Abstract:
The present invention is directed to a process and system for removing artifacts resulting from the interactions of a preparation sequence and a gradient echo sequence. A process for suppressing stimulated echoes for spatial preparation sequences without affecting sequence timing or image contrast is disclosed. Stimulated echoes often form when gradient crushers of a preparation sequence are constant in magnitude and direction throughout the imaging sequence. Changing the direction or effective areas of these gradient crushers throughout imaging acquisition prevents stimulated echoes from forming, thereby eliminating ghosting and blurring artifacts. By applying unbalanced gradients or sets of unbalanced gradients between selective RF pulses of an RF pulse sequence, signals attributable to spin transverse magnetization are suppressed resulting in reduced ghosting and/or artifacts. Additionally, the process varies the phase of each RF pulse with each application of the RF pulse to minimize any artifacts generated by stimulated echoes.

Description:
BACKGROUND OF INVENTION 
     The present invention relates generally to MR imaging and, more particularly, to a method and system to remove artifacts caused by interactions of a preparation sequence and a gradient echo sequence in MR imaging. 
     Typically, MR systems apply chemical shift preparation sequences commonly referred to as “fat suppression” to an imaging space to suppress signals attributable to fat within an imaging subject. Reconstructing images after employing a chemical shift preparation sequence often produces a cleaner and more discernable image for diagnostic analysis. There are several known techniques of performing chemical shift imaging including a saturation technique. 
     With the saturation method, a frequency selective saturation pulse is applied before standard RF pulses of an imaging sequence, e.g., a spin-echo sequence. The saturation pulse operates to set the magnetization along the transverse plane of the component to be suppressed to zero. As a result, the application of the standard imaging sequence thereafter significantly reduces any signal from the suppressed component. Therefore, most signals from the suppressed component, i.e. fat, are not detected and used in final image reconstruction. A spatial saturation preparation sequence using slice selective RF pulses instead of chemically selective RF pulses may also be used to suppress spatial components of an imaging subject. Unfortunately, application of either a fat suppression sequence or a spatial saturation sequence may interact with a subsequent gradient echo sequence to generate unwanted stimulated echo artifacts. 
     Multi-sliced gradient echo images acquired with fat or spatial suppression preparation sequences often exhibit ghosts and/or blurring from the suppressed signal. These artifacts, which are typically generated by stimulated echoes, allow the suppressed signal from one slice to be refocused into another slice and are created through a combination of chemically selective and/or spatially selective RF pulses with a gradient echo sequence. If there is a constant phase between the suppression sequence RF pulses and the gradient echo RF pulses, then the suppressed signal appears to be blurred from slice to slice. If there is a linear phase shift from view to view between RF pulses, then the refocused signal will oftentimes appear as a severe ghost in the final reconstructed image. 
     Referring to FIG. 1, a schematic representation of an imaging space  2  with refocused stimulated echoes is shown. Specifically, FIG. 1 illustrates the refocusing of a signal excited by a preparation sequence from one slice into another. A protocol was prescribed such that half of the prescribed slices were outside the phantom and half the slices were inside the phantom. Prescribing slices outside the phantom provides a convenient method to observe ghosts generated by the stimulated echoes. FIG. 1 is a collection of axial images acquired with a gradient echo sequence with a spatial saturation applied through the center of the phantom. As shown, the signal excited by the intersection  4  of the preparation sequence  6  and the slice selective RF pulses  8  is refocused from one slice to another slice. This cross-talk results in severe ghosting occurring in a final reconstructed image. Ultimately, signals from stimulated echoes are refocused from one slice into another yielding a contaminated reconstructed image. 
     It would therefore be desirable to design a system implementing a technique that removes artifacts caused by interactions of a preparation sequence and a gradient echo sequence in MR imaging. 
    
