Abstract:
A volumetric APT imaging sequence is provided that acquires multi-slice images immediately after a single long continuous wave (CW) RF irradiation, wherein the relaxation-induced loss of CEST contrast is compensated for during post-processing. Thus, a fast volumetric pH-weighted APT imaging technique is provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on, incorporates herein by reference, and claims the benefit of U.S. Provisional Application Ser. No. 61/039,478, filed on Mar. 26, 2008, and entitled “METHOD FOR RELAXATION-COMPENSATED FAST MULTI-SLICE CHEMICAL EXCHANGE SATURATION TRANSFER MRI.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with United States government support awarded by the following agency: NIH/NINDS 5R01NSO38477-09. The United States has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The field of the invention is magnetic resonance imaging (MRI) and systems. More particularly, the invention relates to a method for chemical exchange saturation transfer MRI. 
         [0004]    When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the excited nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B 1 ) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M z , may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M t . A signal is emitted by the excited nuclei or “spins”, after the excitation signal B 1  is terminated, and this signal may be received and processed to form an image. 
         [0005]    When utilizing these “MR” signals to produce images, magnetic field gradients (G x , G y , and G z ) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. 
         [0006]    The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically proven pulse sequences and they also enable the development of new pulse sequences. 
         [0007]    The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space”. Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a roster scan-like pattern sometimes referred to as a “spin-warp”, a “Fourier”, a “rectilinear”, or a “Cartesian” scan. The spin-warp scan technique is discussed in an article entitled “Spin-Warp MR Imaging and Applications to Human Whole-Body Imaging” by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (G y ) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (G x ) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G y  is incremented (ΔG y ) in the sequence of measurement cycles, or “views” that are acquired during the scan to produce a set of k-space MR data from which an entire image can be reconstructed. 
         [0008]    An image is reconstructed from the acquired k-space data by transforming the k-space data set to an image space data set. There are many different methods for performing this task and the method used is often determined by the technique used to acquire the k-space data. With a Cartesian grid of k-space data that results from a 2D or 3D spin-warp acquisition, for example, the most common reconstruction method used is an inverse Fourier transformation (“2DFT” or “3DFT”) along each of the 2 or 3 axes of the data set. With a radial k-space data set and its variations, the most common reconstruction method includes “regridding” the k-space samples to create a Cartesian grid of k-space samples and then perform a 2DFT or 3DFT on the regridded k-space data set. In the alternative, a radial k-space data set can also be transformed to Radon space by performing a 1DFT of each radial projection view and then transforming the Radon space data set to image space by performing a filtered backprojection. 
         [0009]    Molecular imaging includes a variety of imaging modalities and employs techniques that detect molecular events such as cell signaling, gene expression, and pathologic biomarkers. These techniques seek to achieve early detection of diseases, better management of therapy treatment, and improved monitoring of cancer recurrence. MRI provides specific advantages for molecular imaging applications, due to its noninvasive nature. Traditional molecular MRI techniques rely on the administration of a contrast agent to a designated location within a subject. Oftentimes, a site-specific contrast agent is employed that interacts with a given molecule of interest. These conventional techniques, however, exhibit poor sensitivity, making the detection of the contrast agents difficult. This is especially true when imaging the brain, which has a natural barrier to exogenous chemicals. 
         [0010]    A new technique known as chemical exchange saturation transfer (CEST), serves as a useful new tool for molecular MRI. The CEST imaging method offers three advantages over traditional molecular MRI techniques. First, in some cases the molecules of interest within the subject can be directly detected. This feature mitigates the need for administering contrast agents to the subject. Second, the image contrast mechanism can be controlled with the RF pulses produced by the MRI system and, as such, can be turned on and off when desired. This allows the location of specific molecules of interest to be detected by comparing images having the desired contrast present to those where it has been turned off. Lastly, the CEST imaging method is far more sensitive than traditional molecular MRI techniques, making it able to detect substantially low concentrations of given molecules. 
