Complex cyst type 3 (a). CEUS demonstrates thick septa of the cyst (b)
Renal clear cell carcinoma is the most common renal malignancy, and it is the lesion most usually studied with CEUS. Solid renal tumors < 3 cm in size appear usually as hypoechoic masses at gray-scale sonography; in 30% of cases, they can be slightly hyperechoic. When the lesion reaches a diameter superior to 3 cm, it shows frequently an inhomogeneous internal structure. At color Doppler imaging, these masses have a mild increased peripheral arterial vascularization, and seldom internal vessels are identified. When the neoplastic lesion is over 4 cm in size, the echostructure is that of an inhomogeneous mass due to intratumoral necrotic or hemorrhagic areas and calcifications. At CEUS, renal carcinoma has a homogeneous or inhomogeneous enhancement in comparison to normal renal parenchyma in the arterial phase, with progressive enhancement reduction in the late phase (Fig. 54.2). The detection of a hypervascular peripheral rim, sometimes observed, is due to the tumoral pseudocapsule . Angiomyolipoma (AML) is a benign tumor and usually appears as a hyperechoic well-defined solid mass at gray-scale sonography. After microbubbles injection, it is homogeneously enhanced and hypoenhanced in the arterial and late phase . Renal oncocytoma, another benign tumor, usually is detected as a homogeneous hypoechoic mass with a central stellate scar. At color power Doppler imaging, they can show a wheel aspect. After contrast injection, the enhancement is diffuse and homogeneous in the arterial phase, with the depiction of the central scar as a hypoechoic area (Fig. 54.3).
Hypoechoic mass of the right kidney (a). CEUS (b) shows enhancement of this tumor (black arrows). It was an adenocarcinoma
Large mass of the right kidney (a) with central vessel at color Doppler (b). After contrast injection, there is a strong enhancement with depiction of a central scar (black arrow), suggestive of oncocytoma (surgery confirmed) (c)
54.2.2 Segmental Renal Ischemia
Thromboembolic disease is the most common cause of renal infarction and is observed in patients with cardiovascular diseases. Gray-scale sonography is not able to detect ischemic areas, and even color power Doppler, commonly used to detect great vessels’ pathology, usually is not useful in old patients with reduced kidney perfusion. The CEUS is particularly useful in the diagnosis of this disease and has the advantages compared to CT and MR which require the use of nephrotoxic contrast agents . The ischemic areas appear as nonperfused areas or show a late enhancement compared to the normally perfused adjacent renal tissue. The ischemic parenchyma shows sometimes a triangular shape, with the base at the renal capsule (Fig. 54.4).
Renal ischemia visible as a hypoechoic area of triangular shape (black arrows)
54.2.3 Acute Cortical Necrosis
Acute cortical necrosis is a rare cause of acute renal failure, and it is the consequence of a diffuse and rapid renal cortical ischemic necrosis, without medullary parenchyma involvement. The process can be multifocal or diffuse and is commonly bilateral. CEUS is proposed as a diagnostic procedure in patients with rapidly evolving renal failure and when an acute tubular necrosis can be excluded. Acute cortical necrosis diagnosis at CEUS is characterized by a markedly hypoperfused cortex (Fig. 54.5) .
Renal cortical necrosis (black arrows)
54.2.4 Acute Pyelonephritis and Renal Abscess
The diagnosis of acute pyelonephritis is based on clinical data, and imaging is usually not necessary. This can be employed when clinical improvement is not observed during therapeutic observation or when a renal abscess is suspected. Gray-scale sonography is not diagnostic or shows nonspecific signs. At CEUS, the involved parenchyma appears as a hypoperfused area of triangular shape (Fig. 54.6) . The renal abscess is characterized as a hypoechoic area with peripheral contrast enhancement (Fig. 54.7).
Acute pyelonephritis visible as hypoechoic area (arrowheads)
54.2.5 Renal Trauma
UCAs can be used in trauma with the aim to detect posttraumatic lesions of abdominal organs not visible on gray-scale sonography . Kidney traumatic lesions show a reduced or absent contrast enhancement, appearing as hypo- or anechoic lesions. Parenchymal changes are more evident in the venous phase, even if they can be detected in all the phases of contrast flow. Parenchymal lacerations appear as hypo-anechoic areas with well-defined margins, while contusions appear as hypo-anechoic areas with shading margins (Fig. 54.8). In subcapsular renal hematomas, an anechoic fluid collection can be seen around the kidney’s profile. In hematuria with persistent bleeding, CEUS is able to detect bubbles flowing into the hematoma, easily identified in the arterial phase of the examination. Traumatic lesions of the main arterial vessels are characterized by absence of typical contrast enhancement of the renal parenchyma. The lesions of the excretory tract cannot be detected with CEUS, because the bubbles are not filtered by the glomeruli and do not pass in the urinary tract. Other imaging techniques are necessary to identify lesions when they are clinically suspected.
Renal trauma with fracture of the posterior labrum (black arrows) (a) confirmed at CT (white arrow) (b)
54.3 Vesicoureteral Reflux
The urine reflux from bladder into the upper urinary tract can be unilateral or bilateral and is secondary to congenital or acquired anatomical or functional anomaly of the ureterovesical junction. The vesicoureteral reflux (VUR) is the most common urologic pathology in children, and it is observed in 1–2% of all pediatric population and in 30–40% of children with urinary tract infections. VUR is the main cause of chronic renal damage in infancy. The cystosonography (bladder sonography with retrograde contrast introduction), named also voiding urosonography (VUS), is performed after bladder catheterization and subsequent transcatheter introduction of the US contrast . Preliminary baseline urinary tract (kidney and bladder) sonography is mandatory to detect possible pathology, followed by endovesical infusion of saline solution and the contrast medium. For this type of investigation, a concentration of 0.5/100 ml of UCA is used. Real-time observation and video registration are mandatory to document the bladder distension and the bubbles’ flow into the ureters and renal pelvis. This observation must be performed during the filling phase and during voiding (Fig. 54.9). With CEUS, it can be applied the International Reflux System for the radiology voiding cystourethrography. Based on the grade of distension of the upper urinary tract, five grades of VUR are recognized.