Moritz Zaiss

Chemical exchange saturation transfer (CEST) and MR Z-spectroscopy in vivo: a review of theoretical approaches and methods.

Chemical exchange saturation transfer (CEST) of metabolite protons that undergo exchange processes with the abundant water pool enables a specific contrast for magnetic resonance imaging (MRI). The CEST image contrast depends on physical and physiological parameters that characterize the microenvironment such as temperature, pH, and metabolite concentration. However, CEST imaging in vivo is a complex technique because of interferences with direct water saturation (spillover effect), the involvement of other exchanging pools, in particular macromolecular systems (magnetization transfer, MT), and nuclear Overhauser effects (NOEs). Moreover, there is a strong dependence of the diverse effects on the employed parameters of radiofrequency irradiation for selective saturation which makes interpretation of acquired signals difficult. This review considers analytical solutions of the Bloch–McConnell (BM) equation system which enable deep insight and theoretical description of CEST and the equivalent off-resonant spinlock (SL) experiments. We derive and discuss proposed theoretical treatments in detail to understand the influence of saturation parameters on the acquired Z-spectrum and how the different effects interfere and can be isolated in MR Z-spectroscopy. Finally, we provide an overview of reported CEST effects in vivo and discuss proposed methods and technical approaches applicable to in vivo CEST studies on clinical MRI systems.

Inverse Z-spectrum analysis for spillover-, MT-, and T1-corrected steady-state pulsed CEST-MRI – application to pH-weighted MRI of acute stroke

Endogenous chemical exchange saturation transfer (CEST) effects are always diluted by competing effects, such as direct water proton saturation (spillover) and semi‐solid macromolecular magnetization transfer (MT). This leads to unwanted T2 and MT signal contributions that lessen the CEST signal specificity to the underlying biochemical exchange processes. A spillover correction is of special interest for clinical static field strengths and protons resonating near the water peak. This is the case for all endogenous CEST agents, such as amide proton transfer, –OH‐CEST of glycosaminoglycans, glucose or myo‐inositol, and amine exchange of creatine or glutamate. All CEST effects also appear to be scaled by the T1 relaxation time of water, as they are mediated by the water pool. This forms the motivation for simple metrics that correct the CEST signal.

Exchange-dependent relaxation in the rotating frame for slow and intermediate exchange -- modeling off-resonant spin-lock and chemical exchange saturation transfer.

Chemical exchange observed by NMR saturation transfer (CEST) and spin‐lock (SL) experiments provide an MRI contrast by indirect detection of exchanging protons. The determination of the relative concentrations and exchange rates is commonly achieved by numerical integration of the Bloch–McConnell equations. We derive an analytical solution of the Bloch–McConnell equations that describes the magnetization of coupled spin populations under radiofrequency irradiation. As CEST and off‐resonant SL are equivalent, their steady‐state magnetization and dynamics can be predicted by the same single eigenvalue: the longitudinal relaxation rate in the rotating frame R1ρ. For the case of slowly exchanging systems, e.g. amide protons, the saturation of the small proton pool is affected by transverse relaxation (R2b). It turns out, that R2b is also significant for intermediate exchange, such as amine‐ or hydroxyl‐exchange or paramagnetic CEST agents, if pools are only partially saturated. We propose a solution for R1ρ that includes R2 of the exchanging pool by extending existing approaches, and verify it by numerical simulations. With the appropriate projection factors, we obtain an analytical solution for CEST and SL for nonzero R2 of the exchanging pool, exchange rates in the range 1–104 Hz, B1 from 0.1 to 20 μT and arbitrary chemical shift differences between the exchanging pools, whilst considering the dilution by direct water saturation across the entire Z‐spectra. This allows the optimization of irradiation parameters and the quantification of pH‐dependent exchange rates and metabolite concentrations. In addition, we propose evaluation methods that correct for concomitant direct saturation effects. It is shown that existing theoretical treatments for CEST are special cases of this approach. Copyright

On the origins of chemical exchange saturation transfer (CEST) contrast in tumors at 9.4 T

