MRI for PCOS (Polycystic Ovary Syndrome)

What Is PCOS?

Polycystic ovary syndrome (PCOS) is the most common endocrine disorder affecting women of reproductive age. Despite its name, the "cysts" are actually immature follicles — small fluid-filled sacs containing eggs that haven't developed fully. They're not true cysts and don't need surgical removal.

PCOS is diagnosed using the Rotterdam criteria when two of the following three features are present: irregular or absent periods (oligo/anovulation), clinical or biochemical signs of excess androgens (acne, excess hair growth, elevated testosterone), and polycystic ovarian morphology on imaging.

PCOS affects approximately 1 in 10 women in the UK — roughly 1.5 million. However, many cases go undiagnosed. A 2023 survey by Verity (the UK PCOS charity) found that the average time to diagnosis was over two years, with many women seeing multiple doctors before getting answers.

Beyond reproductive health, PCOS is associated with insulin resistance, type 2 diabetes, cardiovascular risk, non-alcoholic fatty liver disease, and mental health conditions including anxiety and depression.

Why MRI Is Used for PCOS

Ultrasound (typically transvaginal) is the standard first-line imaging for PCOS. So when does MRI come into the picture?

• Inconclusive ultrasound: In women with a high BMI, ultrasound image quality can be limited. MRI provides clearer visualisation of the ovaries regardless of body habitus.
• Adolescents: Transvaginal ultrasound may not be appropriate for younger patients. MRI offers a non-invasive alternative that doesn't require vaginal probe insertion.
• Distinguishing PCOS from other ovarian pathology: If ultrasound shows a complex or atypical ovarian appearance, MRI can differentiate polycystic ovarian morphology from ovarian tumours, endometriomas, or other cystic lesions.
• Assessing PCOS complications: MRI can evaluate associated conditions — particularly non-alcoholic fatty liver disease and pelvic pathology — in a single examination.
• Pre-surgical planning: Before ovarian drilling (a laparoscopic procedure for PCOS-related infertility), MRI maps ovarian anatomy in detail.
• Research and accurate follicle counting: MRI provides more accurate antral follicle counts than ultrasound, particularly for deeply positioned ovaries or when follicles overlap on ultrasound images.

MRI Sequences Used

• T2-weighted axial and sagittal: The primary diagnostic sequence. Ovarian follicles appear as bright (high-signal) round structures against the darker ovarian stroma. T2 images show follicle number, size, and distribution with excellent contrast.
• T1-weighted axial: Provides anatomical reference and helps characterise any atypical ovarian lesions. Blood (as in endometriomas) appears bright on T1.
• T1 fat-saturated (pre- and post-contrast if used): Helps identify fat-containing lesions (dermoid cysts) and, if gadolinium is given, shows ovarian stromal vascularity — increased in PCOS.
• DWI (Diffusion-Weighted Imaging): Not routinely used for PCOS diagnosis but adds value if there's concern about a solid ovarian lesion.
• 3D volumetric sequences: Allow precise measurement of ovarian volume, which is one of the diagnostic criteria.

What Radiologists Look For

• Follicle count: The 2018 international evidence-based guideline recommends a threshold of 20 or more follicles (measuring 2-9mm) per ovary on ultrasound using modern probes (≥8MHz). On MRI, similar criteria apply, though MRI typically counts slightly more follicles due to its superior resolution.
• Follicle distribution: In PCOS, follicles characteristically arrange in a peripheral "string of pearls" pattern around a dense central stroma. On MRI T2 images, this pattern is clearly visible.
• Ovarian volume: Greater than 10ml per ovary (calculated as length × width × depth × 0.523). Enlarged ovaries support the diagnosis but aren't required — follicle count alone is sufficient.
• Stromal appearance: In PCOS, the ovarian stroma (the tissue between follicles) is often increased in volume and shows lower T2 signal than normal. On contrast-enhanced images, increased stromal vascularity may be seen.
• Dominant follicle or corpus luteum: The presence of a recent dominant follicle or corpus luteum suggests recent ovulation — relevant for assessing anovulation.
• Exclusion of other pathology: Endometriomas, dermoid cysts, cystadenomas, and rarely ovarian tumours need to be differentiated from polycystic ovarian morphology.

How to Prepare for the Scan

• No fasting required for a standard pelvic MRI
• A moderately full bladder improves image quality — drink a glass or two of water about an hour before the scan, but you don't need to be uncomfortably full
• Timing in the menstrual cycle isn't strictly necessary for PCOS assessment, but if you're still having periods, days 3-5 of the cycle (early follicular phase) provides the most standardised comparison
• Remove all metal jewellery, body piercings, and clothing with metal components
• An antispasmodic injection (Buscopan) may be offered to reduce bowel movement during the scan

What Happens During the Scan

You'll lie on your back on the scanner table. A surface coil (a flat, flexible pad) is placed over your lower abdomen and pelvis. The scan takes approximately 25-35 minutes.

You'll be asked to hold your breath for short periods (10-20 seconds) during some sequences to minimise movement from breathing. The radiographer will talk you through each breath-hold over the intercom.

Contrast injection (gadolinium) is not always needed for PCOS assessment. If used, a cannula is placed in your arm before the scan.

The scan is painless, and unlike transvaginal ultrasound, nothing is inserted. For many women who've found ultrasound assessments uncomfortable or distressing, MRI offers a welcome alternative.

What MRI Can't Show

• Hormone levels: MRI shows ovarian morphology, not biochemistry. You still need blood tests for testosterone, SHBG, LH, FSH, insulin, HbA1c, and thyroid function to complete the PCOS assessment.
• Ovulation in real-time: MRI is a snapshot. Tracking whether and when you ovulate requires serial ultrasound or hormonal monitoring.
• Fallopian tube patency: If you're investigating fertility, MRI doesn't reliably assess whether your fallopian tubes are open. A hysterosalpingogram (HSG) or HyCoSy ultrasound is needed for that.
• Endometrial receptivity: While MRI shows endometrial thickness and appearance, it doesn't assess endometrial receptivity for implantation — relevant for fertility planning.

Treatment Pathways After Diagnosis

• Lifestyle modification: Weight management through diet and exercise improves insulin sensitivity, can restore ovulation, and reduces cardiovascular risk. Even a 5-10% weight loss can produce meaningful improvements in symptoms and fertility.
• Combined oral contraceptive pill: For women not trying to conceive, the pill regulates periods and reduces androgen-driven symptoms (acne, hirsutism). Certain pills containing anti-androgenic progestogens (e.g. co-cyprindiol) are particularly effective.
• Metformin: An insulin-sensitising drug that can help with weight management, menstrual regularity, and ovulation. Often used alongside lifestyle changes.
• Fertility treatment: Letrozole is now recommended as first-line ovulation induction (over clomifene) by international guidelines. If this fails, gonadotrophin injections or IVF may be considered.
• Anti-androgen treatments: Spironolactone for hirsutism and acne (must be used with contraception). Topical treatments for acne and eflornithine cream for facial hair.
• Psychological support: PCOS significantly affects mental health. NICE recommends screening for anxiety and depression and offering psychological support as part of routine PCOS management.

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