Abdominal Ultrasound

Abdominal ultrasound uses high frequency sound to image the abdominal organs. It uses 3 to 5 MHz curvilinear transducers, and 7 to 15 MHz linear transducers for superficial structures and the bowel wall. It is the first line imaging for most abdominal symptoms.

It images the liver, gallbladder, bile ducts, pancreas, spleen, kidneys, adrenal regions, aorta, inferior vena cava, retroperitoneum, bladder, peritoneal cavity and abdominal wall.

Real time grey scale imaging shows the size, shape, echo pattern and contour of organs. Colour and spectral Doppler assess blood flow in the portal, hepatic and renal vessels and in lesions. Contrast enhanced ultrasound with microbubbles characterises liver and renal lesions, and elastography measures liver stiffness.

The examination takes 15 to 30 minutes. It uses no ionising radiation and no contrast (except specialised studies), is portable and low cost, and can be repeated.

Abdominal pain, jaundice, abnormal liver tests, haematuria, renal failure, palpable masses, trauma and fever are among the commonest reasons for imaging.

Ultrasound has high accuracy for gallstones (sensitivity 95%, specificity 95 to 98%), hydronephrosis (over 90%), ascites, and abdominal aortic aneurysm (near 100%). It has moderate accuracy for appendicitis (sensitivity 75 to 90%) and focal liver lesions (sensitivity 40 to 80%).

Accuracy is low in obese patients, gas filled bowel and deep structures such as the pancreas (visible in 50 to 80% of adults).

Indications. Right upper quadrant pain, jaundice, suspected gallstones, cholecystitis or biliary obstruction. Abnormal liver tests, hepatomegaly, suspected fatty liver, cirrhosis, portal hypertension, or focal liver lesion (and surveillance for hepatocellular carcinoma in cirrhosis every 6 months).

Flank pain, haematuria, suspected renal colic, hydronephrosis, renal failure, renal masses, and cysts. Suspected abdominal aortic aneurysm (screening in men over 65 where programmes exist). Suspected appendicitis (especially in children and pregnancy), intussusception, pyloric stenosis, and bowel obstruction. Abdominal mass, distension, ascites, or suspected free fluid.

Splenomegaly or splenic injury. Pancreatic disease (pancreatitis, mass, pseudocyst). Abdominal trauma (focused assessment). Fever with abdominal source, abscess, and control of drainage and biopsy.

Not for suspected bowel perforation (free gas is not seen reliably). Not first line for suspected pancreatic cancer or small bowel disease, for which CT or MR is used.

Contraindications and precautions. None absolute. Fasting for 6 to 8 hours before the examination (clear fluids allowed) distends the gallbladder and reduces bowel gas. A full bladder helps pelvic views. A recent barium study, large amounts of bowel gas, obesity, open wounds, dressings and surgical drains reduce views.

Latex allergy for probe covers is considered in endocavitary studies. Gel is applied to the skin. For contrast enhanced studies, the microbubble contrast is avoided in severe cardiac or pulmonary hypertension and allergy to sulphur hexafluoride components.

Preparation and technique. The patient lies supine, with the right and left lateral decubitus, and sometimes erect or prone, positions used to move gas and shift organs.

A systematic scan covers the following, with measurements and Doppler as needed:

• Liver (all segments, hepatic veins, portal vein, bile ducts).
• Gallbladder (wall, stones, polyps, Murphy sign).
• Pancreas and spleen.
• Both kidneys (length, cortex, collecting system).
• Aorta and inferior vena cava, bladder, and peritoneum.

Breath holding in deep inspiration brings the liver and spleen clear of the ribs.

Normal appearances. Liver: homogeneous echotexture, equal to or slightly brighter than the renal cortex, smooth edge, with a right lobe span of up to 15 cm (craniocaudal 15 to 16 cm in the mid-clavicular line), portal vein of up to 13 mm (hepatopetal flow), hepatic veins that taper toward the periphery with triphasic flow.

Gallbladder: pear shaped anechoic lumen, length up to 10 cm, width under 4 cm, wall under 3 mm after fasting. Common bile duct: up to 6 mm in adults (add 1 mm for each decade over 60, up to 10 mm after cholecystectomy). Pancreas: homogeneous, equal to or brighter than liver, head up to 3 cm, body 2.5 cm, tail 2 to 3 cm, main pancreatic duct under 3 mm. Spleen: homogeneous, length up to 12 cm.

