The diagnosis of acute pyelonephritis is based on clinical evaluation and laboratory findings [1]. Conventional baseline US can demonstrate increased size of the kidney and cortical scarring, suggestive of previous episodes of infections. CEUS plays an important role when the patient is still febrile after 72 h despite of antibiotic treatment and a complicated pyelonephritis is suspected. As CE-MDCT, CEUS can show focal parenchymal areas of pyelonephritis that appear as wedge-shaped areas of reduced enhancement because of the parenchymal oedema (Fig. 52.2).

Sometimes pyelonephritis can complicate with parenchymal abscessualization: a focal inhomogeneous nonenhancing area with intense peripheral uptake (Fig. 52.3).

Purulent material in pelvicalyceal system can be easily detected as echogenic material with no contrast uptake, since contrast agents are not concentrated in the collecting system. This finding is useful to differentiate pus from uro-endothelial tumours [5].

The kidney has a rich blood flow but can undergo a variety of vascular injuries.

CEUS demonstrates high accuracy in detecting kidney parenchymal ischaemia, comparable to CE-MDCT. CEUS shows a higher sensibility in comparison to colour of power Doppler by detecting smaller blood vessels with slower blood flow. Microbubbles reach the microvasculature and amplify the US signal, allowing a direct evaluation of parenchymal perfusion.

Renal ischaemia appears as single or multiple focal triangular or wedge-shaped area of absent, diminished or delayed contrast enhancement, easily detectable in comparison with the surrounding normal parenchyma (Fig. 52.4).

CEUS may also provide more precise information about tissue vitality: it can differentiate infarcts from areas of diminished perfusion. Even if both injuries appear at colour Doppler as non-vascularised areas, only infarcts show complete lack of contrast enhancement after injection of microbubbles.

The renal transplant represents the ideal application of CEUS because the organ is superficial and well vascularised. An important advantage is that microbubbles are not nephrotoxic and do not compromise the renal function, conversely to CT contrast agents. Renal transplant can undergo a wide range of possible complications in the early postoperative period. The main important is the acute rejection. The first-line evaluation is typically performed with spectral Doppler measurements in order to assess abnormal values in resistance index (RI).

Spectral Doppler assessment only provides indirect information about the parenchymal perfusion, whereas microbubble contrast agents allow a direct visualisation of microcirculation. CEUS findings are also earlier than abnormal RI [8–10].

In acute rejection, the parenchymal perfusion is delayed. The time-intensity curves can demonstrate a diffuse delayed and slow contrast enhancement of the renal parenchyma. In a later phase, CEUS can also show perfusional defects (Fig. 52.5). CEUS is also useful in monitoring the antirejection therapy, by assessing an improved parenchymal perfusion [11].

Renal cysts are a common finding, but any cyst that does not show the typical features of a benign cyst is by definition “complicated” and requires further assessment.

CEUS can be useful in differentiating benign cysts from cystic tumours. Even if the Bosniak classification system was developed on the basis of contrast-enhancement findings of cystic renal masses on CE-MDCT [2, 3], CEUS can provide useful information for the management of these lesions: surgical treatment or observation.

CEUS is acquiring an increasing role in the assessment of indeterminate cystic lesions (Bosniak IIF and III) by detecting the presence and the enhancement of solid components. Recent comparative studies [12, 13] between CEUS and CT revealed that CEUS imaging was superior to CT in terms of detecting additional septa, thickness of the wall or septa and solid components. Microbubble contrast agents circulate in the microvessels of septa and walls, and CEUS provides the evaluation of sophisticated internal structures of cystic renal masses with a higher resolution than CE-MDCT. In particular, the demonstration of solid components is the key factor in differential with the categories III and IV that are considered malignant and must be surgically removed (Figs. 52.6 and 52.7).