Radiofrequency Amplitude-Based Ratiometric Guanidinium CEST MRI for Tumor pH-Sensitive Imaging

Altered pH is a hallmark of metabolic disruption in the tumor microenvironment. Chemical exchange saturation transfer (CEST) MRI has emerged as a valuable technique for pH-sensitive imaging. However, the pH specificity of conventional CEST measurements is usually compromised by T₁ relaxation and labile proton concentration. Ratiometric CEST MRI can eliminate these contaminations. This study aimed to investigate the feasibility of radiofrequency (RF) amplitude-based ratiometric CEST MRI for tumor pH-sensitive imaging. Six adult rats implanted with C6 glioma cells underwent CEST MRI under four RF saturation (B₁) levels of 0.75, 1.0, 1.5, and 2.0 μT at an 11.7 T scanner. A quasi-steady-state (QUASS) algorithm was used to reconstruct equilibrium Z-spectra. Guanidinium CEST (GuanCEST) signal at 2 ppm was isolated from inverse Z-spectra using multi-pool Lorentzian fitting. The chemical exchange rate (k_sw) of guanidinium protons was quantified with a spillover- and magnetization transfer-corrected omega plot model. The RF amplitude-based guanidinium CEST ratio (R_GuanCEST) was quantified by ratioing GuanCEST signals under B₁ of 1.5 μT over that under 0.75 μT and was correlated with k_sw using Pearson correlation analysis. Paired Student's t tests were performed to evaluate differences between tumor and contralateral normal regions. p < 0.05 was considered statistically significant. Strong correlations between R_GuanCEST and k_sw were observed both at the pixel level (R = 0.863, p < 0.001) and across animals (R = 0.817, p = 0.001). Tumor regions exhibited significantly lower R_GuanCEST (0.90 ± 0.16) compared with contralateral normal tissues (1.08 ± 0.21, p = 0.001), consistent with corresponding reductions in k_sw (298.04 ± 61.71 s^-1 in tumors vs. 418.86 ± 79.45 s^-1 in contralateral normal tissues, p < 0.001), indicating an acidic microenvironment in gliomas. In conclusion, this study demonstrates the feasibility of endogenous GuanCEST ratiometric MRI for pH-sensitive imaging of gliomas, providing an effective way for evaluating tumor acidity.