    
     BRIEF DESCRIPTION OF INVENTION 
     The present invention is directed to a process and system for removing artifacts resulting from the interactions of a preparation sequence and a gradient echo sequence in MR imaging. The present invention includes a process for suppressing stimulated echoes created from preparation sequences without affecting sequence timing or image contrast. Stimulated echoes often form when gradient crushers of a preparation sequence are constant in magnitude and direction throughout the imaging sequence. Changing the direction of these gradient crushers throughout the imaging acquisition prevents stimulated echoes from forming, thereby eliminating ghosting and blurring artifacts. By applying unbalanced gradients or sets of unbalanced gradients between selective RF pulses of an RF pulse sequence, signals attributable to spin transverse magnetization are suppressed resulting in a reconstructed image with reduced ghosting and/or artifacts. Additionally, the present invention includes a process that varies the phase of each RF pulse with each application of the RF pulse to minimize any artifacts generated by stimulated echoes. 
     Therefore, in accordance with one aspect of the invention, a method is provided to remove artifacts caused by interactions of a preparation sequence and a gradient echo sequence in MR imaging. The method includes the steps of applying a first train of RF pulses to an imaging space and applying a set of gradients having an effective gradient area G 1  across the imaging space. The method also includes the steps of applying a second train of RF pulses to the imaging space followed by application of another set of gradients having an effective gradient area G 2 , wherein G 1 ≠|G 2 |. 
     In accordance with another aspect of the present invention, a computer program includes a set of instructions that when executed by a computer causes the computer to apply a first preparation sequence of RF pulses to an imaging subject. The transverse magnetization generated by the RF pulses is then crushed by applying a gradient factor, G 1 , before a second spatial saturation pulse sequence. The computer program further causes the computer to create a gradient field having a gradient factor G 2  across the imaging space so as to suppress the stimulated echo caused by the first preparation pulse sequence. The gradient fields are applied such that the gradient factor G 1  of the first applied gradient field and the gradient factor G 2  of the second applied gradient field are unbalanced with respect to one another. 
     In accordance with yet another aspect of the present invention, an MR apparatus includes a magnetic resonance imaging system having a plurality of grading coils positioned about a bore of a magnet to impress a polarizing magnetic field and an RF transceiver system. The MR apparatus further includes an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images. The apparatus also includes a computer programmed to determine a number of RF pulses of a preparation sequence and then determine a phase increment φ o  for the preparation sequence. The computer is further programmed to determine a phase variation factor φ n  and then apply an RF pulse having a phase variation factor of φ n  to an imaging space. 
     Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings. 
     BRIEF DESCRIPTION OF DRAWINGS 
     The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
     In the drawings: 
     FIG. 1 is a schematic representation of refocused signals extending from one slice into another as a result of interactions between a preparation sequence and a gradient echo sequence using conventional imaging techniques. 
     FIG. 2 is a schematic block diagram of an MR imaging system for use with the present invention. 
     FIG. 3 is a flow chart setting forth the steps of a process for removing artifacts caused by interactions of a preparation sequence and a gradient echo sequence in accordance with the present invention. 
     FIG. 4 is a flow chart setting forth the steps of a process for minimizing artifacts in MR imaging in accordance with the present invention. 
     FIG. 5 is a flow chart setting forth the steps for a comprehensive process to suppress artifacts caused by interactions of a preparation sequence and a gradient echo sequence employing the processes set forth in FIGS.  3 - 4 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 2, the major components of a preferred magnetic resonance imaging (MRI) system  10  incorporating the present invention are shown. The operation of the system is controlled from an operator console  12  which includes a keyboard or other input device  13 , a control panel  14 , and a display  16 . The console  12  communicates through a link  18  with a separate computer system  20  that enables an operator to control the production and display of images on the screen  16 . The computer system  20  includes a number of modules which communicate with each other through a backplane  20   a . These include an image processor module  22 , a CPU module  24  and a memory module  26 , known in the art as a frame buffer for storing image data arrays. The computer system  20  is linked to disk storage  28  and tape drive  30  for storage of image data and programs, and communicates with a separate system control  32  through a high speed serial link  34 . The input device  13  can include a mouse, joystick, keyboard, track ball, touch activated screen, light wand, voice control, or any similar or equivalent input device, and may be used for interactive geometry prescription. 
