Patent Publication Number: US-10775462-B2

Title: System and method for direct saturation-corrected chemical exchange saturation transfer (DISC-CEST)

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is based on, claims priority to, and incorporates herein by reference in its entirety U.S. Provisional Application Ser. No. 62/528,753, filed Jul. 5, 2017, and entitled, “DIRECT SATURATION-CORRECTED CHEMICAL EXCHANGE SATURATION TRANSFER (DISC-CEST).” 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under 1R01NS083654 and R21NS085574 awarded by the National Institutes of Health. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the 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 ) that is in the x-y plane and that 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 xy . 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. 
     When utilizing these “MR” (magnetic resonance) 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. 
     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 magnetic resonance imaging (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. 
     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 raster scan-like pattern sometimes referred to as a “spin-warp,” a “Fourier,” a “rectilinear,” or a “Cartesian” scan. The spin-warp scan technique 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. 
     MRI can be used to measure the exchange of magnetization between molecules to provide unique information about the chemical and molecular environment of samples or tissues. One type of such exchange measurement is broadly referred to in the field as magnetization transfer. This technique is capable of measuring the exchange of magnetization from spin species that have short transverse relaxation times (T 2 ). Because many different molecules have short T 2 , this technique is not particularly sensitive to specific molecules. 
     A second type of magnetization exchange occurs between water protons and a molecule with long enough T 2  that its difference in frequency from water can be observed. Saturation of the magnetization from this molecule will generally decrease the measurable signal from water. This effect is generally referred to in the field as chemical exchange saturation transfer (“CEST”). Two different types of molecules can generate CEST effects: endogenous, or naturally occurring, molecules and exogenous contrast agents. In either instance, the molecules whose chemical exchange with water produces the CEST effect are generally referred to as so-called “exchangeable protons.” 
     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 control 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. 
     Thus, CEST MRI is a sensitive imaging technique for detecting compounds containing exchangeable protons. Such labile protons can be selectively saturated by an RF pulse, and the saturation subsequently transferred to the bulk water signal via proton chemical exchange, resulting in substantial sensitivity enhancement. CEST imaging has been demonstrated in mapping low-concentration metabolites such as creatine (Cr), glucose, glutamate, and changes in microenvironment properties such as temperature and pH, promising a host of in vivo applications such as imaging of ischemic stroke and tumor. One example is amide proton transfer (APT), a specific form of CEST imaging that is sensitive to pH and mobile protein/peptide content. Recently, APT imaging has shown promising results in characterizing tissue acidosis after ischemic stroke, tumor detection, grading, and differentiation of tumor recurrence from radiation necrosis. 
     Despite holding promises in stroke and tumor imaging, CEST MRI suffers from its qualitative nature, which depends on many factors, including the chemical exchange rate, concentration of exchangeable protons, longitudinal relaxation time, and RF saturation power. To quantify the CEST effect relative to, for example APT, an asymmetry analysis (MTR asym ) is most commonly used to suppress interference from non-linear direct water saturation (RF spillover) and broadband magnetization transfer (MT) effects by taking the difference between a reference image (e.g. −3.5 ppm) and label imagine (e.g. +3.5 ppm). Note the term ‘direct water saturation’ is used interchangeably with ‘RF spillover’ or ‘direct saturation’ or ‘spillover’ in this context. However, in vivo MTR asym  is contaminated by asymmetric magnetization transfer contrasts (MTC) from semisolid macromolecules and nuclear overhauser enhancement (NOE) effects, which result in a negative and inhomogeneous shift across the brain. As such, in vivo MTR asym  has a mixed contribution from APT, asymmetric NOE and MTC effects. If the asymmetric background signals are not removed the result is a substantial underestimation of the CEST effect. Another drawback associated with the MTR asym  method is that MTR asym  analysis is subjected to static magnetic (B 0 ) field inhomogeneity, which needs additional acquisition of densely sampled Z-spectrum to correct for B 0  inhomogeneity using interpolation approach. The dominant B 0  inhomogeneity effect also stems from the non-linear direct saturation effect. 
     Therefore, it would be desirable to develop quantitative methods that can determine in vivo CEST effects without the need of time-consuming Z-spectrum acquisition while controlling for confounding contaminations. 
     SUMMARY OF THE PRESENT DISCLOSURE 
     The present disclosure addresses the aforementioned drawbacks by providing systems and methods for performing analysis using a direct saturation effect-corrected analysis, which can simplify decoupling of CEST effects from information that is contaminating for in vivo quantifications. In particular, the systems and methods provided herein can be used to control against B0 inhomogeneity effects on CEST data and suppress the concomitant water signal during CEST imaging studies, which results in CEST images with higher contrast and less background contamination. Furthermore, the systems and methods can be used to provide reliable in vivo CEST quantification with three CEST images, which can be used to reduce the data acquisition times. 
     In accordance with one aspect of the disclosure, a method is provided for producing a magnetic resonance (MR) image of a subject. The method includes applying at least one radiofrequency (RF) saturation pulse with an MRI system at a range of frequencies to substantially saturate magnetization corresponding to an exchangeable proton in the subject, acquiring chemical exchange saturation transfer (CEST) data with the MRI system, and generating an acquired Z-spectrum (Z) from the CEST data. The method also includes computing an estimated direct water saturation (Z′) based using at least one of relaxation measurements derived from the CEST data or imaging parameters used to acquire the CEST data with the MRI system, computing a direct saturation corrected Z-spectrum (ΔZ) using the acquired Z-spectrum (Z) and the estimated direct water saturation (Z′), and generating a CEST image of the subject using the direct saturation corrected Z-spectrum (ΔZ). 
     In accordance with another aspect of the present disclosure, system is provided that includes a magnet system configured to generate a polarizing magnetic field about at least a portion of a subject, a magnetic gradient system including a plurality of magnetic gradient coils configured to apply at least one magnetic gradient field to the polarizing magnetic field, and a radio frequency (RF) system configured to apply an RF field to the subject and to receive magnetic resonance signals from the subject using a coil array. The system also includes a computer system programmed to control the magnetic gradient system and the RF system to perform a pulse sequence that includes at least one RF saturation pulse at a range of frequencies to substantially saturate magnetization corresponding to an exchangeable proton in the subject. The computer system is further programmed to acquire chemical exchange saturation transfer (CEST) data from the subject, generate an acquired Z-spectrum (Z) from the CEST data, and compute an estimated direct water saturation (Z′) based using at least one of relaxation measurements derived from the CEST data or imaging parameters used to acquire the CEST data with the MRI system. The computer system is further programmed to determine a direct saturation corrected Z-spectrum (ΔZ) using the Z-spectrum (Z) and the estimated direct water saturation (Z′); and reconstruct a CEST image of the subject using the direct saturation corrected Z-spectrum (ΔZ). 
     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 
       The patent or patent application file contains at least one drawing in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee. 
         FIG. 1A  is a schematic illustration of a small quantity of metabolite dissolved in a solvent, where each of the metabolite and the solvent include exchangeable protons in chemical exchange. 
         FIG. 1B  is a schematic illustration of an RF pulse being applied to the exchangeable proton on the metabolite to form a saturated proton, where the saturated proton is in chemical exchange with the solvent. 
         FIG. 1C  is a schematic illustration of the saturated proton being transferred to the solvent resulting in a loss of solvent signal over time. 
         FIG. 2A  is a graphic illustration of an exemplary frequency spectrum that includes a water proton resonance peak and an amide proton resonance peak, which relates to one specific example of the more general group of other labile protons groups and semisolid macromolecules. 
