Renal ultrasonography (synonyms: ultrasound of the kidneys; renal ultrasound) is an important diagnostic procedure in internal medicine, particularly nephrology (kidney medicine), which can be used for real-time imaging of the kidneys to identify and, if necessary, classify pathologic processes of the kidneys. Renal sonography is an exclusively non-invasive procedure that poses no risk to either the patient or the treating physician. The advantage of renal sonography is that it is relatively easy to perform. In addition, no preparatory measures are necessary on the patient’s premises, so that sonography can be used practically anywhere. With the help of sonography it is possible to determine the size of the kidneys as well as the precise anatomical localization of the kidneys. To exclude tumor disease in the renal area, precise screening for tumors, cysts and stones is performed using ultrasound. In particular, painful nephrolithiasis (kidney stone disease) can be sensitively detected with the aid of sonography of the kidneys. Furthermore, the procedure also serves to determine kidney function and the possible determination of renal dysfunction. To assess renal function, urinary retention must be ruled out, as the presence of this symptomatology indicates a nephrological or urological dysfunction. The procedure is also of great importance in the detection of so-called renoparenchymatous disease (pathological changes in kidney tissue), which, in addition to the detection of an existing pathological change, usually also allows the change to be correctly named. Due to the high transplantation rates, it is necessary to have a diagnostic procedure with which the function and morphology of the transplanted kidney can be checked. By using renal sonography as a standard examination procedure, various complications and risks after transplantation can be identified. In particular, renal vein thrombosis, which is relatively common, can be detected by sonographic assessment early after transplantation. In addition, precise detection of pathologically altered vessels can be performed by specification using the Doppler effect. With the aid of sonographic imaging of the renal vessels, primary vascular diseases (vascular diseases) can be detected, including hypertensive nephrosclerosis (pathologically high blood pressure due to thickening of the renal artery) and diabetic glomerulosclerosis (pathological remodeling processes of the renal tissue that occur due to diabetes mellitus). Furthermore, vascular inflammation of the renal arteries can also be detected by the procedure. However, in order to detect the presence of renal artery stenosis, color Doppler sonography (color-coded Doppler sonography, FKDS; see table “Normal values in renal sonography” below) must be used. The use of sonography as a diagnostic method can also be used as a so-called parametric method. Parametric sonography describes the specification of specific numerical values, for example, to perform a description of tissue patterns. Factors such as the brightness and homogeneity (structural uniformity of the renal tissue) of the tissue are included in the calculation, as well as computer-aided manipulation of the image quality. With the help of these calculations, it is possible to increasingly dispense with methods that involve complications, such as taking a biopsy (tissue sample).
Indications (areas of application)
Congenital kidney disease
- Renal hypoplasia – renal hypoplasia is a significantly reduced kidney, but not necessarily accompanied by a loss of function. Usually, however, the opposite kidney is enlarged as an adaptation reaction. However, it must be noted that a reduction in kidney size with a compensatory enlargement of the opposite kidney can also be caused by a stenosis (narrowing) of the renal arteries.
- Ectopic kidneys – ectopy is a defective development of the kidneys associated with abnormal localization of the kidneys.
- Horseshoe kidney – this anatomical change is characterized by a fusion of the lower renal poles (lower end of the kidney). This bridge, also called an isthmus, consists of either renal parenchyma (kidney tissue) or connective tissue. There is a risk of confusion with enlarged lymph nodes due to the size and consistency of the bridge. Often this developmental disorder of the kidney is accompanied by nephrolithiasis and renal pelvic obstruction (obstructions of the renal pelvis).
- Double kidney – this very common malformation of the kidney is first noticed on ultrasound by a parenchymal bridge. However, to be able to prove a double kidney, an intravenous urogram must be made. In the urogram, a double renal pelvis and two ureters (ureters) must be seen to confirm the diagnosis.
