Suspected appendicitis at 26 weeks' gestation: MRI, CT, or repeat ultrasound?

MRI of the abdomen and pelvis without intravenous gadolinium.

Reasoning:

• Ultrasound is the correct first test but is non-diagnostic here, which happens in a substantial minority of pregnant patients, particularly in the third trimester when the appendix is displaced superiorly by the gravid uterus.
• MRI uses no ionising radiation, has excellent soft-tissue contrast, and in pregnancy has reported sensitivity and specificity in the mid-90s for appendicitis. It also gives alternative diagnoses - urinary obstruction, adnexal torsion, inflammatory bowel disease, placental complications - which is valuable because the differential in pregnancy is wide.
• It is available within 45 minutes. Time matters: perforation risk rises with delay, and both maternal and fetal morbidity rise sharply with perforated appendicitis.
• Repeat ultrasound by a second operator is a legitimate alternative if MRI were not available, but here it would add delay for a modest gain.

CT is reserved for the situation where MRI is unavailable or non-diagnostic and the result will change management.

Justification

• A focused appendiceal CT delivers a fetal dose broadly in the range of 10–35 mGy, far below the 100 mGy threshold for deterministic effects (malformation, intellectual disability). The relevant risk is a small stochastic increase in childhood cancer, and it has to be weighed against the real risk of a perforated appendix to both mother and fetus.
• The examination is justified by the referrer and the radiologist together, documented, and performed because the answer will change what happens next. 'To be safe' is not a justification.

Optimisation

• Single portal venous phase only - no unenhanced, no delayed series.
• Limit the z-axis to the region of interest rather than scanning the whole abdomen and pelvis.
• Low kVp with automatic exposure control and iterative reconstruction; keep the pitch high enough to shorten the scan.
• No oral contrast (adds delay and dose without diagnostic gain); IV contrast can be given if needed - pregnancy is not a contraindication to iodinated contrast.
• Radiologist present or immediately available; review on the console before the patient leaves the department.

Gadolinium: no - not needed for this question.

• Gadolinium crosses the placenta and enters the fetal circulation and amniotic fluid; the free ligand can persist in the amniotic sac. No human teratogenicity has been demonstrated, but the data are limited, and the ACR advises that gadolinium be used in pregnancy only when it is essential and would change management.
• For suspected appendicitis, non-contrast MRI (T2, SSFSE, DWI, T1) is sufficient; an inflamed appendix is visible without gadolinium. I would only use it if I were looking for something that genuinely requires enhancement and there were no alternative.

Iodinated contrast: acceptable if CT is performed.

• The ACR does not regard pregnancy as a contraindication to iodinated contrast; the fetal thyroid does not concentrate iodine until around 13 weeks and clinically significant neonatal hypothyroidism after a single maternal dose has not been established.
• Breastfeeding does not need to be interrupted after iodinated contrast.
• Both decisions should be recorded in the notes with the reasoning, since the commonest failing in this scenario is a reflexive 'no contrast because pregnant' that produces a non-diagnostic scan and a repeat exposure.