Quantitative description of radiofrequency (RF) power‐based ratiometric chemical exchange saturation transfer (CEST) pH imaging

Chemical exchange saturation transfer (CEST) MRI holds great promise for the imaging of pH. However, routine CEST measurement varies not only with the pH‐dependent chemical exchange rate, but also with CEST agent concentration, providing pH‐weighted information. Conventional ratiometric CEST imaging normalizes the confounding concentration factor by analyzing the relative CEST effect from different exchangeable groups, requiring CEST agents with multiple chemically distinguishable labile proton sites. Recently, a radiofrequency (RF) power‐based ratiometric CEST MRI approach has been developed for concentration‐independent pH MRI using CEST agents with a single exchangeable group. To facilitate quantification and optimization of the new ratiometric analysis, we quantified the RF power‐based ratiometric CEST ratio (rCESTR) and derived its signal‐to‐noise and contrast‐to‐noise ratios. Using creatine as a representative CEST agent containing a single exchangeable site, our study demonstrated that optimized RF power‐based ratiometric analysis provides good pH sensitivity. We showed that rCESTR follows a base‐catalyzed exchange relationship with pH independent of creatine concentration. The pH accuracy of RF power‐based ratiometric MRI was within 0.15–0.20 pH units. Furthermore, the absolute exchange rate can be obtained from the proposed ratiometric analysis. To summarize, RF power‐based ratiometric CEST analysis provides concentration‐independent pH‐sensitive imaging and complements conventional multiple labile proton group‐based ratiometric CEST analysis. Copyright © 2015 John Wiley & Sons, Ltd.     

Thiol‐water proton exchange of glutathione,cysteine, and N‐acetylcysteine: Implications for CEST MRI

Amide‐, amine‐, and hydroxyl‐water proton exchange can generate MRI contrast through chemical exchange saturation transfer (CEST). In this study, we show that thiol‐water proton exchange can also generate quantifiable CEST effects under near‐physiological conditions (pH = 7.2 and 37°C) through the characterization of the pH dependence of thiol proton exchange in phosphate‐buffered solutions of glutathione, cysteine, and N‐acetylcysteine. The spontaneous, base‐catalyzed, and buffer‐catalyzed exchange contributions to the thiol exchange were analyzed. The thiol‐water proton exchange of glutathione and cysteine was found to be too fast to generate a CEST effect around neutral pH due to significant base catalysis. The thiol‐water proton exchange of N‐acetylcysteine was found to be much slower, yet still in the fast‐exchange regime with significant base and buffer catalysis, resulting in a 9.5% attenuation of the water signal at pH 7.2 in a slice‐selective CEST NMR experiment. Furthermore, the N‐acetylcysteine thiol CEST was also detectable in human serum albumin and agarose phantoms.     

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. Based on eigenspace theory, we propose a novel magnetization transfer ratio (MTR_Rex), employing the inverse Z‐spectrum, which eliminates spillover and semi‐solid MT effects. This metric can be simply related to R_ex, the exchange‐dependent relaxation rate in the rotating frame, and k_a, the inherent exchange rate. Furthermore, it can be scaled by the duty cycle, allowing for simple translation to clinical protocols. For verification, the amine proton exchange of creatine in solutions with different agar concentrations was studied experimentally at a clinical field strength of 3 T, where spillover effects are large. We demonstrate that spillover can be properly corrected and that quantitative evaluation of pH and creatine concentration is possible. This proves that MTR_Rex is a quantitative and biophysically specific CEST‐MRI metric. Applied to acute stroke induced in rat brain, the corrected CEST signal shows significantly higher contrast between the stroke area and normal tissue, as well as less B₁ dependence, than conventional approaches. Copyright © 2014 John Wiley & Sons, Ltd.     

