Acute flank pain — suspected renal colic

ACR AC Acute Onset Flank Pain (2023)

The rare indication where contrast is not merely unnecessary but actively destroys the diagnosis: opacified urine has the same density as a stone. Low-dose unenhanced CT is the protocol, and the forks are pregnancy and childhood, where ultrasound comes first.

Acute unilateral flank pain radiating to the groin, usually with haematuria, in a patient in whom a ureteric calculus is the leading diagnosis.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

Fever or sepsis

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

CT KUB — Low Dose Unenhanced

CT KUB (Non-contrast Stone Protocol)

What we'd amend, and why

A calculus is detected purely by its intrinsic attenuation against low-density urine, so the unenhanced acquisition is not a compromise, it is the diagnostic technique. Intravenous contrast is anti-signal for this question: excreted contrast raises the density of the urine around the stone to the same level and conceals it, which is why adding contrast to a stone protocol is a protocol error rather than a cautious extra. The same scan measures stone size and density, both of which determine whether it will pass and whether lithotripsy will work, and shows the secondary signs of obstruction.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwayadult — Adults
rulerule-paeds-dose — Child-sized technique and contrast dose; checked by Radiographer at the scanner
rulerule-pregnancy-ionising — Pregnancy status before an ionising exposure; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(2)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

Child-sized technique and contrast dose
Confirm that a size- or weight-based protocol is selected — child-sized kV and mAs against size-based diagnostic reference ranges — and that contrast volume is calculated by weight rather than taken from an adult default. Weight-based iodinated contrast volumes of roughly 1.5–2.0 mL/kg are widely used in paediatric CT.
Radiographer at the scannerAt the scanner
Flags back if: No paediatric or size-based protocol exists on the scanner for the requested examination, or the requested coverage or number of phases exceeds what the clinical question needs — for example a multiphase study where a single phase answers it, or whole-body coverage for a focal question.
Pregnancy status before an ionising exposure
Make the pregnancy enquiry immediately before the exposure and record the answer. In the UK this is a statutory operator duty discharged at the time of exposure under the employer’s written procedures required by IR(ME)R 2017 — it is not something the vetting radiologist can perform or pre-empt, and a request is complete without it.
Radiographer at the scannerAt the scanner
Flags back if: The patient states that she is, or may be, pregnant AND the uterus is in or near the primary beam. The exposure is then paused for re-justification by the IR(ME)R practitioner before it proceeds.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

Pregnant with fever — suspected obstructed infected system

Role	Study & protocol	Why this answers the question
First line	Ultrasound renal tract Ultrasound renal tract usually appropriate	Ultrasound stays first for the same reason it is first in any pregnancy: no radiation to the fetus, immediate availability, and it answers the management question directly — is the system obstructed. With fever that question is the whole examination, because an obstructed infected system is a decompression emergency rather than an imaging problem, and the treatment is a nephrostomy or a stent. Read it knowing the usual caveats still apply: the calculus itself is often not seen, and physiological right-sided hydronephrosis of pregnancy complicates the interpretation, so an equivocal study in a febrile patient escalates rather than reassures.
Second line	CT KUB (Non-contrast Stone Protocol) CT KUB — Low Dose Unenhanced	What fever changes is the rung after ultrasound. The card's pregnancy arm already places unenhanced MR urography between ultrasound and CT, and already states that CT is reserved for the pregnant patient in whom a clinically significant question is unresolved, particularly suspected obstructed infection where delay is dangerous. In a febrile patient the MRI rung costs time that the same sentence says is the greater risk: fetal dose from a low-dose unenhanced acquisition sits well below the deterministic threshold, an untreated obstructed infected system does not. So where ultrasound has not resolved obstruction in a febrile pregnant patient, go to the low-dose unenhanced CT rather than queueing for a magnet — and make the decompression pathway part of the same conversation.

Unenhanced MR urography remains a good study in a pregnant patient who is not septic, and it is where the standard pregnancy arm sends them. This arm drops it only because fever makes the delay the dominant risk.

Pregnant

Role	Study & protocol	Why this answers the question
First line	Ultrasound renal tract Ultrasound renal tract usually appropriate	No radiation to the fetus, and it answers the management question directly: is the system obstructed. The stone itself is often not seen, and physiological right-sided hydronephrosis of pregnancy complicates interpretation, so the study should be read alongside symptoms rather than as a stone hunt.
Second line	MRI abdomen and pelvis MRI abdomen/pelvis — non-contrast rapid protocol	Unenhanced MR urography shows the level of obstruction and the ureteric dilatation without ionising radiation, and distinguishes physiological from pathological dilatation better than ultrasound. It does not show the calculus itself, which is a signal void rather than a bright object.
Problem solving	CT KUB (Non-contrast Stone Protocol) CT KUB — Low Dose Unenhanced	Reserved for the pregnant patient in whom ultrasound and MRI have not resolved a clinically significant question, particularly suspected obstructed infection where delay is dangerous. Fetal dose from a low-dose unenhanced acquisition is well below the deterministic threshold, and an untreated obstructed infected system is the greater risk.

Children and adolescents

Role	Study & protocol	Why this answers the question
First line	Ultrasound renal tract Ultrasound renal tract	The slim body habitus that makes paediatric ultrasound informative also makes CT dose relatively more consequential over a lifetime. Ultrasound identifies hydronephrosis and many renal stones, and a substantial proportion of paediatric stone disease is managed on that basis alone.
Second line	CT KUB (Non-contrast Stone Protocol) CT KUB — Low Dose Unenhanced	Where ultrasound is non-diagnostic and the answer will change management, a size-adapted low-dose unenhanced acquisition is appropriate. Paediatric dose parameters are the point: an adult protocol on a child is the failure mode.

