🔬 Abdominal Ultrasound

A non-invasive, radiation-free imaging modality used primarily to evaluate the hepatobiliary system, kidneys, and abdominal aorta.

Indications include right upper quadrant (RUQ) pain, jaundice, suspected gallstones, or abnormal liver function tests. It is used to screen for abdominal aortic aneurysms (AAA) and to investigate palpable abdominal masses. It is also used to assess the kidneys for hydronephrosis in acute kidney injury or to evaluate the spleen and bladder.

A non-invasive imaging technique using high-frequency sound waves emitted from a transducer. A coupling gel is applied to the skin to eliminate air pockets. The probe is moved across the abdomen in various planes (longitudinal, transverse, and coronal) while the patient may be asked to hold their breath or change position. Real-time images are displayed on a monitor, and Doppler can be used to assess blood flow in the portal vein or hepatic arteries.

Normal findings include a liver of uniform echotexture and smooth borders, a thin-walled (~2mm) gallbladder free of stones, and a common bile duct of normal diameter. The kidneys should show preserved corticomedullary differentiation with no evidence of dilatation (hydronephrosis). The abdominal aorta should be of normal calibre (usually <3cm) without aneurysmal dilatation.

Interpretation involves assessing the size, shape, and echotexture of solid organs and the patency of vessels and ducts. Dilatation of the common bile duct (>6-7mm) suggests distal obstruction, such as a stone or pancreatic head mass. Renal size and cortical thickness help differentiate acute from chronic kidney disease. Results must always be correlated with the clinical picture, such as Murphy's sign during the scan (sonographic Murphy's).

Abnormal findings include cholelithiasis (gallstones), which typically appear as hyperechoic structures with posterior acoustic shadowing. Cholecystitis is suggested by gallbladder wall thickening (>3mm) and pericholecystic fluid. Liver findings may include increased echogenicity (steatosis), focal lesions (cysts, haemangiomas, or metastases), or features of cirrhosis such as a nodular surface and splenomegaly. Hydronephrosis or renal calculi may also be identified.

Abdominal ultrasound is often the first-line imaging modality for RUQ pain, jaundice, and suspected biliary disease due to its high sensitivity for gallstones. It is also the primary screening tool for Abdominal Aortic Aneurysm (AAA) in the UK national screening programme. Because it does not involve ionising radiation, it is the preferred initial imaging for children and pregnant women with abdominal symptoms.

A significant pitfall is failing to visualise the entire pancreas or aorta due to bowel gas, which may lead to a false-negative report. Additionally, ultrasound is relatively poor at detecting small stones within the common bile duct (choledocholithiasis) compared to MRCP. Small renal stones or those located in the ureter are also frequently missed.

The quality of the image is highly operator-dependent and can be severely limited by patient factors such as high Body Mass Index (BMI) or excessive overlying bowel gas, which obscures deeper structures like the pancreas and aorta. It is less sensitive than CT for detecting small bowel pathology or retroperitoneal lymphadenopathy. It cannot reliably distinguish between certain types of solid tumours without further imaging or biopsy.

Focus on the classic findings of gallstones and the sonographic signs of cholecystitis. Understand its role as the first-line investigation in the 'jaundice' pathway to differentiate between obstructive (surgical) and non-obstructive (medical) causes.