Advantages of chemical exchange‐sensitive spin‐lock (CESL) over chemical exchange saturation transfer (CEST) for hydroxyl– and amine–water proton exchange studies

The chemical exchange (CE) rate of endogenous hydroxyl and amine protons with water is often comparable to the difference in their chemical shifts. These intermediate exchange processes have been imaged by the CE saturation transfer (CEST) approach with low‐power and long‐duration irradiation. However, the sensitivity is not optimal and, more importantly, the signal is contaminated by slow magnetization transfer processes. Here, the properties of CEST signals are compared with those of a CE‐sensitive spin‐lock (CESL) technique irradiating at the labile proton frequency. First, using a higher power and shorter irradiation in CE‐MRI, we obtain: (i) an increased selectivity to faster CE rates via a higher sensitivity to faster CEs and a lower sensitivity to slower CEs and magnetization transfer processes; and (ii) a decreased in vivo asymmetric magnetization transfer contrast measured at ±15 ppm. The sensitivity gain of CESL over CEST is higher for a higher power and shorter irradiation. Unlike CESL, CEST signals oscillate at a very high power and short irradiation. Second, time‐dependent CEST and CESL signals are well modeled by analytical solutions of CE‐MRI with an asymmetric population approximation, which can be used for quantitative CE‐MRI and validated by simulations of Bloch–McConnell equations and phantom experiments. Finally, the in vivo amine–water proton exchange contrast measured at 2.5 ppm with ω₁ = 500 Hz is 18% higher in sensitivity for CESL than CEST at 9.4 T. Overall, CESL provides better exchange rate selectivity and sensitivity than CEST; therefore, CESL is more suitable for CE‐MRI of intermediate exchange protons. Copyright © 2014 John Wiley & Sons, Ltd.     

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 k_sw 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 k_sw(pH, T). The defining parameters were identified as the effective base‐catalyzed rate constant k_b,eff(298.15 K) = (3.009 ± 0.16) × 10⁹ Hz l/mol and the effective activation energy E_A,b,eff = (32.27 ± 7.43) kJ/mol at a buffer concentration of c_buffer = (1/15) M. As expected, a strong dependence of k_sw 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 k_sw = 1499 Hz. With the explicit function k_sw(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 k_sw to any desired value by preparing creatine model solutions with a specific pH and temperature. Copyright © 2014 John Wiley & Sons, Ltd.     

Relaxation‐compensated CEST‐MRI at 7 T for mapping of creatine content and pH – preliminary application in human muscle tissue in vivo

The small biomolecule creatine is involved in energy metabolism. Mapping of the total creatine (mostly PCr and Cr) in vivo has been done with chemical shift imaging. Chemical exchange saturation transfer (CEST) allows an alternative detection of creatine via water MRI. Living tissue exhibits CEST effects from different small metabolites, including creatine, with four exchanging protons of its guanidinium group resonating about 2 ppm from the water peak and hence contributing to the amine proton CEST peak. The intermediate exchange rate (≈ 1000 Hz) of the guanidinium protons requires high RF saturation amplitude B₁. However, strong B₁ fields also label semi‐solid magnetization transfer (MT) effects originating from immobile protons with broad linewidths (~kHz) in the tissue. Recently, it was shown that endogenous CEST contrasts are strongly affected by the MT background as well as by T₁ relaxation of the water protons. We show that this influence can be corrected in the acquired CEST data by an inverse metric that yields the apparent exchange‐dependent relaxation (AREX). AREX has some useful linearity features that enable preparation of both concentration, and – by using the AREX‐ratio of two RF irradiation amplitudes B₁ – purely exchange‐rate‐weighted CEST contrasts. These two methods could be verified in phantom experiments with different concentration and pH values, but also varying water relaxation properties. Finally, results from a preliminary application to in vivo CEST imaging data of the human calf muscle before and after exercise are presented. The creatine concentration increases during exercise as expected and as confirmed by ³¹P NMR spectroscopic imaging. However, the estimated concentrations obtained by our method were higher than the literature values: to . The CEST‐based pH method shows a pH decrease during exercise, whereas a slight increase was observed by ³¹P NMR spectroscopy. Copyright © 2015 John Wiley & Sons, Ltd.     

