Gynaecological Ultrasound & MRI
Radiology · Genitourinary · lean revision notes
Gynaecological Ultrasound & MRI
Imaging of the female pelvis is dominated by ultrasound (USG) as the first-line modality, with MRI reserved as the problem-solving and staging tool. This topic blends radiological pattern recognition (fibroid types, endometrioma, PCOS) with the FIGO-relevant staging of endometrial and cervical carcinoma — a recurring NEET PG cross-over between Radiology, Obstetrics & Gynaecology, and Pathology.
Modalities & basic principles
The two workhorse ultrasound approaches are transabdominal (TAS) and transvaginal (TVS).
| Feature | Transabdominal (TAS) | Transvaginal (TVS) |
|---|---|---|
| Probe frequency | Low (3–5 MHz) | High (5–9 MHz) |
| Bladder | Full bladder needed (acoustic window) | Empty bladder |
| Field of view | Wide / panoramic | Limited but high resolution |
| Resolution | Lower | Higher (probe closer to organs) |
| Best for | Large masses, overview, virgins/children | Endometrium, ovaries, early pregnancy |
High-yield: TVS is the investigation of choice for the uterus and ovaries, early intrauterine pregnancy, ectopic pregnancy, and endometrial assessment. Use TAS first for very large masses that extend beyond the focal range of the vaginal probe.
MRI offers superior soft-tissue contrast and is the gold standard for local staging of cervical and endometrial carcinoma, characterising adnexal masses indeterminate on USG, mapping fibroids before myomectomy/embolisation, and diagnosing adenomyosis and Müllerian anomalies. CT has a limited role in gynaecology except for distant metastatic and nodal staging.
Uterine fibroids (leiomyoma)
Leiomyomas are the commonest benign uterine tumours, arising from smooth muscle. Classification by location is the most tested concept.
FIGO / location classification
| Type | Location | Clinical relevance |
|---|---|---|
| Submucosal | Bulge into endometrial cavity | Most symptomatic — menorrhagia, infertility, recurrent miscarriage |
| Intramural | Confined to myometrium | Commonest type; bulk symptoms |
| Subserosal | Project from outer surface | Pressure effects; can be pedunculated |
| Cervical / broad-ligament | Off-midline | May mimic adnexal mass |
High-yield: Submucosal fibroids are the most likely to cause abnormal uterine bleeding and infertility — assessed best by saline infusion sonohysterography (SIS) or hysteroscopy. A pedunculated subserosal fibroid can mimic an adnexal/ovarian mass; demonstrating a vascular bridging pedicle/stalk sign on Doppler confirms uterine origin.
USG appearance: well-defined, hypoechoic, whorled, heterogeneous myometrial mass causing posterior acoustic shadowing (calcific edge shadowing). Calcification gives a "popcorn" pattern; cystic change indicates degeneration.
Degenerations (classic exam list):
- Hyaline degeneration — commonest.
- Red (carneous) degeneration — typically in pregnancy; acute pain; on MRI shows T1 hyperintense rim due to haemorrhagic infarction.
- Cystic, myxoid, and calcific degeneration in older fibroids.
MRI: classically T2 hypointense sharply marginated masses — used to count, size, and localise fibroids before uterine artery embolisation (UAE) or myomectomy. A T2-hyperintense or rapidly enlarging "fibroid" should raise concern for leiomyosarcoma.
High-yield: A leiomyoma that is markedly T2 hyperintense, ill-defined, with restricted diffusion and rapid growth suggests malignant transformation to leiomyosarcoma — a key MRI red flag.
Adenomyosis (key differential of fibroid)
Adenomyosis = ectopic endometrial glands within the myometrium. It is a common cause of an enlarged, tender uterus with menorrhagia and dysmenorrhoea in a multiparous woman.
| Adenomyosis | Fibroid | |
|---|---|---|
| Margins | Ill-defined, infiltrative | Well-defined, round |
| Uterus | Diffusely bulky, globular | Lobulated, focal mass |
| USG | Heterogeneous myometrium, venetian-blind shadowing, myometrial cysts | Whorled mass, edge shadowing |
| MRI | Junctional zone > 12 mm (T2 dark), T2 bright foci | Discrete T2-dark mass |
High-yield: On MRI, a thickened junctional zone ≥ 12 mm is diagnostic of adenomyosis (normal < 5 mm; 8–12 mm equivocal). MRI is the investigation of choice to differentiate adenomyosis from a fibroid pre-operatively.
Endometrial assessment & thickness criteria
Endometrial thickness (ET) is measured on a midline sagittal TVS as the double-layer thickness (anterior + posterior endometrium together), excluding any intracavitary fluid.
Endometrial thickness cut-offs
| Setting | Endometrial thickness cut-off |
|---|---|
| Postmenopausal + bleeding | ≤ 4 mm → reassuring; > 4 mm → sample |
| Postmenopausal, no bleeding (incidental) | Up to ~8–11 mm may be observed (less consensus) |
| On tamoxifen | Often thickened/cystic → use bleeding, not thickness, to trigger biopsy |
High-yield: In a postmenopausal woman with bleeding, an endometrium > 4 mm mandates tissue sampling (pipelle/endometrial biopsy; hysteroscopy if focal). An ET ≤ 4 mm carries a very low risk of endometrial carcinoma.