     The system control  32  includes a set of modules connected together by a backplane  32   a . These include a CPU module  36  and a pulse generator module  38  which connects to the operator console  12  through a serial link  40 . It is through link  40  that the system control  32  receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module  38  operates the system components to carry out the desired scan sequence and produces data which indicates the timing, strength and shape of the RF pulses produced, and the timing and length of the data acquisition window. The pulse generator module  38  connects to a set of gradient amplifiers  42 , to indicate the timing and shape of the gradient pulses that are produced during the scan. The pulse generator module  38  can also receive patient data from a physiological acquisition controller  44  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes attached to the patient. And finally, the pulse generator module  38  connects to a scan room interface circuit  46  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  46  that a patient positioning system  48  receives commands to move the patient to the desired position for the scan. 
     The gradient waveforms produced by the pulse generator module  38  are applied to the gradient amplifier system  42  having G x , G y , and G z  amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly generally designated  50  to produce the magnetic field gradients used for spatially encoding acquired signals. The gradient coil assembly  50  forms part of a magnet assembly  52  which includes a polarizing magnet  54  and a whole-body RF coil  56 . A transceiver module  58  in the system control  32  produces pulses which are amplified by an RF amplifier  60  and coupled to the RF coil  56  by a transmit/receive switch  62 . The resulting signals emitted by the excited nuclei in the patient may be sensed by the same RF coil  56  and coupled through the transmit/receive switch  62  to a preamplifier  64 . The amplified MR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  58 . The transmit/receive switch  62  is controlled by a signal from the pulse generator module  38  to electrically connect the RF amplifier  60  to the coil  56  during the transmit mode and to connect the preamplifier  64  to the coil  56  during the receive mode. The transmit/receive switch  62  can also enable a separate RF coil (for example, a surface coil) to be used in either the transmit or receive mode. 
     The MR signals picked up by the RF coil  56  are digitized by the transceiver module  58  and transferred to a memory module  66  in the system control  32 . A scan is complete when an array of raw k-space data has been acquired in the memory module  66 . This raw k-space data is rearranged into separate k-space data arrays for each image to be reconstructed, and each of these is input to an array processor  68  which operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  34  to the computer system  20  where it is stored in memory, such as disk storage  28 . In response to commands received from the operator console  12 , this image data may be archived in long term storage, such as on the tape drive  30 , or it may be further processed by the image processor  22  and conveyed to the operator console  12  and presented on the display  16 . 
     The present invention includes a system and process for suppressing stimulated echoes that result from the interaction of a preparation sequence with a gradient echo sequence. Stimulated echoes often form when gradient crushers of the preparation sequence are constant in magnitude and direction throughout an imaging sequence. A stimulated echo may be created when spins of nuclei within the imaging space are excited by three RF pulses and no gradients are applied. Stimulated echoes may also be created in the presence of applied gradients if the gradient area between a first and second RF pulse and after a third RF pulse is balanced. That is, stimulated echoes may form if the gradient area between the first and second RF pulse equals the gradient area after the third RF pulse. For example, prior to a first RF pulse, the spins are in equilibrium. That is, the spins are oriented along a longitudinal equilibrium. Application of a first RF pulse excites the spins from their longitudinal equilibrium position into the transverse plane wherein the spins have a flip angle of τ/2 and a phase of y. This condition may be represented by:                  I   Z          →       π              /   2          |   y              I   X       =       1   /   2          (       I   -     -     I   +       )               (Eqn.  1)                                
     wherein I Z  represents the orientation of the spins along a longitudinal equilibrium whereas I X  is representative of spin position within the transverse plane. 
     Using Cartesian based product operators I X , I Y , I Z  it becomes clear that the RF pulse rotates the spins into the transverse plane, which is similar to the classical interpretation of the effect of an RF pulse on a spin. While it is certainly possible to represent the effect of gradients and/or RF pulses as they are applied to a spin system using Cartesian expressions, the present invention implements Annihilation and Creation (AC) product operators, I+ and I−, to represent the effect of gradients and RF pulses on a spin system. Simple transformations between Cartesian and AC expressions are set forth in the table below. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Cartesian 
                 Annihilation and Creation 
               