         FIG. 2B  is a graphic illustration of the effect of the application of radio frequency (“RF”) energy at a labeling frequency that is around the resonance frequency of an exchangeable proton, such as an amide proton, on detectable signal from water protons adjacent the exchangeable proton. 
         FIG. 2C  is a graphic illustration of the effect of the application of RF energy at a reference frequency, equal to the negative of the labeling frequency, on detectable signal from water protons adjacent the exchangeable proton. 
         FIG. 3  is a flowchart setting forth the steps of applying a concomitant direct saturation correction (DISC) procedure according to one aspect of the present disclosure. 
         FIG. 4  is a block diagram of an example magnetic resonance imaging (“MRI”) system that is configured to implement methods described here. 
         FIG. 5A  is a graph that shows the experimentally measured Z-spectrum for creatine at 5 concentrations (Z, solid lines), the analytic estimation of direct water saturation effect using relaxation measurements (Z′, dash lines) and their differences which is direct saturation corrected (ΔZ, dotted lines). 
         FIG. 5B  is a graph that shows a comparison between experimentally measured Z (solid lines), direct saturation estimation (dash lines), and the spillover corrected ΔZ at varied RF saturation levels. 
         FIG. 6A  is a graph that shows a comparison of the MTR asym  method, three-offset method, and the non-linear correction of the direct water saturation effect in a creatine phantom with minimal NOE and MTC effects to illustrate the concept of the method according to one aspect of the present disclosure. 
         FIG. 6B  is a set of correlated images from an identical creatine phantom that, together, provide a comparison of the MTR asym  method, three-offset method, and the non-linear direct saturation correction method for generating CEST images specific to metabolite such as creatine in this non-limiting example. 
         FIG. 6C  is a graph showing results of a regression analysis to analyze correlation of MTR asym  to both the three-offset method and the non-linear direct saturation correction method. 
         FIG. 7A  is a graph showing a comparison of Z, Z′, and spillover corrected ΔZ of white matter (WM) and gray matter (GM) from a normal rat brain. 
         FIG. 7B  is a chart showing a comparison of MTR asym , APT* and NOE* effects estimated from three-offset method on Z, APT′ and NOE′ effects from DISC-CEST method on ΔZ between WM and GM of normal brains (N=6). 
         FIG. 8A  is a paired set of images that show the regions of interest (ROIs) of the contralateral normal brain tissue and the tumor for subsequent comparison of Z, Z′ and ΔZ. 
         FIG. 8B  is a graph showing averaged Z, Z′ and ΔZ from the ROIs from the ROIs illustrated in  FIG. 8A . 
         FIG. 9  is a comparison of relaxation maps, MTR asym  maps and APT*, APT′, NOE*, NOE′ maps from two typical slices of a representative rat brain with glioma. 
         FIG. 10  is a set of APT images from a representative rat brain with acute ischemic stroke without and with correction of B0 inhomogeneity by incorporating the frequency shifts measure from the B0 field map in the estimation of direct water saturation. 
         FIG. 11A  is a multi-pool Lorentzian fitting of an averaged Z-spectrum (Z) or direct saturation corrected Z-spectrum (ΔZ) from contralateral normal tissue (green) or ipsilateral ischemic tissue (red) from a representative rat brain with focal ischemic stroke. Individual decoupled CEST pools, including water, APT, amine, NOE and MT are also shown (normal: solid lines; ischemic: dotted lines). 
         FIG. 11B  are diffusion and perfusion MRI maps of one ischemic rat brain as well as the decoupled APT, amine, NOE maps using multi-pool Lorentzian fitting of the direct saturation corrected ΔZ. 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENT DISCLOSURE 
     Chemical Exchange Saturation Transfer (CEST) MRI emerges as a sensitive molecular MR imaging technique that detects exchangeable groups via their interactions with bulk tissue water. Referring now to  FIGS. 1A-1C , a schematic illustration is shown to illustrate magnetization transfer via chemical exchange between a metabolite and a bulk solvent solution, which is exploited to achieve CEST contrast.  FIG. 1A  shows a metabolite  100  dissolved in a solvent  102  where both the metabolite  100  and the solvent  102  comprise an exchangeable proton  104 .  FIG. 1B  shows an exchangeable proton  104  being selectively saturated by an RF pulse  106  to produce a saturated proton  108 . Magnetic saturation will spontaneously be transferred to the solvent  102  overtime due to chemical exchange of the saturated proton  108  with the exchangeable protons  104 . 
     This process continues to produce a reduction in the solvent  102  signal over time, which may be detected using MR imaging. The loss of solvent  102  signal provides an indirect measure for the concentration of the metabolite  100  in the solution, which may be visualized from the variation in the solvent  102  signal as a function of offset frequency of the irradiation pulse, known as a Z-spectrum. CEST imaging has been demonstrated in mapping low-concentration endogenous metabolites  100  with exchangeable protons  104  such as metabolites  100  with amide (—NH), amine (NH 2 ) and hydroxyl (—OH) functional groups. Typically, the solvent  102  comprises water, but could conceivably be any solvent  102  that includes an exchangeable proton  104 . CEST imaging has also been used to map dilute metabolites and to track changes in microenvironment properties such as temperature and pH, promising a host of in vivo applications such as imaging of ischemic stroke and tumor. 
     Referring now to  FIGS. 2A-2C , a graphic illustration of an exemplary method for producing the CEST Z-spectrum is shown. An exemplary Z-spectrum is illustrated in  FIG. 2A , the spectrum including a spectral peak  200  corresponding to water protons and a spectral peak  202  corresponding to amide protons. The amide proton peak  202  exists at a frequency shift relative to the water peak  200 . For example, there is a frequency shift of around +3.5 parts per million (“ppm”) between the water peak  200  and the amide proton peak  202 . Thus, a so-called “labeling spectral line”  204 , or “labeling frequency,” is centered at or around the resonance frequency of the exchangeable proton, which for an amide proton is shifted about +3.5 ppm relative to the water peak  200 . In general, for CEST imaging, the labeling spectral line is selected as a frequency at or around the resonance frequency of the exchangeable proton. A so-called “reference spectral line”  206 , or “reference frequency,” also exists, and is equal to the negative of the labeling frequency relative to the water peak  200 . 
     To obtain a Z-spectrum, a series of image data are acquired with an MRI system by applying RF energy at the labeling spectral line changing incrementally, for example from down-field  204  to up-field  206  of water resonance. If the labeling spectral line is applied at the resonance frequency of the exchangeable proton, the saturation of the exchangeable protons is transferred through chemical exchange processes to nearby water protons, as indicated by line  208  in  FIG. 2B . As a result, the detectable signal from these solvent protons is reduced. Referring now to  FIG. 2C , there is no saturation transferred to the adjacent water spins and, therefore, no resultant decrease in detectable signal. In this manner, a so-called “Z-spectrum” is acquired. 
     For in vivo CEST imaging, the CEST signal at a selected frequency at or around the resonance frequency of the exchangeable proton also contains concomitant contributions from magnetization transfer contrast (MTC), direct water saturation effect, and the nuclear overhauser enhancement (NOE) effects. MTC is similar to CEST, where saturated protons are transferred to non-saturated protons in water; however, MTC involves macromolecular magnetization transfer that have very short T2* and a broad spectrum. The direct water saturation effects, or RF spillover, includes an application of a saturation pulse that comprises side lobes, where the presence side lobes during applications of RF irradiations inevitably affects adjacent offset frequencies, such as the water-proton magnetization. The NOE effect may contribute to concomitant effects through intra- or inter-molecular dipolar cross-relaxation. Each of these factors contributes to measured CEST signals across the Z-spectrum to certain degree, limiting the specificity of CEST quantification. 
     Typically a CEST contrast image is assessed using a MTR asym  map to suppress direct water saturation, which is the normalized difference between the labelling frequency  204  (i.e. 3.5 ppm) and the reference frequency  206  (i.e. −3.5 ppm) as shown below: 
                       MTR   asym     =         s   ⁡     (     -   ω     )       -     s   ⁡     (   ω   )           s   0         ;           (     Eq   .           ⁢   1     )               
where S(−ω) is the signal intensity at the references frequency, S(ω) is the signal intensity at the labelling frequency, and S 0  is the signal intensity without irradiation. However, asymmetric background signals contributed by MTC and NOE cannot be properly accounted for using this simplistic method. Other drawbacks associated with the MTR asym  method is that MTR asym  analysis is subjected to static magnetic (B 0 ) field inhomogeneity, which needs additional acquisition of densely-sampled Z-spectrum centered around the water resonance to correct for B 0  inhomogeneity.
 
