Off-resonant radiofrequency irradiation in tissue indirectly lowers the water signal by saturation transfer processes: On the one hand, there are selective chemical exchange saturation transfer (CEST) effects originating from exchanging endogenous protons resonating a few ppm from water; on the other hand, there is the broad semi-solid magnetization transfer (MT) originating from immobile protons associated with the tissue matrix with kHz line-widths. Recently it was shown that endogenous CEST contrasts can be strongly affected by the MT background so that corrections are needed to derive accurate estimates of CEST effects. Herein we show that a full analytical solution of the underlying Bloch-McConnell equations for both MT and CEST provides insights into their interaction and suggests a simple means to isolate their effects. The presented analytical solution, based on the eigenspace solution of the Bloch-McConnell equations, extends previous treatments by allowing arbitrary line-shapes for the semi-solid MT effects and simultaneously describing multiple CEST pools in the presence of a large MT pool for arbitrary irradiation. The structure of the model indicates that semi-solid MT and CEST effects basically add up inversely in determining the steady-state Z-spectrum, as previously shown for direct saturation and CEST effects. Implications for existing previous CEST analyses in the presence of a semi-solid MT are studied and discussed. It turns out that to accurately quantify CEST contrast, a good reference Z-value, the observed longitudinal relaxation rate of water, and the semi-solid MT pool size fraction, must all be known.

Chemical exchange saturation transfer (CEST) provides an indirect means to detect exchangeable protons within tissues through their effects on the water signal. Previous studies have suggested that amide proton transfer (APT) imaging, a specific form of CEST, detects endogenous amide protons with a resonance frequency offset 3.5 ppm downfield from water, and thus may be sensitive to variations of mobile proteins/peptides in tumors. However, since CEST measurements are influenced by various confounding effects, such as spillover saturation, magnetization transfer (MT) and MT asymmetry, the mechanism or degree of increased APT signal in tumors are not certain. In addition to APT, nuclear Overhauser enhancement (NOE) effects upfield from water may also provide distinct information about tissue composition. In the current study, APT, NOE and several other magnetic resonance parameters were measured and compared comprehensively in order to elucidate the origins of APT and NOE contrasts in tumors at 9.4T. In addition to conventional CEST methods, a new intrinsic inverse metric was applied to correct for relaxation and other effects. After corrections for spillover, MT and T1 effects, corrected APT in tumors was found not significantly different from normal tissues, but corrected NOE effects in tumors showed significant decreases compared with normal tissues. Biochemical measurements verified that there is no significant enhancement of protein contents in the tumors studied, consistent with corrected APT measurements and previous literature while qMT data showed decreases in the fractions of immobile macromolecules in tumors. Our results may assist better understanding the contrast depicted by CEST imaging in tumors, and the development of improved APT and NOE measurements for cancer imaging.

Accurate quantification of chemical exchange saturation transfer (CEST) effects, including dipole-dipole mediated relayed nuclear Overhauser enhancements (rNOE) saturation transfer, is important for applications and studies of molecular concentration and transfer rate (and thereby pH or temperature). Although several quantification methods, such as Lorentzian difference (LD) analysis, multiple-pool Lorentzian fits, and the three-point method, have been extensively used in several preclinical and clinical applications, the accuracy of these methods has not been evaluated. Here we simulated multiple-pool Z-spectra containing the pools that contribute to the main CEST and rNOE saturation transfer signals in brain, and numerically fit them using the different methods, and then compared their derived CEST metrics with the known solute concentrations and exchange rates. Our results show that the LD analysis overestimates contributions from amide proton transfer (APT) and intermediate exchanging amine protons; the three-point method significantly underestimates both APT and rNOE saturation transfer at −3.5 ppm (NOE(−3.5)). In contrast, the multiple-pool Lorentzian fit is more accurate than the other two methods, but only at lower irradiation powers (< 1 μT at 9.4 T) within the range of our simulations. At higher irradiation powers, this method is also inaccurate because of the presence of a fast exchanging CEST signal that has a non-Lorentzian lineshape. Quantitative parameters derived from in vivo images of rodent brain tumor obtained using an irradiation power of 1 μT were also compared. Our results demonstrate that all three quantification methods show similar contrasts between tumor and contralateral normal tissue for both APT and the NOE(−3.5). However, the quantified values of the three methods are significantly different. Our work provides insight into the fitting accuracy obtainable in a complex tissue model and provides guidelines for evaluating other newly developed quantification methods.

In this study, we introduce a new method for amide proton transfer imaging based on chemical exchange rotation transfer. It avoids several artifacts that plague conventional chemical exchange saturation transfer approaches by creating label and reference scans based on varying the irradiation pulse rotation angle (π and 2π radians) instead of the frequency offset (3.5 and −3.5 ppm). Specifically, conventional analysis is sensitive to confounding contributions from magnetic field (B0) inhomogeneities and, more problematically, inherently asymmetric macromolecular resonances. In addition, the lipid resonance at −3.5 ppm complicates the interpretation of the reference scan and decreases the resulting contrast. Finally, partial overlap of the amide signal by nearby amines and hydroxyls obscure the results. By avoiding these issues, our new method is a promising approach for imaging endogenous protein and peptide content and mapping pH.