Approach to a thickened endometrium / focal lesion:
TVS shows thickening → SIS or hysteroscopy to distinguish diffuse vs focal → focal lesion (polyp/submucosal fibroid) → hysteroscopic resection; diffuse thickening → endometrial biopsy → if malignant → MRI for local staging.
Endometrial polyps appear as echogenic intracavitary lesions with a single feeding vessel ("pedicle artery sign") on colour Doppler; they are well seen on SIS as the fluid outlines them.
Polycystic ovary syndrome (PCOS)
Diagnosis is by the Rotterdam criteria (2 of 3): oligo/anovulation, clinical or biochemical hyperandrogenism, and polycystic ovarian morphology (PCOM) on USG.
Ultrasound morphology (PCOM):
- ≥ 20 follicles per ovary (2–9 mm) on a modern high-frequency TVS probe OR ovarian volume ≥ 10 mL (in absence of a dominant follicle/corpus luteum).
- Peripherally arranged follicles around dense central stroma → "string of pearls" / necklace sign.
- Increased echogenic central stroma.
High-yield: The necklace / string-of-pearls sign = peripheral cysts around bright central stroma. The follicle-number threshold was revised upward to ≥ 20 (older texts say ≥ 12) because of improved probe resolution; ovarian volume ≥ 10 mL remains valid. Ultrasound is not required for diagnosis in adolescents (within 8 years of menarche).
Mnemonic — PCOS features: "HAIR-AN" overlap → Hyperandrogenism, Anovulation, Insulin Resistance, Acanthosis Nigricans. Imaging contributes the morphology arm of Rotterdam.
Adnexal / ovarian masses
Endometrioma ("chocolate cyst")
- USG: unilocular cyst with homogeneous low-level internal echoes = classic "ground-glass" appearance; no internal vascularity; may have hyperechoic wall foci.
- MRI: T1 hyperintense (and stays bright on T1 fat-sat — distinguishing it from a dermoid which suppresses) and "T2 shading" (loss of signal on T2 due to chronic blood products).
High-yield: Endometrioma = T1 bright + T2 shading. A mature cystic teratoma (dermoid) is also T1 bright but suppresses with fat saturation (contains fat) and shows a Rokitansky nodule + the "tip of the iceberg" / dermoid mesh sign on USG. This T1-fat-sat behaviour is the classic discriminator.
Mature cystic teratoma (dermoid)
- Fat-fluid level, hyperechoic mural nodule (Rokitansky / dermoid plug), and dense echogenic focus with shadowing ("tip of the iceberg" sign). Contains fat ± calcification (teeth).
Haemorrhagic cyst
- Reticular/"fishnet/lace-like" internal strands (fibrin) with a retracting clot; avascular. Typically resolves on follow-up.
O-RADS (risk stratification)
The O-RADS US lexicon stratifies adnexal lesions 0–5 by malignancy risk to standardise reporting and triage to MRI/oncology. Doppler features matter: central vascularity, thick septa, solid components, and ascites raise suspicion of malignancy.
Ovarian torsion — the emergency
Torsion of the ovary (± tube) on its vascular pedicle is a gynaecological emergency; delay risks ovarian necrosis and loss.
Imaging features (TVS + Doppler):
- Unilaterally enlarged, oedematous ovary (often > 4 cm), displaced to midline.
- Peripherally displaced follicles with central stromal oedema.
- "Whirlpool sign" — twisted vascular pedicle (twisting of ovarian vessels).
- Free pelvic fluid.
High-yield: Doppler is supportive but not definitive — because of dual ovarian blood supply (ovarian + uterine arteries), arterial flow may still be present in a viable, torsed ovary. Absent flow is specific but a late, ominous sign. Torsion is therefore a clinical + grey-scale diagnosis; normal Doppler does NOT exclude it. Definitive treatment = emergency laparoscopic detorsion (ovary-sparing, even if it looks dusky).
The commonest lead point in adults is a benign mass (often a mature cystic teratoma); in young girls torsion can occur with a normal ovary.
Other key entities
- Hydrosalpinx / pyosalpinx (PID/tubo-ovarian abscess): dilated tubular adnexal structure, incomplete septa, "cogwheel" sign (thickened folds in acute salpingitis), and "beads on a string" (mural nodules) in chronic disease.
- Ovarian (theca lutein/follicular) cysts, PCOS, and functional cysts — simple anechoic, thin-walled, posterior enhancement.
- Asherman syndrome: intrauterine adhesions, thin endometrium; SIS/hysteroscopy diagnostic.
- Müllerian anomalies: 3D USG and MRI are the modalities of choice to differentiate septate (good obstetric prognosis after metroplasty) from bicornuate uterus (external fundal contour cleft).
