Suspected intra-abdominal abscess or collection

ACR AC Acute Nonlocalized Abdominal Pain; ACR/NKF 2020; Surviving Sepsis Campaign 2021; WSES intra-abdominal infections 2017; Image Gently current guidance

Confirmation and mapping of a suspected intra-abdominal collection, usually to answer whether there is a drainable target. The vetting decision is almost always about contrast, because the diagnosis depends on it.

Febrile or septic patient with abdominal signs, raised inflammatory markers, often days after surgery or an episode of diverticulitis.

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 Abdomen and Pelvis — Portal Venous Phase

CT Abdomen and Pelvis

What we'd amend, and why

Portal-venous IV contrast is what makes an abscess visible: the rim enhances while the contents do not, and that difference is the diagnosis. An unenhanced study frequently cannot separate a collection from adjacent unopacified bowel, which is the single commonest reason this scan gets repeated. Two things follow that are routinely got wrong. A single portal-venous pass at roughly sixty to eighty seconds answers the question — arterial and delayed series add dose without yield for a collection. And routine positive oral contrast is not part of this protocol: intravenous contrast alone is of equivalent accuracy, the sixty to ninety minute preparation is a real delay in a patient who may be septic or heading for theatre, and dense luminal contrast forecloses converting the study to an angiographic or urographic protocol if the picture changes. Oral contrast is a deliberate choice for a named question — a suspected enteric leak or fistula, or separating an interloop collection from unopacified bowel in a complex post-operative abdomen — and it is withheld entirely from anyone with a reduced conscious level or an unprotected airway.

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

How was this decided?

pathwayadult — Adults
rulerule-contrast-reaction-premed — Prior contrast reaction and elective premedication; checked by Nurse before the scan
rulerule-metformin — Metformin and iodinated contrast; checked by Radiographer at the scanner
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
rulerule-renal-iodinated — Kidney function and intravenous iodinated contrast; checked by Radiographer at the scanner
rulerule-iv-access — Intravenous access adequate for the planned injection; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(4)nothing for you to do

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

Metformin and iodinated contrast
Confirm whether the patient takes metformin or a metformin-containing combination, and if so whether ACR Category II applies (eGFR below 30, known or suspected AKI, or an arterial catheter study likely to cause renal embolisation). If Category I — that is, no AKI and eGFR at or above 30 — no action of any kind is needed.
Radiographer at the scannerAt the scanner
Flags back if: The patient takes metformin AND meets ACR Category II — eGFR below 30 mL/min/1.73 m2, known or suspected acute kidney injury, or an arterial catheter procedure with likely renal arterial embolisation. Metformin plus a normal or mildly reduced eGFR is explicitly NOT a flag-back: there is no need to stop metformin before or after intravenous iodinated contrast in Category I patients, and no need to re-check creatinine afterwards.
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.
Intravenous access adequate for the planned injection
Site and test a cannula that supports the protocol flow rate, preferring an antecubital or large forearm vein, and observe the injection for extravasation. A 20-gauge or larger cannula is preferred for flow rates of 3 mL/s or more.
Radiographer at the scannerAt the scanner
Flags back if: No cannula can be sited that supports the protocol flow rate — for example only a 22-gauge hand or foot cannula for a CT angiogram needing 4–5 mL/s; or the only available access is a central line or port that is not labelled power-injectable; or an extravasation occurs.

