Simplified and scalable numerical solution for describing multi-pool chemical exchange saturation transfer (CEST) MRI contrast
Introduction
The indirect detection mechanism of chemical exchange saturation transfer (CEST) MRI confers it with an enormous sensitivity enhancement so that dilute labile proton and microenvironment properties such as pH and temperature can be estimated [1], [2]. Specifically, CEST contrast is approximately proportional to the labile proton concentration and exchange rate, hence, provides information that complements the conventional imaging methods [3], [4], [5], [6]. For instance, CEST MRI is capable of measuring metabolites and their byproducts such as glucose, glycogen and lactate [7], [8], [9]. In addition, amide proton transfer (APT) imaging, a specific form of CEST MRI, is pH dependent and remains promising for detecting ischemic tissue acidosis beyond the commonly used perfusion and diffusion scans [10], [11], [12], [13], [14]. Nevertheless, CEST MRI contrast is complex; it not only varies with labile proton concentration, exchange rate and relaxation time, but also depends on the experimental parameters such as the magnetic field strength, RF power, duration and scheme [15], [16], [17], [18], [19]. In addition, when there are multiple exchangeable proton groups within a single CEST system, quantitative description of CEST contrast becomes even more complex [13], [20], [21], [22], [23]. Moreover, semisolid macromolecular magnetization transfer (MT) and nuclear overhauser effect (NOE)-mediated saturation transfer may also become non-negligible for in vivo CEST MRI, particularly so if large RF irradiation power is applied [22], [24], [25], [26], [27].
Mathematical models have been developed to describe CEST contrast [27], [28], [29], [30], [31]. Specifically, an empirical solution based on the 2-pool exchange model provides simple yet reasonably accurate quantification of CEST contrast [15], [19]. The solution has also been modified to describe in vivo pH-weighted APT imaging of acute ischemia [31]. While on the other hand, Bloch equations coupled with exchange terms (Bloch–McConnell equations) can also numerically simulate 2-pool chemical exchange, and has been recently extended for quantifying 3-pool and 4-pool CEST contrast [27], [30]. However, conventional Bloch–McConnell equations are not easily scalable, and it is somewhat tedious to apply it to describe multi-pool CEST contrast beyond three labile proton groups. While on the other hand, over ten saturation transfer sites have been identified in biological systems, and therefore, it is necessary to develop a simplified solution for describing complex multi-pool CEST contrast [13], [22], [23], [32].
To address this need, our study investigated whether the multi-pool CEST contrast can be simulated using the commonly used 2-pool exchange model. It is important to note that because the direct RF saturation should be taken into account only once, multi-pool CEST Z-spectrum cannot be obtained by simply superimposing multiple Z-spectra estimated independently. Fortunately, the direct RF saturation can be quantified using a single set of Bloch equations [30], [33]. In addition, whereas direct exchange among labile proton groups is negligible due to their dilute concentration, labile protons may be coupled indirectly through their saturation transfer with bulk water protons. Particularly, when the RF irradiation amplitude becomes comparable with their chemical shifts, multiple labile protons may be simultaneously saturated, thus, compete for CEST contrast. As a result, simple linear superposition of individual CEST contrast will overestimate the multi-pool CEST contrast. Our study here derived the coupling term and developed a simplified solution, based on the classic 2-pool model, to quantify multi-pool CEST MRI contrast. We first confirmed that the proposed method is in good agreement with the conventional multi-pool CEST simulation using a representative 3-pool CEST system. We then showed that the proposed method is scalable and can be easily extended to simulate multi-pool CEST contrast with only minimal modification. In summary, the proposed numerical solution method provides simplified yet reasonably accurate modeling of multi-pool CEST contrast, suitable for quantifying complex CEST contrast.
Section snippets
Theory
Bloch–McConnell equations are often used to describe CEST contrast, and for a representative 2-pool chemical exchange, we have [28], [30],in which Mwx,y,z and Msx,y,z are the x, y and z magnetization components for bulk water and labile protons, respectively, with T1,2w and T1,2s being their
Materials and methods
Numerical simulation was conducted in Matlab 7.4 (Mathworks, Natick MA). The coupling term was first derived, and the proposed simplified solution was compared with the conventional numerical solution using a representative 3-pool CEST model. We assumed that the T1 and T2 for bulk water are 3 s and 100 ms, respectively, and being 1 s and 15 ms for two dilute labile protons. In addition, the chemical shifts for two labile protons were set to be 4 and 5 ppm, for the magnetic field strength of 4.7 T (200
Results
Fig. 1 compares the proposed scalable solution with the conventional simulation of multi-pool CEST contrast. The conventional simulation (Fig. 1a) utilizes multiple sets of Bloch equations coupled by the exchange terms, with each set representing a single labile proton group [28]. The Bloch matrix size, for a general N-saturation transfer sites, is 3 N by 3 N (O(N2)). In comparison, the proposed simplified solution utilizes multiple 2-pool models, with a correction term that takes into account of
Discussion
This study developed a simplified numerical solution for describing multi-pool CEST contrast. The proposed method is scalable, and can be easily extended to describe CEST imaging of a large number of labile protons, potentially useful for quantifying in vivo CEST applications [13], [22]. Whereas for the representative 3-pool CEST contrast, the computation time of the proposed method is comparable with that of conventional solution (∼0.2 s, Dell Dimension 9100), the advantage in computation time
Conclusion
A flexible numerical solution based on the classic 2-pool exchange model was developed to describe multi-pool CEST contrast, in good agreement with the conventional numerical simulation method. The proposed method is scalable and can be easily extended to describe CEST contrast of a large number of saturation transfer sites. As such, our work may facilitate quantitative understanding of complex CEST contrast, complementing the conventional numerical simulation methods.
Acknowledgments
This study was supported in part by Grants from AHA/SDG 0835384N, NIH/NIBIB 1K01EB009771-01 and NIH/R21NS061119-02.
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