MRI staging of endometrial carcinoma
MRI is the best imaging for local (T) staging of endometrial cancer, principally to assess depth of myometrial invasion and cervical stromal involvement, which guide surgery and the need for lymphadenectomy.
| FIGO stage | MRI finding |
|---|---|
| IA | Tumour confined to endometrium / < 50% myometrial invasion |
| IB | ≥ 50% myometrial invasion |
| II | Cervical stromal invasion (not just endocervical glandular) |
| III | Serosa/adnexa, vaginal/parametrial, or nodal spread |
| IVA / IVB | Bladder/bowel mucosa; distant metastases |
High-yield: The single most important MRI determinant in endometrial cancer is whether myometrial invasion is < 50% vs ≥ 50% (IA vs IB) — best seen on dynamic contrast-enhanced (DCE) T1 and DWI. Deep (≥ 50%) invasion predicts nodal metastasis and worse prognosis. The tumour is typically intermediate signal on T2, disrupting the dark junctional zone.
MRI staging of cervical carcinoma
Cervical carcinoma is clinically staged (FIGO), but MRI is the best imaging modality for measuring tumour size, parametrial extension, and selecting surgery vs chemoradiation. FIGO 2018 incorporated imaging and pathology.
| FIGO 2018 stage | Key feature | MRI role |
|---|---|---|
| IA | Microscopic, ≤ 5 mm depth | Often occult on MRI |
| IB1–IB3 | Confined to cervix; subdivided by size (≤2, 2–4, >4 cm) | Tumour size measurement |
| II A | Upper 2/3 vagina, no parametrium | Vaginal extension |
| II B | Parametrial invasion | Disrupted low-signal cervical stromal ring |
| III A/B | Lower 1/3 vagina / pelvic wall / hydronephrosis | Ureteric/wall involvement |
| III C | Pelvic (IIIC1) or para-aortic (IIIC2) nodes | Nodal staging |
| IVA/IVB | Bladder/rectum mucosa; distant | Organ invasion |
High-yield: The intact low-signal (T2 dark) fibrous cervical stromal ring rules out parametrial invasion (high negative predictive value). Tumour breaching this ring = parametrial invasion (≥ IIB) → patient is inoperable, treated with chemoradiation rather than radical hysterectomy. This single MRI sign decides surgery vs radiotherapy.
Cervical tumour is T2 hyperintense against the dark stroma; DWI improves detection of small tumours and nodes.
Recently asked / exam angle
- Endometrioma vs dermoid on MRI → both T1 bright; dermoid suppresses on fat-sat, endometrioma shows T2 shading. (Repeated AIIMS/INI-CET flavour.)
- Investigation of choice style questions: TVS for uterus/ovaries & early pregnancy; MRI for endometrial/cervical staging and adenomyosis; SIS/hysteroscopy for focal endometrial lesions.
- Junctional zone ≥ 12 mm = adenomyosis — direct one-liner.
- Necklace/string-of-pearls = PCOS; updated follicle count ≥ 20.
- Whirlpool sign + enlarged oedematous ovary = torsion; Doppler flow may be preserved (favourite trap).
- Postmenopausal bleeding, ET > 4 mm → biopsy — recurring cut-off.
- Cervical cancer: intact stromal ring excludes parametrial invasion; breach → chemoradiation.
- Endometrial cancer: < 50% vs ≥ 50% myometrial invasion is the crucial MRI call.
- "Bridging vascular pedicle sign" → distinguishes pedunculated subserosal fibroid from ovarian mass.
- Red (carneous) degeneration of fibroid in pregnancy with T1 hyperintense rim.
Rapid revision
- TVS is the investigation of choice for uterus, ovaries, and early/ectopic pregnancy; empty bladder, high-frequency probe.
- Submucosal fibroids cause the most bleeding and infertility; assess with saline infusion sonohysterography.
- Bridging vascular pedicle sign confirms a pedunculated subserosal fibroid (uterine, not ovarian, origin).
- Red (carneous) degeneration of a fibroid occurs in pregnancy → T1 hyperintense rim; rapid T2-bright growth → suspect leiomyosarcoma.
- Junctional zone ≥ 12 mm on MRI = adenomyosis; uterus globular with venetian-blind shadowing.
- Postmenopausal bleeding + endometrium > 4 mm → endometrial sampling.
- PCOS: ≥ 20 follicles/ovary or volume ≥ 10 mL; peripheral necklace / string-of-pearls sign.
- Endometrioma: ground-glass echoes on USG; T1 bright + T2 shading; does NOT fat-suppress.
- Dermoid: Rokitansky nodule, fat-fluid level, "tip of the iceberg" sign; suppresses on fat-sat MRI.
- Ovarian torsion: enlarged oedematous ovary, peripheral follicles, whirlpool sign; arterial flow may persist — diagnosis is clinical, treatment is urgent detorsion.
- Endometrial cancer MRI: key call is < 50% (IA) vs ≥ 50% (IB) myometrial invasion using DCE + DWI.
- Cervical cancer MRI: intact dark stromal ring excludes parametrial invasion; breach (≥ IIB) → inoperable, chemoradiation.