Pathways

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

Children and adolescents under 16

Role	Study & protocol	Why this answers the question
First line	Paediatric abdominal ultrasound Paediatric abdominal ultrasound usually appropriate	Ultrasound is the first study in a child, and this is not a token gesture towards dose. A child has a slim abdomen, little intervening fat and a compliant belly wall, so the collections this card is about — pelvic, right iliac fossa, subphrenic, and the post-appendicectomy collection that is the commonest paediatric version of this request — are precisely the ones ultrasound sees well. It also guides the drain, needs no cannula, no sedation and no iodine, and can be repeated the next morning without a second dose conversation. What it must not do is close the question by default: a technically limited or negative ultrasound in a child who is still septic is a non-diagnostic study, and the pathway escalates that same day rather than waiting to see. Interloop, retroperitoneal and deep pelvic collections, and a gassy post-operative abdomen, are where it fails.
Second line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase	Contrast-enhanced CT is the answer for the child whose ultrasound is non-diagnostic, whose source is not localised, or who is deteriorating — and in that child it happens now, not tomorrow. Nothing about paediatric dose optimisation justifies withholding a study that is going to change whether a collection gets drained, and a septic child who has had a negative ultrasound and no CT is the failure mode of a dose-conscious pathway. Where it is done it is a single portal-venous pass with child-sized kV and mAs against size-based reference ranges and a weight-calculated contrast volume — never a multiphase study, and never with routine positive oral contrast, which adds an hour and a half of preparation to a septic child and must be withheld outright from any child who may go to theatre.
Problem solving	MRI abdomen and pelvis MRI abdomen/pelvis — non-contrast rapid protocol	MRI resolves an equivocal ultrasound without radiation in a stable child, and diffusion-weighted imaging separates a sterile post-operative seroma from pus about as well as anything does. Its limits are practical rather than diagnostic: access out of hours, acquisition length, and the sedation or anaesthesia a young child needs to hold still for it. Those limits are exactly why it is not the answer for a septic or deteriorating child — do not queue a sick child for an MRI slot when a CT would have answered the question in the next ten minutes.

This arm re-sequences; it never withholds. A child who is septic with a non-diagnostic ultrasound has the CT the same day, and saying so is the point of separating the two rungs.
A child compensates and then decompensates abruptly. Tachycardia, perfusion and the trend in inflammatory markers drive the escalation, not the blood pressure.

Febrile or septic, source not localised

Role	Study & protocol	Why this answers the question
First line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase usually appropriate	Same acquisition as for any suspected collection, and for the same reason — portal-venous IV contrast is what makes an abscess visible, because the rim enhances while the contents do not, and an unenhanced study frequently cannot separate a collection from adjacent unopacified bowel. What a declared fever or sepsis changes is the coverage and the point of the report. The source has not been localised, so the study is the whole abdomen and pelvis in one portal-venous pass rather than a look at the quadrant someone suspects, and the question the report has to answer is drainability: whether there is a target, where it is, and what the safe route to it would be. Saying that on the request brings interventional radiology in earlier, which is the part that changes what happens to the patient — in a septic patient the imaging exists to enable source control, not to describe anatomy. Three protocol details decide whether the scan actually answers it. Do not add routine positive oral contrast: intravenous contrast alone gives equivalent accuracy for a collection, the sixty to ninety minutes it takes to opacify distal small bowel is pure delay in a patient whose mortality is measured against the clock to source control, and it must be withheld outright from anyone with a reduced conscious level, an unprotected airway or a likely trip to theatre, where aspiration is the harm. Positive oral contrast remains a deliberate choice for a specific question — a suspected enteric leak or fistula, or separating an interloop collection from unopacified bowel in a complex post-operative abdomen — and where it is chosen the interval is agreed at vetting, not assumed at the scanner. Second, where the operation was a colorectal or ileorectal anastomosis and the real question is a leak, the discriminating manoeuvre is dilute water-soluble contrast per rectum, gravity-instilled immediately before the acquisition and never barium: it costs minutes rather than the oral route's hour and a half, and a portal-venous study without it can show a presacral collection and still not say whether the anastomosis is the source. Third, coverage runs diaphragm to symphysis, so the lung bases are seen but the chest is not — a septic post-operative patient's source is thoracic often enough that if empyema, or telling a subphrenic collection from a pleural one, is part of the question, say so and the coverage is extended rather than repeated.
Problem solving	CT Abdomen and Pelvis CT Abdomen and Pelvis — Rectal Contrast (Anastomotic Leak)	The leak study, and it is a different examination rather than a variation in emphasis. Where the patient has a colorectal, ileorectal or coloanal anastomosis and the question is whether it has given way, dilute water-soluble contrast is gravity-instilled per rectum — or through the stoma or defunctioned limb — immediately before a portal-venous acquisition covering diaphragm to perineum, so that a low pelvic leak and any presacral collection are both in the field. Extraluminal luminal contrast is the diagnosis; the intravenous phase is there for the surrounding inflammation and bowel perfusion. Three things are non-negotiable: never barium, because extraluminal barium causes a granulomatous peritonitis that water-soluble contrast does not; gravity rather than a pressurised system, stopping at the point of discomfort, because creating a leak while looking for one is the failure mode; and adequate filling, because insufficient instillation is the standard false negative. It is not a routine addition to every post-operative CT — using it reflexively costs preparation time and patient discomfort for no diagnostic gain.
Problem solving	Ultrasound abdomen Ultrasound abdomen — full survey	Ultrasound keeps its two jobs and loses one. It is still the right test for a clinically localised superficial, right upper quadrant or pelvic collection, still the study that guides a drain, and still first-line where iodinated contrast genuinely must be avoided. What it cannot do in a febrile patient with no localising sign is exclude a source it was never pointed at — the retroperitoneum, the interloop spaces and a gassy post-operative abdomen are exactly where it fails — so a negative targeted scan here is a non-diagnostic study rather than a negative one, and must not close the question.

