Ultrasound of the Urinary Tract

Ultrasound is a cornerstone imaging modality for evaluating the urinary tract due to its noninvasive nature, absence of ionizing radiation, broad availability, no contrast agents required, point-of-care usage, and real-time diagnostic capabilities. This imaging modality enables effective visualization of the kidneys, ureters (when dilated), urinary bladder, and scrotum, playing a pivotal role in the initial and follow-up assessment of a wide range of urological conditions. Common clinical indications include hematuria, suspected obstruction, urinary tract infections, nephrolithiasis, renal and testicular neoplasms, testicular pain (torsion), and lower urinary tract dysfunction, including urinary retention.  Ultrasonographic imaging is based on the principle that low-density (hypoechoic) structures reflect less high-frequency sound waves than surrounding tissue, whereas high-density (hyperechoic) elements and organs are highly reflective. Hollow structures, such as simple cysts, are considered anechoic and do not reflect sound waves.  The amount and magnitude of the returning sound waves are directly related to the density of the tissue scanned. The elapsed time since the sound was generated corresponds to the depth or distance from the transducer of the structure causing the reflection.The ultrasound machine assembles an image based on these reflections, with highly dense entities such as bones or stones appearing white, whereas anechoic structures such as simple cysts appear black. Color Doppler presents vascular flow velocity and directional data. A computer uses red to indicate flow towards the transducer, whereas blue indicates flow in the opposite direction. Brighter colors indicate a higher flow rate and velocity. . A third color, typically green or yellow, can be used to indicate turbulence. Pulsed wave Doppler is a way of presenting flow velocity data on a timeline. Duplex ultrasound is the continuous presentation of pulsed-wave Doppler and standard ultrasound imaging. The transducer shape and frequency are selected based on the depth of the structure being studied, the desired field width, and the resolution required. Higher frequencies do not penetrate as deeply but provide better resolution. As a first-line imaging tool, ultrasound is especially valuable in populations where minimizing radiation exposure is critical, including children, pregnant individuals, and patients with recurrent renal stone disease. Advances in sonographic technology, including high-resolution probes, color Doppler imaging, contrast-enhanced ultrasound, and elastography, have improved the modality's sensitivity and specificity for various pathologies. Although computed tomography (CT) remains the gold standard for many diagnostic scenarios, ultrasound is often the initial imaging study performed, particularly in emergency, primary care, and outpatient settings. Point-of-care ultrasound (POCUS) further enhances accessibility and diagnostic speed, especially in acute presentations such as flank pain, urinary retention, suprapubic discomfort, bladder fullness, hematuria, and in patients with acute kidney injury. When performed by trained healthcare providers, urinary tract ultrasound facilitates timely diagnosis, reduces unnecessary imaging, and supports safer, more efficient clinical decision-making. This review describes the sonographic assessment of the urinary tract, including indications, techniques, findings, and applications in general, emergency, and point-of-care contexts. Please see StatPearls' companion resource, "Transrectal Ultrasonography and Image-Guided Biopsies of the Prostate," for further information (see Transrectal Ultrasound Guided Biopsy of the Prostate).