Endometriosis is a disease that affects 1 in 10 women and is associated with debilitating pain and infertility. Endometriosis is where cells similar to those lining the womb (the 'endometrium') grow elsewhere in the body, forming 'lesions'. Most commonly the lesions grow on the lining of the pelvic cavity, called 'peritoneal' endometriosis. Lesions can also grow on the ovary, this is called 'ovarian' endometriosis, or form nodules, called 'deep' endometriosis.

At present the only way to confidently identify endometriosis is through surgery, this exposes patient to the risks of surgery and contributes to the diagnostic delay associated with endometriosis.

PET/CT is a specialist scan that is commonly used to identify cancers which cannot be seen on other types of scans. PET/CT uses a 'tracer', a substance given into a vein which then temporarily accumulates in areas of disease.

This project will determine if a new specialist scan, total body PET/CT, and novel tracers that were developed for other conditions can be used to identify some of the key pathways in endometriosis: bleeding and scarring. Being able to identify these processes in endometriosis lesions and being able to track how they change over time would improve our understanding of endometriosis. The investigators also want to know if these pathways are different between superficial, deep and ovarian endometriosis, and what the impact is of the hormones related to the menstrual cycle.

In this study up to 30 people who have suspected endometriosis and are already due to undergo diagnostic surgery will be asked to undergo two total-body PET/CT scans in Edinburgh, one at one visit and one another visit. Participants will also have a PET/MRI scan at visit. Participants will have a different tracer at each visit. The investigators will then compare the scan findings with their subsequent surgical findings.

This will be an observational cohort study using a prospectively recruited cohorts for both cross-sectional and longitudinal comparison across the spectrum of endometriosis subtypes.

Up to 30 participants with a clinical/radiological diagnosis of likely endometriosis and due to undergo confirmatory laparoscopy in the subsequent four months will be recruited.

Data collected at baseline includes demographics, clinical history including previous imaging. Participants will complete the WERF EPHect questionnaire

Participants will attend two visits for imaging. They will undergo advanced imaging by total-body positron emission tomography and computed tomography (PET/CT), followed by PET/MRI for anatomical reconciliation of lesions and additional information about tracer dynamics. At one visit 68Ga-FAPI will be administered, and at the other 18F-GP1. IV buscopan will be given prior to the PET/MRI.

Following completion of imaging studies, participants will be invited to complete an online acceptability questionnaire, and complete and optional qualitative interview about their experience.

Subsequent surgical findings and associated histopathology will be correlated with tracer uptake

Inclusion Criteria:

• Chronic pelvic pain
• Clinical/radiological diagnosis of likely endometriosis and due to undergo confirmatory laparoscopy in the subsequent four months

Exclusion Criteria:

• Inability or unwilling to give informed consent
• Actively trying to conceive, pregnant or breastfeeding
• Post menopausal (no periods for >12 months and not taking hormonal treatments to prevent periods or bilateral oophorectomy performed)
• Previous hysterectomy
• Confirmed or suspected pelvic malignancy
• Contraindication to MRI (ferromagnetic material in the body, metallic device implantation or claustrophobia)
• Contraindication to buscopan (e.g. glaucoma, unstable cardiac disease, arrythmias, myasthenia, previous hypersensitivity to buscopan)
• Taking part in a CTIMP or interventional non-CTIMP study
• Previous severe pelvic inflammatory disease
• Previous peritonitis

30 patients in NHS Lothian with clinically diagnosed endometriosis due to undergo confirmatory laparoscopy in the subsequent four months will be recruited. All subjects recruited for the study will be able to withdraw from the study at any time. The patients will be referred to the study by their treating physicians to investigate clinically diagnosed endometriosis. We will not recruit or enroll healthy volunteers in this study.

Requests for anonymised data sharing will be considered on a case by case basis by the trial management team with appropriate data sharing contracts in place