               
                   
                   
               
             
             
               
                   
                 I x   
                 ½ (I −  − I + ) 
               
               
                   
                 I y   
                 ½ (I −  + I + ) 
               
               
                   
                 I x  − i I y   
                 I −   
               
               
                   
                 I x  + I y   
                 I +   
               
               
                   
                   
               
             
          
         
       
     
     As indicated previously, after application of the first RF pulse, the spins within the imaging space rotate into the transverse plane. As a result, the transverse magnetization, I Z , of the spins is converted into its AC equivalent. After applying a set of gradient magnetic fields across the imaging space and after the first RF pulse, the transverse magnetization of the spins is suppressed or “crushed”. This “crushing” effect may be represented by the following equation:                  1   /   2          (       I   -     -     I   +       )                         →     G   1              1   /   2          (                -   2π                   γ                     G1   _     ·     r   _                I   -       -                               +   2π                   γ                     G1   _     ·     r   _                I   +         )                 (Eqn.  2)                                
     Application of a second RF pulse of a train of RF pulses excites half of the crushed transverse magnetization back to the longitudinal position and may be defined by the following equation:                  1   /   2          (       I   -     -     I   +       )                         →     G   1                1   /   2          (                -   2π                   γ                     G1   _     ·     r   _                I   -       -                               +   2π                   γ                     G1   _     ·     r   _                I   +         )                       →       π   /   2          |   y                  1   /   4          (              -   2π                   γ                     G1   _     ·     r   _           +            +   2π                   γ                     G1   _     ·     r   _             )          I   -       -       1   /   4          (              -   2π                   γ                     G1   _     ·     r   _           +            +   2π                   γ                     G1   _     ·     r   _             )          I   +       +                  1   /   2          (            2π                 γ                     G1   _     ·     r   _           -            -   2π                   γ                     G1   _     ·     r   _             )          I   Z                     (Eqn.  3)                                
     This condition following application of the second RF pulse is commonly referred to as “storing magnetization along z”. Since the stored magnetization is longitudinal, application of a set of gradients after the second RF pulse has no effect on this magnetization. However, if the second set of gradient pulses has the same effective area as the first set of gradient pulses, the spins will be refocused to form an echo, i.e., a Hahn echo. Conversely, however, if the second set of gradient pulses has an effective area unequal to the effective area of the first set of gradient pulses, the transverse magnetization will be further crushed. 
     Following a third RF pulse, the spins that were stored along z in the transverse plane return and may be defined by the following equation:                    1   /   2          (            2π                 γ                     G1   _     ·     r   _           -            -   2π                   γ                     G1   _     ·     r   _             )          I   Z                       →       π   /   2          |   y                  1   /   4          (              +   2π                   γ                     G1   _     ·     r   _           -            -   2π                   γ                     G1   _     ·     r   _             )          I   -       +       1   /   4          (              -   2π                   γ                     G1   _     ·     r   _           -            +   2π                   γ                     G1   _     ·     r   _             )          I   +                              (Eqn.  4)                                