Nothing here withholds or delays anything. The fork exists to stop a septic patient with an unlocalised source being booked for a targeted ultrasound that cannot answer the question.
Where iodinated contrast must be avoided, this becomes the unenhanced CT or ultrasound conversation in the rule override below — a modified study, never a cancelled one.

Adults

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	CT Abdomen and Pelvis CT Abdomen and Pelvis — Portal Venous Phase usually appropriate	Portal-venous IV contrast is what makes an abscess visible: the rim enhances while the contents do not, and that difference is the diagnosis. An unenhanced study frequently cannot separate a collection from adjacent unopacified bowel, which is the single commonest reason this scan gets repeated. Two things follow that are routinely got wrong. A single portal-venous pass at roughly sixty to eighty seconds answers the question — arterial and delayed series add dose without yield for a collection. And routine positive oral contrast is not part of this protocol: intravenous contrast alone is of equivalent accuracy, the sixty to ninety minute preparation is a real delay in a patient who may be septic or heading for theatre, and dense luminal contrast forecloses converting the study to an angiographic or urographic protocol if the picture changes. Oral contrast is a deliberate choice for a named question — a suspected enteric leak or fistula, or separating an interloop collection from unopacified bowel in a complex post-operative abdomen — and it is withheld entirely from anyone with a reduced conscious level or an unprotected airway.
Problem solving	CT Abdomen and Pelvis CT Abdomen and Pelvis — Rectal Contrast (Anastomotic Leak)	Where the patient has a colorectal, ileorectal or coloanal anastomosis and the question is whether it is leaking, the portal-venous study alone can show a presacral collection and still not answer it. Dilute water-soluble contrast is gravity-instilled per rectum, or through the stoma or defunctioned limb, immediately before the acquisition, with coverage taken down to the perineum so a low anastomosis is included. Never barium where a leak is suspected; never a pressurised system; and stop at the point of discomfort, because converting a contained leak into a free one is the harm this study can do. It is not a routine addition to a post-operative CT.
Reasonable alternative	Ultrasound abdomen Ultrasound abdomen — full survey	Targeted ultrasound is reasonable for a clinically localised superficial, right upper quadrant or pelvic collection, it is the study that guides the drain, and it becomes first-line where iodinated contrast must genuinely be avoided or where the patient is pregnant and the collection is localised. Its failure zones are fixed and worth naming on the request: the retroperitoneum, the interloop spaces and a gassy post-operative abdomen. A negative targeted scan in a patient with no localising sign is non-diagnostic rather than negative.