         [0011]    CEST imaging renders MRI, which usually detects only bulk water signal, sensitive to metabolites and their byproducts, such as glucose and lactate. In particular, the chemical exchange between bulk water and amide protons from endogenous proteins and peptides has been shown to be sensitive to ischemic tissue acidosis, and as a result has given rise to an imaging technique referred to as amide proton transfer (APT) imaging. Since tissue pH decreases in response to abnormal glucose/oxygen metabolism during acute ischemia, pH-sensitive APT imaging may serve as a surrogate metabolic imaging marker for stroke. In that it complements perfusion and diffusion MRI, APT imaging may allow better characterization of penumbra for predicting ischemic tissue outcome in acute stroke. Moreover, APT imaging may eventually help guide thromobolytic and/or neuroprotective therapies for acute stroke. 
         [0012]    Unfortunately, CEST and APT imaging techniques are limited to acquiring single slices of image data. This limitation is largely a factor of the very long continuous wave (CW) RF irradiation, typically a few seconds, required to reach the steady state for quantitative APT imaging. CEST and APT imaging methods typically require lipid suppression techniques, which often take tens of milliseconds. As a result, multi-slice imaging techniques suffer from decreased CEST contrast as a result of longitudinal relaxation during preparation and image acquisition. Additionally, repeating RF irradiation for each slice is a very time-consuming procedure, and thus not feasible for in vivo applications. One potential alternative is to use a concatenated short irradiation RF pulse and image readout for each k-space line, similar to methods used in conventional magnetization transfer (MT) imaging. However, the saturation frequency bandwidth for short RF irradiation pulses can be very large and, as such, they may reduce the pH-specificity of APT imaging by introducing concomitant direct RF saturation (spillover) and conventional macromolecular MT effects. Moreover, given that the endogenous in vivo APT contrast is only about 3%, multiple signal averaging is often necessary to enhance the contrast-to-noise ratio (CNR), which requires the duration for each APT scan to be minimized. 
         [0013]    Therefore, it would be desirable to have a system and method that enables feasible multi-slice molecular imaging using MRI system. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention overcomes the aforementioned drawbacks by providing a method for fast, multi-slice chemical exchange saturation transfer (CEST) imaging. More specifically, the present invention provides a method for compensating for relaxation-induced losses of the CEST contrast mechanism. 
         [0015]    In accordance with one aspect of the invention, relaxation compensation is provided by correcting produced CEST ratio (CESTR) images for longitudinal relaxation. A T 1  map is produced and utilized to compensate the produced CESTR image. 
         [0016]    In accordance with another aspect of the invention, a pulse sequence is employed that intrinsically compensates for relaxation-induced losses by recovering saturation of a labile spin species after each image slice is acquired. 
         [0017]    The foregoing and other advantages of the invention will appear from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a block diagram of an MRI system which employs the present invention; 
           [0019]      FIG. 2A  is a graphic illustration of one pulse sequence for directing the MRI system of  FIG. 1  to acquire imaging data in accordance with the present invention; 
           [0020]      FIG. 2B  is a graphic illustration of another pulse sequence for directing the MRI system of  FIG. 1  to acquire imaging data in accordance with the present invention; 
           [0021]      FIG. 3  is a flowchart setting forth the steps of a method for correcting chemical exchange saturation transfer image data in accordance with the present invention; and 
           [0022]      FIG. 4  is another flowchart depicting the steps of another method for correcting chemical exchange saturation transfer image data in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0023]    Referring particularly to  FIG. 1 , the preferred embodiment of the invention is employed in an MRI system. The MRI system includes a workstation  10  having a display  12  and a keyboard  14 . The workstation  10  includes a processor  16  that is a commercially available programmable machine running a commercially available operating system. The workstation  10  provides the operator interface that enables scan prescriptions to be entered into the MRI system. The workstation  10  is coupled to four servers: a pulse sequence server  18 ; a data acquisition server  20 ; a data processing server  22 , and a data store server  23 . The workstation  10  and each server  18 ,  20 ,  22  and  23  are connected to communicate with each other. 
