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.     

CEST imaging of fast exchanging amine pools with corrections for competing effects at 9.4 T

Chemical exchange saturation transfer (CEST) imaging of fast exchanging amine protons at 3 ppm offset from the water resonant frequency is of practical interest, but quantification of fast exchanging pools by CEST is challenging. To effectively saturate fast exchanging protons, high irradiation powers need to be applied, but these may cause significant direct water saturation as well as non‐specific semi‐solid magnetization transfer (MT) effects, and thus decrease the specificity of the measured signal. In addition, the CEST signal may depend on the water longitudinal relaxation time (T_1w), which likely varies between tissues and with pathology, further reducing specificity. Previously, an analysis of the asymmetry of saturation effects (MTR_asym) has been commonly used to quantify fast exchanging amine CEST signals. However, our results show that MTR_asym is greatly affected by the above factors, as well as asymmetric MT and nuclear Overhauser enhancement (NOE) effects. Here, we instead applied a relatively more specific inverse analysis method, named AREX (apparent exchange‐dependent relaxation), that has previously been applied only to slow and intermediate exchanging solutes. Numerical simulations and controlled phantom experiments show that, although MTR_asym depends on T_1w and semi‐solid content, AREX acquired in steady state does not, which suggests that AREX is more specific than MTR_asym. By combining with a fitting approach instead of using the asymmetric analysis to obtain reference signals, AREX can also avoid contaminations from asymmetric MT and NOE effects. Animal experiments show that these two quantification methods produce differing contrasts between tumors and contralateral normal tissues in rat brain tumor models, suggesting that conventional MTR_asym applied in vivo may be influenced by variations in T_1w, semi‐solid content, or NOE effect. Thus, the use of MTR_asym may lead to misinterpretation, while AREX with corrections for competing effects likely enhances the specificity and accuracy of quantification to fast exchanging pools.     

Assessment of ischemic penumbra in patients with hyperacute stroke using amide proton transfer (APT) chemical exchange saturation transfer (CEST) MRI

Chemical exchange saturation transfer (CEST)‐derived, pH‐weighted, amide proton transfer (APT) MRI has shown promise in animal studies for the prediction of infarction risk in ischemic tissue. Here, APT MRI was translated to patients with acute stroke (1–24 h post‐symptom onset), and assessments of APT contrast, perfusion, diffusion, disability and final infarct volume (23–92 days post‐stroke) are reported. Healthy volunteers (n = 5) and patients (n = 10) with acute onset of symptoms (0–4 h, n = 7; uncertain onset <24 h, n = 3) were scanned with diffusion‐ and perfusion‐weighted MRI, fluid‐attenuated inversion recovery (FLAIR) and CEST. Traditional asymmetry and a Lorentzian‐based APT index were calculated in the infarct core, at‐risk tissue (time‐to‐peak, TTP; lengthening) and final infarct volume. On average (mean ± standard deviation), control white matter APT values (asymmetry, 0.019 ± 0.005; Lorentzian, 0.045 ± 0.006) were not significantly different (p > 0.05) from APT values in normal‐appearing white matter (NAWM) of patients (asymmetry, 0.022 ± 0.003; Lorentzian, 0.048 ± 0.003); however, ischemic regions in patients showed reduced (p = 0.03) APT effects compared with NAWM. Representative cases are presented, whereby the APT contrast is compared quantitatively with contrast from other imaging modalities. The findings vary between patients; in some patients, a trend for a reduction in the APT signal in the final infarct region compared with at‐risk tissue was observed, consistent with tissue acidosis. However, in other patients, no relationship was observed in the infarct core and final infarct volume. Larger clinical studies, in combination with focused efforts on sequence development at clinically available field strengths (e.g. 3.0 T), are necessary to fully understand the potential of APT imaging for guiding the hyperacute management of patients. Copyright © 2013 John Wiley & Sons, Ltd.     

