15.1 Kidneys & Ureters

Scanning Technique and Normal Renal Anatomy

The kidneys are retroperitoneal organs imaged through acoustic windows provided by the liver on the right and the spleen on the left. A 2-5 MHz curvilinear transducer is standard. Begin with the patient supine, using a coronal or subcostal oblique approach during suspended respiration; rotate the patient into the decubitus position to shift bowel gas, and use the prone position for a direct posterior approach or biopsy guidance. Measure renal length pole-to-pole in the longest coronal plane.

The normal adult kidney measures 9-12 cm in length; the right kidney lies 1-2 cm lower than the left due to the liver. From outside to inside, recognize:

• Capsule - thin echogenic line surrounding the kidney
• Cortex - mid-level echoes, normally slightly less echogenic than adjacent liver or spleen; cortical thickness should exceed 1 cm
• Medullary pyramids - hypoechoic, triangular structures arranged around the sinus; do not mistake them for cysts or hydronephrosis
• Arcuate vessels - tiny echogenic dots at the corticomedullary junction
• Central renal sinus - brightly echogenic because of fat, plus the collecting system and vessels

The ureters are not routinely visualized unless dilated, but the distal ureters and ureterovesical junctions can be evaluated through a distended bladder.

Normal Variants

Hydronephrosis and Obstruction

Hydronephrosis is dilation of the collecting system, seen as fluid separating the echogenic central sinus complex. Grading follows severity: mild shows simple splaying of the sinus echoes; moderate adds rounded, dilated calyces connecting to the renal pelvis with preserved parenchymal thickness; severe shows ballooned calyces with cortical thinning. The most common acute cause is a ureteral stone; other causes include ureteropelvic junction (UPJ) obstruction, stricture, extrinsic compression from tumor or retroperitoneal fibrosis, and the physiologic dilation of pregnancy. Document the level of obstruction when visible.

Stones and the Twinkle Sign

Renal calculi appear as echogenic foci with clean posterior acoustic shadowing, although shadowing depends on stone size and surface - stones under roughly 5 mm may not shadow. On color Doppler, stones produce the twinkle artifact, a rapidly alternating red-blue mosaic posterior to the stone; lowering the color scale increases sensitivity. At the bladder trigone, ureteral jets can be imaged with color Doppler as urine spurts into the bladder; an absent or markedly asymmetric jet on the symptomatic side supports, but does not prove, obstruction.

Cysts Versus Solid Masses

A simple cyst meets strict criteria: anechoic contents, an imperceptible wall, round or oval shape, posterior acoustic enhancement, and no internal vascularity. The Bosniak classification stratifies cystic lesions: category I is a simple cyst; category II has thin septations or fine calcification; higher categories with thick septa, nodularity, or solid enhancing components are surgical lesions. Any cyst failing simple criteria is complex and needs further characterization.

Solid masses demand a different differential. Renal cell carcinoma (RCC) is the most common primary renal malignancy in adults; small RCCs are often hyperechoic, becoming isoechoic to hypoechoic and heterogeneous as they enlarge, sometimes with cystic areas. Always interrogate the renal vein and inferior vena cava (IVC) for tumor thrombus. In contrast, the angiomyolipoma (AML) is a benign hamartoma of fat, vessels, and smooth muscle that appears brightly echogenic because of its fat content, sometimes with partial shadowing. Multiple bilateral AMLs suggest tuberous sclerosis, and lesions larger than about 4 cm carry a hemorrhage risk.

Medical Renal Disease and Renal Doppler

Chronic medical renal disease produces bilaterally small kidneys (under 9 cm) with increased cortical echogenicity - cortex equal to or brighter than liver or spleen - loss of corticomedullary differentiation, and parenchymal thinning. These findings are nonspecific and must be correlated with laboratory values such as elevated blood urea nitrogen (BUN) and creatinine.

For renal Doppler, sample the arcuate or interlobar arteries and calculate the resistive index (RI): (peak systolic velocity minus end-diastolic velocity) divided by peak systolic velocity. Normal RI is roughly 0.6, with an upper limit of about 0.7. An elevated RI is nonspecific and occurs with medical renal disease, obstruction, transplant dysfunction, and renal vein thrombosis; a side-to-side difference greater than about 0.1 is also suspicious. Optimize the Doppler angle at 60 degrees or less and use a low wall filter to avoid losing diastolic flow.