Abdominal ultrasound, CT, MRI, MRCP and ERCP

01Purpose and principlesWhat the assessment is for and the core concepts behind it.

Imaging choice begins with a management question, not a hierarchy of sophistication. Ultrasound excels at gallbladder stones, duct calibre, portal flow, ascites and guided procedures. CT rapidly surveys the abdomen, retroperitoneum, bowel and vessels and can acquire arterial, portal-venous or delayed phases, but phase and contrast choices must be specified. MRI characterises liver, pancreas, pelvis and bowel with multiple tissue sequences. MRCP uses static fluid as natural contrast to display ducts but can miss tiny stones, ampullary lesions or dynamic functional problems.

Radiation, contrast and access alter the decision. In an emergency, clinically necessary contrast CT should not be delayed solely because a current creatinine is unavailable when the expected benefit is high; assess risk and involve radiology. For planned iodine use, identify kidney disease, acute deterioration, prior reaction, thyroid issues and metformin according to local policy. Gadolinium agents differ in nephrogenic systemic fibrosis risk, and renal function, pregnancy and agent choice require radiology review. MRI safety requires precise implant identification, not simply asking whether the patient has metal.

ERCP combines endoscopy and fluoroscopy to access the biliary or pancreatic ducts. It offers immediate source control but is invasive and operator-dependent. A high-quality pathway establishes indication, reviews prior imaging, anticipates difficulty, addresses anticoagulation and antibiotics, plans sedation or anaesthesia and provides radiological or surgical rescue. Post-ERCP pancreatitis prevention commonly includes rectal NSAID when not contraindicated and additional measures for selected high-risk patients under the local protocol.

Key points

Ultrasound is first-line for gallstones, biliary dilatation, ascites and many liver questions; it is dynamic and radiation-free but limited by bowel gas, body habitus and operator dependence.
Contrast-enhanced CT is fast and broad for acute abdomen, perforation, obstruction, ischaemia, bleeding, sepsis, trauma and cancer staging, but uses ionising radiation and often iodine.
MRI offers high soft-tissue contrast without ionising radiation; liver protocols and enterography answer focused questions but take longer and require implant, claustrophobia and motion assessment.
MRCP is a non-invasive, heavily fluid-sensitive MRI of bile and pancreatic ducts; it maps obstruction but cannot drain, extract a stone or obtain every required tissue sample.
ERCP is primarily therapeutic: sphincterotomy, stone extraction, stenting and drainage. Diagnostic uncertainty is usually addressed first with ultrasound, MRCP, EUS or CT.
In suspected common-bile-duct stones, NICE recommends liver blood tests and ultrasound, then MRCP when ducts are not cleared by ultrasound but are dilated or tests remain abnormal; consider EUS if MRCP is non-diagnostic.
Acute cholangitis with obstruction needs resuscitation, antibiotics and urgent drainage; waiting for elective MRCP can be unsafe in a deteriorating patient.
ERCP complications include pancreatitis, bleeding, infection, perforation and sedation injury; consent and a post-procedure escalation plan are mandatory.

02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.

Acute cholangitisRed flag

Fever or sepsis, right upper-quadrant pain, jaundice and cholestatic tests with obstruction suggest infected biliary pressure. Hypotension, confusion or organ dysfunction requires emergency drainage planning.

Acute abdomen needing CTRed flag

Peritonism, shock, pain out of proportion, obstruction, major gastrointestinal bleeding or postoperative deterioration usually requires prompt contrast CT protocolled to the suspected emergency.

Post-ERCP pancreatitisRed flag

New persistent epigastric pain radiating to the back with enzyme elevation and clinical pancreatitis after ERCP requires fluid, analgesic, severity and complication assessment.

Post-ERCP perforation or bleedingRed flag

Severe pain, subcutaneous emphysema, peritonism, tachycardia, fever, haematemesis, melaena or falling haemoglobin warrants urgent CT and endoscopy, surgery and interventional-radiology coordination.

MRI or contrast hazard

Unknown implanted device, prior severe contrast reaction, acute kidney injury, pregnancy or unstable physiology does not automatically forbid imaging, but requires explicit radiology risk-benefit planning and an alternative if needed.

