Clinical imaging: Abdominal imaging — AXR, ultrasound, CT and MRI

🩻 Abdominal imaging - AXR, ultrasound, CT and MRI - page 3 of the Clinical Imaging chapter. CT has transformed the assessment of the acute abdomen and is now the main test in most adults; the abdominal X-ray has a much narrower role than it once did. Ultrasound remains first line for the gallbladder, kidneys, aorta, pelvis, pregnancy and children. Knowing which test to choose - and recognising the few AXR patterns that matter - is the core skill.
Your four questions for every abdominal imaging decision: What is the likely diagnosis - and the dangerous one I mustn't miss? Is ultrasound, CT or MRI the best test for this patient (age, pregnancy, kidney function, stability)? Does the patient need resuscitation or surgery before imaging? Is there a clear plan for the result?

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Choosing Abdominal Imaging

Clinical question	Usual first test
Acute abdomen in an adult (perforation, obstruction, ischaemia, diverticulitis, appendicitis)	CT abdomen and pelvis with IV contrast (erect chest X-ray for free gas if CT is delayed)
Right upper quadrant pain, jaundice, suspected gallstones	Ultrasound
Renal colic	Non-contrast low-dose CT KUB (ultrasound in pregnancy and children)
AKI - exclude obstruction	Renal ultrasound (within 24 hours if obstruction suspected or cause unclear)
Pelvic pain in a woman of reproductive age	Pregnancy test, then pelvic (transvaginal) ultrasound
Suspected AAA	Ultrasound (stable); CT angiography if rupture suspected and stable enough
Suspected appendicitis in a child	Ultrasound
Abdominal pain in pregnancy	Ultrasound, then MRI (without gadolinium) if needed
Bile duct stones, biliary anatomy	MRCP
Crohn's disease activity	MR enterography (or bowel ultrasound)
Rectal cancer staging, perianal fistula	MRI pelvis
Active GI bleeding in an unstable patient	CT angiography → interventional radiology
Suspected bowel cancer in frail patients	CT colonography (or CT with contrast)

• Unstable patients - resuscitate; some (ruptured AAA, haemorrhagic shock in trauma) go straight to theatre or interventional radiology.
• Don't delay CT for kidney function results when ischaemia, bleeding or perforation is suspected.

When to Request an Abdominal X-ray

• Limited indications (RCR iRefer):
  • Suspected bowel obstruction (CT is often preferred and gives cause and complications).
  • Acute flare of inflammatory bowel disease - toxic megacolon.
  • Sigmoid volvulus (often diagnostic).
  • Ingested or inserted foreign bodies (radiopaque) - e.g. button batteries and magnets (urgent), body packers.
  • Renal tract stone follow-up (KUB film for known radiopaque stones).
  • Monitoring position of devices (feeding tubes in some settings, catheters).
• Not useful for - non-specific abdominal pain, constipation (clinical diagnosis), gastroenteritis, palpable masses, suspected appendicitis, pancreatitis, UTI, haematuria, GI bleeding.
• Perforation - erect chest X-ray is better than AXR for free gas; CT is most sensitive.
• Dose - an AXR is about 35 times a chest X-ray (about 0.4–0.7 mSv) - avoid unnecessary films.

Reading an Abdominal X-ray

• Check details - patient, date, projection (usually supine AP).
• Systematic approach:
  • Bowel gas pattern - distribution and calibre of small and large bowel.
  • Gas outside the bowel - free gas (Rigler's sign), biliary gas, portal venous gas, gas in the bowel wall, gas in the urinary tract.
  • Soft tissues and organs - liver, spleen, kidneys, bladder, psoas outlines.
  • Calcification - stones, vessels, pancreas.
  • Bones - spine, pelvis, hips (fractures, metastases, Paget's, sacroiliitis).
  • Lines, tubes and foreign bodies.
• Small vs large bowel:

  	Small bowel	Large bowel
  Position	Central	Peripheral ("picture frame")
  Folds	Valvulae conniventes - cross the whole width	Haustra - partially cross the width
  Upper limit of normal diameter	3 cm	6 cm (caecum 9 cm)
  Content	Fluid and gas, no faeces	Faeces (mottled)

• "3–6–9 rule" - small bowel 3 cm, colon 6 cm, caecum 9 cm (risk of caecal perforation above about 10–12 cm).

