Kidney Ultrasound

(Renal Ultrasound, Kidney Ultrasonography, Kidney Echography)

Procedure overview

What is a kidney ultrasound?

A kidney ultrasound is a noninvasive (the skin is not pierced) procedure used to assess the size, shape, and location of the kidneys. Ultrasound technology allows quick visualization of the kidneys and related structures from outside the body. Ultrasound may also be used to assess blood flow to the kidneys.

A kidney ultrasound uses a handheld probe called a transducer that sends out ultrasonic sound waves at a frequency too high to be heard. When the transducer is placed on the abdomen at certain locations and angles, the ultrasonic sound waves move through the skin and other body tissues to the organs and structures of the abdomen. The sound waves bounce off the organs like an echo and return to the transducer. The transducer picks up the reflected waves, which are then converted into an electronic picture of the organs.

Different types of body tissues affect the speed at which sound waves travel. Sound travels the fastest through bone tissue, and moves most slowly through air. The speed at which the sound waves are returned to the transducer, as well as how much of the sound wave returns, is translated by the transducer as different types of tissue.

Prior to the procedure, clear, water-based gel is applied to the skin to allow for smooth movement of the transducer over the skin and to eliminate air between the skin and the transducer.

By using an additional mode of ultrasound technology during an ultrasound procedure, blood flow to the kidney can be assessed. An ultrasound transducer capable of assessing blood flow contains a Doppler probe. The Doppler probe within the transducer evaluates the velocity and direction of blood flow in the vessel by making the sound waves audible. The degree of loudness of the audible sound waves indicates the rate of blood flow within a blood vessel. Absence or faintness of these sounds may indicate an obstruction of blood flow.

Ultrasound may be safely used during pregnancy or in the presence of allergies to contrast dye, because no radiation or contrast dyes are used.

Other related procedures that may be performed to evaluate the kidneys include kidney, ureters, and bladder (KUB) X-ray, computed tomography (CT scan) of the kidneys, kidney scan, antegrade pyelogram, intravenous pyelogram (IVP), and kidney angiogram. Please see these procedures for more information.

How do the kidneys work?

The body takes nutrients from food and converts them to energy. After the body has taken the food that it needs, waste products are left behind in the bowel and in the blood.

The kidneys and urinary system keep chemicals, such as potassium and sodium, and water in balance, and remove a type of waste, called urea, from the blood. Urea is produced when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea is carried in the bloodstream to the kidneys.

Two kidneys, a pair of purplish-brown organs, are located below the ribs toward the middle of the back. Their function is to:

  • Remove liquid waste from the blood in the form of urine

  • Keep a stable balance of salts and other substances in the blood

  • Produce erythropoietin, a hormone that aids the formation of red blood cells

  • Regulate blood pressure

The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule.

Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney. 

Reasons for the procedure

A kidney ultrasound may be used to assess the size, location, and shape of the kidneys and related structures, such as the ureters and bladder. Ultrasound can detect cysts, tumors, abscesses, obstructions, fluid collection, and infection within or around the kidneys. Calculi (stones) of the kidneys and ureters may be detected by ultrasound.

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