     After the third RF pulse, the spins that have been returned to the transverse plane are still crushed by the first set of gradients, however, if a third set of gradients has the same effective area as the first set of gradients, the spins are refocused and a stimulated echo results. This condition may be defined by the following equation:                    1   /   4          (              +   2π                   γ                     G1   _     ·     r   _           -            -   2π                   γ                     G1   _     ·     r   _             )          I   -       +                  1   /   4          (              -   2π                   γ                     G1   _     ·     r   _           -            +   2π                   γ                     G1   _     ·     r   _             )          I   +                           →   G3              1   /   4          (              +   2π                       γ   (                    G   1     _     -       G   3     _       )     ·     r   _           -            -   2π                   γ                     (         G   1     _     +       G   3     _       )     ·     r   _             )          I   -       +       1   /   4          (              -   2π                       γ   (                    G   1     _     -       G   3     _       )     ·     r   _           -            +   2π                   γ                     (           G   1     _               +       G   3     _       )     ·     r   _             )          I   +                   (Eqn.  5)                                
     Simply, a stimulated echo is formed when G 1 =|G 3 |. However, application of a third set of gradients having an effective gradient area G 3  wherein G 1 ≠|G 3 | will be sufficient to suppress ghosting of artifact causing signals. 
     Accordingly, the present invention includes a process for applying sets of gradients having unbalanced effective gradient areas to an imaging space to reduce artifacts caused by interactions between a preparation sequence and a gradient echo sequence. An effective gradient area may be defined as the area sum of all crusher, readout, phase encode, and slice encode gradients necessary to acquire MR data. Referring to FIG. 3, a process  100  for suppressing artifacts caused by the interaction between a preparation pulse sequence and a gradient echo sequence begins at  102  with the application of a preparation sequence with n RF pulses and effective gradient GSAT 1  at  103 . The preparation sequence gradients GSAT 1  and the imaging gradients combine to have an effective gradient G 1 . At  104 , gradient echoes are acquired for slice  1  of a first slice group  104 ( a ) following a slice selective RP pulse of the gradient echo imaging sequence. The present invention contemplates selective application of gradient crushers. Therefore, at  105  the process determines if another preparation sequence should be applied for slice group  1 . Typically, a slice group comprises four slices. That is, in one preferred embodiment, a preparation sequence with n RF pulses is applied every fourth slice. Therefore, if no additional preparation sequences need to he applied  105 ,  106 , the slice is incremented at  107  by one for subsequent echo acquisition at  104 . If however, another preparation sequence is to be applied  105 ,  108 , another preparation sequence with n RF pulses and an effective gradient GSAT 2  is applied at  109 . Imaging gradients from slices after application of the slice selective gradient at  104  and before application of the preparation sequence with n RF pulses at  109 , combine to form an effective gradient G 2 . This effective gradient includes all readout, phase and slice encode gradients necessary to acquire data. If G 2  equals G 1 , the spins of the nuclei within the imaging space will be refocused to form a Hahn echo. However, if G 2  does not equal G 1 , half of the previously crushed transverse magnetization will be excited back to a longitudinal position, i.e., storing magnetization along z. 
     At  110 , gradient echoes are acquired for a first slice of a second slice group  110 ( a ). In one embodiment, the slices of slice group  1  must be at a different location than the slices of slice group  2 . Similar to acquisition of gradient echoes for slice group  1 , acquisition of gradient echoes from slice group  2  continues at  111  with a determination as to whether another preparation sequence should be applied. If not  111 ,  112 , the slice is incremented at  113  for additional gradient echo acquisition at  110 . However, if another preparation sequence is to be applied  114 , i.e., another slice group, the process continues to  115  to determine if all data necessary for image reconstruction has been acquired. If not  116 , the process returns to  103  for further data acquisition with gradient crushing whenever necessary. 
     Preparation sequence gradients GSAT 2  and imaging gradients applied after application of the preparation sequence at  109  combine to have an effective gradient G 3 . This effective gradient includes all crusher, readout, phase encode, and slice encode gradients necessary to acquire data. Moreover, to suppress stimulated echoes forming as a result of interaction between the preparation sequence and the gradient echo sequences, the effective gradient area G 3 ≠G 1 . Selection/determination of the appropriate gradient area to crush the stimulated echoes may be determined by a number of methods including a direct analysis of the spin of the nucleus or a historical analysis of the previously applied gradients. 
     Still referring to FIG. 3 if all data has been acquired for image reconstruction  115 ,  117 , an image is reconstructed at  118  whereupon the process concludes at  120 . 
