MRI in Obstetrics and Gynaecology
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Direct answer
MRI is a cross-sectional imaging method that uses magnetic fields rather than ionising radiation, and in obstetrics and gynaecology it is mainly used to map placenta accreta spectrum, characterise adenomyosis and stage gynaecological cancers. Ultrasound remains the first-line test, and MRI is added only when its answer will change management. Unenhanced MRI is considered safe in pregnancy, whereas gadolinium contrast is generally avoided unless essential.
What you must remember
- Ultrasound first: MRI complements transabdominal and transvaginal ultrasound with Doppler and is requested when ultrasound is equivocal or when the extent of disease must be mapped.
- Placenta accreta spectrum: MRI shows dark intraplacental T2 bands, placental bulging, loss of the normal dark myometrial interface and interruption of the bladder wall, which help define depth of invasion.
- Adenomyosis: a junctional zone thickness above roughly 12 mm on T2-weighted images, together with ill-defined or heterogeneous myometrial foci, supports the diagnosis.
- Cervical cancer staging: MRI is the preferred imaging method for local extent, assessing tumour size, parametrial invasion, vaginal involvement and pelvic nodes.
- Endometrial cancer: MRI assesses the depth of myometrial invasion and detects cervical stromal involvement, both of which change surgical planning.
- Pregnancy safety: MRI uses no ionising radiation and no proven fetal harm has been documented at 1.5 T or below, but gadolinium crosses the placenta and is reserved for essential cases.
- Fetal MRI: performed after mid-pregnancy to refine anomalies first seen on ultrasound, mainly of the central nervous system, and sedation is rarely needed.
How MRI is chosen for a pelvic question
A pelvic mass or a suspected placenta accreta is the usual starting point. The request follows a fixed sequence, and each step decides whether MRI is worth doing at all.
- Ultrasound first: Transabdominal and transvaginal ultrasound with colour Doppler answers most first-line questions and needs no contrast or special access.
- Define the question: Name the unanswered point, such as the depth of placental invasion, parametrial spread or myometrial involvement in adenomyosis.
- Check safety: Confirm the gestation and screen for pacemakers, ferromagnetic implants and metallic foreign bodies before the scan.
- Choose the sequences: Use T2-weighted images for anatomy and tumour extent, add T1 and diffusion sequences for cancer work-up, and give contrast only if the answer cannot be obtained otherwise.
- Report for the decision: State the findings in the terms the clinician needs, such as tumour extent for cervical cancer or the depth of invasion in accreta.
- Plan the team: Use the result to arrange delivery, interventional radiology support or the surgical approach before treatment begins.
Where the exam tries to catch you
Vignettes often add an MRI option to a case that ultrasound already answers, or a gadolinium option to a pregnant patient.
- Trap: Choosing MRI as the first test for a pregnant woman with suspected accreta. Correct: Ultrasound with Doppler is first-line, and MRI is added to refine depth and extent when the ultrasound result is inconclusive.
- Trap: Assuming gadolinium is routine in pregnancy. Correct: Gadolinium crosses the placenta, so it is avoided unless the clinical question cannot be answered without it.
- Trap: Treating a thick junctional zone as diagnostic of adenomyosis on its own. Correct: A thick junctional zone supports the diagnosis, but fibroids and other myometrial lesions can mimic it, so the finding is read with the clinical picture.
Frequently asked questions
Is MRI safe during pregnancy?
MRI without gadolinium contrast is considered safe in pregnancy at all trimesters, because it uses no ionising radiation. Gadolinium contrast is generally avoided in pregnancy because it crosses the placenta, so it is reserved for cases where the clinical question cannot be answered otherwise.
Which MRI findings suggest placenta accreta spectrum?
MRI findings suggesting placenta accreta spectrum include dark intraplacental T2 bands, a bulging placenta, loss of the normal dark myometrial interface and interruption of the bladder wall. These features help assess the depth of invasion alongside ultrasound and guide delivery planning.
What junctional zone thickness on MRI suggests adenomyosis?
A junctional zone thickness above roughly 12 mm on T2-weighted MRI is the commonly used threshold for adenomyosis. Ill-defined myometrial foci and a heterogeneous appearance add weight to the diagnosis, which the radiologist interprets alongside the clinical history.
Why is MRI preferred for cervical cancer staging?
MRI is preferred for cervical cancer because soft-tissue contrast shows tumour size, parametrial invasion, vaginal involvement and pelvic nodes clearly. The local extent it defines helps decide between surgery and chemoradiation.
Does fetal MRI need maternal sedation?
Fetal MRI is usually performed without maternal sedation, because the sequences are fast and sedation is avoided in pregnancy where possible. It is typically done after mid-pregnancy to refine anomalies first detected on ultrasound, mainly of the central nervous system.