Bowel Obstruction and Volvulus

• Small bowel obstruction - dilated central loops (over 3 cm) with valvulae conniventes, little or no colonic gas; "string of beads" on erect films.
  • Causes - adhesions (commonest - previous surgery), hernias (always examine the groins), tumours, Crohn's strictures, gallstone ileus, intussusception.
  • CT - confirms obstruction, finds the transition point and cause, and detects closed-loop obstruction and strangulation (reduced bowel wall enhancement, mesenteric fluid, pneumatosis) - emergency surgery.
  • Water-soluble contrast (Gastrografin) challenge in adhesional obstruction - contrast reaching the colon within 24 hours predicts resolution without surgery.
• Large bowel obstruction - dilated peripheral colon (over 6 cm) with haustra.
  • Causes - colorectal cancer (commonest), volvulus, diverticular stricture, hernia.
  • Competent ileocaecal valve - closed-loop obstruction - caecal dilatation and perforation risk.
  • Pseudo-obstruction (Ogilvie's syndrome) - dilated colon without mechanical cause - older, unwell patients (post-operative, electrolyte disturbance, drugs) - CT excludes a mechanical cause.
  • CT with contrast - cause, stage of tumour, perforation.
• Volvulus:
  • Sigmoid volvulus - large "coffee bean" loop arising from the pelvis, pointing to the right upper quadrant; older, constipated, institutionalised patients - flexible sigmoidoscopic decompression and flatus tube; surgery if ischaemia or recurrence.
  • Caecal volvulus - dilated caecum displaced towards the left upper quadrant; with small bowel dilatation - usually surgery.
• Gallstone ileus - Rigler's triad - small bowel obstruction, pneumobilia (gas in the biliary tree), ectopic gallstone (often in the right iliac fossa) - older women.
• Intussusception (children) - target sign on ultrasound; air enema reduction.

Free Gas and Abnormal Gas

• Pneumoperitoneum - perforated peptic ulcer, diverticulum, tumour, ischaemic bowel, iatrogenic (endoscopy), recent surgery or laparoscopy (normal for several days), peritoneal dialysis.
  • Erect chest X-ray - gas under the diaphragm.
  • Supine AXR - Rigler's sign (both sides of the bowel wall visible - "double wall" sign), outline of the falciform ligament, triangular gas between loops, "football sign" in infants.
  • CT - the most sensitive test; shows the likely site of perforation.
• Pneumobilia (gas in the biliary tree - central, branching, near the liver hilum) - after ERCP and sphincterotomy, biliary stents or surgery (common and benign), gallstone ileus, emphysematous cholecystitis.
• Portal venous gas (peripheral branching gas reaching the liver edge) - bowel ischaemia (ominous in adults), necrotising enterocolitis (neonates), severe sepsis.
• Pneumatosis intestinalis - gas in the bowel wall - ischaemia, NEC; also benign forms (COPD, steroids).
• Emphysematous infections - gas in the kidney (emphysematous pyelonephritis - diabetes - urgent drainage or nephrectomy), bladder wall, gallbladder wall.
• Retroperitoneal gas - duodenal perforation (e.g. after ERCP).

Colitis, Toxic Megacolon and Ischaemia

• Toxic megacolon - transverse colon over 6 cm with systemic toxicity in severe colitis (ulcerative colitis, Crohn's colitis, C. difficile, CMV) - loss of haustra, mucosal islands, thumbprinting - risk of perforation - daily AXR in acute severe colitis; urgent colorectal surgical review.
• Colitis signs on AXR - oedematous, thickened bowel wall; thumbprinting (thickened haustral folds - ischaemia or severe colitis); absence of faeces in the inflamed segment (extent of colitis); "lead pipe" featureless colon (chronic UC).
• Acute mesenteric ischaemia:
  • Pain out of proportion to signs, AF or vascular disease, raised lactate (late).
  • AXR is often normal early (late - thumbprinting, pneumatosis, portal venous gas).
  • CT angiography (arterial and portal venous phases) - arterial occlusion (SMA embolus or thrombosis), venous thrombosis, bowel wall changes; don't wait for creatinine.
  • Treatment - revascularisation (endovascular or surgical) and resection of dead bowel.
• Ischaemic colitis - usually left-sided ("watershed" - splenic flexure) - older patients; CT shows wall thickening; colonoscopy.
• CT is the main test for colitis complications (perforation, abscess) and diverticulitis.

