Influence of phosphate concentration on amine, amide, and hydroxyl CEST contrast

Abstract

Purpose To evaluate the influence of phosphate on amine, amide, and hydroxyl CEST contrast using Bloch‐McConnell simulations applied to physical phantom data. Methods Phantom solutions of 4 representative metabolites with exchangeable protons—glycine (α‐amine protons), Cr (η‐amine protons), egg white protein (amide protons), and glucose (hydroxyl protons)—were prepared at different pH levels (5.6 to 8.9) and phosphate concentrations (5 to 80 mM). CEST images of the phantom were collected with CEST‐EPI sequence at 3 tesla. The CEST data were then fitted to full Bloch‐McConnell equation simulations to estimate the exchange rate constants. With the fitted parameters, simulations were performed to evaluate the intracellular and extracellular contributions of CEST signals in normal brain tissue and brain tumors, as well as in dynamic glucose‐enhanced experiments. Results The exchange rates of α‐amine and hydroxyl protons were found to be highly dependent on both pH and phosphate concentrations, whereas the exchange rates of η‐amine and amide protons were pH‐dependent, albeit not catalyzed by phosphate. With phosphate being predominantly intracellular, CEST contrast of α‐amine exhibited a higher sensitivity to changes in the extracellular microenvironment. Simulations of dynamic glucose‐enhanced signals demonstrated that the contrast between normal and tumor tissue was mostly due to the extracellular CEST effect. Conclusion The proton exchange rates in some metabolites can be greatly catalyzed by the presence of phosphate at physiological concentrations, which substantially alters the CEST contrast. Catalytic agents should be considered as confounding factors in future CEST‐MRI research. This new dimension may also benefit the development of novel phosphate‐sensitive imaging methods.

Mapping intracellular pH in tumors using amide and guanidyl CEST‐MRI at 9.4 T

“…[77][78][79] The amide and guanidyl signals have already been shown to be independent of buffer concentration and to be not catalyzed by phosphate. 13,80 Regarding the dependency on the average molecular size, the average composition of the proteome and the protein folding state, we used porcine brain lysates in order to mimic, as good as possible, the in vivo composition of proteins and peptides during calibration. The transfer of the in vitro-calibrated k c for in vivo examinations is supported by the physiologically plausible pH i values of around 7.2 found in tumorbearing mice (n = 19), independent of size and that are in good agreement with literature (i.e.…”

“…buffers or salts), the average molecular size, 76 the average composition of the proteome, and the protein folding state 77–79 . The amide and guanidyl signals have already been shown to be independent of buffer concentration and to be not catalyzed by phosphate 13,80 . Regarding the dependency on the average molecular size, the average composition of the proteome and the protein folding state, we used porcine brain lysates in order to mimic, as good as possible, the in vivo composition of proteins and peptides during calibration.…”

Assessment of cellular responses in three‐dimensional cell cultures through chemical exchange saturation transfer and 1H MRS

“…Therefore, changes in CEST magnitude could monitor changes in physiological states 6,8–15 . The origin of the CEST signals is mostly validated using buffer phantoms, 19,23–28 and some attempts have been made by incorporating MRS to correlate the metabolites to the in vivo CEST images 49,54,55 . Metabolites extracted from cells could be correlated to the in vivo CEST signals using 1 H NMR 56 .…”

“…The pH values of the control and DOX-treated 2D, 3D, and 3D-SP cultures validated using buffer phantoms, 19,[23][24][25][26][27][28] and some attempts have been made by incorporating MRS to correlate the metabolites to the in vivo CEST images. 49,54,55 Metabolites extracted from cells could be correlated to the in vivo CEST signals using 1 H NMR.…”

“…Metabolite phantoms prepared as buffer solutions 19,[23][24][25][26] or gels 27,28 are commonly used to validate a CEST contrast. Biologically derived phantoms with complex compositions, such as tissues, 29 tissue homogenates, 8,30 or cells, 31,32 can be prepared and investigated before animal studies are conducted.…”