         [0024]    The pulse sequence server  18  functions in response to instructions downloaded from the workstation  10  to operate a gradient system  24  and an RF system  26 . Gradient waveforms necessary to perform the prescribed scan are produced and applied to the gradient system  24  that excites gradient coils in an assembly  28  to produce the magnetic field gradients G x , G y , and G z  used for position encoding MR signals. The gradient coil assembly  28  forms part of a magnet assembly  30  that includes a polarizing magnet  32  and a whole-body RF coil  34 . 
         [0025]    Radio frequency excitation waveforms are applied to the RF coil  34  by the RF system  26  to perform the prescribed magnetic resonance pulse sequence. Responsive MR signals detected by the RF coil  34  or a separate local coil (not shown in  FIG. 1 ) are received by the RF system  26 , amplified, demodulated, filtered and digitized under direction of commands produced by the pulse sequence server  18 . The RF system  26  includes an RF transmitter for producing a wide variety of RF pulses used in MR pulse sequences. The RF transmitter is responsive to the scan prescription and direction from the pulse sequence server  18  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  34  or to one or more local coils or coil arrays (not shown in  FIG. 1 ). 
         [0026]    The RF system  26  also includes one or more RF receiver channels. Each RF receiver channel includes an RF amplifier that amplifies the MR signal received by the coil to which it is connected and a detector that detects and digitizes the I and Q quadrature components of the received MR signal. The magnitude of the received MR signal may thus be determined at any sampled point by the square root of the sum of the squares of the I and Q components, as given by: 
         [0000]        M=√ {square root over ( I   2+   Q   2 )},   Eqn. 1; 
         [0027]    and the phase of the received MR signal may also be determined: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0028]    The pulse sequence server  18  also optionally receives patient data from a physiological acquisition controller  36 . The controller  36  receives signals from a number of different sensors connected to the patient, such as EGG signals from electrodes or respiratory signals from a bellows. Such signals are typically used by the pulse sequence server  18  to synchronize, or “gate”, the performance of the scan with the subject&#39;s respiration or heart beat. 
         [0029]    The pulse sequence server  18  also connects to a scan room interface circuit  38  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  38  that a patient positioning system  40  receives commands to move the patient to desired positions during the scan. 
         [0030]    The digitized MR signal samples produced by the RF system  26  are received by the data acquisition server  20 . The data acquisition server  20  operates in response to instructions downloaded from the workstation  10  to receive the real-time MR data and provide buffer storage such that no data is lost by data overrun. In some scans the data acquisition server  20  does little more than pass the acquired MR data to the data processor server  22 . However, in scans that require information derived from acquired MR data to control the further performance of the scan, the data acquisition server  20  is programmed to produce such information and convey it to the pulse sequence server  18 . For example, during prescans MR data is acquired and used to calibrate the pulse sequence performed by the pulse sequence server  18 . Also, navigator signals may be acquired during a scan and used to adjust RF or gradient system operating parameters or to control the view order in which k-space is sampled. And, the data acquisition server  20  may be employed to process MR signals used to detect the arrival of contrast agent in an MRA scan. In all these examples the data acquisition server  20  acquires MR data and processes it in real-time to produce information that is used to control the scan. 
         [0031]    The data processing server  22  receives MR data from the data acquisition server  20  and processes it in accordance with instructions downloaded from the workstation  10 . Such processing may include, for example: Fourier transformation of raw k-space MR data to produce two or three-dimensional images; the application of filters to a reconstructed image; the performance of a backprojection image reconstruction of acquired MR data; the calculation of functional MR images; the calculation of motion or flow images, and the like. 