Calcification and Foreign Bodies

• Renal tract stones - most (about 80–90%) are radiopaque (calcium oxalate and phosphate); uric acid stones are radiolucent (seen on CT); cystine faintly opaque; staghorn calculus (struvite - Proteus infection) fills the collecting system.
• Gallstones - only about 10–20% are visible on X-ray; porcelain gallbladder (calcified wall - cancer risk).
• Pancreatic calcification - chronic pancreatitis (alcohol).
• Vascular calcification - aortic wall (an AAA may be outlined), splenic artery; phleboliths (small round pelvic vein calcifications with lucent centres - normal; don't mistake for ureteric stones).
• Other - calcified lymph nodes, appendicolith, adrenal calcification (TB, haemorrhage), fibroids ("popcorn"), dermoid cyst (teeth, fat), nephrocalcinosis (medullary sponge kidney, hyperparathyroidism, RTA type 1).
• Foreign bodies:
  • Button batteries - "double ring" or halo on AP view - oesophageal battery is an emergency (endoscopic removal within 2 hours - caustic burns; give honey to children over 1 year en route if swallowed within 12 hours); batteries beyond the oesophagus are usually monitored.
  • Multiple magnets - can attract through bowel walls (perforation, fistula) - urgent surgical review.
  • Coins (oesophagus vs trachea - coin seen face-on in the oesophagus on AP), sharp objects.
  • Body packing - drug packets (CT is more sensitive) - risk of rupture and toxicity.
  • Inserted rectal foreign bodies.

Liver, Gallbladder and Biliary Ultrasound

• Preparation - fast for about 6 hours (gallbladder distended).
• Gallstones - echogenic foci with posterior acoustic shadowing that move with position - ultrasound sensitivity over 95%.
• Acute cholecystitis - gallstones plus gallbladder wall thickening (over 3 mm), pericholecystic fluid, distension, sonographic Murphy's sign (maximal tenderness under the probe); complications - empyema, gangrene, perforation, emphysematous cholecystitis.
• Biliary dilatation - common bile duct over about 6 mm (larger normal after cholecystectomy and in older people); intrahepatic duct dilatation - obstructive jaundice:
  • Dilated ducts → find the cause - MRCP (stones), CT pancreas protocol (pancreatic head cancer - "double duct sign"), EUS.
  • Non-dilated ducts in jaundice → hepatocellular or intrahepatic cholestasis (hepatitis, drugs, PBC, sepsis).
• Liver:
  • Fatty liver - bright (echogenic) liver compared with kidney - very common (MASLD); assess fibrosis with blood tests (FIB-4, ELF) and transient elastography (FibroScan).
  • Cirrhosis - nodular surface, coarse texture, caudate enlargement, splenomegaly, ascites, portal hypertension (reversed portal flow), varices.
  • HCC surveillance in cirrhosis - ultrasound every 6 months (with or without AFP).
  • Focal lesions - simple cysts (anechoic), haemangiomas (bright), metastases (often multiple, target lesions), abscess - characterise with contrast-enhanced ultrasound, CT or MRI.
  • Doppler - portal vein thrombosis, Budd–Chiari syndrome (hepatic vein occlusion), TIPS patency.
• Spleen - splenomegaly (over about 13 cm); pancreas - often obscured by bowel gas (CT preferred).
• Ascites - ultrasound detects small volumes and guides paracentesis (diagnostic tap - SBP).

