🔬 Renal Ultrasound

Renal ultrasound is a non-invasive, radiation-free imaging modality used to evaluate the anatomy of the kidneys and bladder. It is essential for detecting obstruction, assessing kidney size in chronic disease, and identifying structural abnormalities or masses. It is the first-line tool for investigating acute kidney injury of unknown cause.

Indications include investigation of unexplained acute kidney injury (AKI) or chronic kidney disease (CKD), screening for polycystic kidney disease, and assessing for renal tract obstruction. It is frequently used to evaluate palpable abdominal masses, persistent haematuria, or recurrent urinary tract infections (UTIs). It is also utilised for guiding invasive procedures such as renal biopsies or the placement of nephrostomy tubes.

The patient is typically scanned in a supine or oblique position using a curvilinear transducer (3.5-5 MHz). High-frequency sound waves are emitted and the reflected echoes are processed to create real-time images of the renal parenchyma, collecting system, and surrounding structures. The examination should include views of both kidneys and the bladder. Patients are often asked to have a full bladder to facilitate assessment of the lower urinary tract and post-micturition residuals.

Normal kidneys are bean-shaped with a smooth contour, measuring between 9cm and 12cm in length. The renal cortex should be less echogenic (darker) than the liver or spleen. The renal medulla contains pyramids which are less echogenic than the cortex. The central renal sinus, containing fat and the collecting system, appears highly echogenic (bright). No evidence of dilatation, calculi, or solid masses should be present.

Interpretation relies on assessing kidney size (typically 9-12cm in adults), cortical thickness, and the presence of focal lesions or dilatation. Hydronephrosis should be graded from mild to severe; however, it must be interpreted alongside clinical context as it does not always equate to acute obstruction (e.g., in pregnancy). Cortical thinning and increased echogenicity compared to the liver or spleen are indicative of parenchymal disease. Bladder volume and post-void residual are often assessed concurrently.

Abnormalities include hydronephrosis (dilatation of the renal pelvis and calyces), suggesting obstructive uropathy, and the presence of echogenic stones with acoustic shadowing. Increased cortical echogenicity or loss of corticomedullary differentiation often signifies chronic kidney disease. Space-occupying lesions may appear as simple cysts (thin-walled, anechoic) or complex masses/solid tumours requiring further characterisation via CT or MRI. Small, shrunken kidneys (<9cm) are characteristic of end-stage renal failure.

Renal ultrasound is the primary imaging modality for investigating acute kidney injury (AKI) to exclude obstruction, particularly in patients with no known cause. It is also the first-line screening tool for autosomal dominant polycystic kidney disease (ADPKD) in at-risk relatives. In the context of haematuria, it helps detect renal cell carcinoma, though it is less sensitive for small urothelial tumours in the ureters or bladder compared to CT urogram or cystoscopy.

A prominent column of Bertin (normal cortical tissue protruding into the sinus) can be mistaken for a renal tumour by inexperienced practitioners. Parapelvic cysts may mimic hydronephrosis; Doppler ultrasound can help distinguish vascular structures from dilated calyces. Conversely, early or partial obstruction may not always show significant hydronephrosis, particularly in dehydrated patients or those with a retroperitoneal fibrosis 'encasing' the ureters.

The utility of renal ultrasound is limited in patients with a high body mass index (BMI) as adipose tissue attenuates the ultrasound beam, reducing image clarity. It is relatively insensitive for detecting small renal stones (<5mm) and ureteric stones, which are often obscured by bowel gas. Furthermore, it cannot differentiate between different types of medical renal disease (e.g., glomerulonephritis vs. interstitial nephritis), which often requires a histopathological diagnosis.

For the MLA, remember that ultrasound is the first-line investigation for AKI of unknown aetiology to rule out obstruction (post-renal failure). Be aware of the 'simple cyst' criteria: anechoic, thin-walled, and posterior acoustic enhancement. Recognise that 'increased echogenicity' is a common descriptor for medical renal disease.