Kidney (Normal, Absent, Large)- Renal (Cysts, Mass)!

Kidney Renal Cysts

Whatever position is used, remember to scan the kidneys in both longitudinal and transverse planes.

When examining any part of the renal area, compare both kidneys in different projections. Variations in size, contour and internal echogenicity may indicate an abnormality.

Normal Kidney 

Measurements made with ultrasound are generally less than those made by radiography: they are more accurate.

Both kidneys should be about the same size. In adults, a difference of more than 2 cm in length is abnormal.

    1. Length: up to 12 cm and not less than 9 cm

    2. Width: normally 4-6 cm but may very a little with the angle of the scan

    3. Thickness: up to 3.5 cm but may vary a little with the angle of the scan 

    4. The central echo complex (the renal sinus) is very echogenic and normally occupies about one-third of the kidney. (The renal sinus includes the pelvis, calyces, vessels and fat.)

In the newborn, the kidney are about 4 cm long and 2 cm wide. 

The renal pyramids are poorly defined hypoechogenic areas in the medulla of the kidney, surrounded by the more echogenic renal cortex. It is easier to see the pyramids in children and young adults. 

When scanning it is important to identify the following:

• The renal capsule. This appears as a bright, smooth, echogenic line around the kidney.
• The cortex. This is less echogenic than the liver but more echogenic than the adjecent renal pyramids.
• The renal medulla. This contain the hypoechogenic renal pyramids which should not be misstaken for renal cysts.
• The renal sinus (the fat, the collecting system and the vessels at the hilus). This is the innermost part of  the kidney and has the greatest echogenicity.
• The uterus. Normal uterus are not always seen: they should be sought where they leave the kidney at the hilus. They may be single or multiple and are often seen in the coronal projections.
• The renal artries and veins. These are best seen at the hilus. They may be multiple and may enter the kidney at different levels. 

Absent kidney

If either kidney cannot be seen, search again. Adjust the gain to show the liver parenchyma and spleen, and scan in different projections. Assess the size of the visible kidney. Hypertrophy of a kidney occurs (at any age) in a few months when the other kidney has been removed or is  not functioning. If there is one large kidney and the other cannot be visualized after a careful search, it is probable that the patient has only one kidney.

If one kidney cannot be demonstrated, consider the following possiblities.  

1. The Kidney may have been removed. check the clinical history and examine the patient for scars.
2. The kidney may be ectopic. Search the kidney area and the whole abdomen, including the pelvis. If no kidney is found, X-ray the chest. A contrast urogram may be necessary.
3. If only one large but normal kidney is demonstrated, and these had not been any surgery, it is likely that there is congenital absence of the other kidney. If only kidney visualized is not enlarged, a failure to demonstrate the other kidney suggests chronic disease .
4. If there is one large but distorted kidney, there may be a developmental abnormality.
5. Apparent absence of both kidneys may be a failure to demonstrate them with ultrasound because of changed echogenicity resulting from chronic disease of the renal parenchyma.
6. Any kidney less than 2 cm thick and 4 cm long can be very difficult to visualize. Locate a renal vessel or ureter; this may help to localize the kidney, especially if the ureter is dilated.  

A pelvic kidney may be confused sonographically with a tubo-ovarian mass or gastrointestinal tumour. Use a contrast urogram to locate the kidney

 

Large kidney

Bilateral enlargement

1. When the kidney are enlarged but normal in shape, with normal, decreased or increased homogeneous echogenicity, the possible causes are:

• Acute or subacute glomerulonephritis or severe pyelonephritis.
• Amyloidosis (probably increased echogenicity).
• The nephrotic syndrome. 

2. When the kidney have a smooth outline and are uniformly enlarged, with non-homogeneous hyperechogenicity, the possible causes are:

• Lymphoma. This may cause multiple areas of low density, especially Burkitt lymphoma I children or in young adults.
• Metastases.
• Polycystic kidneys.

Unilateral enlargement

If one kidney appears to be enlarged but has normal echogenicity, and the other kidney is small or absent, the enlargement may be due to compensatory hypertrophy. When no other kidney is seen, exclude crossed ectopic and other developmental abnormality.

The kidney may be slightly enlarged because of persistent segmentation (duplication) with two or even three ureters. Search for the renal hilus: there are likely to be two or more vessels and ureters. A contrast urogram may be necessary.

What is Renal cysts?

When ultrasound shows multiple, echo-free, well circumscribed areas throughout the kidney, suspect multi-cystic kidney. This condition is usually unilateral, Whereas congenital polycystic kidney disease is almost always bilateral (although the cysts may not be symmetrical).

1. Simple cysts can be single or multiple. On ultrasound the walls are smooth and rounded without internal echoes, but with a clearly defined back wall. Such cysts are usually unilocular and, when multiple, will differ in size. Rarely, these cysts become infected or haemorrhage, producing internal echoes. When this occurs or when the outline of any cyst is irregular, further investigation is required.

2. Hydatid cysts usually contain debris and are often loculated or septate. When clacified, the wall appears as a bright, echogenic convex line with acoustic shadowing. Hydatid cyst may be multiple or bilateral. Scan the liver for other cysts and X-ray the chest.

3. If there are multiple cysts in one kidney, the kidney is usually enlarged. This may indicate alveolar echinococcosis. If the patient is less than 50years old and clinically well, check the other kidney to exclude polycystic disease: congenital cysts are echo-free and without mural calcification. Both kidneys are always enlarged.

More than 70% of all renal cysts are due to benign cystic disease. These cysts are very common over the age of 50 years and may be bilateral. They seldom cause symptoms.

Renal Masses

There are two exceptions to the above statement:

1. In the early stages, a renal angiomyolipoma has ultrasound characteristics that allow accurate recognition. These tumour can occur at any age and may be bilateral. Ultrasound images show a well circumscribed, hyperechogenic and homogenous mass, and as the tumour grow there will be back wall attenuation. However, some tumours will undergo central necrosis and there will be strong back wall echoes. At this stage differentiation by ultrasound is no longer possible, but abdominal X-ray may show fat within the tumour, which is unlikely to occur in any other type of Renal mass.

2. when a renal tumour spreads into the inferior vena cava or into the perirenal tissues, there is no doubt that the tumour is malignant.

Ultrasound cannot reliable differentiate between benign renal tumours (other than renal cysts) and malignant renal tumours, and cannot always accurately differentiate malignant tumours from renal abscesses.

Bullet points!

Ultrasound cannot assess renal functions.

What ever position is used, remember to scan the kidneys in both longitudinal and transverse planes.

When examine any part of the renal area, compare both kidneys in different projections. Variations in size, contour and internal echogenicity may indicate an abnormality.