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Fig. 9.66 Renal contour distortion. a In fetal lobation.

Abscess

Abscesses in and around the kidney can result from local abscess formation (pyelonephritis, usually E. coli) or from the hematogenous spread of infection (usually staphylococcal). They can spread beyond the renal capsule to form a perinephritic abscess, or they may spread into the perirenal fat, forming a paranephritic abscess.

b Physiological, hypoechoic dromedary hump in the left renal border, indistinguishable from a tumor (see 9.4a). M = spleen; N = kidney.

Their sonographic features are diverse. A simple abscess produces a mass effect creating a bulge in the renal outline. It shows a hypoechoic to heterogeneous echo structure and usually has ill-defined margins. The sonographer should look for gas bubbles, which are a common phenomenon and appear as focal

c Regular vessels in CDS, consequently no tumor.

echogenic inclusions with or without reverberations (Fig. 9.30, Fig. 9.67).

Perirenal abscesses appear as ill-defined masses that may spread in an inferior or lumbar direction. Emphysematous and xanthogranulomatous pyelonephritis are special forms.

Fig. 9.67 Renal abscess, emphysematous pyelonephritis. L = liver; N = kidney.

a Echogenic bacterial gas bubbles with reverberations (arrows, W) before medical treatment.

b After antibiotic treatment: hypoechoic mass (arrows), contour distortion, ill-defined margins, and scattered echogenic gas bubbles.

While some cyst criteria such as smooth margins and a round shape are maintained in a hemorrhagic cyst, the bleeding causes the initial anechoic mass to become more reflective. This makes the cyst difficult to distinguish from a tumor. The diagnosis can be established by power Doppler or CEUS, with the latter delivering the more accurate results (Fig. 9.68,

Fig. 9.77).

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Circumscribed Changes

Fig. 9.68 Renal cyst with tumorous mass (images courtesy of Professor C. Goerg, University Hospital Giessen and Marburg, Marburg, Germany).

a Gray-scale image: hypoechoic mass within a cystic lesion.

b CEUS: no enhancement; probably hemorrhagic lesion, Bosniak II.

Fresh Renal Infarct

retractions and focal rarefied areas in the parenchyma due to scarring and fibrous contracture (Fig. 9.69).

Fig. 9.69 Fresh renal infarct.

a Survey scan shows increased echogenicity in the upper pole of the right kidney.

b Color Doppler: the wedge-shaped avascular region (arrows) confirms the infarct. The patient presented clinically with flank pain.

c Extensive renal infarction, power Doppler: avascular area of the upper pole of the right kidney depicting the infarction.

Hematoma

Like renal infarcts and hematomas at other locations, a renal hematoma or contusion undergoes structural changes over time. It appears initially as a hyperechoic or isoechoic

mass that causes a bulge in the renal outline. The echo structure of the mass is somewhat irregular compared with the normal renal parenchyma. Later the hematoma becomes less

echogenic and may culminate in a residual secondary cyst or pseudocyst. Diagnosis, extent, and follow-up are the domain of color Doppler ultrasound and CEUS (Fig. 9.70).

Fig. 9.70 Renal hemorrhage caused by renal puncture.

a Irregular predominantly hypoechoic structure, dissolved architecture.

Sonographic features. The ultrasound appearance of RCC varies greatly with its stage and histological classification. The most common and characteristic appearance of RCC is that of a round, isoechoic mass in the renal parenchyma that creates a bulge in the renal outline. Hypoechoic tumors are also seen. Atypical hyperechoic tumors are found in 30% of cases; most of these are early stage tumors (see Fig. 9.82, Fig. 9.83). Cystic RCC displays cystic features along with solid elements or septations. Small RCC are homogeneous and ordinarily present a hyperechoic structure. They become a more hypoechoic or irregular structure as they grow (Fig. 9.55a, Fig. 9.78).

Advanced tumors show regressive changes in the form of liquefaction, hyperechoic areas, and calcifications (9.4 d). With its wide spectrum of features, RCC can be characterized as chameleon-like in its sonographic appearance ( 9.4).

