Quantitative tissue pH measurement during cerebral ischemia using amine and amide concentration-independent detection (AACID) with MRI

Tissue pH is an indicator of altered cellular metabolism in diseases including stroke and cancer. Ischemic tissue often becomes acidic due to increased anaerobic respiration leading to irreversible cellular damage. Chemical exchange saturation transfer (CEST) effects can be used to generate pH-weighted magnetic resonance imaging (MRI) contrast, which has been used to delineate the ischemic penumbra after ischemic stroke. In the current study, a novel MRI ratiometric technique is presented to measure absolute pH using the ratio of CEST-mediated contrast from amine and amide protons: amine/amide concentration-independent detection (AACID). Effects of CEST were observed at 2.75 parts per million (p.p.m.) for amine protons and at 3.50 p.p.m. for amide protons downfield (i.e., higher frequency) from bulk water. Using numerical simulations and in vitro MRI experiments, we showed that pH measured using AACID was independent of tissue relaxation time constants, macromolecular magnetization transfer effects, protein concentration, and temperature within the physiologic range. After in vivo pH calibration using phosphorus ( 31P) magnetic resonance spectroscopy (31P-MRS), local acidosis is detected in mouse brain after focal permanent middle cerebral artery occlusion. In summary, our results suggest that AACID represents a noninvasive method to directly measure the spatial distribution of absolute pH in vivo using CEST MRI. © 2014 ISCBFM All rights reserved

Temperature and pH Imaging using Chemical Exchange Saturation Transfer (CEST) MRI Contrast

Chemical exchange saturation transfer (CEST) is a novel mechanism used to generate contrast in magnetic resonance imaging (MRI). Recently, CEST contrast was proposed to noninvasively measure physiological parameters including temperature and pH. Tissue temperature and pH are known markers of pathological processes in many diseases including stroke and cancer. CEST contrast can be generated using endogenous proteins and peptides (endogenous CEST) or using exogenous paramagnetic lanthanide agents (PARACEST). The general problem of optimizing applications of endogenous CEST and PARACEST contrast to measure temperature and pH is addressed in this thesis. Highlights of the thesis include a novel application of PARACEST contrast to measure extracellular pH and temperature in-vivo and a novel ratiometric approach that uses endogenous CEST contrast to measure intracellular pH in-vivo. Using a Tm3+-based PARACEST agent (Tm3+-DOTAM-Gly-Lys), the PARACEST amide peak chemical shift and linewidth were shown to depend on pH and temperature in a deterministic manner. Quantitative temperature and pH maps were simultaneously measured in a normal mouse leg following agent injection using empirical relations derived in-vitro. A ratio of endogenous amide and amine proton CEST effects was developed to measure absolute tissue pH that is heavily weighted to the intracellular compartment. The technique called amine and amide concentration-independent detection (AACID) was developed using in-vitro phantoms and numerical simulations. Following in-vivo pH-calibration using 31P-magnetic resonance spectroscopy (MRS), tissue pH measurement was demonstrated in mice following focal cerebral ischemia. Local acidosis was measured in ischemic regions and found to correlate with regions of tissue damage. Finally, two endogenous CEST metrics including the AACID ratio were used to monitor cancer treatment using an anticancer drug called lonidamine. Lonidamine selectively acidifies cancer cells. In-vivo experiments demonstrate that endogenous CEST imaging is sensitive to intracellular acidification by lonidamine in a glioblastoma brain tumor mouse model. Overall, the results presented in this thesis demonstrate quantitative measurement of pH and temperature using CEST and/or PARACEST contrast in-vivo. Some of the novel techniques developed in this thesis were demonstrated in stroke and cancer mouse models. Future work should focus on 1) development of PARACEST agents with higher sensitivity in-vivo to improve accuracy of temperature and pH maps; 2) application of AACID for absolute pH measurement to differentiate high- and low-grade tumors in-vivo; and 3) application of endogenous CEST measurement to monitor tumor response to different clinically approved chemotherapy treatments

Characterization of amine proton exchange for analyzing the specificity and intensity of the CEST effect: from humans to fish

Chemical exchange saturation transfer (CEST) at about 2.8 ppm downfield from water is characterized besides other compounds by exchanging amine protons of relatively high concentration amino acids and is determined by several physiological (pH, T) and experimental (B0, B1, tsat) parameters. Although the weighting of the CEST effect observed in vivo can be attributed mainly to one compound depending on the organism and organ, there are still several other amino acids, proteins and molecules that also contribute. These contributions in turn exhibit dependences and thus can lead to possible misinterpretation of the measured changes in the CEST effect. With this in mind, this work aimed to determine the exchange rates of six important amino acids as a function of pH and temperature, and thus to create multi-pool models that allow the accurate analysis of the CEST effect concerning different physiological and experimental parameters for a wide variety of organisms. The results show that small changes in the above parameters have a significant impact on the CEST effect at about 2.8 ppm for the chosen organisms, i.e. the human brain (37 °C) and the brain of polar cod (1.5 °C), furthermore, the specificity of the CEST effect observed in vivo can be significantly affected. Based on the exchange rates ksw(pH, T) determined for six metabolites in this study, it is possible to optimize the intensity and the specificity for the CEST effect of amino acids at about 2.8 ppm for different organisms with their specific physiological characteristics. By adjusting experimental parameters accordingly, this optimization will help to avoid possible misinterpretations of CEST measurements. Furthermore, the multi-pool models can be utilized to further optimize the saturation

What do we know about dynamic glucose-enhanced (DGE) MRI and how close is it to the clinics? Horizon 2020 GLINT consortium report

Cancer is one of the most devastating diseases that the world is currently facing, accounting for 10 million deaths in 2020 (WHO). In the last two decades, advanced medical imaging has played an ever more important role in the early detection of the disease, as it increases the chances of survival and the potential for full recovery. To date, dynamic glucose-enhanced (DGE) MRI using glucose-based chemical exchange saturation transfer (glucoCEST) has demonstrated the sensitivity to detect both D-glucose and glucose analogs, such as 3-oxy-methyl-D-glucose (3OMG) uptake in tumors. As one of the recent international efforts aiming at pushing the boundaries of translation of the DGE MRI technique into clinical practice, a multidisciplinary team of eight partners came together to form the "glucoCEST Imaging of Neoplastic Tumors (GLINT)" consortium, funded by the Horizon 2020 European Commission. This paper summarizes the progress made to date both by these groups and others in increasing our knowledge of the underlying mechanisms related to this technique as well as translating it into clinical practice