Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) detects molecules in their natural forms in a sensitive and non-invasive manner. This makes it a robust approach to assess brain tumors and related molecular alterations using endogenous molecules, such as proteins/peptides, and drugs approved for clinical use. In this review, we will discuss the promises of CEST MRI in the identification of tumors, tumor grading, detecting molecular alterations related to isocitrate dehydrogenase (IDH) and O-6-methylguanine-DNA methyltransferase (MGMT), assessment of treatment effects, and using multiple contrasts of CEST to develop theranostic approaches for cancer treatments. Promising applications include (i) using the CEST contrast of amide protons of proteins/peptides to detect brain tumors, such as glioblastoma multiforme (GBM) and low-grade gliomas; (ii) using multiple CEST contrasts for tumor stratification, and (iii) evaluation of the efficacy of drug delivery without the need of metallic or radioactive labels. These promising applications have raised enthusiasm, however, the use of CEST MRI is not trivial. CEST contrast depends on the pulse sequences, saturation parameters, methods used to analyze the CEST spectrum (i.e., Z-spectrum), and, importantly, how to interpret changes in CEST contrast and related molecular alterations in the brain. Emerging pulse sequence designs and data analysis approaches, including those assisted with deep learning, have enhanced the capability of CEST MRI in detecting molecules in brain tumors. CEST has become a specific marker for tumor grading and has the potential for prognosis and theranostics in brain tumors. With increasing understanding of the technical aspects and associated molecular alterations detected by CEST MRI, this young field is expected to have wide clinical applications in the near future.

Imaging hydrogel-based local drug delivery to the brain after tumor resection has implications for refining treatments, especially for brain tumors with poor prognosis and high recurrence rate. Here, we developed a series of self-healing chitosan–dextran (CD)-based hydrogels for drug delivery to the brain. These hydrogels are injectable, self-healing, mechanically compatible, and detectable by chemical exchange saturation transfer magnetic resonance imaging (CEST MRI). CD hydrogels have an inherent CEST contrast at 1.1 ppm, which decreases as the stiffness increases. We further examined the rheological properties and CEST contrast of various chemotherapeutic-loaded CD hydrogels, including gemcitabine (Gem), doxorubicin, and procarbazine. Among these formulations, Gem presented the best compatibility with the rheological (G′: 215.3 ± 4.5 Pa) and CEST properties of CD hydrogels. More importantly, the Gem-loaded CD hydrogel generated another CEST readout at 2.2 ppm (11.6 ± 0.1%) for monitoring Gem. This enabled independent and simultaneous imaging of the drug and hydrogel integrity using a clinically relevant 3 T MRI scanner. In addition, the Gem-loaded CD hydrogel exhibited a longitudinal antitumor efficacy of Gem over a week in vitro. Furthermore, the CD hydrogel could be visualized by CEST after brain injection with a contrast of 7.38 ± 2.31%. These natural labels on both the chemotherapeutics and hydrogels demonstrate unique image-guided local drug delivery for brain applications.

Imaging pHe of the tumor microenvironment has paramount importance for characterizing aggressive, invasive tumors, as well as therapeutic responses. Here, a robust approach to image pH changes in the tumor microenvironment longitudinally and during sodium bicarbonate treatment was reported. The pH-sensing microbeads were designed and prepared based on materials approved for clinical use, i.e., alginate microbeadcontaining computed tomography (CT) contrast-agent (iopamidol)-loaded liposomes (Iop-lipobeads). This Iop-lipobead prepared using a customized microfluidic device generated a CEST contrast of 10.6% at 4.2 ppm at pH 7.0, which was stable for 20 days in vitro. The CEST contrast decreased by 11.8% when the pH decreased from 7.0 to 6.5 in vitro. Optimized Iop-lipobeads next to tumors showed a significant increase of 19.7 ± 6.1% (p < 0.01) in CEST contrast at 4.2 ppm during the first 3 days of treatment and decreased to 15.2 ± 4.8% when treatment stopped. Notably, percentage changes in Iop-lipobeads were higher than that of amide CEST (11.7% and 9.1%) in tumors during and after treatment. These findings demonstrated that the Iop-lipobead could provide an independent and sensitive assessment of the pHe changes for a noninvasive and longitudinal monitoring of the treatment effects using multiple CEST contrast.

Dynamic glucose enhanced (DGE) MRI can detect glucose-related events in the brain, however, the influence of oxygen levels on DGE signal remains unknown. Here, we investigated the DGE signal changes under normoxia and hyperoxia on mouse brain, using on-resonance variable delay multi-pulse (onVDMP) MRI. Significantly higher signal change under normoxia than that under hyperoxia was observed in parenchyma but not in cerebrospinal fluid (CSF). Without glucose infusion, a signal increment of about 3% was found in both parenchyma and CSF from hyperoxia to normoxia, interpreted as related to BOLD effect. These data provide insight into the origin of glucoCEST contrast.

Glioblastoma is a malignant form of brain tumors, which has a high recurrence even with combined treatments. Here, we developed anti-cancer drugs loaded liposomal hydrogels for sustainable drug release to treat brain tumors locally under MRI guidance. These anti-cancer drugs are CEST-detectable. We observed a continuous decrease in tumor size and CEST contrast at 3.5 and -3.5 ppm compared to control group, where drug-free liposomal hydrogel was injected. The therapeutic efficacy of local drug delivery using liposomal hydrogel was evidenced and can be monitored using CEST multiple-contrast approach. This could serve a robust theranostic application for local brain tumor treatment.