Genitourinary Imaging

Renal/Bladder Ultrasound

David A. Hatch, M.D.

Loyola University Stritch School of Medicine

Ultrasound is performed by generating high frequency sound waves (typically 5 - 10 kHz) and directing them through body tissues using a probe held on the skin. The probe also contains a receiver to detect sound waves (called echoes) reflected from tissues. When the sound waves travel easily through uniform substances (water, oil, urine, etc.), no echoes are generated. The ultrasound image seen on the screen is, therefore, black; there are no echoes. When the sound waves encounter a tissue that absorbs or transmits the sound, a wave is reflected back to the probe. The ultrasound image is white or gray depending on the intensity of the reflection. Unlike x-rays or CAT scans, ultrasound doesn't detect tissue density. Rather, it detects sonotransmission (the passage or reflection of sound). Highly dense tissues such as bone or kidney stones readily reflect echoes and, therefore, appear bright white on an ultrasound. Air, such as in the bowel, also readily reflects echoes. The edge of the bowel, therefore, appears white on an ultrasound. Therefore substances with widely differing densities (air - bone) may both appear bright white on an ultrasound. Remember, ultrasound does not detect tissue density.

You need to know a few conventions used in ultrasound imaging in order to read ultrasounds. First, try to get as much information about the ultrasound image as you can from the frame. You will see that this image has a label: "RIGHT LONG FLANK POST VOID." This means that you are looking at a longitudinal image of the right flank. The left side of a longitudinal image is always the cephalad side of the image. The right side of a longitudinal image is the caudad side. The kidney is the bean shaped structure marked at either end by a small cursor [+]. Superior to the kidney and superficial (toward the top of the image) is the liver. It is rather homogeneous (fairly regular grey pattern). Notice that the kidney is not so homogeneous. In the center of the kidney is an area of increased echoes (light grey or white). This is the hilum of the kidney also called the central sinus. It shows increased echogenicity because there are several structures (pelvis of the kidney, blood vessels, nerves, fat and lymphatics) that transmit sound differently. As the sound wave hits the interface between two such structures, an echo is generated.

This is a transverse image of a kidney. In a transverse image, the left side of the image represents the right side of the body, just as in a standard x-ray or CT scan.

Usefulness: Ultrasound is the most useful pediatric urologic imaging test. It is easily performed without any preparation of the child and it causes no pain. It can differentiate solid from cystic masses and it is very helpful in defining renal and bladder anatomy. Using sophisticated computer analysis, current ultrasound machines can detect movement. By this means they are able to detect the velocity of blood flow in vessels. This is very useful in allowing one to measure perfusion to a kidney or testis. Ultrasound is also a relatively economical exam.

Limitations: Ultrasound detects shape and sonodensity. It does not, however, measure function of kidneys. Nuclear renograms are the best G/U imaging study to measure renal function.

Indications: Children with urine infections, abdominal masses, suspected hydronephrosis or hematuria.

Examples: Normal kidney. Hydronephrosis. Kidney tumor.

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©David A. Hatch, M.D., 1996