     A stimulated echo forms when the gradients before acquisition of gradient echoes for slice  1  and after application of the preparation sequence and effective gradient G 2  are balanced with respect to one another. The effective gradient G 1  is composed of the preparation suppression sequence, GSAT 1 , and all imaging gradients before acquisition of the gradient echo from slice  1  at  104 . Since the signal for acquisition of gradient echoes from slice  1  at  104  represents any slice in slice group  1 , the effective gradient G 1  will change according to which slice is represented by the signal  104 . Effective gradient G 2  is composed of all imaging gradients after application of the slice selective signal at  104  and before application of the preparation sequence at  109 . The effective crushing area of G 2  will also change depending upon which slice selective RF pulse is implemented or used at  104 . The effective gradient at G 3  is composed of the gradients after application of the preparation sequence at  109  and before application of the slice selective RF pulse at  110 . The slice selective RF pulse at  110  may represent any slice selective RF pulse in slice block  2 . However, the slice selective RF pulse from step  104  and the slice selective pulse from step  110  must represent the same n RF pulse in their respective slice group. That is, if the slice selective pulse applied at  104  represents the second slice selective RF pulse in slice group  1 , then the slice selective pulse applied at  110  must represent the second slice selective RF pulse in slice group  2 . This condition is necessary so the contribution of the imaging gradients for the effective gradient G 1  and G 2  are balanced. This allows the preparation sequence gradient to be modulated to crush the stimulated echoes. Essentially, the condition that G 1 ≠G 3  may be met if GSAT 1 ≠GSAT 2 . 
     In a preferred embodiment, three RF pulses are applied followed by three sets of gradient crushers. However, the present invention is not limited to application with a suppression preparation sequence having three RF pulses and/or application of three sets of gradients. That is, the present invention is applicable with a preparation sequence having n RF pulses and application of n sets of gradients. It remains important however, that the effective gradient areas be unbalanced. For example, the effective gradient area of an n th set of gradients must not equal the absolute value of gradient area G 1 . Simply, G n ≠|G 1 |. 
     In another embodiment of the present invention and referring to FIG. 4, a process  130  for minimizing artifacts generated by stimulated echoes begins at  132  by determining a number of RF pulses of a preparation sequence at  134 . A phase increment φ o  for the preparation sequence is then determined at  136 . A phase increment θ 0  for an imaging sequence is then determined at  138  and compared to the phase increment φ o  for the preparation sequence at  140 . If the phase increment φ o  is equal to the phase increment θ 0    142 , the process  130  terminates at  144 . Note that in an alternate embodiment as illustrated by the dashed lines, if φ o =θ 0  at  140 ,  142   a , the phase increment φ o  for the preparation sequence or the phase increment θ 0  for the imaging sequence may be reset such that φ o ≠θ 0  at  142   b . If φ o ≠θ 0    140 ,  144 , a phase varying factor φ n  is determined at  146 . In one embodiment, the phase varying factor φ n  is defined by the linear function φ n =n φ o , where n is the number of RF preparation pulses determined at  134 , and φ o  is the phase increment for the preparation sequence determined at  138 . The phase varying factor φ n  need not be defined at all times by this linear function. That is, any function may be used to define the phase varying factor φ n . 
     Following determination of the phase varying factor φ n  at  146 , each phase of an RF pulse of a preparation sequence is varied by the phase varying factor φ n  at  148 . For example, the phase of a first RF pulse would have a phase Φ 1  equal to Φ o . However, the phase of a second RF pulse would be defined as twice Φ o  or simply, Φ n = 2 Φ o . Following this pattern, the phase of a third RF pulse of a preparation sequence has a phase Φ 3 =to  3 Φ 0 . Ultimately, an n th  pulse would have phase Φ n =nΦ o . Altering the phase of each RF pulse each time the RF pulse is applied effectively spoils the preparation sequence and thereby minimizes artifacts generated by stimulated echoes which form as a result of interactions between a gradient echo sequence and a preparation sequence. To fully minimize the artifacts, the phase increment for the preparation sequence Φ o  must be independent of the phase increment for an imaging sequence θ 0 . That is, Φ o ≠θ 0 . Process  130  ends at  144  following phase varying of the last RF pulse. 
