A kidney ultrasound is a non-invasive diagnostic test using high-frequency sound waves to produce real-time images of the kidneys and bladder. Because it does not use ionizing radiation, it is a safe imaging method suitable for a wide range of patients, including pregnant women and children. The technology allows medical professionals to assess the size, shape, and location of the kidneys and observe blood flow, which is important for diagnosing and managing various urinary conditions.
Reasons for a Kidney Ultrasound
A physician may order a kidney ultrasound to investigate symptoms or abnormal test results. Common reasons include pain in the flank—the side of the body between the ribs and hip—blood in the urine (hematuria), or recurrent urinary tract infections. These symptoms can signal issues such as kidney stones, infections, or blockages.
The test is also used to evaluate the kidneys when blood tests show poor function or to investigate the cause of high blood pressure. It allows for the detection and monitoring of cysts, tumors, abscesses, or fluid buildup. An ultrasound can also identify scarring from past infections or monitor the health of a transplanted kidney.
What to Expect During a Kidney Ultrasound
Preparation for a kidney ultrasound requires a full bladder. You will need to drink about one liter (32 ounces) of water an hour before the appointment and avoid urination. A full bladder provides a clear “acoustic window,” pushing the intestines aside for an unobstructed view of the urinary system.
The procedure is painless and lasts between 20 to 45 minutes. You will lie on an examination table while a clear, water-based gel is applied to your abdomen or back. This gel helps the transducer, a small handheld device, maintain contact with the skin and transmit sound waves effectively as the sonographer moves it over your skin.
To get the clearest images, you may be asked to hold your breath or change positions. The bladder is examined while full, and then you may be asked to empty it for post-void images. After the scan, the gel is wiped off, and you can resume normal activities.
Key Kidney Structures Seen on a Labeled Ultrasound
A labeled kidney ultrasound provides a map of the organ’s anatomy, using text or pointers to identify its components. On the image, a normal adult kidney appears as a bean-shaped structure, 10-14 cm long. The kidney is enclosed by a thin, bright white line known as the renal capsule, which separates it from surrounding fat.
The outermost functional layer is the renal cortex, which appears as a darker, grainy-textured area just inside the capsule. The cortex should have a uniform thickness, measuring between 7-10 mm. Deeper within the kidney is the renal medulla, composed of several triangular sections called medullary pyramids, which are darker than the surrounding cortex.
In the center of the kidney is the renal sinus, a bright, fatty area that contains the urine collecting system. This system includes the calyces, which collect urine from the pyramids, and the renal pelvis, a funnel-shaped structure that channels urine into the ureter. The renal hilum, where the renal artery, vein, and ureter enter and exit, is also located within this central region.
Common Conditions Visualized on Labeled Ultrasounds
Labeled ultrasounds are effective at identifying abnormalities within the kidney. Kidney stones, for instance, appear as very bright (hyperechoic) spots on the ultrasound. They often cast a dark “acoustic shadow” behind them, as sound waves cannot pass through the dense stone. Labels specify the stone’s location, such as “stone in the lower pole calyx,” which is important for treatment planning.
Fluid-filled structures like simple cysts are also easily identified. They appear as round, dark (anechoic) sacs with well-defined borders and are common, especially in people over 50. A label might read “cyst in the upper pole renal cortex” to document its position.
Another common finding is hydronephrosis, the swelling of the kidney’s collecting system due to a blockage. On an ultrasound, this appears as an expansion of the dark, fluid-filled renal pelvis and calyces. In moderate cases, this can be described as having a “bear’s paw” appearance.
Solid masses or tumors can also be detected, appearing as distinct areas with a different texture or brightness. Labels are used to describe the mass’s location to guide further diagnostic steps, such as a biopsy or CT scan. Changes in the kidney’s overall size or cortical thickness, seen in chronic kidney disease, are also documented.