         [0032]    Images reconstructed by the data processing server  22  are conveyed back to the workstation  10  where they are stored. Real-time images are stored in a data base memory cache (not shown) from which they may be output to operator display  12  or a display that is located near the magnet assembly  30  for use by attending physicians. Batch mode images or selected real time images are stored in a host database on disc storage  44 . When such images have been reconstructed and transferred to storage, the data processing server  22  notifies the data store server  23  on the workstation  10 . The workstation  10  may be used by an operator to archive the images, produce films, or send the images via a network to other facilities. 
         [0033]    As noted above, the chemical exchange saturation transfer (CEST) imaging method is built upon the method of conventional magnetization transfer. The contrast mechanism of CEST imaging allows for the detection of exchangeable proteins and peptides. This is achieved by the selective irradiation of labile protons, which in turn attenuate the bulk water signal through saturation transfer. Labile protons are saturated using either an off-resonance pulse or continuous wave of RF irradiation. This saturation process is known as “RF labeling” or simply “labeling”. 
         [0034]    Because chemical exchange is often dependent on pH, CEST imaging methods can be sensitive to microenvironment pH. A specific variant of CEST imaging methods, referred to as amide proton transfer (APT) MRI, utilizes labile amide protons from endogenous proteins and peptides to provide the CEST contrast mechanism. 
         [0035]    The following definitions and terms are provided to better define the present invention and to guide those of ordinary skill in the art in the practice of the present invention: 
         [0036]    k ws  is the chemical exchange rate from bulk water to the labile proton pool; 
         [0037]    k sw  is the chemical exchange rate from the labile proton pool to bulk water; 
         [0038]    R 1w  is the longitudinal relaxation rate of bulk water; 
         [0039]    R 1s  is the longitudinal relaxation rate of the labile proton pool; 
         [0040]    R 2w  is the transverse relaxation rate of bulk water; 
         [0041]    R 2s  is the transverse relaxation rate of the labile proton pool; 
         [0042]    r 1w =R 1w +k ws  is a term that describes the effects of longitudinal relaxation and chemical exchange on bulk water; 
         [0043]    r 1s =R 1w +k sw  is a term that describes the effects of longitudinal relaxation and chemical exchange on the labile proton pool; 
         [0044]    r 2w =R 2w +k ws  is a term that describes the effects of transverse relaxation and chemical exchange on bulk water; and 
         [0045]    r 2s =R 2s +k sw  is a term that describes the effects of transverse relaxation and chemical exchange on the labile proton pool. 
         [0046]    CEST imaging is often described using a simple 2-pool chemical exchange model, with the magnetizations for labile protons and bulk water described by two distinct sets of Bloch equations that are coupled by chemical exchange. In this model, the bulk water Bloch equation is given by: 
         [0000]        dM   zw   /dt=−r   1w ( M   zw   −M   0w )+ k   sw ( M   zs   −M   0s ),   Eqn. 3; 
         [0047]    where M zw , M zs , M zw , and M 0s  are the transient and equilibrium z-magnetizations for the bulk water and labile proton pools, respectively. Likewise, the labile proton Bloch equation is given by: 
         [0000]        dM   zs   /dt=−r   1s ( M   zs   −M   0s )+ k   ws ( M   zw   −M   0w ),   Eqn 4. 
         [0048]    After RF irradiation, coupling between the bulk water and labile proton groups cause their respective magnetizations to recover towards their equilibrium states in accordance with the above physical model. If the spillover effects are negligible the z-magnetization for the bulk water pool can determined by: 
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         [0049]    where: 
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         [0000]        B =( r   1s   −r   1w ) 2 +4 k   sw   k   ws ; and 
         [0050]    α is a labeling coefficient, which for slow to intermediate chemical exchange has the form: 
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         [0052]    and ω 1  is the RF irradiation power. 
         [0053]    Additionally, f is the proton ratio between bulk water and the labile proton and t is the duration between the end of the RF irradiation and the echo time. Generally, data from a reference scan is acquired before a label scan, in which saturation of the labile proton group occurs. For the reference scan, the irradiation pulse is applied at a reference frequency that is symmetric around the bulk water resonance frequency and opposite to the labile proton frequency. As a result, the labeling coefficient, α, is minimal and the z-magnetization of the bulk water is given by: 
         [0000]        M   zw ( t )= M   0w ,   Eqn. 6. 
         [0054]    A parameter called the CEST ratio (CESTR) can be derived by taking the difference between the reference and label scans, and normalized by a control signal without RF irradiation as: 
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                                                       s 
                                                     