Renal Tract Imaging

• Renal ultrasound:
  • Normal kidneys about 9–12 cm; small, echogenic kidneys suggest chronic kidney disease; asymmetry over 1.5 cm - renovascular disease or scarring.
  • Hydronephrosis - dilated collecting system - obstruction (stones, prostate, pelvic tumour, retroperitoneal fibrosis); also pregnancy (physiological, right more than left).
  • AKI - ultrasound within 24 hours if obstruction suspected or no clear cause (NICE NG148); within 6 hours if pyonephrosis suspected.
  • Cysts (simple vs complex), polycystic kidneys, masses, bladder volume and post-void residual, prostate size.
• CT KUB (non-contrast, low dose) - renal colic (NICE NG118) - within 24 hours of presentation (immediately if fever, solitary kidney, or diagnostic uncertainty) - stone size and position, hydronephrosis, alternative diagnoses (AAA, appendicitis); ultrasound first in pregnancy and children.
  • Stones 5 mm or less usually pass spontaneously; larger stones and proximal stones less likely.
  • Infected obstructed kidney - urological emergency - urgent nephrostomy or ureteric stent.
• CT urogram (multiphase with delayed excretory images) - visible haematuria (with flexible cystoscopy) - upper tract urothelial cancer, renal cancer, stones.
• Renal masses:
  • Solid enhancing mass - renal cell carcinoma until proven otherwise (often incidental).
  • Angiomyolipoma - contains fat (negative HU) - benign; bleed risk if large (over 4 cm); tuberous sclerosis.
  • Complex cysts - Bosniak classification (I–IV).
• Renal artery stenosis - CT or MR angiography, Doppler (resistive index); fibromuscular dysplasia in young women.
• Prostate - multiparametric MRI first line for suspected prostate cancer before biopsy (NICE NG131 - PI-RADS 1–5).
• Testicular ultrasound - solid intratesticular mass - cancer until proven otherwise; epididymitis (increased flow); varicocele; torsion is a clinical diagnosis - don't delay surgical exploration for a scan.
• Children - ultrasound after UTI (NICE NG224 criteria); DMSA for scarring; MCUG for reflux.

The Abdominal Aorta and AAA

• Abdominal aortic aneurysm - infrarenal aorta 3 cm or more.
• NHS AAA Screening Programme - one-off ultrasound for men in the year they turn 65 (men over 65 can self-refer):

  Diameter	Action
  Under 3.0 cm	Normal - discharged
  3.0–4.4 cm (small)	Annual ultrasound surveillance
  4.5–5.4 cm (medium)	Every 3 months
  5.5 cm or more (large)	Refer to a vascular surgeon - seen within 2 weeks

• Repair (NICE NG156) - consider for aneurysms 5.5 cm or more, symptomatic aneurysms, or those over 4 cm growing more than 1 cm a year - open repair or EVAR (CT angiography for planning).
• Cardiovascular risk reduction for all - stop smoking (the main risk factor for growth and rupture), BP control, statin, antiplatelet.
• Ruptured AAA - abdominal or back pain, collapse, pulsatile mass, shock:
  • Unstable - permissive hypotension, straight to theatre (or EVAR suite) with vascular surgery - don't delay for CT.
  • Stable - CT angiography to confirm and plan (EVAR suitability).
  • Bedside ultrasound can show an aneurysm but can't exclude rupture.
• Mimics - renal colic in an older man with no history of stones - consider AAA (CT KUB will show it).

⚠️ Safety Pearl: "Renal colic" in a man over 60 who has never had a stone may be a leaking AAA. Examine for an aneurysm, check BP and pulse, and make sure imaging includes the aorta - a missed rupture is often fatal.

Pelvic, Gynaecological and Obstetric Ultrasound

• Transabdominal (full bladder) and transvaginal (better detail - empty bladder) ultrasound.
• Early pregnancy:
  • Pregnancy test first in every woman of reproductive age with abdominal pain or bleeding.
  • Ectopic pregnancy - empty uterus with an adnexal mass, free fluid (haemoperitoneum); pregnancy of unknown location - serial hCG (rise of over 63% in 48 hours suggests a viable intrauterine pregnancy) and repeat scans; haemodynamically unstable - surgery.
  • Miscarriage - criteria for non-viability (e.g. crown–rump length 7 mm or more without heartbeat, or mean sac diameter 25 mm or more without a fetal pole) - repeat scan if uncertain (NICE NG126).
  • Molar pregnancy - "snowstorm" appearance, very high hCG.
• Routine pregnancy scans - dating scan (10–14 weeks, with nuchal translucency), anomaly scan (18–20+6 weeks), growth scans for higher-risk pregnancies; low-lying placenta at the anomaly scan - rescan at 32 weeks; Doppler (umbilical artery) for growth restriction (see Obstetrics chapter).
• Gynaecology:
  • Postmenopausal bleeding - transvaginal ultrasound - endometrial thickness 4 mm or more needs hysteroscopy or biopsy (NICE NG12 - direct access ultrasound for women 55 and over with PMB).
  • Suspected ovarian cancer - CA125 35 IU/mL or more → ultrasound (NICE CG122); risk of malignancy index (RMI) or O-RADS / IOTA rules to triage adnexal masses.
  • Ovarian torsion - enlarged ovary with oedema; Doppler flow may be preserved - clinical diagnosis, don't delay laparoscopy.
  • Fibroids, endometriomas, polycystic ovaries, tubo-ovarian abscess, IUD position.
• MRI pelvis - endometriosis mapping, adenomyosis, cervical and endometrial cancer staging, complex adnexal masses, fistulae.