Chemical exchange rotation transfer imaging of intermediate‐exchanging amines at 2 ppm

Chemical exchange saturation transfer (CEST) imaging of amine protons exchanging at intermediate rates and whose chemical shift is around 2 ppm may provide a means of mapping creatine. However, the quantification of this effect may be compromised by the influence of overlapping CEST signals from fast‐exchanging amines and hydroxyls. We aimed to investigate the exchange rate filtering effect of a variation of CEST, named chemical exchange rotation transfer (CERT), as a means of isolating creatine contributions at around 2 ppm from other overlapping signals. Simulations were performed to study the filtering effects of CERT for the selection of transfer effects from protons of specific exchange rates. Control samples containing the main metabolites in brain, bovine serum albumin (BSA) and egg white albumen (EWA) at their physiological concentrations and pH were used to study the ability of CERT to isolate molecules with amines at 2 ppm that exchange at intermediate rates, and corresponding methods were used for in vivo rat brain imaging. Simulations showed that exchange rate filtering can be combined with conventional filtering based on chemical shift. Studies on samples showed that signal contributions from creatine can be separated from those of other metabolites using this combined filter, but contributions from protein amines may still be significant. This exchange filtering can also be used for in vivo imaging. CERT provides more specific quantification of amines at 2 ppm that exchange at intermediate rates compared with conventional CEST imaging.     

Continuous‐wave saturation considerations for efficient xenon depolarization

The combination of hyperpolarized Xe with chemical exchange saturation transfer (Hyper‐CEST) is a powerful NMR technique to detect highly dilute concentrations of Xe binding sites using RF saturation pulses. Crucially, that combination of saturation pulse strength and duration that generates the maximal Hyper‐CEST effect is a priori unknown. In contrast to CEST in proton MRI, where the system reaches a steady‐state for long saturation times, Hyper‐CEST has an optimal saturation time, i.e. saturating for shorter or longer reduces the Hyper‐CEST effect. Here, we derive expressions for this optimal saturation pulse length. We also found that a pulse strength, B₁, corresponding to five times the Xe exchange rate, k_BA (i.e. B₁ = 5 k_BA/γ with the gyromagnetic ratio of ¹²⁹Xe, γ), generates directly and without further optimization 96 % of the maximal Hyper‐CEST contrast while preserving spectral selectivity. As a measure that optimizes the amplitude and the width of the Hyper‐CEST response simultaneously, we found an optimal saturation pulse strength corresponding to times the Xe exchange rate, i.e. . When extremely low host concentration is detected, then the expression for the optimum saturation time simplifies as it approaches the longitudinal relaxation time of free Xe. Copyright © 2015 John Wiley & Sons, Ltd.     

MR imaging of protein folding in vitro employing Nuclear‐Overhauser‐mediated saturation transfer

MR Z‐spectroscopy allows enhanced imaging contrast on the basis of saturation transfer between the proton pools of cellular compounds and water, occurring via chemical exchange (chemical exchange saturation transfer, CEST) or dipole–dipole coupling (nuclear Overhauser effect, NOE). In previous studies, signals observed in the aliphatic proton region of Z‐spectra have been assigned to NOEs between protons in water molecules and protons at the surface of proteins. We investigated a possible relationship between the signal strength of NOE peaks in Z‐spectra obtained at B₀ = 7 T and protein structure. Here, we report a correlation of NOE‐mediated saturation transfer with the structural state of bovine serum albumin (BSA), which was monitored by fluorescence spectroscopy. Encouraged by CEST signal changes observed in tumor tissue, our observation also points to a possible contrast mechanism for MRI sensitive to the structural integrity of proteins in cells. Therefore, protein folding should be considered as an additional property affecting saturation transfer between water and proteins, in combination with the microenvironment and physiological quantities, such as metabolite concentration, temperature and pH. Copyright © 2013 John Wiley & Sons, Ltd.     