                                                   
                                                   - 
                                                   
                                                     r 
                                                     
                                                       1 
                                                        
                                                       
                                                           
                                                       
                                                        
                                                       w 
                                                     
                                                   
                                                 
                                               
                                                
                                               
                                                 k 
                                                 ws 
                                               
                                             
                                           
                                           ) 
                                         
                                       
                                        
                                       t 
                                     
                                   
                                 
                                 2 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                   , 
                 
               
               
                 
                   Eqn 
                   . 
                   
                       
                   
                    
                   7. 
                 
               
             
           
         
       
     
         [0055]    When the labile proton group is dilute, the proton ratio, f, has the form: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     = 
                     
                       
                         
                           k 
                           ws 
                         
                         
                           k 
                           sw 
                         
                       
                       ≈ 
                       
                         
                           r 
                           
                             1 
                              
                             
                                 
                             
                              
                             w 
                           
                         
                         
                           r 
                           
                             1 
                              
                             
                                 
                             
                              
                             s 
                           
                         
                       
                        
                       1 
                     
                   
                   , 
                 
               
               
                 
                   
                     Eqn 
                     . 
                     
                         
                     
                      
                     8 
                   
                   ; 
                 
               
             
           
         
       
     
         [0056]    and as such, CESTR can be further simplified as: 
         [0000]    
       
         
           
             
               
                 
                   
                     CESTR 
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         k 
                         ws 
                       
                       
                         r 
                         
                           1 
                            
                           
                               
                           
                            
                           w 
                         
                       
                     
                     · 
                     α 
                     · 
                     
                       
                          
                         
                           
                             ( 
                             
                               
                                 - 
                                 
                                   r 
                                   
                                     1 
                                      
                                     
                                         
                                     
                                      
                                     w 
                                   
                                 
                               
                               + 
                               
                                 
                                   
                                     k 
                                     sw 
                                   
                                   
                                     
                                       r 
                                       
                                         1 
                                          
                                         
                                             
                                         
                                          
                                         s 
                                       
                                     
                                     - 
                                     
                                       r 
                                       
                                         1 
                                          
                                         
                                             
                                         
                                          
                                         w 
                                       
                                     
                                   
                                 
                                  
                                 
                                   k 
                                   ws 
                                 
                               
                             
                             ) 
                           
                            
                           t 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   Eqn 
                   . 
                   
                       
                   
                    
                   9 
                 
               
             
           
         
       
     
         [0057]    In addition, for dilute labile protons, 
         [0000]    
       
         
           
             
               
                 k 
                 sw 
               
               
                 
                   r 
                   
                     1 
                      
                     
                         
                     
                      
                     s 
                   
                 
                 - 
                 
                   r 
                   
                     1 
                      
                     
                         
                     
                      
                     w 
                   
                 
               
             
             ≈ 
             1. 
           
         
       
     
         [0000]    Therefore, CESTR can be even further simplified to: 
         [0000]    
       
         
           
             
               
                 
                   
                     CESTR 
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         k 
                         ws 
                       
                       
                         r 
                         
                           1 
                            
                           
                               
                           
                            
                           w 
                         
                       
                     
                     · 
                     α 
                     · 
                     
                       
                          
                         
                           
                             - 
                             
                               R 
                               
                                 1 
                                  
                                 
                                     
                                 
                                  
                                 w 
                               
                             
                           
                            
                           t 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   Eqn 
                   . 
                   
                       
                   
                    
                   10. 
                 
               
             
           
         
       
     
