Background In Crohn disease, differentiation between active intestinal inflammation and fibrosis has implications for treatment, but current imaging modalities are not reliably accurate. Purpose To evaluate the predictive value of gallium 68 (⁶⁸Ga)-labeled fibroblast activation protein inhibitor (FAPI) PET/MR enterography for the assessment of bowel wall fibrosis in Crohn disease. Materials and Methods In this prospective single-center study, consecutive participants with Crohn disease and obstructive symptoms underwent preoperative ⁶⁸Ga-FAPI PET/MR enterography from May 2021 to January 2022. Histopathologic analysis of resected bowel segments was performed to grade active inflammation (A0-A2) and fibrosis (F0-F2), which served as the reference standard. The fibroblast activation protein (FAP) expression in bowel wall layers was analyzed immunohistochemically for each layer. ⁶⁸Ga-FAPI-derived maximum standardized uptake value (SUV_max) was compared with histopathologic results by using mixed-model analysis of variance and Bonferroni-corrected post hoc tests. Results In 14 participants (mean age, 45 years ± 9 [SD]; 10 men), fibrosis was diagnosed histopathologically in 28 of 51 bowel segments (grade F1, n = 14; grade F2, n = 14). Mean SUV_max was higher in segments with fibrosis than without (7.6 vs 2.0; P < .001). In severe fibrosis, mean SUV_max was higher than in mild to moderate fibrosis (8.9 ± 0.9 vs 6.2 ± 0.9; P = .045). Bowel segments with isolated active inflammation had lower mean ⁶⁸Ga-FAPI uptake than segments with combined active inflammation and fibrosis (SUV_max, 3.2 ± 0.4 vs 8.1 ± 0.1; P = .005). With an SUV_max cutoff value of 3.5, the area under the receiver operating characteristic curve for the prediction of fibrosis was 0.94 (95% CI: 0.9, 1.0), with sensitivity of 26 of 28 segments (93%) and specificity of five of six segments (83%). ⁶⁸Ga-FAPI-derived SUV_max correlated with FAP expression across all bowel layers (R² = 0.50, P < .001). Conclusion Higher gallium 68 fibroblast activation protein inhibitor uptake at PET/MR enterography was associated with histopathologically assessed bowel wall fibrosis in participants with Crohn disease, suggesting diagnostic potential for treatment decisions. © RSNA, 2023 Supplemental material is available for this article. See also the editorial by O'Shea in this issue.

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