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 T 1 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 (B 1 ) 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 B 1 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.