Renal Ultrasonography

In renal ultrasonography, high-frequency sound waves are transmitted from a transducer to the kidneys and perirenal structures. The resulting echoes are displayed on a monitor as anatomic images.

Renal ultrasonography can be used to detect abnormalities or clarify those detected by other tests. It's especially useful in cases in which excretory urography is ruled out. Unlike excretory urography, this test isn't dependent on renal function and therefore may be useful in patients with renal failure. Ultrasonography of the ureter, bladder, and gonads also may be used to evaluate urologic disorders.

Purpose

• To determine the size, shape, and position of the kidneys, their internal structures, and perirenal tissues
• To evaluate and localize urinary obstruction and abnormal fluid accumulation
• To assess and diagnose complications after kidney transplantation

Patient preparation

• Explain to the patient that this test is used to detect kidney abnormalities.
• Inform him that he needn't restrict food or fluids before the test.
• Tell him who will perform the test and where, that it takes about 30 minutes, and that it's safe and painless.

Procedure and posttest care

• The patient is placed in prone position, the area to be scanned is exposed, and conductive gel is applied to the area.
• The longitudinal axis of the kidneys is located by using measurements from excretory urography or by performing transverse scans through the upper and lower renal poles.
• These points are marked on the skin and connected with straight lines. Sectional images 3/8" to 3/4 " (1 to 2 cm)apart can then be obtained.
• During the test, the patient may be asked to breathe deeply to assess the kidneys' movement during respiration.
• Remove the conductive gel from the patient's skin.

Normal findings

The kidneys are located between the superior iliac crests and the diaphragm. The renal capsule should be outlined sharply; the cortex should produce more echoes than the medulla. In the center of each kidney, the renal collecting systems appear as irregular areas of higher density than surrounding tissue. The renal veins and, depending on the scanner, some internal Structures can be visualized. If the bladder is also being evaluated, its size, shape, position, and urine content can be determined.

Abnormal findings

Cysts are usually fluid-filled, circular structures that don't reflect sound waves. Tumors produce multiple echoes and appear as irregular shapes. Abscesses found within or around the kidneys usually echo sound waves poorly; their boundaries are slightly more irregular than those of cysts. A perirenal abscess may displace the kidney anteriorly.

Generally, acute pyelonephritis and glomerulonephritis aren't detectable unless the renal parenchyma is significantly scarred and atrophied. In such patients, the renal capsule appears irregular and the kidney may appear smaller than normal; also, an increased number of echoes may arise from the parenchyma due to fibrosis.

In patients with hydronephrosis, renal ultrasonography may show a large, echo-free, central mass that compresses the renal cortex. Calyceal echoes are usually circularly diffused and the pelvis significantly enlarged. This test can also be used to detect congenital anomalies, such as horseshoe, ectopic, or duplicated kidneys. Ultrasonography clearly detects renal hypertrophy.

Following renal transplantation, compensatory hypertrophy of the transplanted kidney is normal but an acute increase in size indicates rejection.

This test allows identification of abnormal accumulations of fluid within or around the kidneys that sometimes arise from an obstruction. It also allows evaluation of perirenal structures and can identify abnormalities of the adrenal glands, such as tumors, cysts, and adrenal dysfunction. However, a normal adrenal gland is difficult to define ultrasonically because of its small size.

Renal ultrasonography can be used to detect changes in the shape of the bladder that result from masses and can assess urine volume. Increased urine volume or residual urine post voiding may indicate bladder outlet obstruction.

Interfering factors

• Retained barium from a previous test (possible poor imaging)
• Obese patient (possible poor imaging)