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 in mobile proteins/peptides in tumors. However, as 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 is not certain. In addition to APT, nuclear Overhauser enhancement (NOE) effects upfield from water may also provide distinct information on tissue composition. In the current study, APT, NOE and several other MR parameters were measured and compared comprehensively in order to elucidate the origins of APT and NOE contrasts in tumors at 9.4 T. 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 to be significantly different from that in normal tissues, but corrected NOE effects in tumors showed significant decreases compared with those in normal tissues. Biochemical measurements verified that there was no significant enhancement of protein contents in the tumors studied, consistent with the corrected APT measurements and previous literature, whereas quantitative MT data showed decreases in the fractions of immobile macromolecules in tumors. Our results may assist in the better understanding of the contrast depicted by CEST imaging in tumors, and in the development of improved APT and NOE measurements for cancer imaging. Copyright

Relaxation-compensated CEST-MRI of the human brain at 7T: Unbiased insight into NOE and amide signal changes in human glioblastoma

Endogenous chemical exchange saturation transfer (CEST) effects of protons resonating near to water protons are always diluted by competing effects such as direct water saturation and semi-solid magnetization transfer (MT). This leads to unwanted T2 and MT signal contributions that contaminate the observed CEST signal. Furthermore, all CEST effects appear to be scaled by the T1 relaxation time of the mediating water pool. As MT, T1 and T2 are also altered in tumor regions, a recently published correction algorithm yielding the apparent exchange-dependent relaxation AREX, is used to evaluate in vivo CEST effects. This study focuses on CEST effects of amides (3.5ppm) and Nuclear-Overhauser-mediated saturation transfer (NOE, -3.5ppm) that can be properly isolated at 7T. These were obtained in 10 glioblastoma patients, and this is the first comprehensive study where AREX is applied in human brain as well as in human glioblastoma. The correction of CEST effects alters the contrast significantly: after correction, the CEST effect of amides does not show significant contrast between contrast enhancing tumor regions and normal tissue, whereas NOE drops significantly in the tumor area. In addition, new features in the AREX contrasts are visible. This suggests that previous CEST approaches might not have shown pure CEST effects, but rather water relaxation shine-through effects. Our insights help to improve understanding of the CEST effect changes in tumors and correlations on a cellular and molecular level.

Correction of B1-inhomogeneities for relaxation-compensated CEST imaging at 7 T

Chemical exchange saturation transfer (CEST) imaging of endogenous agents in vivo is influenced by direct water proton saturation (spillover) and semi‐solid macromolecular magnetization transfer (MT). Lorentzian fit isolation and application of the inverse metric yields the pure CEST contrast AREX, which is less affected by these processes, but still depends on the measurement technique, in particular on the irradiation amplitude B1 of the saturation pulses. This study focuses on two well‐known CEST effects in the slow exchange regime originating from amide and aliphatic protons resonating at 3.5 ppm or −3.5 ppm from water protons, respectively. A B1‐correction of CEST contrasts is crucial for the evaluation of data obtained in clinical studies at high field strengths with strong B1‐inhomogeneities. Herein two approaches for B1‐inhomogeneity correction, based on either CEST contrasts or Z‐spectra, are investigated. Both rely on multiple acquisitions with different B1‐values. One volunteer was examined with eight different B1‐values to optimize the saturation field strength and the correction algorithm. Histogram evaluation allowed quantification of the quality of the B1‐correction. Finally, the correction was applied to CEST images of a patient with oligodendroglioma WHO grade 2, and showed improvement of the image quality compared with the non‐corrected CEST images, especially in the tumor region. Copyright

A combined analytical solution for chemical exchange saturation transfer and semi-solid magnetization transfer

Off‐resonant RF 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 parts per million 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 kilohertz linewidths. Recently it was shown that endogenous CEST contrasts can be strongly affected by the MT background, so 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 lineshapes 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. Copyright

Analytical solution for the depolarization of hyperpolarized nuclei by chemical exchange saturation transfer between free and encapsulated xenon (HyperCEST).

We present an analytical solution of the Bloch-McConnell equations for the case of chemical exchange saturation transfer between hyperpolarized nuclei in cavities and in solvent (HyperCEST experiment). This allows quantitative investigation of host-guest interactions by means of nuclear magnetic resonance spectroscopy and, due to the strong HyperCEST signal enhancement, even NMR imaging. Hosts of interest can be hydrophobic cavities in macromolecules or artificial cages like cryptophane-A which was proposed as a targeted biosensor. Relevant system parameters as exchange rate and host concentration can be obtained from the monoexponential depolarization process which is shown to be governed by the smallest eigenvalue in modulus. For this dominant eigenvalue we present a useful approximation leading to the depolarization rate for the case of on- and off-resonant irradiation. It is shown that this rate is a generalization of the longitudinal relaxation rate in the rotating frame. We demonstrate for the free and cryptophane-A-encapsulated xenon system, by comparison with numerical simulations, that HyperCEST experiments are precisely described in the valid range of this widely applicable analytical approximation. Altogether, the proposed analytical solution allows optimization and quantitative analysis of HyperCEST experiments but also characterization and optimal design of possible biosensors.

