{"paper_id":"068c9dc7-5220-45fc-bc12-cd69f807d4cd","body_text":"Abstract\nPurpose To develop and validate a novel chemical exchange saturation transfer (CEST) MRI method to map skeletal muscle OXPHOS (Oxidative Phosphorylation CEST or OXCEST).\nTheory and Methods Our proposed OXCEST method acquires creatine (Cr)-weighted CEST maps by applying RF saturation (B1) at only two frequency offsets: +1.8 ppm (targeting the Cr amine resonance) and −1.8 ppm (to calculate MTRasym at 1.8 ppm). The pre-exercise MTRasym is modeled as a second-order polynomial function (f) of B0. Next, the post-exercise alteration in MTRasym is hypothesized to be affected by both an exercise-induced increase in Cr and changes in B0 inhomogeneity. By inputting post-exercise B0 values into f, the change in MTRasym due to B0 variation alone was estimated. Thus, the Cr-related post-exercise MTRasym could be isolated and quantified. OXCEST and 31P-MRS were performed in seven subjects across two sessions to compare the OXCEST-derived Cr recovery time constant (TCr) with the ground-truth phosphocreatine recovery time constant (TPCr).\nResults A second-order polynomial function f could reliably describe the relationship between pre-exercise MTRasym and B0 (R2=0.87±0.07 in the lateral gastrocnemius (LG); R2=0.98±0.01 in the medial gastrocnemius (MG); R2=0.96±0.03 in the soleus). The mean pre-exercise MTRasym was approximately 6-7% for all muscle groups. Following exercise, MTRasym increased by 11.4±4.5% in LG and 8±2.4% in MG, and showed mono-exponential recovery (R2>0.97). The combined TCr of LG and MG was found to be significantly correlated with TPCr (R²=0.83, p=0.005).\nConclusion OXCEST enables reliable assessment of post-exercise Cr recovery and demonstrated strong agreement with 31P-MRS.\nCompeting Interest Statement\nThe authors have declared no competing interest.\nFootnotes\nNew analysis have been done and updated in the manuscript. The manuscript now shows why B0 correction is important. The aim of the revised manuscript is to establish OXCEST as a new technique development.","source_license":"CC-BY-4.0","license_restricted":false}