This is a prospective, single-center, diagnostic study designed to compare the accuracy of [68Ga]Ga-CTR-FAPI PET/MRI and [18]F-FDG PET/MRI for the detection of lymph node metastasis in patients with initially untreated, resectable middle to high rectal cancer. The study aims to evaluate whether [68Ga]Ga-CTR-FAPI PET/MRI provides higher sensitivity and specificity than traditional 18F-FDG PET/MRI at both the patient level and the lymph node level.

Accurate preoperative staging of colorectal cancer is crucial for formulating treatment plans and improving patient prognosis. While conventional MRI and 18F-FDG PET/MRI are utilized, they face limitations such as false positives due to inflammatory lymph nodes or fibrotic scars. Fibroblast activating protein inhibitor (FAPI) specifically targets tumor-associated fibroblasts, potentially reducing false-positive rates by avoiding non-specific uptake in inflammatory areas.

This study aims to cross-validate the diagnostic efficacy of these two probes through a precise imaging-pathology matching technique. All enrolled patients will sequentially undergo both [68Ga]Ga-CTR-FAPI PET/MRI and 18F-FDG PET/MRI within 30 days prior to radical surgery. Following surgery, fresh mesorectal specimens will be precisely sectioned and matched to preoperative anatomical imaging maps. By leveraging a well-established node-by-node matching and identification methodology with high accuracy, this study ensures the execution of spatial correlation between preoperative radiologic imaging and postoperative microscopic pathology. Through mapping individual lymph nodes, the study will analyze the root causes of false positives and negatives, and extract radiomics features to evaluate their correlation with the tumor microenvironment (e.g., fibroblast density, Ki-67 index).

Inclusion Criteria:

• Confirmed rectal adenocarcinoma by pathological biopsy.
• Lower margin of the tumor is ≥ 5 cm and ≤15 cm from the anal verge.
• Planned to undergo radical surgery.
• ECOG performance status score ≤ 2.
• Patient is willing to undergo PET/MRI examination.

Exclusion Criteria:

• History of pelvic radiotherapy or chemotherapy.
• Concurrent with other malignant tumors or active infections.
• Pregnant or unable to sign the informed consent form.
• Claustrophobia or inability to tolerate MRI examinations due to other reasons.