Computed tomography of the kidneys

Congenital anomalies of the kidneys

Density of renal parenchyma on native images during CT scanning is about 30 HU. Kidney sizes vary widely. If the outer contour of the kidney is smooth and the parenchyma is uniformly thinned, unilateral renal hypoplasia is likely. A reduced kidney is not necessarily diseased.

If the kidney is adjacent to the ilium, this is not always a sign of ectopia. There may be a transplanted kidney there. Its vessels are connected to the iliac, and the ureter to the bladder.

The location and number of renal arteries are highly variable. They must be examined carefully to confirm stenosis as a cause of renal hypertension. Complete or partial duplication of the ureter occurs. Complete duplication of the kidney is characterized by duplication of the renal pelvis.

Sometimes the low-density fatty tissue in the hilum has an unclear border with the surrounding renal parenchyma due to the course of X-ray hardness or the partial volume effect. In this case, comparison of adjacent sections will show that only the fatty tissue of the renal hilum is visualized. And the true tumor in this example is adjacent to the posterior edge of the right lobe of the liver.

Kidney cysts

Kidney cysts in adults are often detected incidentally. They can be located in any part of the parenchyma. Cysts located near the renal pelvis resemble hydronephrosis. Benign cysts usually contain serous transparent fluid with a density of -5 to +15 HU. There is no enhancement after CB injection, since cysts are avascular. Cyst density measurement may not always be accurate due to the partial volume effect on a given section or an eccentrically located window of the region of interest. Only the correct location of the region of interest in the center of the cyst allows us to determine its true density (about 10 HU). In rare cases, when hemorrhage occurs in benign cysts, an increase in the density of its contents is determined on non-enhanced images. There is no change in density after the introduction of a contrast agent.

Increased density or calcification of kidney formations indicate previous tuberculosis, hydatid invasion or renal cell carcinoma. The difference between the images before and after contrast enhancement also provides information on kidney function. With good perfusion, after approximately 30 s, the first phase of contrast accumulation is determined, which begins with the cortex. After another 30 - 60 s, the contrast agent is excreted into more distal tubules, causing enhancement of the medulla - a homogeneous enhancement of the entire renal parenchyma occurs.

The appearance of kidneys with multiple cysts in children with congenital autosomal recessive polycystic disease differs markedly from cysts in adults, which are usually an incidental finding. Polycystic kidney disease in adults is an autosomal dominant disease, accompanied by multiple cysts in the liver, bile ducts, less often in the pancreas, and the presence of aneurysms of the brain or abdominal vessels.

Hydronephrosis

Cysts near the renal pelvis can be confused with stage 1 hydronephrosis, which is characterized by dilation of the pelvis and ureter on native images. In stage 2 hydronephrosis, the boundaries of the renal calyces become unclear. In stage 3, renal parenchyma atrophy occurs.

Computed tomography of the kidneys should not be used to diagnose nephrolithiasis alone, as this is associated with significant radiation exposure to the patient. In nephrolithiasis, as in hydronephrosis, the method of choice is ultrasound.

In stage 3 chronic hydronephrosis, the parenchyma volume decreases and is defined as a narrow strip of tissue, atrophy develops, and the kidney does not function. In doubtful cases, the detection of a dilated ureter distinguishes hydronephrosis from a peripelvic cyst. The contrast agent accumulates in the dilated renal pelvis, but not in cysts.

Solid tumor formations of the kidneys

Contrast enhancement can often help differentiate the private volume effect of a benign cyst from a hypodense renal tumor. However, CT imaging does not provide specific information about the etiology of the lesion, especially when the renal parenchymal neoplasm has unclear borders. Inhomogeneous enhancement, infiltration of surrounding structures, and invasion into the pelvis or renal vein are signs of malignancy.

If the formation is solid, has a heterogeneous structure and contains fatty inclusions, one should think about angiomyolipoma. Benign hamartomas contain fatty tissue, atypical muscle fibers and blood vessels. Tumor invasion into the vessel wall often occurs, which leads to intratumoral or retroperitoneal bleeding (not shown here).

Kidney pathology associated with blood vessels

If fresh blood in the abdominal cavity is detected by ultrasound examination in case of penetrating injury or blunt abdominal trauma, it is necessary to determine the source of bleeding as soon as possible. The differential diagnosis should include not only a ruptured spleen or a large vessel, but also kidney damage. On non-enhanced images, signs of kidney rupture are a blurred kidney outline in the area of injury and bleeding, as well as the presence of a hyperdense fresh hematoma located in the retroperitoneal space. In this case, enhanced images demonstrate renal parenchyma with a still good blood supply and preserved function.

After extracorporeal shock wave lithotripsy (ESWL), kidney damage sometimes occurs with the formation of small hematomas or urine leakage from the ureter. If persistent pain or hematuria occurs after ESWL, a control CT scan should be performed. After intravenous administration of a contrast agent and its excretion by the kidneys, leakage of the contrast agent with urine into the retroperitoneal space is determined.

On CT imaging, renal infarction usually has a triangular shape in accordance with the angioarchitecture of the kidney. The broad base is adjacent to the capsule, and the triangular cone gradually narrows toward the pelvis. A typical sign is the absence of enhancement with intravenous contrast in both the early perfusion and late excretory phases. Emboli usually form in the left heart or in the aorta with its atherosclerotic lesion or aneurysmal dilation.

If, after the injection of a contrast agent, a low-density area is detected in the lumen of the renal vein, one can think of aseptic thrombosis or tumor thrombosis in kidney cancer. In the presented case, the thrombus extends into the inferior vena cava.

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