Imaging of amide proton transfer and nuclear Overhauser enhancement in ischemic stroke with corrections for competing effects

Chemical exchange saturation transfer (CEST) potentially provides the ability to detect small solute pools through indirect measurements of attenuated water signals. However, CEST effects may be diluted by various competing effects, such as non‐specific magnetization transfer (MT) and asymmetric MT effects, water longitudinal relaxation (T₁) and direct water saturation (radiofrequency spillover). In the current study, CEST images were acquired in rats following ischemic stroke and analyzed by comparing the reciprocals of the CEST signals at three different saturation offsets. This combined approach corrects the above competing effects and provides a more robust signal metric sensitive specifically to the proton exchange rate constant. The corrected amide proton transfer (APT) data show greater differences between the ischemic and contralateral (non‐ischemic) hemispheres. By contrast, corrected nuclear Overhauser enhancements (NOEs) around ?3.5 ppm from water change over time in both hemispheres, indicating whole‐brain changes that have not been reported previously. This study may help us to better understand the contrast mechanisms of APT and NOE imaging in ischemic stroke, and may also establish a framework for future stroke measurements using CEST imaging with spillover, MT and T₁ corrections. Copyright © 2014 John Wiley & Sons, Ltd.     

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 © 2014 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.     

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.     

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.     

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.     

Extracranial measurements of amide proton transfer using exchange-modulated point-resolved spectroscopy (EXPRESS)

Chemical exchange saturation transfer (CEST) imaging has been used experimentally in a large range of applications. However, full quantification of CEST effects in vivo using standard imaging sequences is time consuming as a large number of saturation frequency offsets, each followed by an imaging readout, are required to define a z spectrum. Furthermore, outside the brain, the presence of fat can confound the interpretation of z spectra. A novel acquisition and post‐processing technique is presented in this study, named exchange‐modulated point‐resolved spectroscopy (EXPRESS), which aims to address these limitations and to enable spatially localised, high signal‐to‐noise measurements of CEST effects in vivo. Using amide proton exchange (APT) measurements in tumours, it is demonstrated that the acquisition of two‐dimensional EXPRESS spectra composed of chemical shift and saturation frequency offset dimensions allows the correction of CEST data containing both fat and water signals, which is a common confounding property of tissues found outside the brain. Copyright © 2011 John Wiley & Sons, Ltd.     

Role of the cholesterol hydroxyl group in the chemical exchange saturation transfer signal at −1.6 ppm

Chemical exchange saturation transfer (CEST) can provide metabolite‐weighted images in the clinical setting; therefore, understanding the origin of each CEST signal is essential to revealing the changes in diseases at the molecular level, which would provide further insight for diagnoses and treatments. The CEST signal at ?1.6 ppm is attributed to the choline methyl group of phosphatidylcholines. The methyl groups have no exchangeable protons, so the corresponding CEST signals must result from the relayed nuclear Overhauser effect (rNOE); however, the detailed mechanism remains unclear. Cholesterol is a major component of biological membranes, and its content is closely related to the dynamics and phases of these lipids. However, cholesterol has a hydroxyl group, which could participate in proton exchange to complete the rNOE process. In this study, we used liposomes containing cholesterol and its analogs (5α‐cholestane and progesterone), which presumably have similar capabilities of influencing lipid bilayers, and found that the steroid hydroxyl group is the key to inducing the rNOE at ?1.6 ppm. Our results suggest that the origin of the rNOE at ?1.6 ppm likely requires an intermolecular NOE between the proton of the choline methyl group and that of the cholesterol hydroxyl group, and a chemical exchange between the cholesterol hydroxyl group and bulk water. However, the phenomenon in which the rNOE at ?1.6 ppm appears when the cholesterol concentration is high seems to contradict the in vivo results, suggesting a more complicated mechanism associated with the rNOE at ?1.6 ppm in biological membranes.     

Towards the complex dependence of MTRasym on T1w in amide proton transfer (APT) imaging

Amide proton transfer (APT) imaging is a variation of chemical exchange saturation transfer MRI that has shown promise in diagnosing tumors, ischemic stroke, multiple sclerosis, traumatic brain injury, etc. Specific quantification of the APT effect is crucial for the interpretation of APT contrast in pathologies. Conventionally, magnetization transfer ratio with asymmetric analysis (MTR_asym) has been used to quantify the APT effect. However, some studies indicate that MTR_asym is contaminated by water longitudinal relaxation time (T_1w), and thus it is necessary to normalize T_1w in MTR_asym to obtain specific quantification of the APT effect. So far, whether to use MTR_asym or the T_1w‐normalized MTR_asym is still under debate in the field. In this paper, the influence of T_1w on the quantification of APT was evaluated through theoretical analysis, numerical simulations, and phantom studies for different experimental conditions. Results indicate that there are two types of T_1w effect (T_1w recovery and T_1w‐related saturation), which have inverse influences on the steady‐state MTR_asym. In situations with no or weak direct water saturation (DS) effect, there is only the T_1w recovery effect, and MTR_asym linearly depends on T_1w. In contrast, in situations with significant DS effects, the dependence of MTR_asym on T_1w is complex, and is dictated by the competition of these two T_1w effects. Therefore, by choosing appropriate irradiation powers, MTR_asym could be roughly insensitive to T_1w. Moreover, in non‐steady‐state acquisitions with very short irradiation time, MTR_asym is also roughly insensitive to T_1w. Therefore, for steady‐state APT imaging at high fields or with very low irradiation powers, where there are no significant DS effects, it is necessary to normalize T_1w to improve the specificity of MTR_asym. However, in clinical MRI systems (usually low fields or non‐steady‐state acquisitions), T_1w normalization may not be necessary when appropriate sequence parameters are chosen.     