     Previous methods have been proposed to ameliorate the negative background signals associated with RF spillover, NOE and asymmetric MTC effects. One such method is a three-offset approach that subtracts the label image from the average of two boundary images to better quantify the background noise. In this method, the negative background signals are approximated using a linear function. As such, the apparent signal associated with the metabolite of interest may be calculated by taking the difference between the Z-spectrum and the linear approximation. The MTR(Δω) in the three-offset method is calculated by: 
                       MTR   ⁡     (     Δ   ⁢           ⁢   ω     )       =     {           S   ⁡     (       Δ   ⁢           ⁢   ω     +   δ     )       +     S   ⁡     (       Δ   ⁢           ⁢   ω     -   δ     )         2     -     S   ⁡     (     Δ   ⁢           ⁢   ω     )         }       ;           (     Eq   .           ⁢   2     )               
where Δω±δ are the frequency offsets of the two boundaries with equal offset shift δ from the peak of interest (Δω). However, the linear correction of the baseline underlying this simplistic approach makes it limited to quantification of CEST peaks far from the water resonance (i.e., large chemical shift or high field) under the condition of weak RF saturation level. If the condition is not met such as at sub-high field strength, the RF spillover effect cannot be approximated by a linear function, which results in substantial underestimation of the CEST effect.
 
     The present disclosure provides a relaxation-based, direct saturation-corrected CEST (DISC-CEST) analysis that addresses the limitations of the previous methods by providing an improved method for reducing concomitant contributions.  FIG. 3  shows a flowchart setting forth the steps of an example method  300  for generating a magnetic resonance (MR) image of a subject using CEST data that is substantially free of concomitant contributions. The method includes applying at least one radiofrequency (RF) saturation pulse with an MRI system at a range of frequencies to substantially saturate magnetization corresponding to an exchangeable proton in the subject at process block  302 . In some aspects, the saturation pulse may include a continuous wave irradiation such as a long rectangular RF pulse. In other aspects, the saturation pulse may include a train of RF saturation pulses such as Fermi or Gaussian pulses. Following saturation, CEST data may be acquired using the MRI system at process block  304 . In some aspects, the CEST data may be acquired using an echo planar imaging (EPI) readout. In other aspects, the CEST data may be acquired using fast image readout such as Fast Low Angle SHot (FLASH) or turbo spin echo (TSE). Next, by incrementally changing the frequency of the RF saturation pulse, a Z-spectrum (Z) is generated at process block  306  from the CEST data. In some aspects, the Z-spectrum (Z) may be acquired by changing the RF saturation frequency using an incremental interval. The frequency range of saturation pulses typically covers from upfield of water resonance (e.g. −6 ppm) to downfield (e.g., +6 ppm) with small incremental intervals (e.g., 0.25 ppm). Following acquisition of the Z-spectrum, relaxation measurements may be obtained at process block  308 . The relaxation measurements may include, as examples, a longitudinal relaxation value (T 1 ) and a transverse relaxation value (T 2 ). 
     The process may then continue at process block  310  by computing an estimated direct water saturation (Z′) based at least in part on the relaxation measurements. In some aspects, the estimated direct water saturation comprises a non-linear function. In some non-limiting examples, the estimated direct water saturation may be based at least in part on parameters such as a relaxation recovery (Tr), saturation times (Ts), a flip angle (FA), a bulk water longitudinal relation rate (R 1w ), a bulk water traverse relaxation rate (R 2w ), and RF irradiation levels. At process block  312 , the process  300  continues by computing a direct saturation corrected Z-spectrum (ΔZ) by taking the difference between the experimentally acquired Z-spectrum from process block  306  and the estimated direct saturation from process block  310 . A CEST image may then be generated at process block  314  from the direct saturation corrected Z-spectrum from process block  312 . 
     In the instance that the solvent is water, a simulated direct water saturation (Z′) signal can be described by: 
                       Z   ′     =           S   sat     ⁡     (       R     1   ⁢           ⁢   p       ,   TR   ,   FA   ,   Ts   ,     B   1       )           S   0     ⁡     (     TR   ,   FA     )         =                   (     1   -     e       -     T   r       ⁢     R     1   ⁢           ⁢   w             )     ·     e       -     R     1   ⁢           ⁢   p         ·     T   s           +                     R     1   ⁢           ⁢   w       ⁢     cos   2     ⁢   θ       r     1   ⁢           ⁢   p         ·     (     1   -     e       -     R     1   ⁢           ⁢   p         ·   Ts         )               1   -       cos   ⁡     (   FA   )       ·     e       -     T   r       ⁢     R     1   ⁢           ⁢   w           ·     e       -     R     1   ⁢           ⁢   p         ·   Ts                 (     1   -     e       -     T   r       ⁢     R     1   ⁢           ⁢   w             )     /     (     1   -       cos   ⁡     (   FA   )       ·     e       -     T   r       ⁢     R     1   ⁢           ⁢   w               )             ;           (     Eq   .           ⁢   3     )               
where R 1p =R 1w  cos 2  θ+R 2w  sin 2  θ, Tr and Ts are the relaxation recovery and saturation times (TR=Tr+Ts), FA is the image excitation flip angle, R 1,2w  are the bulk water longitudinal and traverse relaxation rates. In addition, θ=tan −1 (ω 1 /Δω), where ω 1  and Δω are the RF irradiation level and offset. In some aspects, the direct saturation corrected Z-spectrum (ΔZ) may be obtained by taking the difference between the experimentally acquired Z-spectrum (Z) created using data acquired from the subject and the estimated direct saturation (Z′), given as:
 