Nuclear Overhauser Enhancement Mediated Chemical Exchange Saturation Transfer Imaging at 7 Tesla in Glioblastoma Patients

Background and Purpose Nuclear Overhauser Enhancement (NOE) mediated chemical exchange saturation transfer (CEST) is a novel magnetic resonance imaging (MRI) technique on the basis of saturation transfer between exchanging protons of tissue proteins and bulk water. The purpose of this study was to evaluate and compare the information provided by three dimensional NOE mediated CEST at 7 Tesla (7T) and standard MRI in glioblastoma patients. Patients and Methods Twelve patients with newly diagnosed histologically proven glioblastoma were enrolled in this prospective ethics committee–approved study. NOE mediated CEST contrast was acquired with a modified three-dimensional gradient-echo sequence and asymmetry analysis was conducted at 3.3ppm (B1 = 0.7 µT) to calculate the magnetization transfer ratio asymmetry (MTRasym). Contrast enhanced T1 (CE-T1) and T2-weighted images were acquired at 3T and used for data co-registration and comparison. Results Mean NOE mediated CEST signal based on MTRasym values over all patients was significantly increased (p<0.001) in CE-T1 tumor (−1.99±1.22%), tumor necrosis (−1.36±1.30%) and peritumoral CEST hyperintensities (PTCH) within T2 edema margins (−3.56±1.24%) compared to contralateral normal appearing white matter (−8.38±1.19%). In CE-T1 tumor (p = 0.015) and tumor necrosis (p<0.001) mean MTRasym values were significantly higher than in PTCH. Extent of the surrounding tumor hyperintensity was smaller in eight out of 12 patients on CEST than on T2-weighted images, while four displayed at equal size. In all patients, isolated high intensity regions (0.40±2.21%) displayed on CEST within the CE-T1 tumor that were not discernible on CE-T1 or T2-weighted images. Conclusion NOE mediated CEST Imaging at 7T provides additional information on the structure of peritumoral hyperintensities in glioblastoma and displays isolated high intensity regions within the CE-T1 tumor that cannot be acquired on CE-T1 or T2-weighted images. Further research is needed to determine the origin of NOE mediated CEST and possible clinical applications such as therapy assessment or biopsy planning.

Characterization of creatine guanidinium proton exchange by water-exchange (WEX) spectroscopy for absolute-pH CEST imaging in vitro

Chemical exchange saturation transfer (CEST) enables indirect detection of small metabolites in tissue by MR imaging. To optimize and interpret creatine‐CEST imaging we characterized the dependence of the exchange‐rate constant ksw of creatine guanidinium protons in aqueous creatine solutions as a function of pH and temperature T in vitro. Model solutions in the low pH range (pH = 5–6.4) were measured by means of water‐exchange (WEX)‐filtered 1H NMR spectroscopy on a 3 T whole‐body MR tomograph. An extension of the Arrhenius equation with effective base‐catalyzed Arrhenius parameters yielded a general expression for ksw(pH, T). The defining parameters were identified as the effective base‐catalyzed rate constant kb,eff(298.15 K) = (3.009 ± 0.16) × 109 Hz l/mol and the effective activation energy EA,b,eff = (32.27 ± 7.43) kJ/mol at a buffer concentration of cbuffer = (1/15) M. As expected, a strong dependence of ksw on temperature was observed. The extrapolation of the exchange‐rate constant to in vivo conditions (pH = 7.1, T = 37 °C) led to the value of the exchange‐rate constant ksw = 1499 Hz. With the explicit function ksw(pH, T) available, absolute‐pH CEST imaging could be realized and experimentally verified in vitro. By means of our calibration method it is possible to adjust the guanidinium proton exchange‐rate constant ksw to any desired value by preparing creatine model solutions with a specific pH and temperature. Copyright