CT in the Acute Abdomen

• Appendicitis - adults: CT (dilated appendix over 6 mm with periappendiceal inflammation, appendicolith, abscess); young women - ultrasound first (and to look at the ovaries); children - ultrasound; pregnancy - ultrasound then MRI. Clinical scores (Alvarado, AIR) guide who needs imaging.
• Diverticulitis - CT with contrast - bowel wall thickening and fat stranding in the sigmoid; complications - abscess (drainage if over 3–5 cm), perforation, fistula (colovesical - gas in the bladder) - Hinchey classification.
• Acute pancreatitis:
  • Diagnosis is clinical plus lipase or amylase over 3 times normal - CT isn't needed for diagnosis unless there's doubt.
  • Ultrasound for gallstones in everyone.
  • CT at 72–96 hours (or if deteriorating) for necrosis and complications.
  • Revised Atlanta collections - acute peripancreatic fluid collection and acute necrotic collection (first 4 weeks); pseudocyst and walled-off necrosis (after 4 weeks).
• Perforation - free gas, fluid, site of perforation; oral water-soluble contrast for upper GI leaks (post-operative, Boerhaave's).
• Bowel obstruction - transition point, cause, closed loop, ischaemia.
• Mesenteric ischaemia - CT angiography.
• Abdominal trauma - CT with contrast in stable patients (solid organ injury grading, active bleeding - "contrast blush"); eFAST in unstable patients (see page 5).
• Post-operative complications - collections, anastomotic leak, bleeding.
• Older adults - low threshold for CT; signs are often subtle.
• Lower GI bleeding - CT angiography in unstable patients or major bleeding (BSG) → embolisation.

MRI of the Abdomen and Pelvis

• MRCP (MR cholangiopancreatography) - non-invasive biliary and pancreatic duct imaging - common bile duct stones (when ultrasound shows dilated ducts without a cause, or intermediate risk), primary sclerosing cholangitis ("beaded" ducts), strictures, pancreatic duct anatomy and cysts; avoids diagnostic ERCP.
• Liver MRI - characterises focal lesions (with liver-specific contrast), HCC in cirrhosis, metastases before surgery, iron overload (haemochromatosis, transfusion), fat quantification.
• MR enterography - Crohn's disease (extent, activity, strictures, fistulae) - no radiation for young patients needing repeated imaging.
• MRI pelvis:
  • Rectal cancer staging - tumour depth, nodes and the circumferential resection margin - decides on neoadjuvant therapy.
  • Perianal fistulae and abscesses (Crohn's).
  • Prostate - multiparametric MRI before biopsy (PI-RADS).
  • Gynaecological - cervical and endometrial cancer, endometriosis, adenomyosis, fibroids before embolisation.
• Adrenal MRI - chemical shift imaging (lipid-rich adenomas lose signal).
• MRI in pregnancy - appendicitis, other acute abdominal causes, placenta accreta spectrum - without gadolinium.
• Pancreatic cysts - MRI surveillance of IPMN.

Liver Lesions and Cancer Imaging

Lesion	Typical imaging
Simple cyst	Anechoic on US; water density on CT; very bright on T2 - no follow-up
Haemangioma	Commonest benign tumour - bright on US; peripheral nodular enhancement filling in centrally on delayed images; very bright on T2
Focal nodular hyperplasia	Young women - arterial enhancement with a central scar; retains liver-specific contrast - benign
Hepatic adenoma	Women on the combined pill, anabolic steroids - bleeding and malignant risk; stop oestrogens; resect if large (over 5 cm) or in men
Hepatocellular carcinoma	In cirrhosis - arterial hyperenhancement with portal venous "washout" and a capsule (LI-RADS) - can be diagnosed on imaging without biopsy
Metastases	Often multiple; colorectal (hypovascular - may be resectable), neuroendocrine and renal (hypervascular)
Liver abscess	Rim-enhancing collection ± gas - drain (pyogenic) or treat amoebic abscess medically
Hydatid cyst	Cyst with daughter cysts and calcified wall - travel or farming history

• Cancer staging - CT chest, abdomen and pelvis with contrast for most solid cancers; MRI for rectum, prostate, cervix, liver; PET-CT for oesophagus, lung, lymphoma, head and neck; EUS for oesophageal and pancreatic T and N staging and biopsy.
• Pancreatic cancer - pancreatic protocol CT (resectability - vessel involvement).
• CT colonography - alternative to colonoscopy in frail patients, after incomplete colonoscopy, or as patient preference; lesions need colonoscopy for biopsy.
• Response assessment - RECIST criteria for solid tumours on CT.

