An adnexal mass is anything growing in or near an ovary or fallopian tube. Most are found by chance on a pelvic ultrasound, done because of pain, bleeding, subfertility or an unrelated scan. The ultrasound answers the first question - is there something there. Pelvic MRI answers the harder second question - what is it, and does it need to come out.
This piece is not written to alarm you. It is what actually happens in the UK in 2026 when an ultrasound flags an adnexal mass, how the MRI fits into the pathway, what your radiologist is looking at on the images, and how the O-RADS MRI score translates into the plan you leave the clinic with.
One-line answer
Ultrasound finds the mass. Pelvic MRI tells you what it is - simple cyst, endometrioma, dermoid, fibroma or something suspicious - and gives the gynaecologist an O-RADS score that decides whether you watch, refer, biopsy or operate.
The ultrasound-first pathway
Every UK pathway for an adnexal mass starts with transvaginal ultrasound. It is quick, radiation-free, and in experienced hands answers the question for most women without any further imaging. The scan is reported using the IOTA (International Ovarian Tumour Analysis) rules, which classify a lesion as benign, malignant or indeterminate on the basis of size, wall thickness, septations, solid components and blood flow.
A simple, thin-walled, unilocular cyst under 5 cm in a pre-menopausal woman is almost always physiological and needs nothing more than a repeat scan in six to twelve weeks. A complex mass with solid tissue, thick septations, ascites or Doppler flow inside solid areas is a different conversation - one that usually now includes pelvic MRI.
| Ultrasound finding | IOTA classification | Typical next step |
|---|---|---|
| Simple thin-walled cyst under 5 cm | Benign | Repeat ultrasound in 6 to 12 weeks |
| Cyst 5 to 7 cm, no worrying features | Benign | Surveillance, or MRI if persistent |
| Cyst with septations or solid area | Indeterminate | Pelvic MRI for characterisation |
| Mass with Doppler flow inside solid tissue | Suspicious | MRI plus gynae-oncology referral |
| Post-menopausal cyst of any size | Indeterminate | MRI and CA-125 blood test |
When MRI is added
Pelvic MRI is not the first-line test, and most women with an ovarian cyst will never need one. It is added when the ultrasound cannot answer the question on its own. The recognised UK triggers in 2026 are:
- Indeterminate ultrasound. The lesion is not clearly benign and not clearly malignant on ultrasound. MRI usually resolves it.
- Large mass, over 7 to 10 cm. Ultrasound cannot capture the whole thing, and the origin (ovarian, tubal, uterine or bowel) needs sorting out before surgery.
- Suspected endometrioma. Blood products inside a cyst have a very specific MRI signature, and MRI is much better than ultrasound for confirming the diagnosis.
- Possible malignancy. MRI with contrast is the accepted problem-solver for lesions the gynae-oncologist is worried about, and it drives the O-RADS score.
- Pre-operative planning. Before surgery for a known complex mass, MRI maps the surrounding anatomy - bladder, bowel, ureters, peritoneum - so the operation can be planned properly.
The pelvic MRI protocol
A pelvic MRI for an adnexal mass is a specific set of sequences, not just a generic scan of the pelvis. When the report you receive lists the sequences, it usually looks like this:
- High-resolution T2-weighted imaging, with and without fat saturation. This is the workhorse sequence. It shows the internal architecture of the mass - fluid, septations, solid nodules, wall thickness. Fat saturation helps distinguish fatty tissue (a dermoid) from other bright material.
- T1-weighted in-phase and out-of-phase imaging. This differentiates fat from blood. A dermoid drops signal on fat-saturated sequences. An endometrioma stays bright on T1 because of blood breakdown products.
- Diffusion-weighted imaging (DWI). Solid areas that restrict diffusion are more likely to be malignant. It is a key input into the O-RADS score.
- Dynamic contrast-enhanced imaging with gadolinium. Added when malignancy is suspected or a solid component needs assessment. The pattern and speed of enhancement is one of the most powerful malignancy predictors on MRI.
You will be asked to lie on your back for 30 to 45 minutes. Most women tolerate the scan well. Bring headphones-friendly music if the clinic offers it, and let the radiographer know if you get claustrophobic - a mild sedative is available at most private centres if you ask ahead.
The five common MRI diagnoses
Nine times out of ten, a pelvic MRI for an adnexal mass will conclude with one of five diagnoses. Knowing what they mean helps you read the report:
- Simple functional cyst. Thin-walled, uniformly fluid, no solid tissue, no enhancement. Almost always benign. Managed with a repeat scan or nothing at all.
- Endometrioma. A cyst filled with old blood, related to endometriosis. The classic MRI sign is high T1 signal that stays bright on fat-saturated sequences (T2 shading). Very common, usually benign, sometimes needs surgery if symptomatic.
- Mature cystic teratoma (dermoid). A benign tumour containing fat, and sometimes hair, teeth or bone. MRI is the best test to confirm it - the fat drops out on fat-saturated sequences in a very characteristic way. Usually removed surgically because of a small risk of ovarian torsion.
- Fibroma or thecoma. Solid, fibrous benign tumour of the ovary. Very dark on T2. Sometimes associated with pelvic fluid. Benign, but often removed for certainty.
- Borderline or malignant epithelial tumour. Complex mass with solid components, thick septations, restricted diffusion and early strong enhancement after contrast. This is where the O-RADS score becomes critical, and where a gynae-oncology referral follows.
Most adnexal masses are not cancer. The job of pelvic MRI is not to find cancer - it is to be honest about which lesions we can safely leave alone, and which ones deserve the operating theatre.