Δ Z=Z′−Z   (Eq. 4).
 
     Using the direct saturation corrected Z-spectrum (ΔZ), individual CEST effects such as APT, NOE and broad MTC effects can be decoupled, for example, with multi-pool Lorentzian fitting method. For example, the ΔZ spectrum can be fitted as the sum of multiple Lorentzian functions using the following equation: 
                       Δ   ⁢           ⁢   Z     =       ∑     i   =   1     N     ⁢       A   i       1   +     4   ⁢       (       ω   -     ω   i         σ   i       )     2               ;           (     Eq   .           ⁢   5     )               
where ω is the frequency offset from the water resonance, A i , ω i  and σ i  are the amplitude, frequency offset, and linewidth, respectively, of the CEST peak for the proton pool i. A five-pool Lorentzian model (i=5), including magnetization transfer (MT), amide, amine, hydroxyl (—OH) and NOE may be employed.
 
     Using the direct saturation corrected Z-spectrum (ΔZ), the concomitant effects from direct saturation, NOE and broad MTC effects in the conventional analysis methods such as MTR asymmetry or three-offset method, may be reduced by using the frequency offset method, dubbed the DISC-CEST method: 
                         MTR     ′   ⁢           ⁡     (     Δ   ⁢           ⁢   ω     )       =             δ   1     ·   Δ     ⁢           ⁢     Z   ⁡     (       Δ   ⁢           ⁢   ω     +     δ   2       )         +         δ   2     ·   Δ     ⁢           ⁢     Z   ⁡     (       Δ   ⁢           ⁢   ω     -     δ   1       )               δ   1     +     δ   2           ;           (     Eq   .           ⁢   6     )               
where Δω is the frequency offset of interest, δ 1  is a first offset shift, and δ 2  is a second offset shift. Different from multi-pool Lorentzian fitting of ΔZ, which requires the acquisition of a Z-spectrum, the DISC-CEST minimally uses three offsets and drastically reduces the scan time. Unlike the previous three-offset method, the DISC-CEST method can use arbitrary boundary offsets defined by a first boundary offset, (Δω+δ 2 ), and a second boundary offset, (Δω−δ 1 ), respectively. In some aspects, the first offset shift may be different than the second offset shift. The B 0  field inhomogeneity effect can be taken into consideration by adjusting the frequency offsets measured from B 0  field map into Δω for subsequent calculation of MTR′. In some aspects, the boundary offsets may be determined by choosing the peak with a minimum residual CEST effect. In other aspects, the boundary offsets can be determined using a first derivative of the Z-spectrum.
 
     As will be detailed further below using Examples, the DISC-CEST correction more accurately models confounding concomitant contributions from direct saturation, NOE and broad MTC effects. Application of the DISC-CEST correction leads to a more accurate prediction of the CEST effect when compared with previous models. In some aspects, the DISC-CEST provides more reliable quantification of in vivo CEST measurement compared to the commonly used asymmetry analysis or three-offset method because the direct saturation is corrected; there is reduced contamination from the upfield NOE effect; and the broad MTC effect is assumed to be linear within a relatively small frequency range and is therefore reasonably accounted for in the DISC-CEST method. 
     In some aspects, the DISC-CEST method includes an acquisition of at least three frequency offsets, which may substantially reduce the scan times. The DISC-CEST method is not restricted to field strength or experimental conditions such as saturation pulses. The DISC-CEST method not only improves scanning efficiency but also has the potential to be applied in cancer and stroke imaging at clinical field strength. 
     In some aspects, the DISC-CEST method may be used to generate CEST images including APT, NOE and MTC maps. The DISC-CEST method more accurately models concomitant effects and results in CEST images with higher contrast. In other aspects, DISC-CEST method may be used in combination with quantitative analytical methods such as quantification of exchange rate with RF saturation time and power (QUEST and QUESP), ratiometric analysis and omega-plot analysis to generate a CEST quantitative metrics of in vivo or in vitro molecule concentrations and/or pH values. The molecule may include an amide, amine, or hydroxyl group that comprises an exchangeable proton. 
     The present disclosure recognizes that CEST imaging can provide clinical utility beyond the brain, such as when imaging other organs, including the heart or kidneys. That is, the present disclosure recognizes that pH estimated from CEST imaging to provide useful metabolic information, for example, to accompany conventional perfusion and diffusion MRI studies. 
     Referring particularly now to  FIG. 4 , an example of a magnetic resonance imaging (“MR”) or nuclear magnetic resonance (NMR) system  400  that can implement the methods described here is illustrated. The MR system  400  includes an operator workstation  402  that may include a display  404 , one or more input devices  406  (e.g., a keyboard, a mouse), and a processor  408 . The processor  408  may include a commercially available programmable machine running a commercially available operating system. The operator workstation  402  provides an operator interface that facilitates entering scan parameters into the MR system  300 . The operator workstation  402  may be coupled to different servers, including, for example, a pulse sequence server  410 , a data acquisition server  412 , a data processing server  414 , and a data store server  416 . The operator workstation  402  and the servers  410 ,  412 , 414 , and  416  may be connected via a communication system  440 , which may include wired or wireless network connections. 
     The pulse sequence server  410  functions in response to instructions provided by the operator workstation  402  to operate a gradient system  418  and a radiofrequency (“RF”) system  420 . Gradient waveforms for performing a prescribed scan are produced and applied to the gradient system  418 , which then excites gradient coils in an assembly  422  to produce the magnetic field gradients G x , G y , and G z  that are used for spatially encoding magnetic resonance signals. The gradient coil assembly  422  forms part of a magnet assembly  424  that includes a polarizing magnet  426  and a whole-body RF coil  428 . 
     RF waveforms are applied by the RF system  420  to the RF coil  428 , or a separate local coil to perform the prescribed magnetic resonance pulse sequence. Responsive magnetic resonance signals detected by the RF coil  428 , or a separate local coil, are received by the RF system  420 . The responsive magnetic resonance signals may be amplified, demodulated, filtered, and digitized under direction of commands produced by the pulse sequence server  410 . The RF system  420  includes an RF transmitter for producing a wide variety of RF pulses used in MR pulse sequences. The RF transmitter is responsive to the prescribed scan and direction from the pulse sequence server  410  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  428  or to one or more local coils or coil arrays. 
     The RF system  420  also includes one or more RF receiver channels. An RF receiver channel includes an RF preamplifier that amplifies the magnetic resonance signal received by the coil  428  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 a 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 )}  (Eq. 7);
 