         [0058]    Thus, the effects of longitudinal relaxation on CESTR can be compensated for and a compensated CESTR (cCESTR) determined in accordance with: 
         [0000]      cCESTR≈CESTR(t)·e R     1w     t    Eqn. 11; 
         [0059]    where t, again, is the time between the end of the RF irradiation and the echo time. 
         [0060]    Referring now to  FIG. 2A , a pulse sequence  250  employed to practice the present invention is shown to have five sections: a labeling section  200 , wherein RF labeling occurs; a preparation section  202 , which includes an outer volume suppression module  201  and fat suppression; a data acquisition section  206 ; and an idle period  208 . The labeling section  200  of the pulse sequence  250  includes the application of continuous wave (CW) RF irradiation at a selected off-resonance frequency to “label” the labile proton pool spins. However, a series repetitive frequency-selective RF irradiation pulses can alternatively be applied to the subject in order to achieve the desired labeling. In one contemplated process, the labile proton group includes amide protons on proteins and peptides; however, the present invention can be practiced in any application that utilizes CEST imaging methods and the labile proton group can include many variations appreciated by those skilled in the art. 
         [0061]    The imaging volume is next prepared during the preparation section  202  of the pulse sequence  250 . First, outer volume suppression is performed through the application of the outer volume suppression module  201 . An RF excitation pulse  212  is applied in the presence of magnetic field gradients that are applied in three orthogonal directions: one along the z-axis  214 , one along the y-axis  216 , and one along the x-axis  218 . As a result, a region in the peripheral outer volume of the subject is selectively excited such that the MR signals originating from the region can be selectively suppressed. The outer volume suppression module  201  is repeated a plurality of times until a desired region of the subject has been selectively excited. Next, the pulse sequence  250  proceeds with a fat suppression section. In one embodiment, fat suppression is achieved with a chemical shift selective CHESS method; however, it is contemplated that any fat suppression technique can be employed. For the CHESS method, a spectrally selective excitation pulse  220  is applied to excite all of the spins corresponding to fat tissue in the imaging volume selectively excited during the outer volume suppression module  201 . In order to maintain a substantially homogeneous B 0  field for effective CHESS fat suppression, higher-order shimming gradients are utilized prior to execution of the pulse sequence  250 . A spoiler gradient  222  is then applied to reduce the signal from the excited fat spins. As a result, substantially no fat molecules will produce a detectable NMR signal during subsequent imaging. Alternatively, it is contemplated that the spoiler gradient  222  may be replaced with a crusher gradient. 
         [0062]    The data acquisition section  204  of the pulse sequence  250  is then performed. In one embodiment, data acquisition occurs by employing a spin-echo, echo planar imaging (EPI) sequence. First, a slice selective gradient  226  is played out during the application of an RF excitation pulse  224 . To mitigate signal loss resultant from phase dispersions introduced by the slice selective gradient  226 , a rephasing lobe  228  is applied after the slice selective gradient  226 . A rephasing RF pulse  230  is then applied in the presence of another slice selective gradient  232 . In order to substantially reduce unwanted phase dispersions, a first crusher gradient  234  bridges the slice selective gradient  232  with a second crusher gradient  236 . A pre-phaser gradient waveform  238  is then applied in the phase encoding direction, and a pre-phaser pulse  240  in the readout direction followed by an alternating readout gradient pulse train  242  is then applied. A spin-echo MR signal is acquired during each positive and negative top of the readout pulse train  242  and a phase-encoding gradient “blip”  244  is applied between each readout to separately phase encode each acquired echo signal, or view. The data acquisition section  204  is repeated a plurality of times with a different first slice selective gradient  226  and second slice selective gradient  232  such that multiple slices of image data are acquired. Each repetition of the pulse sequence  250  ends with an idle period  206 . 
         [0063]    Referring particularly to  FIG. 3 , the pulse sequence  250  described above is employed to acquire MR image data that is sensitive to the CEST contrast mechanism. Furthermore, from the image data a CEST ratio map is produced and subsequently corrected for relaxation-induced losses of CEST contrast. First, T 1  measurement data is acquired in step  300 . In one embodiment, a Look-Locker EPI pulse sequence is employed to rapidly acquire T 1  measurements as the transverse magnetization recovers to equilibrium. From the acquired T 1  measurements, a T 1  map is produced, as indicated by step  302 . Next, reference image data is acquired from the subject by performing the imaging pulse sequence  250  described above in reference to  FIG. 2A , as indicated at step  304 . In order to acquire reference image data, the imaging pulse sequence  250  is performed by applying the continuous wave RF irradiation  210  at a reference frequency, −Δf s . Labeled image data is then acquired at step  306  through the performance of the imaging pulse sequence  250 , wherein now the continuous wave RF  210  is applied at a labeling frequency, Δf s . It should be noted that the reference frequency, −Δf s , and the labeling frequency, Δf s , are chosen to be symmetric about the bulk water resonance frequency. Moreover, the labeling frequency is preferably chosen as the chemical shift difference between bulk water and the labile protons. Next, reference and labeled images are reconstructed in step  308  through conventional Fourier transform methods. 
         [0064]    From the reconstructed reference and labeled images, a CEST ratio (CESTR) map is produced by subtracting the labeled images from the corresponding reference images, as indicated at step  310 . At step  312 , a compensated CESTR (cCESTR) map is produced from the CESTR map and using the T 1  map by correcting for longitudinal magnetization relaxation effects on a voxel-by-voxel basis and is carried out in accordance with equation 11 described above, such that: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       cCESTR 
                        
                       
                         ( 
                         
                           i 
                           , 
                           j 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         CESTR 
                          
                         
                           ( 
                           
                             i 
                             , 
                             j 
                           
                           ) 
                         
                       
                       · 
                       
                          
                         
                           t 
                           
                             
                               T 
                               
                                 1 
                                  
                                 
                                     
                                 
                                  
                                 w 
                               
                             
                              
                             
                               ( 
                               
                                 i 
                                 , 
                                 j 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   
                     Eqn 
                     . 
                     