Abdominal Imaging at a Glance

Problem	First test	Key sign
Small bowel obstruction	CT (AXR)	Central loops over 3 cm, valvulae conniventes; transition point
Large bowel obstruction	CT	Peripheral loops over 6 cm, haustra; caecum over 9 cm
Sigmoid volvulus	AXR	Coffee bean from pelvis to RUQ
Perforation	Erect CXR / CT	Free gas; Rigler's sign
Toxic megacolon	AXR	Transverse colon over 6 cm
Mesenteric ischaemia	CT angiography	Vessel occlusion; pneumatosis; portal venous gas (late)
Gallstones / cholecystitis	Ultrasound	Shadowing stones; wall thickening; sonographic Murphy's
Obstructive jaundice	Ultrasound → MRCP / CT	CBD over 6 mm; double duct sign
Renal colic	CT KUB within 24 hours	Stone with hydronephrosis
AKI	Renal ultrasound	Hydronephrosis; small echogenic kidneys (CKD)
Visible haematuria	CT urogram + cystoscopy	Urothelial or renal tumour
AAA	Ultrasound (screening); CTA (rupture)	Aorta 3 cm or more; repair at 5.5 cm
Ectopic pregnancy	hCG + TVUS	Empty uterus, adnexal mass, free fluid
Postmenopausal bleeding	TVUS	Endometrium 4 mm or more → biopsy
Appendicitis	CT (adult), US (child, young woman), MRI (pregnancy)	Appendix over 6 mm, inflammation
Pancreatitis	Lipase + US (gallstones); CT at 72–96 hours	Necrosis, collections

Spot the Diagnosis

Clue	Diagnosis
Central dilated loops with folds crossing the whole width; previous laparotomy	Adhesional small bowel obstruction
Large inverted U-shaped loop from the pelvis to the RUQ in a care home resident	Sigmoid volvulus
Small bowel obstruction, gas in the biliary tree and a stone in the RIF	Gallstone ileus
Both sides of the bowel wall visible on a supine film	Rigler's sign - free gas
Branching gas reaching the liver periphery with severe abdominal pain and AF	Portal venous gas - mesenteric ischaemia
Transverse colon 8 cm in a febrile patient with severe UC	Toxic megacolon
Thumbprinting at the splenic flexure in an older woman with bloody diarrhoea	Ischaemic colitis
Calcification across the upper abdomen in a heavy drinker with steatorrhoea	Chronic pancreatitis
Double-ring opacity in the oesophagus of a toddler	Button battery - urgent endoscopic removal
Shadowing stones, wall thickening and probe tenderness over the gallbladder	Acute cholecystitis
Painless jaundice with dilated CBD and pancreatic ducts	Pancreatic head cancer (double duct sign)
Bright liver on ultrasound in an obese man with raised ALT	Fatty liver (MASLD) - assess fibrosis
Arterial enhancement with washout in a cirrhotic liver	Hepatocellular carcinoma
Liver lesion with peripheral nodular enhancement filling in on delayed images	Haemangioma
Young woman on the pill with a liver lesion that bled	Hepatic adenoma
Renal mass with negative Hounsfield units	Angiomyolipoma (fat)
Fever, loin pain and hydronephrosis with a ureteric stone	Infected obstructed kidney - nephrostomy
Gas in the renal parenchyma of a diabetic with sepsis	Emphysematous pyelonephritis
Small echogenic kidneys in a patient with high creatinine	Chronic kidney disease
5.8 cm infrarenal aorta on screening	Large AAA - vascular referral within 2 weeks
Positive pregnancy test, empty uterus and free fluid in the pelvis	Ectopic pregnancy
Endometrial thickness 9 mm in a 62-year-old with PMB	Needs hysteroscopy and biopsy
Gas in the bladder and sigmoid diverticular disease	Colovesical fistula
Solid mass within the testis of a 28-year-old	Testicular cancer until proven otherwise

Exam-style Scenarios

1. A 78-year-old man has colicky abdominal pain, distension and absolute constipation for 3 days. He had a right hemicolectomy 10 years ago.
Answer: Suspected bowel obstruction.