Kidneys: length 9 to 12 cm, cortex thinner than 1.5 cm and less echogenic than liver or spleen, with a central echogenic sinus, and cortical thickness of 7 to 10 mm. Aorta: under 3 cm in diameter (under 2 cm at the bifurcation). Inferior vena cava: collapses with inspiration. No free fluid is seen except a trace in the pelvis in women. Normal bowel wall is under 3 mm.

Interpretation.

• Liver, fatty liver: diffusely bright echotexture with blurred vessel margins and posterior attenuation.
• Liver, cirrhosis: coarse, nodular echotexture, irregular surface, caudate enlargement, atrophy of the right lobe, splenomegaly, ascites, portal vein over 13 mm with slow or reversed flow, varices.
• Liver, acute hepatitis: hypoechoic liver with bright portal triads, "starry sky" pattern, and gallbladder wall thickening. Congestion: dilated hepatic veins and inferior vena cava.
• Liver, focal lesions: simple cysts (anechoic, thin walled, with posterior enhancement), haemangioma (well defined, hyperechoic lesion under 3 cm), hepatocellular carcinoma (a hypoechoic or heterogeneous mass in cirrhosis, with portal vein invasion), metastases (target or bright lesions).
• Liver, other: abscess (hypoechoic complex lesion with debris), hydatid cyst (cyst with daughter cysts, a "water lily" sign with detached membrane), and portal vein thrombosis (echogenic material, cavernous transformation).
• Biliary, gallbladder: gallstones (echogenic foci with posterior acoustic shadowing that move with position), acute cholecystitis (stones, wall thickening over 3 mm, pericholecystic fluid, positive sonographic Murphy sign), biliary sludge, polyps, adenomyomatosis, gallbladder carcinoma (mass, thick wall).
• Biliary, ducts: biliary dilatation (common duct over 6 mm, intrahepatic ducts parallel to portal veins as "double barrel" sign), choledocholithiasis (stones in the duct), choledochal cyst, and cholangiocarcinoma.
• Pancreas: acute pancreatitis (enlarged, hypoechoic gland, peripancreatic fluid; gas obscured), chronic pancreatitis (atrophy, calcification, ductal dilatation), pseudocyst, and pancreatic mass (hypoechoic lesion with ductal obstruction and a dilated common duct and pancreatic duct, "double duct" sign).
• Spleen: splenomegaly (over 12 cm; portal hypertension, infections including malaria and kala azar, haematological disease), infarcts (wedge shaped), abscess, cysts, accessory spleen, and traumatic laceration or haematoma.
• Kidneys and urinary tract: hydronephrosis (grades by calyceal and pelvic dilatation, cortical thinning), stones (echogenic with shadow), cysts, solid masses (renal cell carcinoma, angiomyolipoma), polycystic kidney disease.
• Kidneys, other: small echogenic kidneys in chronic disease (length under 9 cm, thin cortex, loss of corticomedullary differentiation), pyelonephritis and abscess, and bladder wall thickening or masses.
• Aorta and vessels: aneurysm (diameter 3 cm or more; risk of rupture rises over 5.5 cm), dissection flap, thrombus, and inferior vena cava thrombosis.
• Gastrointestinal: appendicitis (non-compressible blind ending tube over 6 mm with periappendiceal fluid or echogenic fat), diverticulitis, bowel wall thickening (Crohn disease, tumour, infection).
• Gastrointestinal, other: intussusception ("target" or "doughnut" sign in children), pyloric stenosis (wall over 3 mm, canal over 15 mm), and dilated fluid filled bowel in obstruction.
• Peritoneum: free fluid (anechoic: ascites; echogenic: blood, pus), loculated collections, and peritoneal masses.
• Lymph nodes and masses: enlarged round hypoechoic nodes with loss of fatty hilum.

Critical values and findings needing immediate action.