Quantitative pulsed CEST‐MRI using Ω‐plots

Chemical exchange saturation transfer (CEST) allows the indirect detection of dilute metabolites in living tissue via MRI of the tissue water signal. Selective radio frequency (RF) with amplitude B₁ is used to saturate the magnetization of protons of exchanging groups, which transfer the saturation to the abundant water pool. In a clinical setup, the saturation scheme is limited to a series of short pulses to follow regulation of the specific absorption rate (SAR). Pulsed saturation is difficult to describe theoretically, thus rendering quantitative CEST a challenging task. In this study, we propose a new analytical treatment of pulsed CEST by extending a former interleaved saturation–relaxation approach. Analytical integration of the continuous wave (cw) eigenvalue as a function of the RF pulse shape leads to a formula for pulsed CEST that has the same structure as that for cw CEST, but incorporates two form factors that are determined by the pulse shape. This enables analytical Z‐spectrum calculations and permits deeper insight into pulsed CEST. Furthermore, it extends Dixon's Ω‐plot method to the case of pulsed saturation, yielding separately, and independently, the exchange rate and the relative proton concentration. Consequently, knowledge of the form factors allows a direct comparison of the effect of the strength and B₁ dispersion of pulsed CEST experiments with the ideal case of cw saturation. The extended pulsed CEST quantification approach was verified using creatine phantoms measured on a 7 T whole‐body MR tomograph, and its range of validity was assessed by simulations. Copyright © 2015 John Wiley & Sons, Ltd.     

Simulation,phantom validation,and clinical evaluation of fast pH‐weighted molecular imaging using amine chemical exchange saturation transfer echo planar imaging (CEST‐EPI) in glioma at 3 T

Acidity within the extracellular milieu is a hallmark of cancer. There is a current need for fast, high spatial resolution pH imaging techniques for clinical evaluation of cancers, including gliomas. Chemical exchange saturation transfer (CEST) MRI targeting fast‐exchanging amine protons can be used to obtain high‐resolution pH‐weighted images, but conventional CEST acquisition strategies are slow. There is also a need for more accurate MR simulations to better understand the effects of amine CEST pulse sequence parameters on pH‐weighted image contrast. In the current study we present a simulation of amine CEST contrast specific for a newly developed CEST echoplanar imaging (EPI) pulse sequence. The accuracy of the simulations was validated by comparing the exchange rates and Z‐spectrum under a variety of conditions using physical phantoms of glutamine with different pH values. The effects of saturation pulse shapes, pulse durations, pulse train lengths, repetition times, and relaxation rates of bulk water and exchangeable amine protons on the CEST signal were explored for normal‐appearing white matter (NAWM), glioma, and cerebrospinal fluid. Last, 18 patients with WHO II‐IV gliomas were evaluated. Results showed that the Z‐spectrum was highly dependent on saturation pulse shape, repetition time, saturation amplitude, magnetic field strength, and T₂ within bulk water; however, the Z‐spectrum was only minimally influenced by saturation pulse duration and the specific relaxation rates of amine protons. Results suggest that a Gaussian saturation pulse train consisting of 3 × 100 ms pulses using the minimum allowable repetition time is optimal for achieving over 90% available contrast across all tissues. Results also demonstrate that high saturation pulse amplitude and scanner field strength (>3 T) are necessary for adequate endogenous pH‐weighted amine CEST contrast. pH‐weighted amine CEST contrast increased with increasing tumor grade, with glioblastoma showing significantly higher contrast compared with WHO II or III gliomas.     

