Ovarian Cyst MRI: Imaging and Diagnosis

Ovarian cysts are extremely common — most women will develop at least one during their lifetime. The vast majority are functional (related to the normal menstrual cycle) and resolve on their own within a few weeks. But when a cyst is large, persistent, complex-looking on ultrasound, or causing symptoms, MRI is the best imaging tool for determining whether it is benign or potentially cancerous — often avoiding unnecessary surgery.

If you have been told you have an ovarian cyst on ultrasound and your doctor wants a closer look, or if you have pelvic pain, bloating, or irregular periods and need an answer, an MRI can characterise the cyst with high accuracy and guide the right next step.

Types of Ovarian Cysts

Functional Cysts (Benign)

• Follicular cysts — the most common type. A follicle grows but does not rupture to release an egg. Usually less than 5cm. Almost always resolve spontaneously within 1-3 menstrual cycles.
• Corpus luteum cysts — form after the egg has been released. Can contain blood (haemorrhagic corpus luteum). Usually resolve within a few weeks. Can cause sudden, severe pain if they rupture or bleed.

Benign Pathological Cysts

• Endometriomas (chocolate cysts) — cysts filled with old blood, caused by endometriosis. Found in 20-40% of women with endometriosis. MRI shows them clearly: bright on T1 (blood products), characteristic "shading" (layered dark signal) on T2. Often bilateral.
• Dermoid cysts (mature cystic teratomas) — contain a bizarre mixture of tissues: fat, hair, teeth, skin, and sometimes bone. The most common ovarian tumour in women under 30. MRI shows fat (bright on T1, signal loss on fat-saturated images), which is the diagnostic clue. Usually benign, but 1-2% can undergo malignant transformation (usually in older women).
• Cystadenomas — serous (thin-walled, clear fluid) or mucinous (thick-walled, mucin-filled, can be very large). Both are usually benign but can occasionally be borderline or malignant.
• Paraovarian/paratubal cysts — arise near the ovary, not from it. Usually simple and benign. MRI can distinguish these from true ovarian cysts by showing a separate normal ovary.

Borderline and Malignant Tumours

• Borderline ovarian tumours — "low malignant potential." Have some abnormal features (papillary projections, thick septations) but do not invade surrounding tissues. Good prognosis. Usually treated with surgery alone.
• Ovarian cancer — affects around 7,500 women in the UK each year. MRI features that raise concern: solid components, thick irregular septations (over 3mm), papillary projections, ascites (fluid in the abdomen), peritoneal nodularity, contrast enhancement of solid components.

How MRI Characterises Ovarian Cysts

MRI excels at "tissue characterisation" — working out what a cyst contains based on its signal characteristics:

• Simple fluid — dark on T1, bright on T2. Benign.
• Blood products — bright on T1 (endometrioma, haemorrhagic cyst). T1 fat-sat confirms blood (stays bright) vs fat (goes dark).
• Fat — bright on T1, goes dark on fat-saturated images. Dermoid cyst.
• Mucin — intermediate T1 signal, variable T2. Mucinous cystadenoma.
• Solid tissue — intermediate signal, enhances with contrast. The presence of enhancing solid components is the most important feature for distinguishing benign from malignant.

O-RADS MRI Scoring

Ovarian cysts are increasingly scored using the O-RADS (Ovarian-Adnexal Reporting and Data System) MRI classification:

• O-RADS 1 — normal ovary
• O-RADS 2 — almost certainly benign (less than 1% malignancy risk)
• O-RADS 3 — low malignancy risk (1-10%)
• O-RADS 4 — intermediate malignancy risk (10-50%)
• O-RADS 5 — high malignancy risk (over 50%)

What to Expect During the Scan

A pelvic MRI for ovarian cysts takes 25-40 minutes. You lie on your back with a coil over your pelvis. A moderately full bladder is helpful. An antispasmodic (Buscopan) may be given. Gadolinium contrast is usually used to assess enhancement of solid components.

Preparation

• Timing — ideally during days 7-14 of your menstrual cycle (when functional cysts are least likely to confuse the picture)
• Fasting for 4-6 hours may be requested if contrast is planned
• Kidney function test (eGFR) may be needed for contrast

What MRI Cannot Show

• Definitive tissue diagnosis — MRI can characterise a cyst with high accuracy but cannot provide a histological diagnosis. If a cyst has suspicious features (O-RADS 4-5), surgery is needed for definitive diagnosis.
• Tiny peritoneal deposits — small peritoneal implants (less than a few mm) from ovarian cancer may be below MRI resolution. CT or staging laparoscopy may be needed for full staging.
• Function — MRI cannot tell you whether a cyst is affecting your fertility or hormone production. Clinical assessment and blood tests (AMH, hormonal profiles) provide this information.

Alternative Diagnostic Approaches

• Transvaginal ultrasound (TVS) — the first-line investigation for ovarian cysts. Available in most gynaecology clinics. Can characterise most cysts accurately (simple cysts, endometriomas, dermoids). The IOTA (International Ovarian Tumour Analysis) rules help distinguish benign from malignant. MRI is typically used when ultrasound findings are indeterminate.
• CA-125 blood test — a tumour marker that can be raised in ovarian cancer but also in endometriosis, fibroids, pregnancy, menstruation, and other conditions. Most useful in post-menopausal women with a complex cyst. The RMI (Risk of Malignancy Index) combines CA-125, ultrasound score, and menopausal status.
• HE4 and ROMA score — newer biomarkers that improve specificity alongside CA-125.
• CT — used for staging known ovarian cancer (chest/abdomen/pelvis) but not for initial characterisation of ovarian cysts.

Treatment Pathways

• Watchful waiting — for simple cysts under 5cm in pre-menopausal women. Repeat ultrasound in 8-12 weeks to confirm resolution. Most disappear without treatment.
• Hormonal treatment — the combined pill does not treat existing cysts but may prevent new functional cysts forming.
• Surgery:
  • Cystectomy — removing the cyst while preserving the ovary. Preferred for benign cysts in women wanting to preserve fertility. Laparoscopic (keyhole) approach for most.
  • Oophorectomy — removing the entire ovary. For larger, complex, or recurrent cysts. For post-menopausal women with complex cysts, bilateral salpingo-oophorectomy (removing both ovaries and tubes) is standard.
  • Cancer surgery — if ovarian cancer is confirmed, staging surgery includes total hysterectomy, bilateral salpingo-oophorectomy, omentectomy, and lymph node sampling. Performed by a specialist gynaecological oncologist.
• Ovarian cancer treatment — surgery followed by platinum-based chemotherapy (carboplatin + paclitaxel) for most patients. PARP inhibitors (olaparib, niraparib) for maintenance therapy, particularly in BRCA-mutated cancers.

UK Statistics

• Ovarian cysts are found in approximately 7% of pre-menopausal and 18% of post-menopausal women on imaging
• The vast majority (over 95%) are benign
• Ovarian cancer affects around 7,500 women per year in the UK
• Ovarian cancer 5-year survival: approximately 45% overall; 93% for stage 1
• Dermoid cysts account for 10-20% of all ovarian tumours
• Endometriomas affect 20-40% of women with endometriosis

Frequently Asked Questions

Related Articles

• Endometriosis MRI
• Fibroids MRI
• Can an MRI Detect Cancer?