Overview of the kidneys and urinary tract
We have two kidneys that lie on each side of the spine just below the ribs in the retroperitoneal space of the abdomen. (or behind the abdominal cavity) The kidneys are bean-shaped and generally 9-12cm in length. They produce hormones that control blood pressure, filter and balance fluids in our blood and remove the waste as urine. The urine collects in the renal pelvis and drains into the bladder through the ureter. Blood flows to the kidney through the renal artery which branches off of the abdominal aorta. Lack of blood flow to the kidneys can have an impact on renal function and blood pressure.
Ultrasound of the kidneys and urinary tract
A renal ultrasound produces images of the kidneys, renal pelvis and can include the bladder if ordered by your physician. Ureters are not normally seen unless there are underlying problems. Information on the size, location and contour (or shape) of the kidney is obtained during the ultrasound to assess renal disease and possible infection. The ultrasound can be carried out regardless of the kidney function including severe renal insufficiency. Image quality can be hindered by increased body habitus. Air in the intestines can make it difficult for the technologist to obtain diagnostic images. For this reason, fasting is recommended as a preparation for the exam.
The urinary bladder however can only be seen and evaluated when full so the prep may include drinking 16-32 oz of water one hour prior to the exam time.
Areas of application
Congenital kidney disease
- Kidney hypoplasia-This is congenital abnormality where part of the kidney does not develop properly in the womb. It can affect one or both kidneys. The small size of the kidney means it is not able to function as well as a normal size kidney would. This may cause problems with age including kidney failure.
- Ectopic kidneys–This is a birth defect where the kidney develops in an abnormal location. Most of the time this is not associated with any symptoms however it can cause the kidney to develop urinary problems such as infection or renal stones.
- Horseshoe kidney-This anatomical change is characterized by a fusion of the lower poles of the kidney (lower end of the kidney). This developmental disorder of the kidney is often accompanied by kidney stones causing obstruction of the renal pelvis.
- Duplex kidney-This common malformation refers to being born with a “double” kidney on one side of the abdomen with a normally developed kidney on the other side. This is characterized by the kidney having a double renal pelvis and ureter. This can be found on ultrasound and confirmed with a CT urogram.
Parenchymal kidney disease
- Diabetic nephropathy– this is the most common kidney disease that can lead to incurable kidney damage.
- Amyloidosis – extracellular (“outside the cell”) deposits of amyloids (degradation-resistant proteins). The sonographic examination of the kidney reveals that the kidney parenchyma is significantly widened.
- Acute kidney failure– the causes of kidney failure are diverse. The differentiation between pre-and intrarenal (in front of and within the kidney) kidney failure is of great importance. A crucial sonographic marker is an echogenicity (short sound wave impulses that are reflected in different degrees depending on the tissue). Echogenic kidneys are considered a poor prognosis.
Kidney disease of the tubular system
- Pyelonephritis – inflammation of the kidney due to bacterial infection.
After Kidney transplantation
- Mainly to rule out frequent complications such as renal vein thrombosis and to determine the function of the transplanted kidney.
Which findings are possible to detect by kidney ultrasound?
Kidney stones can be detected with ultrasound, whereby the CT is the better examination in this context. Stones smaller than 5 mm can easily be overlooked with ultrasound.
By blocking the urinary tract, urography and ultrasound are equally good methods of examining the expansion of the collection system, but urography provides a more precise picture of the causes of occlusions. The predominant cause of blockage is kidney stones. Other causes are tumors, scars after infections, congenital disorders, and changes in the posterior abdominal wall. The ultrasound can show the size of the kidneys and the width of the kidney tissue and determine if you have hydro-nephrosis (enlargement of the renal pelvis). Blockages higher up in the ureter can often be detected, while other blockages are more difficult to detect due to the air-filled bowel above.
If there are constrictions in the renal arteries, this can be represented as decreased blood flow in the kidney using the Doppler technique.
When a tumor or other mass is found in the kidney, ultrasound is used to determine whether it is a cyst (a cavity filled with fluid, almost always benign) or a solid tumor. In the case of a solid tumor, the doctor examines whether the tumor has grown into the surrounding tissue and whether it has spread in the lymph nodes or the liver. In rare cases, abscesses (cavity filled with pus) can be found inside or outside the kidney.