03Method and interpretationA systematic approach to the test and its findings.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Abdominal ultrasoundFirst step
Why
Assess gallstones, bile-duct calibre, liver texture and lesions, portal or hepatic flow, kidneys, pelvic organs and ascites; guide safe aspiration or drainage.
Interpretation and limitations
A normal study does not exclude early duct obstruction, small distal duct stones, bowel pathology or mild steatosis. State whether the common duct was actually visualised.
02
Contrast-enhanced CT abdomen and pelvis
Why
Evaluate acute inflammation, perforation, obstruction, ischaemia, haemorrhage, abscess, trauma, malignancy and vascular anatomy using a question-specific phase.
Interpretation and limitations
Non-contrast, portal-venous, arterial and multiphase liver studies are not interchangeable. Negative imaging requires reassessment if physiology continues to worsen.
03
MRI abdomen
Why
Characterise focal liver, pancreatic, adrenal, pelvic and small-bowel disease where soft-tissue contrast, diffusion or a specialised protocol adds value.
Interpretation and limitations
Motion, iron, fat and sequence selection alter appearance. The report should be interpreted within a liver, pancreas or enterography pathway rather than as generic MRI.
04
MRCP
Why
Map intrahepatic and extrahepatic bile ducts and the pancreatic duct non-invasively after equivocal ultrasound or when intervention is not yet established.
Interpretation and limitations
It identifies level and possible cause of obstruction but offers no drainage. Tiny stones, sludge and ampullary lesions may need EUS or ERCP after specialist review.
05
ERCP
Why
Provide biliary or pancreatic therapy through sphincterotomy, extraction, dilation, stenting, tissue brushing or drainage when the expected benefit justifies procedural risk.
Interpretation and limitations
Successful cannulation is not the same as complete drainage or stone clearance. The report must document intervention, residual disease, stent plan and follow-up.
06
Contrast and MRI safety screen
Why
Identify kidney or acute illness risk, metformin, prior reactions, pregnancy, thyroid disease, implants, foreign bodies, claustrophobia and ability to lie flat or cooperate.
Interpretation and limitations
Risk is agent-, route-, urgency- and patient-specific. Radiology may modify the protocol, use another agent or recommend non-contrast or alternative imaging.

04Clinical next stepsHow the result changes management or prompts escalation.

01BiliaryJaundice or suspected duct stoneFirst stepCholestatic tests, jaundice, biliary pain or ductal concern prompts imaging.

1Assess for sepsis and organ dysfunction, obtain liver tests and ultrasound, and start urgent cholangitis treatment when clinically indicated.
2If ultrasound does not show a duct stone but the duct is dilated or liver tests remain abnormal, use MRCP; consider EUS if MRCP remains non-diagnostic and suspicion persists.
3DefinitiveUse ERCP for therapeutic duct clearance or drainage before or at definitive surgical management, according to stability and local expertise.
4Document stent removal or exchange and cholecystectomy planning so temporary treatment does not become permanent by omission.

02EmergencyTime-critical abdominal CTPeritonism, shock, suspected ischaemia, perforation, haemorrhage, obstruction or deep sepsis.

1Resuscitate in parallel, state the suspected diagnosis and urgency clearly to radiology, and select the appropriate contrast and phase protocol.
2Do not delay a life-saving scan solely for routine renal testing; provide known risk, pregnancy and allergy information and use senior radiology judgement.
3EscalationEscalate immediately when CT shows a source needing surgery, endoscopy, drainage or embolisation.
4If the scan is non-diagnostic but physiology deteriorates, re-examine, re-discuss and reconsider timing, modality or operative assessment.

03ERCPSafe therapeutic ERCPImaging and clinical assessment establish a likely benefit from duct intervention.

1Confirm indication, anatomy, prior imaging, anticoagulant plan, blood results, antibiotic need, consent and sedation or anaesthesia plan through the ERCP team.
2Give post-ERCP pancreatitis prophylaxis, commonly rectal NSAID when not contraindicated, and other high-risk measures according to the current local protocol.
3After treatment monitor pain, observations, bleeding, fever and recovery; obtain labs and imaging promptly when a complication is suspected.
4Provide a written result, residual-stone or stricture plan, pathology ownership and a dated stent-removal or exchange arrangement.

