Repeatability and reproducibility of APT and GluCEST imaging at 5 T: phantom validation and clinical evaluation in patients with brain tumors

PURPOSE: To evaluate the repeatability and reproducibility of amide proton transfer (APT) and glutamate chemical exchange saturation transfer (GluCEST) imaging at 5 T and to investigate their clinical value in patients with brain tumors. METHODS: Phantom experiments under varying pH conditions and bovine serum albumin (BSA) concentrations were performed to assess the within-session repeatability, between-session and between-day reproducibility of APT and GluCEST measurements using intraclass correlation coefficients (ICCs), mean absolute difference, coefficient of repeatability (CoR) and coefficient of variation (CV). In vivo APT and GluCEST imaging was performed in 96 patients with brain tumors. Intra- and inter-observer agreements of APTmean and GluCESTmean measurements were evaluated using ICCs and Bland-Altman analysis across tumor, peritumoral edema, and normal brain tissue regions. Quantitative tumor-region APT and GluCEST metrics (APT/GluCESTmean, APT/GluCEST difference, and APT/GluCEST change ratio) were compared among major tumor subtypes using the Kruskal-Wallis test, followed by post hoc pairwise comparisons using Dunn's test with Bonferroni correction. Receiver operating characteristic (ROC) analysis was performed to evaluate the diagnostic performance for differentiating high-grade gliomas from low-grade gliomas. RESULTS: In phantom studies, both APT and GluCEST demonstrated robust repeatability and reproducibility with high ICCs (> 0.96) and low mean absolute difference, CoR and CV. Both APT and GluCEST-weighted signals showed nonlinear pH-dependent changes and increased with higher BSA concentrations. In patients, intra- and inter-observer agreements were excellent for APTmean and GluCESTmean in tumor, peritumoral edema, and normal brain regions (ICCs > 0.93). Tumor and peritumoral edema regions showed higher APT and GluCEST values than normal brain tissue (p < 0.05). After multiple-comparison correction, high-grade gliomas showed significantly higher APTmean, APT difference, GluCESTmean, GluCEST difference and GluCEST change ratio than low-grade gliomas. GluCEST-derived metrics showed favorable performance for glioma grading, with GluCEST change ratio achieving the highest area under the curve (AUC) of 0.846, followed by GluCEST difference (AUC = 0.838) and GluCESTmean (AUC = 0.836). CONCLUSION: APT and GluCEST imaging at 5 T showed excellent repeatability and reproducibility in phantom experiments and robust measurement agreement in patients with brain tumors. APT and GluCEST measurements demonstrated subtype-related differences, and GluCEST-derived metrics showed favorable performance for differentiating high-grade from low-grade gliomas.