Fibroids MRI: Uterine Imaging Guide

Uterine fibroids (leiomyomas) are the most common benign tumours in women of reproductive age, affecting up to 70-80% of women by age 50 — though many never cause symptoms. They are non-cancerous growths of the uterine muscle wall. MRI is the most accurate imaging test for fibroids, showing their number, size, exact location within the uterine wall, blood supply, and tissue composition — all of which directly determine treatment options.

If you have been told you have fibroids on ultrasound, or you are experiencing heavy periods, pelvic pressure, frequent urination, or fertility problems, an MRI provides the detailed map your gynaecologist needs to plan the best approach.

Understanding Fibroids

Fibroids are growths of smooth muscle and connective tissue in or on the uterus. They range from pea-sized to larger than a grapefruit. Their location determines which symptoms they cause:

• Intramural — within the uterine wall. The most common type. Can cause heavy periods, pelvic pressure, and pain when large.
• Submucosal — projecting into the uterine cavity. Even small submucosal fibroids can cause heavy, prolonged periods and fertility problems. Classified using the FIGO system: type 0 (entirely within the cavity), type 1 (less than 50% within the wall), type 2 (more than 50% within the wall).
• Subserosal — projecting outward from the uterine surface. Tend to cause pressure symptoms (bladder, bowel) rather than bleeding. Pedunculated subserosal fibroids hang from a stalk.
• Cervical — in the cervix. Uncommon but can cause difficulty with cervical smears and, rarely, obstruct delivery.

Risk Factors

• Age — most common in 30s and 40s, shrink after menopause
• Ethnicity — Black women are 2-3 times more likely to develop fibroids, develop them earlier, and have larger/more symptomatic fibroids
• Family history — 2-3x risk if a first-degree relative has fibroids
• Obesity — increased oestrogen levels promote fibroid growth
• Early menarche (first period before age 10)

What Fibroids Look Like on MRI

MRI provides superior fibroid characterisation compared to ultrasound:

• T2-weighted images — the primary diagnostic sequence. Typical fibroids appear dark (low signal) against the brighter uterine muscle. The radiologist documents each fibroid's location (intramural, submucosal, subserosal), size (three dimensions), and relationship to the endometrial cavity.
• Signal characteristics — typical fibroids are uniformly dark on T2. But fibroids can undergo degeneration:
  • Hyaline degeneration — the most common type. Fibroid appears homogeneously dark.
  • Cystic degeneration — areas of bright fluid signal within the fibroid.
  • Red (carneous) degeneration — haemorrhagic degeneration, common in pregnancy. Shows bright T1 signal.
  • Calcification — appears very dark on all sequences. Better seen on CT or X-ray.
• Contrast enhancement — gadolinium shows the blood supply to fibroids. Well-vascularised fibroids (enhancing brightly) respond better to uterine artery embolisation (UAE). Non-enhancing areas suggest degeneration or necrosis.
• Adenomyosis — often coexists with fibroids. MRI distinguishes between the two (important because treatment differs). Adenomyosis shows a thickened junctional zone with ill-defined borders, while fibroids have well-defined capsules.

Why MRI Before Treatment

MRI is specifically recommended before:

• Uterine artery embolisation (UAE) — MRI confirms fibroid location, rules out other pathology (adenomyosis, sarcoma), and assesses vascularity. NICE recommends MRI before UAE.
• MRI-guided focused ultrasound (MRgFUS/HIFU) — treatment planning relies entirely on MRI to target the fibroid and protect surrounding structures.
• Complex myomectomy — when multiple fibroids or difficult locations are involved, MRI mapping helps the surgeon plan the approach.
• Distinguishing fibroid from sarcoma — uterine sarcomas (malignant) are rare but can mimic fibroids. MRI features that raise concern include rapid growth, irregular margins, high T2 signal, areas of necrosis, and restricted diffusion on DWI. However, definitive distinction from a degenerate fibroid on imaging alone is not always possible.

What MRI Cannot Show

• Definitive exclusion of sarcoma — while MRI features can suggest or raise concern for sarcoma, definitive diagnosis requires tissue examination. However, uterine sarcomas are very rare (3-7 per 100,000 women per year).
• Functional impact on fertility — MRI shows fibroid location relative to the uterine cavity but cannot predict the specific impact on fertility. Fertility specialists combine MRI findings with hysteroscopy and other assessments.

What to Expect During the Scan

A pelvic MRI for fibroids takes 25-40 minutes. You lie on your back with a coil over your pelvis. A moderately full bladder is helpful. An antispasmodic injection (Buscopan) may be given to reduce bowel movement. If contrast is used, a cannula is placed in your arm.

Treatment Pathways

• Watchful waiting — many fibroids are asymptomatic and need no treatment. Monitoring with periodic ultrasound or MRI. Fibroids typically shrink after menopause.
• Medical management — hormonal treatments to manage symptoms:
  • Tranexamic acid and mefenamic acid for heavy bleeding
  • Combined oral contraceptive pill
  • Levonorgestrel IUS (Mirena coil) — effective for controlling bleeding from intramural fibroids
  • GnRH agonists (goserelin) — shrink fibroids temporarily by inducing a temporary menopause. Used pre-operatively to reduce fibroid size.
  • Ulipristal acetate — previously used but availability restricted due to liver safety concerns
• Uterine artery embolisation (UAE) — a minimally invasive radiological procedure. Tiny particles are injected through a catheter into the arteries feeding the fibroids, cutting off their blood supply. Fibroids shrink by 40-60% over 3-6 months. Day case or overnight. Good option for women wanting to avoid surgery. Not recommended when future pregnancy is a priority.
• Myomectomy — surgical removal of fibroids while preserving the uterus. Options include hysteroscopic (for submucosal fibroids), laparoscopic, or open myomectomy depending on fibroid size and location. Best option for women wanting future pregnancies.
• MRI-guided focused ultrasound (MRgFUS) — non-invasive. High-intensity ultrasound waves are focused on the fibroid under MRI guidance, heating and destroying the tissue. Available at limited UK centres.
• Hysterectomy — definitive treatment. Around 30,000 hysterectomies for fibroids are performed in the UK each year. Options include vaginal, laparoscopic, or open approach. Eliminates recurrence but permanently removes the uterus.

UK Statistics

• Fibroids affect up to 70-80% of women by age 50
• Around 30-50% of women with fibroids experience symptoms
• Fibroids are the most common indication for hysterectomy in the UK
• Black women are 2-3 times more likely to develop fibroids
• Around 30,000 hysterectomies for fibroids annually in England
• Average age at diagnosis: 30-40 years

Frequently Asked Questions

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