create a varying flip-angle weighting between excitations and therefore introduce inaccurate contrast in reconstructed images. These difficulties are overcome by designing multiple pulses exciting multiple slices. That is, the RF pulses designed using the method of the present invention are implemented in multi-slice acquisition strategies using slice selective excitation pulses. In this approach, a given slice is always excited using the same pulse and is therefore free of ghosting artifacts and inaccurate image contrast. 
     Sub-optimality refers to the fact that the currently available RF pulse design techniques yield pulses that, in general, do not achieve the best possible tradeoff between local SAR and excitation fidelity. This is because these techniques do not explicitly control local SAR in the pulse design process, nor do they design all pulses simultaneously within a single optimization scheme. The method of the present invention, however, is optimal because it designs multiple pulses simultaneously while explicitly constraining the local SAR of the average of all pulses. Because the resulting optimization problem is convex and a convergent algorithm is used to solve it, the pulses computed with this technique achieve an optimal tradeoff between the local SAR of all the pulses and excitation fidelity. 
     It is noted again that an advantageous feature of the present invention is that local SAR is constrained explicitly in the pulse design process. This is in contrast with most previous techniques, which have focused on regularization of global SAR or average pulse power. This feature of the present invention is achieved in part by compressing the SAR matrices at all locations in the body to a dramatically smaller set of compressed SAR matrix points, which may be VOPs. Using this more manageable set of SAR matrices, pulses can be designed with explicit local SAR constraints within reasonable computation time. Note that a constrained optimization algorithm can be used to calculate least square pulses that satisfy simultaneous constraints on local SAR (via compressed SAR matrix points), global SAR, average power on every channel, and peak power on every channel. This is in contrast with regularized pulse design strategies that indirectly control the tradeoff between SAR, power, and fidelity via regularization parameters that the user needs to determine (soft constraints). The method of the present invention does not depend on regularization parameters; instead, SAR and power limits are explicitly enforced (hard constraints). This feature of the pulse design algorithm considerably simplifies the task of the operator during in vivo scans and improves the quality of images by automatically finding optimal tradeoffs between local SAR, global SAR, peak and average pulse power, and excitation fidelity. 
     The two pulse design modules described herein (explicit local SAR control and “SAR hopping”) can also be used independently. Used individually, they allow reducing local SAR at constant excitation error by twenty percent (“SAR hopping”) and sixty-four percent (local SAR control). Used in combination, they allow reducing local SAR at constant excitation error by seventy-seven percent compared to only constraining global SAR. 
     The present invention has been described in terms of one or more preferred embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention.