The reciprocal linear QUEST analysis method facilitates the measurements of chemical exchange rates with CEST MRI

Rapid and quantitative CEST-MRI sequence using water presaturation

PURPOSE

Despite the significant potential for in vivo metabolic imaging in preclinical and clinical applications, CEST MRI suffers from long scan time and inaccurate quantification. This study aims to suppress the contaminations among signals under different frequencies, which could shorten the TR and thereby facilitate CEST imaging acceleration and quantification.

METHODS

A novel sequence is proposed by applying a water-presaturation (WPS) module at the beginning of each TR. WPS CEST quickly knocks down the residual signal from previous TRs so that the magnetization of all TRs recovers from zero, which aligns well with the formula of quasi-steady-state theorem and enables accurate quantification within shorter TR. WPS CEST was assessed by simulations, creatine phantom, and healthy human brain scans at 3 T.

RESULTS

In simulation and phantom experiment, WPS CEST allows accurate estimation of exchange rate (ksw) using omega plot and using shorter delay time (Td) and saturation time (Ts) (e.g., 1 s/1 s) compared with the conventional CEST. Simulations further showed that WPS CEST could obtain consistent spin-lock relaxation (R1ρ) values over varied Tds and Tss. Six human scans indicated that R1ρ collected from conventional sequences showed significant differences between two groups with Td and Ts of (1 s/1 s) and (2 s/2 s) (amide: 1.721 ± 0.051 s-1 vs. 1.622 ± 0.050 s-1, p = 0.001; nuclear Overhauser enhancement: 1.792 ± 0.046 s-1 vs. 1.687 ± 0.053 s-1, p = 0.004), whereas WPS CEST scans using these 2 Td/Ts values obtained the same mean R1ρ (amide: 1.616 ± 0.053 s-1 vs. 1.616 ± 0.048 s-1, p = 0.862; nuclear Overhauser enhancement: 1.688 ± 0.064 s-1 vs. 1.684 ± 0.054 s-1, p = 0.544).

CONCLUSION

WPS CEST demonstrated accurate quantitation within shorter TR compared with conventional sequences, and thereby may allow rapid quantitative CEST scans in various situations.

Motion and magnetic field inhomogeneity correction techniques for chemical exchange saturation transfer (CEST) MRI: A contemporary review

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) has emerged as a powerful imaging technique sensitive to tissue molecular composition, pH, and metabolic processes in situ. CEST MRI uniquely probes the physical exchange of protons between water and specific molecules within tissues, providing a window into physiological phenomena that remain invisible to standard MRI. However, given the very low concentration (millimolar range) of CEST compounds, the effects measured are generally only on the order of a few percent of the water signal. Consequently, a few critical challenges, including correction of motion artifacts and magnetic field (B0 and B1 +) inhomogeneities, have to be addressed in order to unlock the full potential of CEST MRI. Motion, whether from patient movement or inherent physiological pulsations, can distort the CEST signal, hindering accurate quantification. B0 and B1 + inhomogeneities, arising from scanner hardware imperfections, further complicate data interpretation by introducing spurious variations in the signal intensity. Without proper correction of these confounding factors, reliable analysis and clinical translation of CEST MRI remain challenging. Motion correction methods aim to compensate for patient movement during (prospective) or after (retrospective) image acquisition, reducing artifacts and preserving data quality. Similarly, B0 and B1 + inhomogeneity correction techniques enhance the spatial and spectral accuracy of CEST MRI. This paper aims to provide a comprehensive review of the current landscape of motion and magnetic field inhomogeneity correction methods in CEST MRI. The methods discussed apply to saturation transfer (ST) MRI in general, including semisolid magnetization transfer contrast (MTC) and relayed nuclear Overhauser enhancement (rNOE) studies.

Induced saturation transfer recovery steady states (iSTRESS) for proton exchange rate mapping with CEST MRI, a preliminary study

Proton exchange underpins essential mechanisms in diverse MR imaging contrasts. Omega plots have proven effective in mapping proton exchange rates (kex) in live human brains, enabling the differentiation of MS lesion activities and characterization of ischemic stroke. However, Omega plots require extended saturation durations (typically 5 to 10 s), resulting in high specific absorption rates (SAR) that can hinder clinical feasibility. In this study, we introduce a novel kex mapping approach, named induced Saturation Transfer Recovery Steady-States (iSTRESS). iSTRESS integrates an excitation flip angle pulse prior to chemical exchange saturation transfer (CEST) saturation, effectively aligning the magnetization with its steady-state value. This innovation reduces saturation times and mitigates SAR concerns. The formula for iSTRESS-based kex quantification was derived theoretically, involving two measurements with distinct excitation flip angles and saturation B1 values. Bloch-McConnell simulations confirmed that iSTRESS-based kex values closely matched input values (R2 > 0.99). An iSTRESS MRI sequence was implemented on a 9.4 T preclinical MRI, imaging protein phantoms with pH values ranging from 6.2 to 7.4 (n = 4). Z-spectra were acquired using excitation flip angles of 30° and 60°, followed by CEST saturation at powers of 30 and 120 Hz respectively, with a total saturation time of <1 s, resulting in two iSTRESS states for kex mapping. kex maps derived from the phantom study exhibited a linear correlation (R2 > 0.99) with Omega plot results. The developed iSTRESS method allows for kex quantification with significantly reduced saturation times, effectively minimizing SAR concerns.

An end-to-end LSTM-Attention based framework for quasi-steady-state CEST prediction

Chemical exchange saturation transfer (CEST)-magnetic resonance imaging (MRI) often takes prolonged saturation duration (Ts) and relaxation delay (Td) to reach the steady state, and yet the insufficiently long Ts and Td in actual experiments may underestimate the CEST measurement. In this study, we aimed to develop a deep learning-based model for quasi-steady-state (QUASS) prediction from non-steady-state CEST acquired in experiments, therefore overcoming the limitation of the CEST effect which needs prolonged saturation time to reach a steady state. To support network training, a multi-pool Bloch-McConnell equation was designed to derive wide-ranging simulated Z-spectra, so as to solve the problem of time and labor consumption in manual annotation work. Following this, we formulated a hybrid architecture of long short-term memory (LSTM)-Attention to improve the predictive ability. The multilayer perceptron, recurrent neural network, LSTM, gated recurrent unit, BiLSTM, and LSTM-Attention were included in comparative experiments of QUASS CEST prediction, and the best performance was obtained by the proposed LSTM-Attention model. In terms of the linear regression analysis, structural similarity index (SSIM), peak signal-to-noise ratio (PSNR), and mean-square error (MSE), the results of LSTM-Attention demonstrate that the coefficient of determination in the linear regression analysis was at least R2 = 0.9748 for six different representative frequency offsets, the mean values of prediction accuracies in terms of SSIM, PSNR and MSE were 0.9991, 49.6714, and 1.68 × 10-4 for all frequency offsets. It was concluded that the LSTM-Attention model enabled high-quality QUASS CEST prediction.

Validation of the presence of fast exchanging amine CEST effect at low saturation powers and its influence on the quantification of APT

PURPOSE

Accurately quantifying the amide proton transfer (APT) effect and the underlying exchange parameters is crucial for its applications, but previous studies have reported conflicting results. In these quantifications, the CEST effect from the fast exchange amine was always ignored because it was considered weak with low saturation powers. This paper aims to evaluate the influence of the fast exchange amine CEST on the quantification of APT at low saturation powers.

METHODS

A quantification method with low and high saturation powers was used to distinguish APT from the fast exchange amine CEST effect. Simulations were conducted to assess the method's capability to separate APT from the fast exchange amine CEST effect. Animal experiments were performed to assess the relative contributions from the fast exchange amine and amide to CEST signals at 3.5 ppm. Three APT quantification methods, each with varying degrees of contamination from the fast exchange amine, were employed to process the animal data to assess the influence of the amine on the quantification of APT effect and the exchange parameters.

RESULTS

The relative size of the fast exchange amine CEST effect to APT effect gradually increases with increasing saturation power. At 9.4 T, it increases from approximately 20% to 40% of APT effect with a saturation power increase from 0.25 to 1 μT.

CONCLUSION

The fast exchange amine CEST effect leads overestimation of APT effect, fitted amide concentration, and amide-water exchange rate, potentially contributing to the conflicting results reported in previous studies.

Detection of tissue pH with quantitative chemical exchange saturation transfer magnetic resonance imaging

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) has emerged as a novel means for sensitive detection of dilute labile protons and chemical exchange rates. By sensitizing to pH-dependent chemical exchange, CEST MRI has shown promising results in monitoring tissue statuses such as pH changes in disorders like acute stroke, tumor, and acute kidney injury. This article briefly reviews the basic principles for CEST imaging and quantitative measures, from the simplistic asymmetry analysis to multipool Lorentzian decoupling and quasi-steady-state reconstruction. In particular, the advantages and limitations of commonly used quantitative approaches for CEST applications are discussed.

Label-Free Assessment of Mannitol Accumulation Following Osmotic Blood-Brain Barrier Opening Using Chemical Exchange Saturation Transfer Magnetic Resonance Imaging

PURPOSE

Mannitol is a hyperosmolar agent for reducing intracranial pressure and inducing osmotic blood-brain barrier opening (OBBBO). There is a great clinical need for a non-invasive method to optimize the safety of mannitol dosing. The aim of this study was to develop a label-free Chemical Exchange Saturation Transfer (CEST)-based MRI approach for detecting intracranial accumulation of mannitol following OBBBO.

METHODS

In vitro MRI was conducted to measure the CEST properties of D-mannitol of different concentrations and pH. In vivo MRI and MRS measurements were conducted on Sprague-Dawley rats using a Biospec 11.7T horizontal MRI scanner. Rats were catheterized at the internal carotid artery (ICA) and randomly grouped to receive either 1 mL or 3 mL D-mannitol. CEST MR images were acquired before and at 20 min after the infusion.

RESULTS

In vitro MRI showed that mannitol has a strong, broad CEST contrast at around 0.8 ppm with a mM CEST MRI detectability. In vivo studies showed that CEST MRI could effectively detect mannitol in the brain. The low dose mannitol treatment led to OBBBO but no significant mannitol accumulation, whereas the high dose regimen resulted in both OBBBO and mannitol accumulation. The CEST MRI findings were consistent with ¹H-MRS and Gd-enhanced MRI assessments.

CONCLUSION

We demonstrated that CEST MRI can be used for non-invasive, label-free detection of mannitol accumulation in the brain following BBBO treatment. This method may be useful as a rapid imaging tool to optimize the dosing of mannitol-based OBBBO and improve its safety and efficacy.

Toward In Vivo MRI of the Tissue Proton Exchange Rate in Humans

Quantification of proton exchange rate (kex) is a challenge in MR studies. Current techniques either have low resolutions or are dependent on the estimation of parameters that are not measurable. The Omega plot method, on the other hand, provides a direct way for determining kex independent of the agent concentration. However, it cannot be used for in vivo studies without some modification due to the contributions from the water signal. In vivo tissue proton exchange rate (kex) MRI, based on the direct saturation (DS) removed Omega plot, quantifies the weighted average of kex of the endogenous tissue metabolites. This technique has been successfully employed for imaging the variation in the kex of ex vivo phantoms, as well as in vivo human brains in healthy subjects, and stroke or multiple sclerosis (MS) patients. In this paper, we present a brief review of the methods used for kex imaging with a focus on the development of in vivo kex MRI technique based on the DS-removed Omega plot. We then review the recent clinical studies utilizing this technique for better characterizing brain lesions. We also outline technical challenges for the presented technique and discuss its prospects for detecting tissue microenvironmental changes under oxidative stress.

Development of a Nanoscale Chemical Exchange Saturation Transfer Magnetic Resonance Imaging Contrast Agent That Measures pH

AcidoCEST MRI can measure the extracellular pH (pHe) of the tumor microenvironment in mouse models of human cancers and in patients who have cancer. However, chemical exchange saturation transfer (CEST) is an insensitive magnetic resonance imaging (MRI) contrast mechanism, requiring a high concentration of small-molecule agent to be delivered to the tumor. Herein, we developed a nanoscale CEST agent that can measure pH using acidoCEST MRI, which may decrease the requirement for high delivery concentrations of agent. We also developed a monomer agent for comparison to the polymer. After optimizing CEST experimental conditions, we determined that the polymer agent could be used during acidoCEST MRI studies at 125-fold and 488-fold lower concentration than the monomer agent and iopamidol, respectively. We also determined that both agents can measure pH with negligible dependence on temperature. However, pH measurements with both agents were dependent on concentration, which may be due to concentration-dependent changes in hydrogen bonding and/or steric hindrance. We performed in vivo acidoCEST MRI studies using the three agents to study a xenograft MDA-MB-231 model of mammary carcinoma. The tumor pHe measurements were 6.33 ± 0.12, 6.70 ± 0.15, and 6.85 ± 0.15 units with iopamidol, the monomer agent, and polymer agent, respectively. The higher pHe measurements with the monomer and polymer agents were attributed to the concentration dependence of these agents. This study demonstrated that nanoscale agents have merit for CEST MRI studies, but consideration should be given to the dependence of CEST contrast on the concentration of these agents.

Quasi-steady-state CEST (QUASS CEST) solution improves the accuracy of CEST quantification: QUASS CEST MRI-based omega plot analysis

PURPOSE

CEST MRI omega plot quantifies the labile proton fraction ratio (f_r ) and exchange rate (k_sw ), yet it assumes long RF saturation time (Ts) and relaxation delay (Td). Our study aimed to test if a quasi-steady-state (QUASS) CEST analysis that accounts for the effect of finite Ts and Td could improve the accuracy of CEST MRI quantification.

METHODS

We modeled the MRI signal evolution using a typical CEST EPI sequence. The signal relaxes toward its thermal equilibrium following the bulk water relaxation rate during Td, and then toward its CEST steady state following the spin-lock relaxation rate during Ts from which the QUASS CEST effect is derived. Both f_r and k_sw were solved from simulated conventional apparent CEST and QUASS CEST MRI. We also performed MRI experiments from a Cr-gel phantom under serially varied Ts and Td times from 1.5 to 7.5 s.

RESULTS

Simulation showed that, although k_sw could be slightly overestimated (3%-15%) for the range of Ts and Td, f_r could be substantially underestimated by as much as 67%. In contrast, the QUASS solution provided accurate k_sw and f_r determination within 2%. The CEST MRI experiments confirmed that the QUASS solution enabled robust quantification of k_sw and f_r , superior over the omega plot analysis based on the conventional apparent CEST MRI measurements.

CONCLUSIONS

The QUASS CEST MRI algorithm corrects the effect of finite Ts and Td times on CEST measurements, thereby allowing robust and accurate CEST quantification.

Pulse sequences for measuring exchange rates between proton species: From unlocalised NMR spectroscopy to chemical exchange saturation transfer imaging

Within the field of NMR spectroscopy, the study of chemical exchange processes through saturation transfer techniques has a long history. In the context of MRI, chemical exchange techniques have been adapted to increase the sensitivity of imaging to small fractions of exchangeable protons, including the labile protons of amines, amides and hydroxyls. The MR contrast is generated by frequency-selective irradiation of the labile protons, which results in a reduction of the water signal associated with transfer of the labile protons' saturated magnetization to the protons of the surrounding free water. The signal intensity depends on the rate of chemical exchange and the concentration of labile protons as well as on the properties of the irradiation field. This methodology is referred to as CEST (chemical exchange saturation transfer) imaging. Applications of CEST include imaging of molecules with short transverse relaxation times and mapping of physiological parameters such as pH, temperature, buffer concentration and chemical composition due to the dependency of this chemical exchange effect on all these parameters. This article aims to describe these effects both theoretically and experimentally. In depth analysis and mathematical modelling are provided for all pulse sequences designed to date to measure the chemical exchange rate. Importantly, it has become clear that the background signal from semi-solid protons and the presence of the Nuclear Overhauser Effect (NOE), either through direct dipole-dipole mechanisms or through exchange-relayed signals, complicates the analysis of CEST effects. Therefore, advanced methods to suppress these confounding factors have been developed, and these are also reviewed. Finally, the experimental work conducted both in vitro and in vivo is discussed and the progress of CEST imaging towards clinical practice is presented.

Detection of Enzyme Activity and Inhibition during Studies in Solution, In Vitro and In Vivo with CatalyCEST MRI

PURPOSE

The detection of enzyme activities and evaluation of enzyme inhibitors have been challenging with magnetic resonance imaging (MRI). To address this need, we have developed a diamagnetic, nonmetallic contrast agent and a protocol known as catalyCEST MRI that uses chemical exchange saturation transfer (CEST) to detect enzyme activity as well as enzyme inhibition.

PROCEDURES

We synthesized a diamagnetic MRI contrast agent that has enzyme responsive and enzyme unresponsive CEST signals. We tested the ability of this agent to detect the activity of kallikrein 6 (KLK6) in biochemical solutions, in vitro and in vivo, with and without a KLK6 inhibitor.

RESULTS

The agent detected KLK6 activity in solution and also detected KLK6 inhibition by antithrombin III. KLK6 activity was detected during in vitro studies with HCT116 colon cancer cells, relative to the detection of almost no activity in a KLK6-knockdown HCT116 cell line and HCT116 cells treated with antithrombin III inhibitor. Finally, strong enzyme activity was detected within an in vivo HCT116 tumor model, while lower enzyme activity was detected in a KLK6 knockdown tumor model and in the HCT116 tumor model treated with antithrombin III inhibitor. In all cases, comparisons of the enzyme responsive and enzyme unresponsive CEST signals were critical for the detection of enzyme activity.

CONCLUSIONS

This study has established that catalyCEST MRI with an exogenous diaCEST agent can evaluate enzyme activity and inhibition in solution, in vitro and in vivo.

Detecting in vivo urokinase plasminogen activator activity with a catalyCEST MRI contrast agent

Urokinase plasminogen activator (uPA) promotes tumor invasion and metastasis. The monitoring of uPA activity using molecular imaging may have prognostic value and be predictive for response to anti-cancer therapies. However, the detection of in vivo enzyme activity with molecular imaging remains a challenge. To address this problem, we designed a nonmetallic contrast agent, GR-4Am-SA, that can be detected with chemical exchange saturation transfer (CEST) MRI. This agent has a peptide that is cleaved by uPA, which causes a CEST signal at 5.0 ppm to decrease, and also has a salicylic acid moiety that can produce a CEST signal at 9.5 ppm, which is largely unresponsive to enzyme activity. The two CEST signals were used to determine a reaction coordinate, representing the extent of enzyme-catalyzed cleavage of the GR-4Am-SA agent during an experimental study. Initial biochemical studies showed that GR-4Am-SA could detect uPA activity in reducing conditions. Subsequently, we used our catalyCEST MRI protocol with the agent to detect the uPA catalysis of GR-4Am-SA in a flank xenograft model of Capan-2 pancreatic cancer. The results showed an average reaction coordinate of 80% ± 8%, which was strongly dependent on the CEST signal at 5.0 ppm. The relative independence of the reaction coordinate on the CEST signal at 9.5 ppm showed that the detection of enzyme activity was largely independent of the concentration of GR-4Am-SA within the tumor tissue. These results demonstrated the advantages of a single CEST agent with biomarker-responsive and unresponsive signals for reliably assessing enzyme activity during in vivo cancer studies.

Diamagnetic Imaging Agents with a Modular Chemical Design for Quantitative Detection of β-Galactosidase and β-Glucuronidase Activities with CatalyCEST MRI

Imaging agents for the noninvasive in vivo detection of enzyme activity in preclinical and clinical settings could have fundamental implications in the field of drug discovery. Furthermore, a new class of targeted prodrug treatments takes advantage of high enzyme activity to tailor therapy and improve treatment outcomes. Herein, we report the design and synthesis of new magnetic resonance imaging (MRI) agents that quantitatively detect β-galactosidase and β-glucuronidase activities by measuring changes in chemical exchange saturation transfer (CEST). Based on a modular approach, we incorporated the enzymes' respective substrates to a salicylate moiety with a chromogenic spacer via a carbamate linkage. This furnished highly selective diamagnetic CEST agents that detected and quantified enzyme activities of glycoside hydrolase enzymes. Michaelis-Menten enzyme kinetics studies were performed by monitoring catalyCEST MRI signals, which were validated with UV-vis assays.

Noninvasive detection of enzyme activity in tumor models of human ovarian cancer using catalyCEST MRI

PURPOSE

We proposed to detect the in vivo enzyme activity of γ-glutamyl transferase (GGT) within mouse models of human ovarian cancers using catalyCEST MRI with a diamagnetic CEST agent.

METHODS

A CEST-FISP MRI protocol and a diamagnetic CEST agent were developed to detect GGT enzyme activity in biochemical solution. A quantitative Michaelis-Menten enzyme kinetics study was performed to confirm that catalyCEST MRI can measure enzyme activity. In vivo catalyCEST MRI studies generated pixel-wise activity maps of GGT activities. Ex vivo fluorescence imaging was performed for validation.

RESULTS

CatalyCEST MRI selectively detected two CEST signals from a single CEST agent, whereby one CEST signal was responsive to GGT enzyme activity and the other CEST signal was an unresponsive control signal. The comparison of these CEST signals facilitated in vivo catalyCEST MRI studies that detected high GGT activity in OVCAR-8 tumors, low GGT activity in OVCAR-3 tumors, and low or no GGT activity in muscle tissues.

CONCLUSION

CatalyCEST MRI with a diamagnetic CEST agent can detect the level of GGT enzyme activity within in vivo tumor models of human ovarian cancers. Magn Reson Med 77:2005-2014, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Detection of Sulfatase Enzyme Activity with a CatalyCEST MRI Contrast Agent

A chemical exchange saturation transfer (CEST) MRI contrast agent has been developed that detects sulfatase enzyme activity. The agent produces a CEST signal at δ=5.0 ppm before enzyme activity, and a second CEST signal appears at δ=9.0 ppm after the enzyme cleaves a sulfate group from the agent. The comparison of the two signals improved detection of sulfatase activity.

Screening CEST contrast agents using ultrafast CEST imaging

A chemical exchange saturation transfer (CEST) experiment can be performed in an ultrafast fashion if a gradient field is applied simultaneously with the saturation pulse. This approach has been demonstrated for studying dia- and para-magnetic CEST agents, hyperpolarized Xe gas and in vivo spectroscopy. In this study we present a simple method for the simultaneous screening of multiple samples. Furthermore, by interleaving a number of saturation and readout periods within the TR, a series of images with different saturation times can be acquired, allowing for the quantification of exchange rates using the variable saturation time (QUEST) approach in a much accelerated fashion, thus enabling high throughput screening of CEST contrast agents.

A single diamagnetic catalyCEST MRI contrast agent that detects cathepsin B enzyme activity by using a ratio of two CEST signals

CatalyCEST MRI can detect enzyme activity by monitoring the change in chemical exchange with water after a contrast agent is cleaved by an enzyme. Often these molecules use paramagnetic metals and are delivered with an additional non-responsive reference molecule. To improve this approach for molecular imaging, a single diamagnetic agent with enzyme-responsive and enzyme-unresponsive CEST signals was synthesized and characterized. The CEST signal from the aryl amide disappeared after cleavage of a dipeptidyl ligand with cathepsin B, while a salicylic acid moiety was largely unresponsive to enzyme activity. The ratiometric comparison of the two CEST signals from the same agent allowed for concentration independent measurements of enzyme activity. The chemical exchange rate of the salicylic acid moiety was unchanged after enzyme catalysis, which further validated that this moiety was enzyme-unresponsive. The temperature dependence of the chemical exchange rate of the salicylic acid moiety was non-Arrhenius, suggesting a two-step chemical exchange mechanism for salicylic acid. The good detection sensitivity at low saturation power facilitates clinical translation, along with the potentially low toxicity of a non-metallic MRI contrast agent. The modular design of the agent constitutes a platform technology that expands the variety of agents that may be employed by catalyCEST MRI for molecular imaging.

Design and optimization of pulsed Chemical Exchange Saturation Transfer MRI using a multiobjective genetic algorithm

Pulsed Chemical Exchange Saturation Transfer (CEST) MRI experimental parameters and RF saturation pulse shapes were optimized using a multiobjective genetic algorithm. The optimization was carried out for RF saturation duty cycles of 50% and 90%, and results were compared to continuous wave saturation and Gaussian waveform. In both simulation and phantom experiments, continuous wave saturation performed the best, followed by parameters and shapes optimized by the genetic algorithm and then followed by Gaussian waveform. We have successfully demonstrated that the genetic algorithm is able to optimize pulse CEST parameters and that the results are translatable to clinical scanners.

A comparison of iopromide and iopamidol, two acidoCEST MRI contrast media that measure tumor extracellular pH

Acidosis within tumor and kidney tissues has previously been quantitatively measured using a molecular imaging technique known as acidoCEST MRI. The previous studies used iopromide and iopamidol, two iodinated contrast agents that are approved for clinical CT diagnoses and have been repurposed for acidoCEST MRI studies. We aimed to compare the performance of the two agents for measuring pH by optimizing image acquisition conditions, correlating pH with a ratio of CEST effects from an agent, and evaluating the effects of concentration, endogenous T1 relaxation time and temperature on the pH-CEST ratio correlation for each agent. These results showed that the two agents had similar performance characteristics, although iopromide produced a pH measurement with a higher dynamic range while iopamidol produced a more precise pH measurement. We then compared the performance of the two agents to measure in vivo extracellular pH (pHe) within xenograft tumor models of Raji lymphoma and MCF-7 breast cancer. Our results showed that the pHe values measured with each agent were not significantly different. Also, iopromide consistently measured a greater region of the tumor relative to iopamidol in both tumor models. Therefore, an iodinated contrast agent for acidoCEST MRI should be selected based on the measurement properties needed for a specific biomedical study and the pharmacokinetic properties of a specific tumor model.

A review of optimization and quantification techniques for chemical exchange saturation transfer MRI toward sensitive in vivo imaging

Chemical exchange saturation transfer (CEST) MRI is a versatile imaging method that probes the chemical exchange between bulk water and exchangeable protons. CEST imaging indirectly detects dilute labile protons via bulk water signal changes following selective saturation of exchangeable protons, which offers substantial sensitivity enhancement and has sparked numerous biomedical applications. Over the past decade, CEST imaging techniques have rapidly evolved owing to contributions from multiple domains, including the development of CEST mathematical models, innovative contrast agent designs, sensitive data acquisition schemes, efficient field inhomogeneity correction algorithms, and quantitative CEST (qCEST) analysis. The CEST system that underlies the apparent CEST-weighted effect, however, is complex. The experimentally measurable CEST effect depends not only on parameters such as CEST agent concentration, pH and temperature, but also on relaxation rate, magnetic field strength and more importantly, experimental parameters including repetition time, RF irradiation amplitude and scheme, and image readout. Thorough understanding of the underlying CEST system using qCEST analysis may augment the diagnostic capability of conventional imaging. In this review, we provide a concise explanation of CEST acquisition methods and processing algorithms, including their advantages and limitations, for optimization and quantification of CEST MRI experiments.