The kidney and bladder ultrasound, often called a renal and bladder ultrasound (RBU) or urinary tract ultrasound, is a non-invasive imaging method. This procedure uses high-frequency sound waves transmitted via a handheld probe to create images of the organs and structures within the urinary tract. The returning echoes are processed by a computer to generate pictures on a screen. This allows healthcare providers to assess the size, shape, and internal structure of the kidneys and the bladder without using ionizing radiation or contrast dyes.
Kidney Structure and Anatomy
The primary goal of a kidney ultrasound is to assess the structure of the renal parenchyma and collecting system. The normal adult kidney is an oval, bean-shaped organ measuring approximately 10 to 12 centimeters in length. Significant deviation from this size or shape is noted; for instance, smaller kidneys with increased brightness (echogenicity) may suggest chronic kidney disease or atrophy, while an enlarged kidney might indicate acute infection or obstruction.
The ultrasound characterizes masses found within the kidney. It differentiates between a simple cyst, which is a benign, fluid-filled sac appearing entirely black (anechoic), and a solid mass, which has internal echoes and a complex, gray appearance. This distinction is important because solid masses, unlike simple cysts, have the potential to be malignant and require further investigation. Complex cysts, which may contain internal debris, septations, or thickened walls, also warrant closer attention.
Kidney stones, or renal calculi, are identified as bright spots with a shadow extending behind them (posterior acoustic shadowing). Their presence and location, whether in the main body of the kidney or the collecting system, are mapped. In cases of acute infection, such as pyelonephritis or an abscess, the normal internal architecture of the kidney may appear distorted or swollen. An abscess, a pocket of pus, typically presents as a fluid collection with irregular margins and internal echoes, indicating tissue breakdown.
Bladder Capacity and Wall Integrity
The ultrasound examines the urinary bladder, assessing its ability to store and empty urine and the condition of its wall. Assessing the bladder requires it to be adequately filled, as this distends the organ and smooths the inner surface for clear viewing. The sonographer measures the bladder’s volume using specific formulas based on three-dimensional measurements to determine its functional capacity.
The thickness of the bladder wall is an indicator of its function. A normal, well-filled bladder wall typically measures between 3 to 5 millimeters. A wall consistently measuring more than 7 millimeters in an adult may indicate chronic irritation or an underlying obstruction, which causes the muscle to thicken and work harder, often due to chronic urinary tract infections or long-standing blockage at the bladder neck.
The ultrasound scans the interior of the bladder for abnormal growths. Bladder stones appear similarly to kidney stones: bright, shadowed objects that shift position with gravity as the patient moves. Masses or polyps growing from the inner wall are visualized and measured, raising suspicion for potential tumors. In male patients, the extent to which an enlarged prostate gland protrudes into the bladder lumen is noted, as this can affect urine flow.
The Post-Void Residual (PVR) volume measures the amount of urine remaining in the bladder after the patient attempts to empty it. A PVR volume exceeding 100 milliliters in adults is generally considered significant. A high residual volume suggests the bladder is not emptying properly, which can be due to a physical obstruction, such as an enlarged prostate, or issues with the bladder muscle or nerve control.
Identifying Blockages and Urinary Flow
The ultrasound identifies blockages in the urinary tract, even if the ureter is not clearly visible. The most recognized sign of obstruction is hydronephrosis, which is the swelling of the kidney’s urine-collecting system caused by a back-up of urine that cannot drain. The severity of hydronephrosis is often graded based on how much the central collecting system and the kidney’s cups (calyces) are dilated.
If the obstruction is located lower down, the ureters, which transport urine from the kidneys to the bladder, may also become dilated on the scan. This dilation supports the diagnosis of impeded urine flow from the kidney. While the entire length of the ureter is challenging to visualize due to overlying bowel gas, the effect of a blockage on the kidney above is easily captured.
Doppler ultrasound technology evaluates the “ureteral jets.” These jets are visible spurts of urine entering the bladder from the ureters at the ureterovesical junction. In a healthy, well-hydrated patient, these jets occur at regular intervals, typically every 15 to 20 seconds. The visualization of jets confirms that urine is traveling from the kidney to the bladder.
If an obstruction is present on one side, such as a stone lodged in the ureter, the frequency or duration of the ureteral jet on that side will be reduced or completely absent. By comparing the jets from the right and left sides, the sonographer gains evidence of a blockage, complementing static structural findings like hydronephrosis. The Doppler technique provides insight into urine transport.