Pitfalls

Accepting the request as written when contrast was not specified — an unenhanced scan reported as showing no drainable collection is the classic false negative here.
Early post-operative collections (roughly the first week) are common and often sterile; correlate with the trend in inflammatory markers rather than a single value.
Unopacified bowel loops mimic collections, which is a further argument for both IV contrast and careful correlation.
Adding routine positive oral contrast to this scan. Intravenous contrast alone is of equivalent accuracy for a collection, and the sixty to ninety minutes it takes to opacify distal small bowel is delay bought at the cost of source control in a septic patient. It is also unsafe in anyone with a reduced conscious level or heading for theatre, where aspiration is the harm. Reserve it for a named question — suspected enteric leak or fistula, or separating an interloop collection from bowel in a complex post-operative abdomen.
Answering "is the anastomosis leaking" with a plain portal-venous scan. Where the operation was a colorectal, ileorectal or coloanal anastomosis, luminal contrast instilled per rectum immediately before the acquisition is what converts "there is a presacral collection" into "the anastomosis is the source", and it costs minutes rather than the oral route's hour and a half. Water-soluble agent only — extraluminal barium causes a granulomatous peritonitis.
Sending a child straight to CT because the request said CT. Ultrasound answers most paediatric versions of this question, and the post-appendicectomy pelvic collection is exactly what it is good at — but a non-diagnostic ultrasound in a child who is still septic must escalate to CT the same day, not the next morning.
Reading the absence of a well-formed enhancing rim as the absence of an abscess in a neutropenic or otherwise immunosuppressed patient. Rim enhancement and the surrounding inflammatory response depend on a host response the patient may not be able to mount, so an ill-defined phlegmon, a few locules of extraluminal gas or fat stranding alone may be the whole finding, and the inflammatory markers and temperature will under-call the picture too.
Cropping the study at the diaphragm when the source has not been localised. Empyema, a subphrenic collection and a basal pneumonia all present as post-operative sepsis with an unremarkable abdomen, and the lung bases on a standard abdominopelvic acquisition are not a chest examination.

Priors — what to pull first

Pull the operation note and date: an apparent abscess may be an expected evolving seroma or haematoma on a normal trajectory.
Compare with the most recent CT for interval change in any known collection.

What makes a good request

A good request states the suspected source, the operation and post-operative day, and current inflammatory markers.
If the real question is drainability, saying so changes reporting emphasis and may bring interventional radiology in earlier.
Saying whether the patient is febrile or septic is not decoration on this card. It is what decides whether a targeted ultrasound is a reasonable first look or an inadequate one, because a scan pointed at one quadrant cannot exclude a source nobody has localised.
Say what the operation was, not just that there was one. A colorectal or ileorectal anastomosis turns the question from "is there a collection" into "is the anastomosis leaking", and that question is answered by adding dilute water-soluble contrast per rectum immediately before the acquisition — a few minutes of preparation, not the hour and a half the oral route costs.
In pregnancy the question is the same and the sequence changes: ultrasound first for a localised collection, MRI without gadolinium where the patient is stable and the scanner is available, and CT where the patient is septic with an unlocalised source or MRI cannot be had in time. The documented harm in this group is delay, not dose — an undrained collection kills the mother and the fetus with her.

Scoring this once it is done

The classification and risk tools this question ends in.

eGFR (CKD-EPI 2021 Creatinine)

How these studies are acquired

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

Paediatric abdominal ultrasound CT Abdomen and Pelvis — Portal Venous Phase MRI abdomen/pelvis — non-contrast rapid protocol CT Abdomen and Pelvis — Rectal Contrast (Anastomotic Leak)Ultrasound abdomen — full survey

Confirm locally

CT Abdomen and Pelvis — Portal Venous Phase: 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.