High‐resolution creatine mapping of mouse brain at 11.7 T using non‐steady‐state chemical exchange saturation transfer

The current study aims to optimize the acquisition scheme for the creatine chemical exchange saturation transfer weighted (CrCESTw) signal on mouse brain at 11.7 T, in which a strong magnetization transfer contrast (MTC) is present, and to further develop the polynomial and Lorentzian line‐shape fitting (PLOF) method for quantifying CrCESTw signal with a non‐steady‐state (NSS) acquisition scheme. Studies on a Cr phantom with cross‐linked bovine serum albumin (BSA) as well as on mouse brain demonstrated that the maximum CrCESTw signal was reached with a short saturation time determined by the rotating frame relaxation time of the MTC pool instead of the steady‐state saturation. The saturation power for the maximal signal was around 1–1.5 μT for Cr with 20% cross‐linked BSA and in vivo applications, but 2 μT was found to be most practical for signal stability. For the CrCEST acquisition with strong MTC interference, the optimal saturation power and length are completely different from those on Cr solution alone. This observation could be explained well using R_1ρ theory by incorporating the strong MTC pool. Finally, a high‐resolution Cr map was obtained on mouse brain using the PLOF method with the NSS CEST acquisition and a cryogenic coil. The Cr map obtained by CEST showed homogenous intensity across the mouse brain except for regions with cerebrospinal fluid.     

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.     

Evaluating the feasibility of creatine‐weighted CEST MRI in human brain at 7 T using a Z‐spectral fitting approach

The current study aims to evaluate the feasibility of creatine (Cr) chemical exchange saturation transfer (CEST)‐weighted MRI at 7 T in the human brain by optimizing the saturation pulse parameters and computing contrast using a Z‐spectral fitting approach. The Cr‐weighted (Cr‐w) CEST contrast was computed from phantoms data. Simulations were carried out to obtain the optimum saturation parameters for Cr‐w CEST with lower contribution from other brain metabolites. CEST‐w images were acquired from the brains of four human subjects at different saturation parameters. The Cr‐w CEST contrast was computed using both asymmetry analysis and Z‐spectra fitting approaches (models 1 and 2, respectively) based on Lorentzian functions. For broad magnetization transfer (MT) effect, Gaussian and Super‐Lorentzian line shapes were also evaluated. In the phantom study, the Cr‐w CEST contrast showed a linear dependence on concentration in physiological range and a nonlinear dependence on saturation parameters. The in vivo Cr‐w CEST map generated using asymmetry analysis from the brain represents mixed contrast with contribution from other metabolites as well and relayed nuclear Overhauser effect (rNOE). Simulations provided an estimate for the optimum range of saturation parameters to be used for acquiring brain CEST data. The optimum saturation parameters for Cr‐w CEST to be used for brain data were around B_1rms = 1.45 μT and duration = 2 seconds. The Z‐spectral fitting approach enabled computation of individual components. This also resulted in mitigating the contribution from MT and rNOE to Cr‐w CEST contrast, which is a major source of underestimation in asymmetry analysis. The proposed modified z‐spectra fitting approach (model 2) is more stable to noise compared with model 1. Cr‐w CEST contrast obtained using fitting was 6.98 ± 0.31% in gray matter and 5.45 ± 0.16% in white matter. Optimal saturation parameters reduced the contribution from other CEST effects to Cr‐w CEST contrast, and the proposed Z‐spectral fitting approach enabled computation of individual components in Z‐spectra of the brain. Therefore, it is feasible to compute Cr‐w CEST contrast with a lower contribution from other CEST and rNOE.     