Parenchymatous kidney disease
- Diabetic nephropathy – this disease is the most common kidney disease that can lead to terminal renal failure (non-curable kidney damage). Due to this, in the endpoint as a therapy dialysis procedure is used for blood purification.
- Glomerulonephritis – although there is no clear sonographic means of detection, an ultrasound examination is still performed because there is usually a compression of the kidney tissue. Glomerulonephritis can also lead to end-stage renal failure.
- Amyloidosis – extracellular (“outside the cell”) deposits of amyloids (degradation-resistant proteins) that can lead to cardiomyopathy (heart muscle disease), neuropathy (peripheral nervous system disease), and hepatomegaly (liver enlargement), among other conditions. Sonographic examination of the kidney reveals that the renal parenchyma is markedly widened.
- Acute renal failure (ANV) – the causes of renal failure are many. Of great importance is the differentiation of pre- and intrarenal (upstream and inside the kidney) renal failure. A crucial sonographic marker is echogenicity (short sound wave pulses that are reflected to different degrees depending on the tissue). If severe echogenicity is present, this should be considered a poor prognosis for further progression.
- Suspected space occupying lesions – cyst; solid tumor (renal cell carcinoma, angiomyolipoma).
Renal diseases of the tubular system
- Pyelonephritis – this disease is an inflammation of the renal pelvis, which can be detected by sonographic assessment.
Renal pelvis
- Urinary stones, urinary retention
Transplant kidneys – as previously described, sonographic examination is performed after transplantation to rule out common complications such as renal vein thrombosis or swelling of thee kidney and to determine the function of the transplanted kidney.
The procedure
Procedure
- When performing the procedure, the first step is to ensure that the patient is positioned correctly. However, the physician must make his or her own judgment as to which positioning variations are most appropriate for kidney assessment. In order to obtain adequate results during ultrasound examination of the kidney, it is necessary to adapt the patient’s positioning to both the patient’s constitution and the structure to be assessed. As a rule, however, the diagnostic procedure is performed with the patient lying flat, holding his arms above his head, so that the flank sections can be made on the left and right during inspiration (inhalation). The resulting longitudinal section is excellent for size assessment of the kidneys.
- After the flank sections have been recorded, the kidney should then be “fanned” in another plane. To do this, the physician rotates the transducer of the ultrasound scanner 90° counterclockwise. The “fanning” of the kidney allows closer differentiation of the localization of renal cysts, so that a demarcation of hydronephrosis (chronic kidney congestion with destruction of kidney tissue) to a renal cyst becomes possible.
- In addition, the change in transducer position serves to visualize the renal cortex in an additional plane, which has the consequence that the detection of neoplasms (benign and malignant neoplasms) in the renal area is easier. If it is difficult to adequately determine pathological changes, there is the option of performing the sonography completely in the lateral position.If this positioning variant does not bring the hoped-for success, an examination with the patient in the prone position is indicated. In the prone examination, the transducer is placed on the patient’s back. To complete the examination after the various transducer positions, an examination of the bladder filling must be performed. This is accompanied by the determination of the volume of urine and, in men, the measurement of the volume of the prostate.
- After each B-scan diagnosis of the kidney (including for the detection of lesions* ) should always be a color-coded Doppler sonography.
* Contrast ultrasonography with injected microbubbles has proven useful for the diagnosis and differential diagnosis of lesions.
Normal values in renal ultrasonography
| Parameters | Standard values |
| Kidney length | 90-125 mm |
| Parenchyma width | 15-20 mm |
| Kidney volume | Body weight [kg] × 2 ± 25 % ml |
| Resistive Index (RI)* | 0.5-0.7 (age-dependent) |
| Maximum flow velocity* | <200 cm/s |
* Determined by color-coded Doppler sonography (FKDS); resistive index (RI) indicates intrarenal vascular resistance; flow velocity provides direct evidence of renal artery stenosis (NAST). Note
- Ultrasonography is limited in solid renal lesions. Differential diagnoses are numerous: tumors, hemorrhagic cysts, abscesses (accumulation of pus), etc..