• Resuscitate - IV fluids, NG tube, catheter; bloods including lactate; examine hernial orifices.
• CT abdomen and pelvis with contrast - small vs large bowel, transition point, cause (adhesions, tumour, hernia), closed loop or ischaemia.
• Adhesional SBO without ischaemia - conservative with water-soluble contrast challenge; surgery if ischaemia or closed loop.

2. A 42-year-old woman has right upper quadrant pain, fever and jaundice. Bilirubin 85, ALP 420.
Answer: Suspected ascending cholangitis (choledocholithiasis).

• Sepsis bundle - blood cultures, IV antibiotics, fluids.
• Urgent ultrasound - gallstones and duct dilatation; MRCP if the cause is unclear.
• ERCP for biliary drainage (urgently if severe or not responding); later cholecystectomy.

3. A 35-year-old man has severe left loin pain radiating to the groin and microscopic haematuria. Temperature 37.2°C.
Answer: Suspected renal colic.

• Analgesia (NSAID first line - NICE NG118); non-contrast low-dose CT KUB within 24 hours.
• Stone 5 mm or less, no infection - likely to pass (medical expulsive therapy with tamsulosin for distal stones 10 mm or less).
• Fever, solitary kidney or AKI with obstruction - urgent urology (decompression).

4. A 28-year-old woman has lower abdominal pain and light vaginal bleeding. Her last period was 7 weeks ago. BP 100/60, HR 105.
Answer: Suspected ectopic pregnancy.

• Urine pregnancy test (positive), serum hCG, FBC, group and save; IV access.
• Transvaginal ultrasound - adnexal mass, free fluid, empty uterus.
• If haemodynamically unstable or ruptured - urgent laparoscopy; otherwise management per NICE NG126.

5. A 72-year-old woman with sudden severe abdominal pain has a soft, mildly tender abdomen. She has AF and stopped warfarin 2 months ago. Lactate 4.8.
Answer: Suspected acute mesenteric ischaemia (SMA embolus).

• Resuscitation; urgent CT angiography - don't wait for creatinine.
• IV heparin; broad-spectrum antibiotics.
• Emergency vascular and general surgical review - revascularisation (endovascular or open) and bowel resection.

Key Points

• CT is the main imaging test for the adult acute abdomen; AXR has limited indications.
• 3–6–9 rule - small bowel 3 cm, colon 6 cm, caecum 9 cm.
• Small bowel - central with valvulae conniventes; large bowel - peripheral with haustra.
• Sigmoid volvulus - coffee bean to the RUQ; gallstone ileus - Rigler's triad.
• Free gas - erect chest X-ray or CT; Rigler's sign on supine AXR.
• Portal venous gas and pneumatosis - think bowel ischaemia; CT angiography without waiting for creatinine.
• Toxic megacolon - transverse colon over 6 cm.
• Ultrasound first for gallbladder, biliary dilatation, kidneys, aorta, pelvis, pregnancy and children.
• CT KUB within 24 hours for renal colic; ultrasound in pregnancy; infected obstructed kidney needs urgent decompression.
• AAA screening for men at 65 - 3.0–4.4 annual, 4.5–5.4 3-monthly, 5.5 cm or more refer within 2 weeks.
• Pregnancy test in all women of reproductive age with abdominal pain; TVUS for ectopic pregnancy.
• PMB - endometrium 4 mm or more needs biopsy; CA125 then ultrasound for suspected ovarian cancer.
• MRCP for duct stones; MRI for rectal cancer, perianal disease, prostate and Crohn's.
• HCC in cirrhosis - arterial enhancement with washout; 6-monthly ultrasound surveillance.

Guidelines and Further Reading

• Royal College of Radiologists: iRefer - gastrointestinal and urological imaging guidance
• NICE NG118: Renal and ureteric stones - assessment and management
• NICE NG156: Abdominal aortic aneurysm - diagnosis and management; NHS AAA Screening Programme
• NICE NG126: Ectopic pregnancy and miscarriage; NICE CG122: Ovarian cancer; NICE NG12: Suspected cancer
• NICE NG131: Prostate cancer - diagnosis and management (MRI first)
• NICE NG148: Acute kidney injury; NICE NG104: Pancreatitis
• British Society of Gastroenterology: Acute lower GI bleeding guideline (2019); BSG/ACPGBI imaging recommendations
• BNF - for current doses