     and the phase of the received magnetic resonance signal may also be determined according to the following relationship: 
     
       
         
           
             
               
                 
                   φ 
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                         tan 
                         
                           - 
                           1 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           Q 
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     The pulse sequence server  410  may receive patient data from a physiological acquisition controller  430 . By way of example, the physiological acquisition controller  430  may receive signals from a number of different sensors connected to the patient, including electrocardiograph (“ECG”) signals from electrodes, or respiratory signals from a respiratory bellows or other respiratory monitoring devices. These signals may be used by the pulse sequence server  410  to synchronize, or “gate,” the performance of the scan with the subject&#39;s heart beat or respiration. 
     The pulse sequence server  410  may also connect to a scan room interface circuit  432  that receives signals from various sensors associated with the condition of the patient and the magnet system. Through the scan room interface circuit  432 , a patient positioning system  434  can receive commands to move the patient to desired positions during the scan. 
     The digitized magnetic resonance signal samples produced by the RF system  420  are received by the data acquisition server  412 . The data acquisition server  412  operates in response to instructions downloaded from the operator workstation  302  to receive the real-time magnetic resonance data and provide buffer storage, so that data is not lost by data overrun. In some scans, the data acquisition server  412  passes the acquired magnetic resonance data to the data processor server  414 . In scans that require information derived from acquired magnetic resonance data to control the further performance of the scan, the data acquisition server  412  may be programmed to produce such information and convey it to the pulse sequence server  410 . For example, during pre-scans, magnetic resonance data may be acquired and used to calibrate the pulse sequence performed by the pulse sequence server  410 . As another example, navigator signals may be acquired and used to adjust the operating parameters of the RF system  420  or the gradient system  418 , or to control the view order in which k-space is sampled. In still another example, the data acquisition server  412  may also process magnetic resonance signals used to detect the arrival of a contrast agent in a magnetic resonance angiography (“MRA”) scan. 
     The data processing server  414  receives magnetic resonance data from the data acquisition server  412  and processes the magnetic resonance data in accordance with instructions provided by the operator workstation  402 . Such processing may include, for example, reconstructing two-dimensional or three-dimensional images by performing a Fourier transformation of raw k-space data, performing other image reconstruction algorithms (e.g., iterative or backprojection reconstruction algorithms), applying filters to raw k-space data or to reconstructed images, generating functional magnetic resonance images, or calculating motion or flow images. Optionally, the method for “Image Downsampling Expedited Adaptive Least-squares” (“IDEAL”) fitting for magnetic resonance (“MR”) image approach may be implemented here, such as described in U.S. Application Ser. No. 62/473,800. 
     Images reconstructed by the data processing server  414  are conveyed back to the operator workstation  402  for storage. Real-time images may be stored in a data base memory cache, from which they may be output to operator display  402  or a display  436 . Batch mode images or selected real time images may be stored in a host database on disc storage  438 . When such images have been reconstructed and transferred to storage, the data processing server  414  may notify the data store server  416  on the operator workstation  402 . The operator workstation  402  may be used by an operator to archive the images, produce films, or send the images via a network to other facilities. 
     The MR system  400  may also include one or more networked workstations  442 . For example, a networked workstation  442  may include a display  444 , one or more input devices  446  (e.g., a keyboard, a mouse), and a processor  448 . The networked workstation  442  may be located within the same facility as the operator workstation  402 , or in a different facility, such as a different healthcare institution or clinic. 
     The networked workstation  442  may gain remote access to the data processing server  414  or data store server  416  via the communication system  440 . Accordingly, multiple networked workstations  442  may have access to the data processing server  414  and the data store server  416 . In this manner, magnetic resonance data, reconstructed images, or other data may be exchanged between the data processing server  414  or the data store server  416  and the networked workstations  442 , such that the data or images may be remotely processed by a networked workstation  442 . 
     EXAMPLES 
     The following examples set forth, in detail, ways in which the present disclosure may be used or implemented. The following examples are presented by way of illustration and are not meant to be limiting in any way. 
     CEST MRI has shown promise in clinical tissue characterization. As detailed above, in vivo CEST imaging such as APT MRI is challenging due to the concomitant factors such as RF spillover, macromolecular MTC, and NOE. The commonly used asymmetry analysis (MTR asym ) suppresses RF spillover effect, yet it is susceptible to up-field NOE and asymmetric MT effects. Recently, an analytical steady-state CEST solution has been derived that solves RF spillover effect. As described above, the present disclosure provides a system and method for a non-linear background correction, dubbed DISC-CEST analysis that is used for quantification of in vivo CEST measurement. 
     The DISC-CEST techniques described above were tested in a classical 2-pool creatine-gel CEST phantom and demonstrated the DISC-CEST contrast had much stronger correlation with MTR asym  than the previously proposed three-offset method, which showed substantial underestimation. The DISC-CEST approach was then evaluated in normal rat brains, revealing significantly stronger APT effect in gray matter and higher NOE effect in white matter. Furthermore, The DISC-CEST approach was then used to derive both APT and NOE maps in a rat model of glioma, and revealed significantly higher APT effect in the tumors than contralateral normal tissue but no apparent difference in NOE. The DISC-CEST techniques, by correction of non-linear spillover effect, serve as an alternative to both the commonly-used MTR asym  and the simplistic three-offset analyses for in vivo quantitative CEST analysis. 
     Phantom Studies: 
     CEST phantoms were prepared with creatine (Cr) and low gelling point agarose (Sigma Aldrich, St. Louis, Mo.). Briefly, 1% agarose was added to phosphate-buffered saline (PBS) solution doped with 30 μM gadolinium (Bayer HealthCare, Whippany, N.J.). The mixture was microwave-heated and immersed in a water bath at 50° C. The mixture was titrated to pH 6.75 (EuTech Instrument, Singapore). Cr was added to the gel solution to reach concentrations of 10, 20, 30, 40 and 50 mM, respectively. Each concentration of solution was transferred into separate 5 mm NMR tubes. Afterward, the tubes were sealed and inserted into a phantom container filled with 1% empty gel. 
     Tumor Model: 
     Animal experiments were performed in accordance with institutional guidelines, as approved by the Institutional Animal Care and Use Committees, Massachusetts General Hospital (IACUC, MGH). 2×10 5  cells of the non-infiltrating D74-rat glioma model were injected into the right frontal lobe of adult male Fischer 344 rats (N=8), as previously described (Fulci et al., 2006). The animals were imaged 11-13 days after tumor implantation. 
     MRI scans were performed on a 4.7 Tesla scanner (Bruker Biospec, Ettlingen, Germany). For the phantom study, CEST MRI was obtained with single-slice, single-shot EPI (field of view (FOV)=50×50 mm 2 , matrix=96×96, slice thickness=5 mm). Z-spectrum was acquired from −3 ppm to 3 ppm with intervals of 0.0625 ppm and a series of RF irradiation power level of 0.5, 0.75, 1, 1.25 and 1.5 μT. The repetition time (TR)/saturation time (TS)/echo time (TE) was 10 s/5 s/50 ms, number of average (NSA)=2. For the in vivo study, multi-slice MRI (FOV=20×20 mm 2 , matrix=64×64, 5 slices, slice thickness/gap=1.8/0.2 mm) was acquired with single-shot EPI. Z-spectrum were obtained from −6 ppm to 6 ppm with intervals of 0.25 ppm and RF irradiation power level of 0.75 μT, TR/TS/TE=10 s/5 s/15 ms, NSA=2 and scan time=8 min 20 s. In addition, T 1 -weighted images were acquired with seven inversion delays ranging from 250 ms to 3,000 ms (TR/TE=6,500/28 ms, NSA=4); T 2 -weigthed images were obtained with two TE of 30 and 100 ms (TR=3,250 ms, NSA=16). 
     Data were processed in MATLAB (MathWorks, Natick, Mass.). Background noise was removed from the acquired CEST data by using the background noise correction as described in Eqns. 3-5. The boundary offsets with minimum residual CEST effect from the peak were chosen. The three-offset method and the MTR asym  method were also calculated for comparison with the DISC-CEST method, as described above. Parametric T 1  maps were derived using least-squares fitting of the signal as functions of inversion time, as shown by:
 