Signature of protein unfolding in chemical exchange saturation transfer imaging

Chemical exchange saturation transfer (CEST) allows the detection of metabolites of low concentration in tissue with nearly the sensitivity of MRI with water protons. With this spectroscopic imaging approach, several tissue‐specific CEST effects have been observed in vivo. Some of these originate from exchanging sites of proteins, such as backbone amide protons, or from aliphatic protons within the hydrophobic protein core. In this work, we employed CEST experiments to detect global protein unfolding. Spectral evaluation revealed exchange‐ and NOE‐mediated CEST effects that varied in a highly characteristic manner with protein unfolding tracked by fluorescence spectroscopy. We suggest the use of this comprehensive spectral signature for the detection of protein unfolding by CEST, as it relies on several spectral hallmarks. As proof of principle, we demonstrate that the presented signature is readily detectable using a whole‐body MR tomograph (B₀ = 7 T), not only in denatured aqueous protein solutions, but also in heat‐shocked yeast cells. A CEST imaging contrast with the potential to detect global protein unfolding would be of particular interest regarding protein unfolding as a marker for stress, ageing, and disease. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

The pH sensitivity of APT‐CEST using phosphorus spectroscopy as a reference method

The pH value is a potential physiological marker for clinical diagnosis as it is altered in pathologies such as tumors. While intracellular pH can be measured noninvasively via phosphorus spectroscopy (³¹P MRSI), Amide Proton Transfer‐Chemical Exchange Saturation Transfer (APT‐CEST) MRI has been suggested as an alternative method for pH quantification. To assess the suitability of APT‐CEST contrast for pH quantification, two approaches (magnetization transfer ratio asymmetry [MTR_asym] and Lorentzian difference analysis [LDA]) for analyzing the Z‐spectrum have been correlated with pH values obtained by ³¹P MRSI. Fourteen patients with glioblastoma and 12 healthy controls were included. In contrast to MTR_asym, the LDA is modeling the direct water saturation and the semi‐solid magnetization transfer, allowing a separate evaluation of the aliphatic nuclear Overhauser effect and the APT‐CEST. The results of our study show that the pH values obtained by ³¹P MRSI correspond well with both methods describing the APT‐CEST contrast. Two‐sample t‐test showed significant differences in MTR_asym, LDA and pH obtained by ³¹P MRSI for regions of interest in glioblastoma, contralateral control areas and normal appearing white matter (P < 0.001). A slightly improved correlation between the amide signal and pH was found after performing LDA (r = 0.78) compared with MTR_asym (r = 0.70). While both methods can be used to monitor pH changes, the LDA approach appears to be better suited.     

Imaging chemical exchange saturation transfer (CEST) effects following tumor‐selective acidification using lonidamine

Increased lactate production through glycolysis in aerobic conditions is a hallmark of cancer. Some anticancer drugs have been designed to exploit elevated glycolysis in cancer cells. For example, lonidamine (LND) inhibits lactate transport, leading to intracellular acidification in cancer cells. Chemical exchange saturation transfer (CEST) is a novel MRI contrast mechanism that is dependent on intracellular pH. Amine and amide concentration‐independent detection (AACID) and apparent amide proton transfer (APT*) represent two recently developed CEST contrast parameters that are sensitive to pH. The goal of this study was to compare the sensitivity of AACID and APT* for the detection of tumor‐selective acidification after LND injection. Using a 9.4‐T MRI scanner, CEST data were acquired in mice approximately 14 days after the implantation of 10⁵ U87 human glioblastoma multiforme (GBM) cells in the brain, before and after the administration of LND (dose, 50 or 100 mg/kg). Significant dose‐dependent LND‐induced changes in the measured CEST parameters were detected in brain regions spatially correlated with implanted tumors. Importantly, no changes were observed in T₁‐ and T₂‐weighted images acquired before and after LND treatment. The AACID and APT* contrast measured before and after LND injection exhibited similar pH sensitivity. Interestingly, LND‐induced contrast maps showed increased heterogeneity compared with pre‐injection CEST maps. These results demonstrate that CEST contrast changes after the administration of LND could help to localize brain cancer and monitor tumor response to chemotherapy within 1 h of treatment. The LND CEST experiment uses an anticancer drug to induce a metabolic change detectable by endogenous MRI contrast, and therefore represents a unique cancer detection paradigm which differs from other current molecular imaging techniques that require the injection of an imaging contrast agent or tracer. Copyright © 2015 John Wiley & Sons, Ltd.     

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.