05Procedure and medicine safetyRelevant preparation, treatment and contraindications.

Reduces post-ERCP pancreatitis risk in patients without a contraindication.

Rectal diclofenac or indometacin

A common adult prophylactic regimen is 100 mg rectally immediately before or after ERCP, chosen under the current BSG or local ERCP protocol.

Avoid or seek specialist advice with NSAID hypersensitivity, active bleeding or ulceration, significant renal injury, relevant pregnancy stage or other contraindication. It does not eliminate pancreatitis risk.

Enhances vessels, bowel wall and solid-organ perfusion to improve CT diagnosis and guide intervention.

Iodinated contrast medium

Agent, concentration, volume and injection rate are selected by radiology for body size, vascular access, examination and scanner protocol; there is no universal dose.

Prior reactions, kidney risk and thyroid disease require assessment. Emergency benefit may outweigh theoretical renal risk; follow the local metformin and post-contrast policy rather than stopping it universally.

Improves MRI characterisation of liver lesions, inflammation, perfusion and vascular anatomy.

Gadolinium-based contrast agent

Radiologist-selected lowest effective licensed dose for the sequence and agent, after renal, pregnancy and indication review.

Agents differ in tissue retention and nephrogenic systemic fibrosis risk. Severe renal impairment, peri-transplant state, pregnancy and previous reaction require agent-specific specialist review.

Treats established cholangitis and may prevent infection when incomplete drainage is anticipated in selected patients.

Antibiotic around ERCP

Use the local biliary sepsis or prophylaxis regimen based on cholangitis, likelihood of complete drainage, prior cultures, allergy and resistance data.

Antibiotics do not decompress an obstructed infected system. Obtain cultures where possible, adjust for renal function and narrow when results and drainage permit.

06Risks, monitoring and follow-upComplications, safety checks and further assessment.

After contrast, monitor clinically significant reactions immediately and follow renal testing only where the patient's risk and local policy indicate.
After ERCP, track pain, pulse, blood pressure, temperature, oxygenation, vomiting and bleeding through recovery and provide explicit delayed-complication advice.
Check FBC, renal and liver profile, CRP and pancreatic enzymes when post-ERCP complication is suspected rather than ordering them routinely without a question.
Track biliary stents with a reliable register and a named owner for removal or exchange.
Review pathology from duct brushings or biopsies and ensure strictures enter the hepatopancreatobiliary MDT pathway.
Audit ERCP cannulation, intended therapy completion, pancreatitis, bleeding, perforation, infection, unplanned admission and mortality using BSG standards.
Reconcile incidental findings and recommended interval imaging so no lesion is lost after the acute question is answered.

07Special situationsVariants, exceptions and circumstances that change the usual approach.

MRCP is a map

It depicts fluid-filled ducts without instrumentation but cannot relieve pressure. A septic obstructed patient needs a drainage plan, not imaging alone.

CT protocol is the test

A generic portal-venous scan may miss active arterial bleeding or subtle ischaemia. The clinical question determines contrast phase and coverage.

Ultrasound is dynamic

The sonographer can assess tenderness, compression, flow and patient position in real time, which makes it more than a static image.

Metal history needs identity

MRI safety depends on the exact device, location and conditions. 'Has metal' and 'MRI safe' are both inadequate undocumented shortcuts.

A stent creates future work

Every temporary biliary or pancreatic stent needs a documented purpose and removal or exchange date; forgotten stents cause obstruction and infection.

08Common pitfallsFrequent interpretation and management errors.

01
Using ERCP as a first-line diagnostic test when MRCP or EUS can answer the question safely.
02
Delaying emergency contrast CT solely because a current creatinine is unavailable despite life-threatening pathology.
03
Assuming a non-dilated duct on ultrasound excludes early obstruction.
04
Requesting generic CT without stating whether perforation, bleeding, ischaemia or cancer is suspected.
05
Giving rectal NSAID without checking relevant contraindications.
06
Discharging after ERCP without a stent plan or clear pancreatitis and bleeding advice.