CEST signal at 2 ppm (CEST@2ppm) from Z‐spectral fitting correlates with creatine distribution in brain tumor

In general, multiple components such as water direct saturation, magnetization transfer (MT), chemical exchange saturation transfer (CEST) and aliphatic nuclear Overhauser effect (NOE) contribute to the Z‐spectrum. The conventional CEST quantification method based on asymmetrical analysis may lead to quantification errors due to the semi‐solid MT asymmetry and the aliphatic NOE located on a single side of the Z‐spectrum. Fitting individual contributors to the Z‐spectrum may improve the quantification of each component. In this study, we aim to characterize the multiple exchangeable components from an intracranial tumor model using a simplified Z‐spectral fitting method. In this method, the Z‐spectrum acquired at low saturation RF amplitude (50 Hz) was modeled as the summation of five Lorentzian functions that correspond to NOE, MT effect, bulk water, amide proton transfer (APT) effect and a CEST peak located at +2 ppm, called CEST@2ppm. With the pixel‐wise fitting, the regional variations of these five components in the brain tumor and the normal brain tissue were quantified and summarized. Increased APT effect, decreased NOE and reduced CEST@2ppm were observed in the brain tumor compared with the normal brain tissue. Additionally, CEST@2ppm decreased with tumor progression. CEST@2ppm was found to correlate with the creatine concentration quantified with proton MRS. Based on the correlation curve, the creatine contribution to CEST@2ppm was quantified. The CEST@2ppm signal could be a novel imaging surrogate for in vivo creatine, the important bioenergetics marker. Given its noninvasive nature, this CEST MRI method may have broad applications in cancer bioenergetics. Copyright © 2014 John Wiley & Sons, Ltd.     

Accuracy in the quantification of chemical exchange saturation transfer (CEST) and relayed nuclear Overhauser enhancement (rNOE) saturation transfer effects

Accurate quantification of chemical exchange saturation transfer (CEST) effects, including dipole–dipole mediated relayed nuclear Overhauser enhancement (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 the brain, 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)). 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.     

Reliable chemical exchange saturation transfer imaging of human lumbar intervertebral discs using reduced‐field‐of‐view turbo spin echo at 3.0 T

The reduced field‐of‐view (rFOV) turbo‐spin‐echo (TSE) technique, which effectively suppresses bowel movement artifacts, is developed for the purpose of chemical exchange saturation transfer (CEST) imaging of the intervertebral disc (IVD) in vivo. Attempts to quantify IVD CEST signals in a clinical setting require high reliability and accuracy, which is often compromised in the conventionally used technique. The proposed rFOV TSE CEST method demonstrated significantly superior reproducibility when compared with the conventional technique on healthy volunteers, implying it is a more reliable measurement. Phantom study revealed a linear relation between CEST signal and glycosaminoglycan (GAG) concentration. The feasibility of detecting IVD degeneration was demonstrated on a healthy volunteer, indicating that the proposed method is a promising tool to quantify disc degeneration. Copyright © 2013 John Wiley & Sons, Ltd.     

Ammonia‐weighted imaging by chemical exchange saturation transfer MRI at 3 T

Hepatic encephalopathy (HE) is triggered by liver cirrhosis and is associated with an increased ammonia level within the brain tissue. The goal of this study was to investigate effects of ammonia on in vitro amide proton transfer (APT)‐weighted chemical exchange saturation transfer (CEST) imaging in order to develop an ammonia‐sensitive brain imaging method. APT‐weighted CEST imaging was performed on phantom solutions including pure ammonia, bovine serum albumin (BSA), and tissue homogenate samples doped with various ammonia concentrations. All CEST data were assessed by magnetization transfer ratio asymmetry. In addition, optical methods were used to determine possible structural changes of the proteins in the BSA phantom. In vivo feasibility measurements were acquired in one healthy participant and two patients suffering from HE, a disease associated with increased brain ammonia levels. The CEST effect of pure ammonia showed a base‐catalyzed behavior. At pH values greater than 5.6 no CEST effect was observed. The APT‐weighted signal was significantly reduced for ammonia concentrations of 5mM or more at fixed pH values within the different protein phantom solutions. The optical methods revealed no protein aggregation or denaturation for ammonia concentrations less than 5mM. The in vivo measurements showed tissue specific and global reduction of the observed CEST signal in patients with HE, possibly linked to pathologically increased ammonia levels. APT‐weighted CEST imaging is sensitive to changes in ammonia concentrations. Thus, it seems useful for the investigation of pathologies with altered tissue ammonia concentrations such as HE. However, the underlying mechanism needs to be explored in more detail in future in vitro and in vivo investigations.     