                         
                     
                      
                     12 
                   
                   ; 
                 
               
             
           
         
       
     
         [0065]    where cCESTr(i,j) indicates a voxel value in the compensated CESTR map at voxel location (i,j), CESTr(i,j) indicates the voxel value in the CESTR map at the corresponding voxel location (i,j), and T 1w (i,j) indicates the voxel value in the produced T 1  map at the corresponding voxel location (i,j). Additionally, t is the duration of time after the RF irradiation at which the signal corresponding to a given image was acquired. 
         [0066]    Alternatively, a control scan can be performed by performing the imaging pulse sequence  250  without the labeling section  200 . A control image is then reconstructed from the acquired control scan data and employed to normalize the produced CESTR image by dividing the CESTR image by the reconstructed control image. 
         [0067]    Referring now to  FIG. 2B , it is contemplated that another pulse sequence  252  may be utilized to compensating for relaxation-induced losses of the CEST contrast mechanism. This pulse sequence  252  is similar to the one described above except that a second labeling section  208  is appended after the data acquisition section  204 . As such, performance of the pulse sequence  252  proceeds much like in the sequence  250  described above with respect to  FIG. 2A . A labeling section  200  acts to saturate the labile proton group, by applying a continuous wave (CW) RF irradiation at a selected off-resonance frequency. The preparation section  202  is then played out. First, an outer volume suppression module  201  repeatedly selectively excites the outer volume regions of the subject being imaged by applying an RF excitation pulse  212  in the presence of an x-gradient  218 , y-gradient  216 , and z-gradient  214 . Lipid suppression follows through the application of a spectrally selective excitation pulse  220  and a spoiler gradient  222  that substantially reduces lipid signal from the previously selected spins. Differing from the previously described pulse sequence  250 , the presently described embodiment only plays out the data acquisition section  204  for one imaging slice at a time. At the end of the data acquisition section  204 , a second labeling section  208  applies an additional saturation pulse  246  at the selected off-resonance frequency that acts to recover saturation of the labile proton group that was lost to longitudinal relaxation. A spoiler gradient  248  is then applied to dephase any residual transverse magnetization. The pulse sequence  252  then repeats the preparation  202 , data acquisition  204 , and second labeling  208  sections, during each repetition altering the first  226  and second  232  slice selective gradients in the data acquisition section  204 . In this manner, the pulse sequence  252  can be employed to rapidly acquire multiple slices of CEST image data that are intrinsically compensated for longitudinal relaxation induced losses of CEST contrast. 
         [0068]    Referring now particularly to  FIG. 4 , a method for practicing the present invention, which employs the pulse sequence  252  shown in  FIG. 2B , is presented. Because the pulse sequence  252  intrinsically compensates for the relaxation-induced loss of CEST contrast, no explicit T 1  measurement is performed. Instead, reference image data is acquired in step  400  by performing the imaging pulse sequence  252  with the application of the continuous wave RF irradiation  210  at a reference frequency, −Δf s . Furthermore, the additional saturation pulse  246  is also applied at the reference frequency, −Δf s . Labeled image data is then acquired at step  402  through the performance of the imaging pulse sequence  252 , wherein now the continuous wave RF  210  and additional saturation pulse  246  are applied at a labeling frequency, Δf s . From these two sets of data, reference and labeled images are reconstructed in step  404 . A CESTR image is then produced in step  406  in a manner similar to that described above in step  310 . However, since the data acquired with the pulse sequence  252  intrinsically compensates for relaxation-induced losses of the CEST contrast mechanism, no additional compensation is performed. Alternatively, a control scan can be performed by performing the imaging pulse sequence  252  without the first  200  and second  208  labeling sections. A control image is then reconstructed from the acquired control scan data and employed to normalize the produced CESTR image by dividing the CESTR image by the reconstructed control image. 
         [0069]    The present invention has been described in terms of one or more 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. For example, the present invention may be practiced with a gradient-recalled echo EPI pulse sequence and the selected slice may be oriented in any direction by proper rotation of the phase-encoding, readout and slice selection gradient fields. Additionally, other variants of the CEST ratio can be produced, such as by dividing the reconstructed labeled images by the corresponding reference images.