A narrow, anechoic rim (displaced vessels) is consistently present around the tumor. If this is accompanied by irregular internal tumor vessels within the mass (color Doppler, pattern 4 (after Jinzaki;3 see Fig. 9.71), the ultrasound diagnosis may be considered established. In some cases spectral analysis demonstrates (predominantly in the peripheral vascularity) a high Doppler shift with high-velocity signals (> 70 cm/s), but also tumor vessels with nearly continuous flow signals and a low systolic– diastolic variation.

In CEUS, a rapidly increasing enhancement from peripheral to central is present within 10–20 s due to arteriovenous shunts, leading in larger carcinomas to a rapid wash-out. But solid renal tumors do not show diagnostic perfusion patterns on CEUS, which is thus usually not able to differentiate between malignant and benign solid renal masses (e. g., carcinoma versus angiomyolipoma). CEUS does not improve the sensitivity because there are no reliable criteria to distinguish malignant from benign masses such as angiomyolipoma, oncocytoma, or leiomyoma.6

The differentiation of hyperechoic small carcinoma from angiomyolipoma is more successful in gray-scale ultrasound combined with power Doppler (accuracy 78%15). Power Dop-

9.4 Renal Cell Carcinoma (RCC)

Varying echogenicity

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Circumscribed Changes

e Complex structured clear-cell carcinoma. Areas of necrobiotic liquefaction and bleeding (histology). N = kidney; T = tumor.

f Magnification: CDS: Intratumoral and marginal vascularization. Complex structured tumor mass with hyperand anechoic (Z) portions of the right kidney (N). P = renal pelvis.

g and h Differential diagnosis: small focal h CDS. mass (arrow, cursors), 9.6 × 17.8 mm.

g Causing cystically enlarged pyramid (known for 3 years; masses < 1.5 cm can be observed). Probably adenoma.

pler ultrasound can add information to that obtained from gray-scale ultrasound for differential diagnosis of small solid renal lesions.3 The rate of correct diagnosis is significantly increased with combined ultrasound (78%) compared to gray-scale or power Doppler ultrasound alone.

Diagnostic procedure. At present, RCC is detected incidentally in routine ultrasound in 70–80% of cases. It is rarely diagnosed by a targeted search in patients who already have metastatic symptoms. Metastasis may occur by the hematogenous route or by contiguous spread into the renal vein and inferior vena cava.16 Tumor thrombi may occur in the right heart. It is essential, therefore, to closely inspect the renal veins, vena cava, and locoregional lymphatics during ultrasound examination. On the left side, a dromedary hump should be included in the differential diagnosis.

CEUS is recommended in the following clinical situations:11

●Suspected vascular disorders, including renal infarction and cortical necrosis

●Differential diagnosis between solid lesions and cysts presenting with equivocal appearance at conventional ultrasound

●Differentiation between renal tumors and anatomical variations mimicking a renal tumor (“pseudotumors”) when conventional ultrasound is equivocal

●Characterization of complex cystic masses as benign, indeterminate, or malignant to provide information for surgical strategy

●Additional help, when necessary, in the fol- low-up of nonsurgical complex masses

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Kidneys

9.5 Differential Diagnosis of Hypoechoic Masses in the Renal Sinus Echo Complex

● Duplicated ureters (visible only with obstruction)

Abscess, pyonephrosis

●Round or polygonal, nonhomogeneous mass, usually with ill-defined margins

●No vascularity

a Hypertrophic renal column (C) next to large, hypoechoic medullary pyramids; renal transplant. NP = parenchyma.

b Tumor-like nub of parenchyma extending into the central echo complex.

c Hypoechoic tumor-like transformation of the sinus echo complex; no vascularity in the mass. N = kidney.

d Renal pelvic abscess: hypoechoic mass with ill-defined borders. Clinically, the patient had poorly controlled diabetes. N = kidney.

e Urocelial carcinoma (TU): hypoechoic mass in the central reflex invading the caliceal system; minimal obstruction.

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Circumscribed Changes