     Referring to FIG. 5, the steps of a process  150  incorporating another embodiment of the present invention are set forth. Process  150  is similar to process  100  of FIG. 3 insofar as applying unbalanced gradients to an imaging space, but incorporates the RF spoiling process  130  of FIG.  4 . Process  150  for suppressing artifacts caused by the interaction between a preparation pulse sequence and a gradient echo sequence begins at  152  with the application of a preparation sequence with n RF pulses and effective gradient GSAT 1  at  153 . The RF pulses are applied with a phase varying factor Φ n , where Φ n =nΦ o . Varying the phase of each RF pulse spreads the energy associated with each pulse throughout the image, thereby reducing artifacts in the final image. The preparation sequence gradients, GSAT 1 , and the imaging gradients combine to have an effective gradient G 1 . At  154 , gradient echoes are acquired for slice  1  of slice group  154 ( a ) following a slice selective RF pulse of the gradient echo imaging sequence. 
     The present invention contemplates selective application of gradient crushers. Therefore, at  155  the process determines if another preparation sequence should be applied for slice group  1 . Typically, a slice group comprises four slices. That is, in one preferred embodiment, a preparation sequence with n RF pulses is applied every fourth slice. Therefore, if no additional preparation sequences need to be applied  155 ,  156 , the slice is incremented at  157  by one for subsequent echo acquisition at  154 . If however, another preparation sequence is to be applied  155 ,  158 , another preparation sequence with n RF pulses and an effective gradient GSAT 2  is applied at  159 . Again, the phase of each RF pulse is varied by φ n  to spread the signal energy throughout the imaging space. Imaging gradients from slices after application of the slice selective gradient at  154  and before application of the preparation sequence with n RF pulses at  159 , combine to form an effective gradient G 2 . This effective grading includes all readout, phase and slice encode gradients necessary to acquire data. If G 2  equals G 1 , the spins of the nuclei within the imaging space will be refocused to form a Hahn echo. However, if G 2  does not equal G 1 , half of the previously crushed transverse magnetization will be excited back to a longitudinal position, i.e., storing magnetization along z. 
     At  160 , gradient echoes are acquired for a first slice of a second slice group  160 ( a ). In one embodiment, the slices of slice group  1  must be at a different location than the slices of slice block  2 . Similar to acquisition of gradient echoes for slice group  1 , acquisition of gradient echoes from slice group  2  continues at  161  with a determination as to whether another preparation sequence should be applied. If not  161 ,  162 , the slice is incremented at  163  for additional gradient echo acquisition at  160 . However, if another preparation sequence is to be applied  164 , the process continues to  165  to determine if all data necessary for image reconstruction has been acquired. If not  166 , the process returns to  153  for further data acquisition with gradient crushing when necessary. 
     Preparation sequence gradients GSAT 2  and imaging gradients applied after application of the preparation sequence at  159  combine to have an effective gradient G 3 . This effective gradient includes all crusher, readout, phase encode, and slicing encode gradients necessary to acquire data. Moreover, to suppress stimulated echoes forming as a result of interaction between the preparation sequence and the gradient echo sequences, is imperative, and in accordance with the present invention, that effect gradient area G 3 ≠G 1 . Selection/determination of the appropriate gradient area to crush the stimulated echoes may be determined by a number of methods including a direct analysis of the spin of the nucleus by itself or a historical analysis of the previously applied gradients. If all data has been acquired for image reconstruction  165 ,  167 , an image is reconstructed at  168  whereupon the process concludes at  170 . 
     In accordance with a further embodiment of the present invention, a computer program includes a set of instructions that when executed by a computer causes the computer to apply a preparation sequence of RF pulses to an imaging subject. The transverse magnetization generated by the RF pulses is then crushed by applying a gradient factor, G 1 . The computer program further causes the computer to create, before a second spatial saturation pulse sequence, a gradient field having a gradient factor G 2  across the imaging space so as to suppress the stimulated echo caused by the first spatial saturation pulse sequence. The gradient fields are applied such that the gradient factor G 1  of the first applied gradient field and the gradient factor G 2  of the second applied gradient field are unbalanced with respect to one another. 
     In a further embodiment of the present invention, an MR apparatus includes a magnetic resonance imaging system having a plurality of grading coils positioned about a bore of a magnet to impress a polarizing magnetic field and an RF transceiver system. The MR apparatus further includes an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly to acquire MR images. The apparatus also includes a computer programmed to determine a number of RF pulses of a preparation sequence and then determine a phase increment φ o  for the preparation sequence. The computer is further programmed to determine a phase variation factor φ n  and then apply an RF pulse having a phase of φ n  to an imaging space. 
     The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.