Endometriosis can take up to 12 years to diagnose, in part because current imaging techniques frequently miss lesions outside the ovaries. New research suggests fibroblast activation protein (FAP)-targeted PET/MRI could help close that gap.
In a pilot study published June 4 in the Journal of Nuclear Medicine, researchers at University Hospital Münster found that FAPI-PET/MRI detected more than twice as many endometriosis-suspicious lesions as MRI alone among all evaluated anatomical segments, 49% versus 19%.
The study was co-led by Dr. Philipp Schindler, EDiR, head of hybrid imaging at University Hospital Münster, and Dr. Jonas Brandt of the same department.
Where conventional imaging falls short
The difference was most pronounced in the peritoneum, a site where endometriosis frequently goes undetected. MRI detects peritoneal endometriosis with sensitivity as low as 14%, combined PET/MRI identified lesions there in five of seven patients versus just one on MRI alone.
Tubo-ovarian findings expanded from two patients to five. Where laparoscopy was performed after imaging, findings showed good agreement with PET/MRI results.
FAPI PET/MRI images from a patient with menstrual cycle-dependent right shoulder pain. PET identified diaphragmatic and pelvic lesions (A, B) later confirmed by laparoscopy (C) and FAP immunostaining (D).Courtesy of Schindler, Brandt et al., Journal of Nuclear Medicine, 2026.
Focal extrauterine FAP uptake was observed in 41% of segments in the endometriosis group versus 13% in the reference group. The mean number of PET-positive segments per patient was 4.9 versus 1.7.
Why FAP is a plausible target
The mechanism behind the findings is biologically plausible. FAP is overexpressed in tissues undergoing active remodeling, precisely the cyclic tissue injury, repair, and fibrosis that characterizes endometriotic lesions. That biological match makes FAP a plausible target for molecular imaging in locations where MRI and transvaginal ultrasound consistently fall short.
The study included 18 premenopausal women, nine with suspected or confirmed endometriosis, nine without, using the radiopharmaceutical [68Ga]BED003, a FAP inhibitor provided by Blue Earth Diagnostics.
Maximum-intensity-projection PET images from the test group (T1–T9, top) and reference group (R1–R9, bottom). Blue arrowheads indicate endometriosis-suspicious findings.Courtesy of Schindler, Brandt et al., Journal of Nuclear Medicine, 2026.
The authors also emphasized the study's limitations. With nine patients in the endometriosis group and laparoscopic confirmation in only three, no definitive conclusions about sensitivity or specificity can yet be drawn. Larger prospective studies with standardized protocols and menstrual cycle timing are needed.
But as among the first systematic evaluations of molecular imaging for endometriosis extent mapping, the Münster findings point toward a diagnostic tool for a disease that has long resisted accurate noninvasive assessment.
The full study can be found here.