Fever with suspected obstructing stone

Role	Study & protocol	Why this answers the question
First line	CT KUB (Non-contrast Stone Protocol) CT KUB — Low Dose Unenhanced usually appropriate	Neither the study nor the protocol changes, and the reason is unchanged: a calculus is detected by its intrinsic attenuation against low-density urine, and excreted contrast raises the urine to the same density and conceals it. What changes is what the scan is for and how fast it has to happen. Fever with an obstructing calculus is an obstructed infected system until proven otherwise, and this card already states the consequence in its pregnancy arm — an untreated obstructed infected system is the greater risk, and delay is dangerous. The scan exists to confirm obstruction, locate the stone and measure it so that a nephrostomy or a retrograde stent can be placed, not to be reported at the end of the list; antibiotics alone do not treat pus under pressure. Two things follow. Book it now and with the decompression pathway already arranged. And if the question has moved from the stone to the kidney — a drainable collection, gas, failure to respond to antibiotics — that is a different question, and it is answered by a contrast-enhanced study rather than by this one.
Reasonable alternative	Ultrasound renal tract Ultrasound renal tract	Ultrasound answers the part of the question that has to be answered first — is the system obstructed — and it answers it at the bedside for a patient who should not be waiting in a queue or travelling far. It will not show most ureteric stones directly and cannot size or characterise them, so where the CT is available promptly it remains the better single study; where it is not, obstruction on ultrasound in a febrile patient is enough to start the decompression conversation.

This arm sits after the pregnancy and paediatric arms deliberately. A febrile pregnant patient and a febrile child both still start with ultrasound; fever changes what follows it rather than the first study.

Adults

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	CT KUB (Non-contrast Stone Protocol) CT KUB — Low Dose Unenhanced usually appropriate	A calculus is detected purely by its intrinsic attenuation against low-density urine, so the unenhanced acquisition is not a compromise, it is the diagnostic technique. Intravenous contrast is anti-signal for this question: excreted contrast raises the density of the urine around the stone to the same level and conceals it, which is why adding contrast to a stone protocol is a protocol error rather than a cautious extra. The same scan measures stone size and density, both of which determine whether it will pass and whether lithotripsy will work, and shows the secondary signs of obstruction.
Reasonable alternative	Ultrasound renal tract Ultrasound renal tract	A reasonable first test in a young adult with a typical history and a previous confirmed stone, where the aim is to confirm obstruction rather than to characterise a new diagnosis, accepting that most ureteric stones will not be seen directly.

Pitfalls

Adding intravenous contrast to a stone protocol, which hides the stone in the opacified system.
Cropping the coverage above the symphysis and missing a vesicoureteric junction calculus.
Calling a phlebolith a distal ureteric stone on axial images alone. Coronal tracking of the ureter resolves most of them.
Using the standard low-dose technique in a large patient, where noise makes a small stone undetectable — the benchmark assumes a standard-sized patient.
Missing a leaking aortic aneurysm presenting as first-episode flank pain in an older patient.
Treating obstruction of a solitary, transplanted or only-functioning kidney, or bilateral obstruction, as a routine stone report because the patient is afebrile. That is a decompression emergency driven by anuria and a rising creatinine, and the report has to say so rather than describing the stone and stopping.
Repeating a CT KUB within the same episode, or scanning every recurrence in a known stone former. Cumulative dose in a young recurrent patient is the dose problem on this card, and ultrasound answers an obstruction question without adding to it.
Poorly attenuating stones, including indinavir and some matrix stones, which can be invisible even on a technically perfect study.

Priors — what to pull first

A previous CT KUB tells you the stone burden, the usual composition and whether the current pain is a recurrence or something new.
Repeat imaging within a short interval for the same episode rarely changes management and is a common source of avoidable dose.

What makes a good request

Low-dose technique is the standard rather than a variant: published effective doses of around 3-4 mSv compare with roughly 10-12 mSv for a conventional unenhanced abdomen and pelvis, with sensitivity in the high nineties for stone disease.
Coverage must extend below the pubic symphysis. Vesicoureteric junction stones are the commonest obstructing site and are the ones cropped out by a study that stops at the symphysis.
A first presentation of flank pain in an older patient should be read with the aorta in mind — a leaking aneurysm presents as renal colic often enough that it is a named pitfall.
Painless visible haematuria is a different question with a different protocol, even though the request wording can look similar.
Fever is not the only thing that turns an obstructing stone into an emergency. Obstruction of a solitary or transplanted kidney, and bilateral obstruction, are decompression emergencies without any sepsis at all — the trigger there is anuria and a rising creatinine, the treatment is still a nephrostomy or a stent, and the report should say which kidney is functioning rather than simply describing the calculus.
Stone disease recurs and the patients are often young, so cumulative dose across repeated presentations is the real radiation problem here rather than any single scan. A patient with a previously confirmed stone presenting with their familiar pain does not automatically need another CT; where the question is obstruction rather than diagnosis, ultrasound answers it, and a repeat CT within the same episode almost never changes management.

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

Ultrasound renal tract CT KUB — Low Dose Unenhanced MRI abdomen/pelvis — non-contrast rapid protocol

Confirm locally

CT KUB — Low Dose Unenhanced: timings are typical — confirm against local protocol.
Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.