 I=I   0 [1−(1+ n )] e   −TI/T     1     (Eq. 9);
 
     where η is the inversion efficiency, TI is the inversion time, and I 0  is the equilibrium signal. Parametric T2 maps were generated using the following: 
                       T   2     =         TE   2     -     TE   1         ln   ⁡     (       I   ⁡     (     TE   1     )         I   ⁡     (     TE   2     )         )           ;           (     Eq   .           ⁢   10     )               
where TE 1,2  were approximately 30 and 100 ms, respectively. The regions of interest (ROIs) at cortex and corpus callosum were used for analyzing the CEST data of brain WM and GM. The ROIs of tumors were defined based on T1 map and mirrored to the contralateral hemisphere after a slight adjustment for the shifted midline.
 
Phantom Study Results:
 
       FIG. 5 a    shows the experimentally measured Z-spectra (solid) for 5 creatine (Cr) concentrations, ranging from 10 mM to 50 mM, (B1=0.75 μT) and the analytic estimation of RF spillover effect (dashed) based on Eq. 3, with their difference (ΔZ, dotted). Both MTR asym  from experimentally measured Z-spectra and ΔZ reveal Cr guanidinium proton CEST effect at 1.9 ppm, which increases with Cr concentration. The ΔZ approach provides satisfactory spillover correction, showing more distinct Cr CEST peaks than the Z-spectra.  FIG. 5 b    compares Z, Z′ and ΔZ of one creatine concentration at varied saturation power. The ΔZ suppressed the RF spillover effectively at different B1. More importantly, the ΔZ revealed more distinguishable Cr peaks than the Z-spectra, even at large B1. 
     The three-offset analysis was analyzed on the experimental data (Z) and the proposed DISC-CEST approach using the spillover corrected ΔZ, and compared them with MTR asym  ( FIGS. 6A-C ). Because creatine has negligible up-field CEST effect, MTR asym  provides reliable CEST effect calculation with minimal confounding contribution. The three-offset method assumes that the spillover effect between the two boundary frequency offsets is approximately linear (blue dotted lines,  FIG. 6A ), which deviates from the non-linear spillover effect estimated using Eq. 5 (dashed line). This discrepancy is minimal for a narrow Cr CEST peak with a flat baseline in ΔZ (red dotted lines,  FIG. 6A ).  FIG. 6B  shows that Cr CEST image from the three-offset method had generally smaller CEST effect than MTR asym  approach, with an underestimation of 55.5±23.4% (B1=0.75 μT). In comparison, the Cr CEST map from the DISC-CEST approach was closer to MTR asym , with their difference being 18.7±2.8%. Regression analysis showed noticeably higher correlation between the DISC-CEST approach and MTR asym  than the original three-offset method ( FIG. 6C ). 
     In Vivo Study: Normal Rats 
     The proposed DISC-CEST method was evaluated in comparison with MTR asym  and the three-offset method in normal rat brains.  FIG. 7A  shows the averaged Z, Z′ and ΔZ from the ROIs of WM and GM. Distinct APT peaks at +3.5 ppm and NOE peaks at −3.5 ppm were observed in Z and ΔZ. The boundary offsets of each peak were found to be +2.8 ppm and +4.5 ppm for APT and −2.3 ppm and −5 ppm for NOE.  FIG. 7B  compares the MTR asym , APT* and NOE* effects estimated from the three-offset method, and APT′ and NOE′ effects from the DISC-CEST approach, between WM and GM of normal brains (N=6). Despite significant contrast between WM and GM, the MTR asym  maps were generally negative, due to confounding strong upfield NOE and slightly asymmetric MT effects. Both the three-offset and DISC-CEST analyses revealed significantly stronger APT effect and smaller NOE effect in GM than WM. In addition, the newly proposed approach provided significantly stronger APT′ and NOE′ than the three-offset approach (i.e., APT* and NOE*). 
     In Vivo Study: Glioma Rats 
     The original three-offset method, DISC-CEST methods, and MTR asym  were compared in a rat glioma model.  FIG. 8A-B  show representative Z, Z′ and ΔZ from ROIs of the contralateral normal brain tissue and the tumor. In addition to the bulk water peak and underlying broad MT effect, apparent APT and NOE peaks were present in both Z and ΔZ. The tumor region shows relatively stronger APT effect than the normal tissue. This contrast became more apparent in the RF spillover corrected ΔZ. Interestingly, no apparent difference was observed for the NOE effect between the normal tissue and tumor. For the three-offset and DISC-CEST analyses, the same boundary offsets of APT or NOE peaks as in the normal WM and GM were selected. 
       FIG. 9  shows relaxation maps, MTR asym  maps and APT*, APT′, NOE*, NOE′ maps from two typical slices of a representative rat brain with glioma. T1 maps show clear contrast between the tumor and normal brain tissue. T2 map shows a relatively lower T2 in the tumor core with elevated T2 in the tumor rim. The MTR asym  maps display a positive contrast in the tumor, but the MTR asym  values were negative in the normal tissue due to contamination from upfield NOE effect and asymmetric MT effect. 
     On the other hand, the cerebrospinal fluid (CSF) appeared hyperintense in MTR asym  maps (black arrow,  FIG. 9 ), lowering the specificity of MTR asym  increase with protein level change in cancerous tissue. In comparison, the tumor appears hyperintense in both APT* and APT′ maps with a substantially stronger contrast observed in the APT′ maps. Meanwhile, no obvious difference between the tumor and normal regions was found in NOE* or NOE′ maps. The results from all eight tumor rats were summarized in Table 1. The APT′ and NOE′ effects were significantly higher than the APT* and NOE* (P&lt;0.001, paired t-tests). More importantly, a significant increase in the APT contrast between normal tissue and tumor was found, being APT* 1.04±0.6% and APT′ 1.26±0.6% (p&lt;0.001, paired t-tests). 
                     TABLE 1                  Comparison of relaxation times, MTR asym  and        CEST effects estimated using three-offset analysis (APT*        and NOE*) and DISC-CEST approach (APT′ and NOE′)        in normal and tumor regions. Mean        standard deviation are shown.                         0.75 μT   Normal   Tumor               T 1       1.54 ± 0.13     1.97 ± 0.10**       T 2       56.7 ± 2.1     53.4 ± 2.4**       MTR asym     −3.79 ± 1.61%   −1.25 ± 1.33%*       APT*     1.28 ± 0.58%     2.32 ± 0.66%**       APT′     1.99 ± 0.56%†††     3.25 ± 0.61%***†††       NOE*     1.80 ± 0.14%     1.71 ± 0.69%       NOE′     3.92 ± 0.25%†††     4.40 ± 0.75%†††               Paired t-test were performed with ***p &lt; 0.001, **p &lt; 0.01, *p &lt; 0.05 indicating significant difference between normal and tumor tissues. and †††p &lt; 0.001 indicating significant difference between the three-offset and DISC-CEST analyses (APT* vs. APT′, NOE* vs. NOE′).            
Discussion:
 