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 © 2015 John Wiley & Sons, Ltd.     

Increased CEST specificity for amide and fast‐exchanging amine protons using exchange‐dependent relaxation rate

Chemical exchange saturation transfer (CEST) imaging of amides at 3.5 ppm and fast‐exchanging amines at 3 ppm provides a unique means to enhance the sensitivity of detection of, for example, proteins/peptides and neurotransmitters, respectively, and hence can provide important information on molecular composition. However, despite the high sensitivity relative to conventional magnetic resonance spectroscopy (MRS), in practice, CEST often has relatively poor specificity. For example, CEST signals are typically influenced by several confounding effects, including direct water saturation (DS), semi‐solid non‐specific magnetization transfer (MT), the influence of water relaxation times (T_1w) and nearby overlapping CEST signals. Although several editing techniques have been developed to increase the specificity by removing DS, semi‐solid MT and T_1w influences, it is still challenging to remove overlapping CEST signals from different exchanging sites. For instance, the amide proton transfer (APT) signal could be contaminated by CEST effects from fast‐exchanging amines at 3 ppm and intermediate‐exchanging amines at 2 ppm. The current work applies an exchange‐dependent relaxation rate (R_ex) to address this problem. Simulations demonstrate that: (1) slowly exchanging amides and fast‐exchanging amines have distinct dependences on irradiation powers; and (2) R_ex serves as a resonance frequency high‐pass filter to selectively reduce CEST signals with resonance frequencies closer to water. These characteristics of R_ex provide a means to isolate the APT signal from amines. In addition, previous studies have shown that CEST signals from fast‐exchanging amines have no distinct features around their resonance frequencies. However, R_ex gives Lorentzian lineshapes centered at their resonance frequencies for fast‐exchanging amines and thus can significantly increase the specificity of CEST imaging for amides and fast‐exchanging amines.     

Aggregation‐induced changes in the chemical exchange saturation transfer (CEST) signals of proteins

Chemical exchange saturation transfer (CEST) is an MRI technique that allows mapping of biomolecules (small metabolites, proteins) with nearly the sensitivity of conventional water proton MRI. In living organisms, several tissue‐specific CEST effects have been observed and successfully applied to diagnostic imaging. In these studies, particularly the signals of proteins showed a distinct correlation with pathological changes. However, as CEST effects depend on various properties that determine and affect the chemical exchange processes, the origins of the observed signal changes remain to be understood. In this study, protein aggregation was identified as an additional process that is encoded in the CEST signals of proteins. Investigation of distinct proteins that are involved in pathological disorders, namely amyloid beta and huntingtin, revealed a significant decrease of all protein CEST signals upon controlled aggregation. This finding is of particular interest with regard to diagnostic imaging of patients with neurodegenerative diseases that involve amyloidogenesis, such as Alzheimer's or Huntington's disease. To investigate whether the observed CEST signal decrease also occurs in heterogeneous mixtures of aggregated cellular proteins, and thus prospectively in tissue, heat‐shocked yeast cell lysates were employed. Additionally, investigation of different cell compartments verified the assignment of the protein CEST signals to the soluble part of the proteome. The results of in vitro experiments demonstrate that aggregation affects the CEST signals of proteins. This observation can enable hypotheses for CEST imaging as a non‐invasive diagnostic tool for monitoring pathological alterations of the proteome in vivo.     

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 R_1ρ. For the case of slowly exchanging systems, e.g. amide protons, the saturation of the small proton pool is affected by transverse relaxation (R_2b). It turns out, that R_2b 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 R_1ρ 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 © 2012 John Wiley & Sons, Ltd.     

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 © 2014 John Wiley & Sons, Ltd.