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(Renal Ultrasound, Kidney Ultrasonography, Kidney Echography)
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?
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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.
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.
A kidney ultrasound may be performed to assist in placement of needles used to biopsy (obtain a tissue sample) the kidneys, to drain fluid from a cyst or abscess, or to place a drainage tube. This procedure may also be used to determine blood flow to the kidneys through the renal arteries and veins.
Kidney ultrasound may be used after a kidney transplant to evaluate the transplanted kidney.
There may be other reasons for your physician to recommend a kidney ultrasound.
There is no radiation used and generally no discomfort from the application of the ultrasound transducer to the skin.
There may be risks depending upon your specific medical condition. Be sure to discuss any concerns with your physician prior to the procedure.
Certain factors or conditions may interfere with the results of the test. These include, but are not limited to, the following:
- Severe obesity
- Barium within the intestines from a recent barium procedure
- Intestinal gas
- Your physician will explain the procedure to you and offer you the opportunity to ask any questions that you might have about the procedure.
- You may be asked to sign a consent form that gives your permission to do the procedure. Read the form carefully and ask questions if something is not clear.
- Generally, no prior preparation, such as fasting or sedation, is required.
- Although the gel applied to the skin during the procedure does not stain clothing, you may wish to wear older clothing, as the gel may not be completely removed from your skin afterwards.
- If the bladder is to be studied, you should not empty your bladder prior to the procedure.
- Based upon your medical condition, your physician may request other specific preparation.
A kidney ultrasound may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your physician's practices.
Generally, a kidney ultrasound follows this process:
- You will be asked to remove any clothing, jewelry, or other objects that may interfere with the scan.
- If asked to remove clothing, you will be given a gown to wear.
- You will lie on an examination table on your stomach.
- A clear gel will be placed on the skin over the area to be examined.
- The transducer will be pressed against the skin and moved around over the area being studied.
- If the blood flow is being assessed, you may hear a "whoosh, whoosh" sound when the Doppler probe is used.
- If the bladder is examined, you will be asked to empty your bladder after scans of the full bladder have been completed. Additional scans will be made of the empty bladder.
- Once the procedure has been completed, the gel will be wiped off.
While the kidney ultrasound procedure itself causes no pain, having to lie still for the length of the procedure may cause slight discomfort, and the clear gel will feel cool and wet. The technologist will use all possible comfort measures and complete the procedure as quickly as possible to minimize any discomfort.
There is no special type of care required after a kidney ultrasound. You may resume your usual diet and activities unless your physician advises you differently.
Your physician may give you additional or alternate instructions after the procedure, depending on your particular situation.