     The above-described, non-limiting studies estimated nonlinear RF spillover effect from relaxation measurements, correction of which improves in vivo CEST quantification, augmenting both the conventional MTR asym  approach and the original three-offset method. Notably, the original three-offset is applicable for cases of high field strength, weak saturation power levels and frequency offsets far from the water resonance, where the spillover effect over the frequency range can be approximated by a linear function. Otherwise, it risks substantial underestimation of CEST effect. The relaxation-based direct saturation correction overcomes these limitations as the results demonstrated improved three-offset determination of CEST effects at 4.7 Tesla for frequency offset as small as 1.9 ppm and saturation power as high as 1.5 μT. The CEST peak after spillover correction has a reasonably linear baseline, which allows delineation of the CEST peak from the baseline that is likely attributable to much broader MT effect. As the linear approximation holds for a much broader frequency range than in the uncorrected Z, the boundary offset shifts, or δ 1,2 , can be slightly larger to minimize residual CEST effect from the peak. 
     In the normal brain, both the three-offset and DISC-CEST analyses showed stronger APT effect in GM and larger NOE effect in WM, consistent with previous findings. However, the difference between the APT* and NOE* from the original three-offset method was substantially lower than MTR asym , indicating non-negligible contribution from macromolecular MTC effect. Interestingly, the DISC-CEST approach yielded much stronger APT′ and NOE′ values, with their difference closer to MTR asym  than that between APT* and NOE*. It is worthwhile to point out that the MT effect induces significant signal drop over a wide frequency range and is slightly asymmetric around the water peak. It can be estimated by fitting wide-offset data with a super-Lorentzian lineshape. Alternatively, the macromolecular MT effect within a small frequency range around the water resonance can be fitted by a Lorentzian line shape without the need of acquiring wide-offset data. However, since the MT is much broader than other major CEST peaks, it can be approximated as a linear function or even a constant over a small frequency range. As such, the residual MT effect has a less impact on the DISC-CEST approach than on MTR asym . In this study, we obtain Z-spectrum from −6 to 6 ppm to demonstrate the concept. The DISC-CEST method allows reliable in vivo APT quantification without acquiring the whole Z-spectrum. In theory, it minimally requires three offsets, which drastically reduces the scan time. Meanwhile, acquisition of more offsets can provide more robust correction of field inhomogeneity. 
     Because tumor has significantly T1 and T2 changes from normal tissue ( FIG. 9  and Table 1), it experiences different spillover effects, correction of which is necessary. Using the three-offset or DISC-CEST analyses, significantly higher APT effect was found in the tumors, similar to previous findings from APT imaging of glioma. The APT changes have been suggested to arise from elevated intracellular mobile proteins/peptides concentration. More importantly, the APT′ maps show significantly stronger contrast between the normal and tumor tissues than APT* map, which is beneficial for tumor detection, tracking tumor growth and monitoring APT changes in response to treatments. Therefore, the proposed DISC-CEST method allows in vivo APT quantification with better accuracy and can serve as a promising alternative to MTR asym  and APT*. Our study found no apparent NOE contrast between the normal tissue and tumor, in line with the previous studies using similar irradiation powers. The NOE quantification has been difficult due to its broad linewidth, which will lead to significant error if the non-linear spillover and macromolecular MT contributions are not well eliminated. Previous NOE measurements with similar saturation powers under the same field strength vary from 8% to 10% in normal brain. In this study, we found the NOE′ values are around 4%, which are approximately two times of the NOE* values (2%) but still lower than the literature values. Note the NOE for aliphatic and olefinic protons ranges from 0 to 5 ppm up-field from the water. Therefore, the underestimation in our NOE′ measurement likely arose from residual NOE effects in the boundary offsets chosen here, especially −2.3 ppm. Nevertheless, NOE′ measurement is sensitive in detecting NOE difference between WM and GM. Further improvement in its accuracy can be achieved by reducing the MT effect from the spillover-corrected Z-spectrum using numerical fitting of a theoretical MT model. Notably, the DISC CEST MRI decuples APT from NOE, which is advantageous when both may alter at the same time. 
     The B 0  correction approach does not require the interpolation approach based on densely-sampled APT images. During the development of the DISC-MRI, we realized that the dominant B 0  inhomogeneity effect is through the non-linear direct saturation effect, which can be estimated from relaxation and B0 field images. For the instance of acute ischemic stroke, as the intact tissue has little pH, the residual inhomogeneity effect can be corrected by including the B 0  field map to the analysis for expedient correction as shown in  FIG. 10 . 
     Using the direct saturation corrected Z-spectrum (ΔZ), individual CEST effects such as APT, NOE and broad MTC effects can be decoupled with multi-pool Lorentzian fitting method with Eq. 5.  FIGS. 11  A-B shows a multi-pool Lorentzian fitting of in vivo CEST Z-spectral images from a representative rat brain with focal ischemic stroke.  FIG. 11A  is the multi-pool Lorentzian fitting of the averaged Z-spectrum (Z) or direct saturation corrected Z-spectrum (ΔZ) from contralateral normal tissue (green) or ipsilateral ischemic tissue (red). Individual decoupled CEST pools, including water, APT, amine, NOE and MT are also shown (normal: solid lines; ischemic: dotted lines).  FIG. 11B  is a diffusion and a perfusion MRI map of one ischemic rat brain as well as the decoupled APT, amine, NOE maps using multi-pool Lorentzian fitting of the direct saturation corrected ΔZ. 
