A kidney ultrasound is a non-invasive diagnostic tool that uses high-frequency sound waves to create detailed images of the kidneys. This procedure allows medical professionals to assess the size, shape, and internal structure of these organs without using radiation. It is a primary method for investigating kidney-related symptoms, evaluating function, and distinguishing between different types of kidney problems.
Visualizing a Healthy Kidney
A healthy adult kidney has a characteristic bean-like shape and a smooth, well-defined outline on an ultrasound image. They measure between 9 and 12 centimeters in length, though this can vary with an individual’s height and age, and a size difference of more than 2 cm between the two kidneys may suggest an issue. The texture of a healthy kidney is uniform, presenting a specific grayscale appearance that provides a baseline for identifying problems.
The internal structure of the kidney is clearly distinguishable on a normal ultrasound. The outer portion, the renal cortex, contains the filtering units called nephrons and appears slightly darker than or as bright as the nearby liver, which serves as a reference point. The inner medulla is made of cone-shaped pyramids that are darker than the cortex. In the center is the renal sinus, which contains fat and the collecting system and appears brightest on the screen.
Identifying Acute Kidney Failure on Ultrasound
When kidney function declines suddenly, a condition known as acute kidney failure or acute kidney injury (AKI), an ultrasound is often one of the first tests performed. In many cases of AKI, especially those caused by issues outside the kidney itself like dehydration, the kidneys may look normal on the scan. Their size, shape, and texture can appear unchanged despite the loss of function.
A primary finding an ultrasound can detect in certain types of AKI is hydronephrosis. This condition is a swelling of the kidney’s central collecting system, where urine gathers before flowing to the bladder. It appears on the ultrasound as enlarged, dark, fluid-filled spaces in the middle of the kidney. Hydronephrosis signals a blockage in the urinary tract from a kidney stone, an enlarged prostate, or another obstruction. In some instances of AKI, the kidneys might also appear swollen or larger than normal.
Hallmarks of Chronic Kidney Failure on Ultrasound
Chronic kidney failure, or chronic kidney disease (CKD), develops over a long period, and the changes it causes are often visible on an ultrasound. Unlike the potential swelling in acute failure, chronic failure is characterized by signs of scarring and shrinking. These changes provide direct visual evidence of the progressive loss of kidney function.
One of the most common signs of CKD is a reduction in kidney size. As the disease progresses, the kidneys atrophy and become smaller than the typical 9-12 cm length due to the loss of functional tissue.
Another indicator is the thinning of the renal cortex. The cortex becomes thinner as nephrons are destroyed and replaced by scar tissue. A normal cortical thickness is between 7 and 10 millimeters, but in advanced CKD, it can become significantly reduced.
A final feature of CKD on ultrasound is increased echogenicity, meaning the kidney tissue appears unusually bright. This brightness is caused by fibrosis and sclerosis—the scarring that replaces healthy tissue. A damaged kidney cortex will often appear brighter than the adjacent liver, which is a reversal of the normal appearance and a sign of chronic disease.
Assessing Blood Flow with Doppler Ultrasound
Beyond creating grayscale images, ultrasound can evaluate blood flow using a technique known as Doppler ultrasound. This function provides dynamic information about circulation and is often displayed with colors overlaid on the image to show the direction and speed of blood.
One primary use of Doppler is to check for renal artery stenosis, a narrowing of the arteries that supply blood to the kidneys. This condition can cause high blood pressure and contribute to kidney damage. The Doppler measures blood flow velocity, and a significant increase in speed through an artery can indicate a stenosis.
The technique also measures the resistance to blood flow within the kidney’s smaller vessels by calculating a resistive index (RI). A high RI can suggest increased resistance within the kidney, which may be a sign of disease affecting the microvasculature. This information about blood flow provides functional data that complements the structural details seen on the standard ultrasound.