• Ruptured or symptomatic abdominal aortic aneurysm (over 5.5 cm or with pain, hypotension or periaortic fluid): immediate vascular surgical referral.
• Free intraperitoneal fluid in trauma or haemodynamic instability: urgent surgical review and CT or theatre.
• Obstructed infected kidney (hydronephrosis with fever and sepsis): urgent decompression.
• Biliary obstruction with cholangitis (dilated ducts with fever, jaundice and pain): antibiotics and urgent biliary drainage.
• Acute cholecystitis, gangrenous or emphysematous change, or perforation: surgical referral and antibiotics.
• Appendicitis with abscess or perforation: surgical referral.
• Intussusception: urgent reduction by enema or surgery.
• Ectopic pregnancy suspected (free fluid with positive pregnancy test): urgent gynaecological review.
• Portal vein thrombosis or hepatic vein thrombosis (Budd-Chiari syndrome): anticoagulation and specialist hepatology.
• Hepatic or splenic rupture or haematoma: resuscitation and surgical or interventional care.
• Suspected malignant mass: urgent staging with CT or MR and referral.

Limitations and artefacts.

• Operator dependence: accuracy depends on the skill of the sonographer and the quality of the machine; a systematic scan and image documentation improve consistency.
• Patient factors: obesity, bowel gas, abdominal wall scars, recent meals and ascites reduce views. The pancreas, retroperitoneum, aorta and bowel are often obscured, and small lesions (under 1 cm) may be missed.
• Ultrasound has low sensitivity for small liver lesions in cirrhotic liver (about 60% for hepatocellular carcinoma), for small bowel disease, for pancreatic tumours, and for ureteric stones. Sensitivity is about 45% for stones in the ureter, and higher for hydronephrosis.
• Artefacts: acoustic shadowing (stones, gas), posterior enhancement (cysts), reverberation (gas, bowel), mirror image, side lobe artefacts (mimicking sludge in the gallbladder), edge shadowing, and anisotropy. A bright reflection may hide structures behind it.
• Normal variants mimic disease: a folded gallbladder (Phrygian cap), prominent column of Bertin, focal fatty sparing near the gallbladder, a dromedary hump of the kidney, duodenal gas or the pylorus mimicking stones or masses, and foetal lobulation.
• A normal ultrasound does not exclude appendicitis, early bowel ischaemia, pancreatic cancer, small renal stones, or small liver metastases. CT or MR is added when suspicion persists.
• Measurement variation (organ size varies with age, sex, body size and ethnicity) should be considered.
• Doppler assessment is angle dependent and limited in deep vessels.

Special groups.

• Children: ultrasound is the first line test (appendicitis, intussusception, pyloric stenosis, masses, hip, kidneys). No sedation is needed with a calm approach, and the bladder is filled for pelvic views.
• Pregnancy: safe (no ionising radiation); first line for abdominal pain (gallstones, appendicitis, renal obstruction) and then MR if needed.
• Obese patients: lower frequency probes, harmonic imaging, and patient positioning improve images; CT or MR may be needed.
• Older adults: aortic aneurysm, gallstones, hydronephrosis from prostate disease, and malignancy.
• Chronic liver disease: surveillance for hepatocellular carcinoma every 6 months with or without alpha fetoprotein.
• Trauma: focused assessment with sonography shows free fluid in minutes.
• Regions: hydatid disease, amoebic and pyogenic liver abscess, schistosomal periportal fibrosis ("pipe stem" fibrosis with thickened portal tracts), tuberculosis (nodes, ascites with debris and septa, bowel wall thickening), hepatosplenomegaly from malaria and visceral leishmaniasis, hepatocellular carcinoma from hepatitis B, and Kaposi sarcoma are common.
• In these regions ultrasound is often the only imaging available.

Follow up tests. CT or MR of the abdomen with contrast for characterisation and staging, MR cholangiopancreatography for the biliary tree and pancreatic duct, contrast enhanced ultrasound for focal liver lesions, elastography or liver biopsy for fibrosis, endoscopic ultrasound for pancreatic and biliary disease, liver function tests, hepatitis B and C serology, alpha fetoprotein,

amylase and lipase, renal function and urinalysis, full blood count, C-reactive protein, tumour markers (as relevant), stool and blood tests for parasites, and referral to surgery, gastroenterology, hepatology, urology or oncology.