     In accordance with one aspect of the disclosure, a method is provided for producing a magnetic resonance (MR) image of a subject. The method includes applying at least one radiofrequency (RF) saturation pulse with an MRI system at a range of frequencies to substantially saturate magnetization corresponding to an exchangeable proton in the subject, acquiring chemical exchange saturation transfer (CEST) data with the MRI system, and generating an acquired Z-spectrum (Z) from the CEST data. The method also includes computing an estimated direct water saturation (Z′) based using at least one of relaxation measurements derived from the CEST data or imaging parameters used to acquire the CEST data with the MRI system, computing a direct saturation corrected Z-spectrum (ΔZ) using the acquired Z-spectrum (Z) and the estimated direct water saturation (Z′), and generating a CEST image of the subject using the direct saturation corrected Z-spectrum (ΔZ). 
     In accordance with another aspect of the present disclosure, system is provided that includes a magnet system configured to generate a polarizing magnetic field about at least a portion of a subject, a magnetic gradient system including a plurality of magnetic gradient coils configured to apply at least one magnetic gradient field to the polarizing magnetic field, and a radio frequency (RF) system configured to apply an RF field to the subject and to receive magnetic resonance signals from the subject using a coil array. The system also includes a computer system programmed to control the magnetic gradient system and the RF system to perform a pulse sequence that includes at least one RF saturation pulse at a range of frequencies to substantially saturate magnetization corresponding to an exchangeable proton in the subject. The computer system is further programmed to acquire chemical exchange saturation transfer (CEST) data from the subject, generate an acquired Z-spectrum (Z) from the CEST data, and compute an estimated direct water saturation (Z′) based using at least one of relaxation measurements derived from the CEST data or imaging parameters used to acquire the CEST data with the MRI system. The computer system is further programmed to determine a direct saturation corrected Z-spectrum (ΔZ) using the Z-spectrum (Z) and the estimated direct water saturation (Z′); and reconstruct a CEST image of the subject using the direct saturation corrected Z-spectrum (ΔZ). 
     In one aspect of the disclosure, a system is provided that includes a magnet system configured to generate a polarizing magnetic field about at least a portion of a subject. The system includes a magnetic gradient system including a plurality of magnetic gradient coils configured to apply at least one magnetic gradient field to the polarizing magnetic field, and a radio frequency (RF) system configured to apply an RF field to the subject and to receive magnetic resonance signals from the subject using a coil array. The system further includes a computer system programmed to apply at least one radiofrequency (RF) saturation pulse at a range of frequencies to substantially saturate magnetization corresponding to an exchangeable proton group in the subject. The system may then apply at least one radiofrequency (RF) saturation pulse using the RF system at a range of frequencies to substantially saturate magnetization corresponding to an exchangeable proton in the subject. The system then acquires chemical exchange saturation transfer (CEST) data, and generates a Z-spectrum (Z) from the CEST data. The system is further programmed to compute an estimated direct water saturation (Z′) based at least in part on relaxation measurements comprising a longitudinal relaxation value (T 1 ) and a traverse relaxation value (T 2 ). The system then generates a direct saturation corrected Z-spectrum (ΔZ) by subtracting the Z-spectrum and the estimated direct water saturation, and generates a CEST image of the subject based at least in part on the direct saturation corrected Z-spectrum using a frequency offset method. 
     In other aspects of the disclosure, a method is provided for producing a magnetic resonance (MR) image of a subject. The method steps comprise applying at least one radiofrequency (RF) saturation pulse with an MRI system at a range of frequencies to substantially saturate magnetization corresponding to an exchangeable proton in the subject. The method further including acquiring chemical exchange saturation transfer (CEST) data with the MRI system, and generating a Z-spectrum (Z) from the CEST data. An estimated direct water saturation (Z′) is then computed based at least in part on relaxation measurements comprising a longitudinal relaxation value T 1  and a traverse relaxation value T 2 . A direct saturation corrected Z-spectrum (ΔZ) is then computed by subtracting the Z-spectrum and the estimated direct water saturation. A CEST image of the subject is then generated based at least in part on the direct saturation corrected Z-spectrum. 
     In other aspects of the disclosure, a method is provided for producing a magnetic resonance (MR) image of a subject. The method steps comprise applying at least one radiofrequency (RF) saturation pulse with an MRI system at a range of frequencies to substantially saturate magnetization corresponding to an exchangeable proton in the subject. The method further including acquiring chemical exchange saturation transfer (CEST) data with the MRI system, and generating a Z-spectrum (Z) from the CEST data. An estimated direct water saturation (Z′) comprising a non-linear function is then computed. A direct saturation corrected Z-spectrum (ΔZ) is then computed by subtracting the Z-spectrum and the estimated direct water saturation. A CEST image of the subject is then generated using a frequency offset method.