CT KUB Request in a 19-Week Pregnant Patient

The request is reasonable but the pathway is not automatic, and "just do it" is not a justification.

Before scanning:

• Speak directly to the emergency physician and the radiologist on duty. Confirm the clinical question (obstructing stone with or without obstruction/infection) and confirm that hCG status and gestational age are documented.
• Record that the discussion happened and who made the decision.

Alternatives, in order of increasing fetal dose:

1. Ultrasound - renal and bladder US first. It is radiation-free and usually available at night; its weakness is that physiological hydronephrosis of pregnancy can mimic obstruction and it may miss a distal ureteric stone.
2. MRI without gadolinium - excellent for hydronephrosis and stone as a dark filling defect; limited by access, time, and a patient who is vomiting.
3. Low-dose CT KUB - the most sensitive and specific test, and often the one that actually ends the diagnostic loop instead of repeating tests.

The wrong answer is a reflex ultrasound that leaves the question unanswered, followed by a CT two days later with a worse clinical picture. The right answer is a documented, deliberate choice.

I answer honestly, in plain language, without either minimising or frightening her.

• The dose is small and there is no deterministic risk. A modern low-dose CT KUB delivers a fetal dose in the order of a few milligray to roughly ten to twenty milligray depending on protocol and maternal habitus - well below the 50–100 mGy region associated with deterministic effects such as malformation. The remainder is a very small stochastic risk that cannot be measured in an individual.
• The bigger risk is the untreated stone - obstruction plus infection in pregnancy causes preterm labour and sepsis. That is why the test is being considered at all.
• Shielding does not do what people think. A lead apron over the abdomen does not stop internal scatter generated inside her own body, and modern practice (ACR and others) no longer uses routine gonadal or abdominal shielding; it can even increase dose by forcing repeat exposures when it degrades the image. I would say this plainly rather than draping lead on to look reassuring.
• I tell her the alternative exists and let her be part of the decision, then document her consent and what she was told.

Protocol - the goal is the answer at the lowest dose, first time:

• Limit the z-axis range to the kidneys through the bladder base; no routine coverage above the upper poles.
• Low fixed mAs / automated exposure control tuned down, low kVp setting within the diagnostic range, and iterative reconstruction - noise is cheaper than dose.
• No oral or IV contrast; stone disease needs an unenhanced scan.
• Single acquisition, no repeat phases, and a scout that is itself tight.
• Positioning supine, arms up, with clear instruction to hold still; motion is the main cause of a repeat.

When I would stop:

• If the requisition or history changes - e.g. she becomes febrile or septic, in which case decompression (nephrostomy or stent) may be the priority and the imaging question changes.
• If the actual scan range must extend far beyond the KUB (e.g. a suspected perforation), because the dose estimate I gave her no longer applies.
• If the radiologist is not contactable and the request is being pushed through by workflow pressure rather than clinical need - that is a governance failure, and I escalate rather than absorb it.