Hepatic abscess is defined as a suppurated cavity caused by the invasion and multiplication of microorganisms within healthy or diseased liver parenchyma, and it may be caused by bacteria, fungi, and parasites [1]. When the causative agents are bacteria, hepatic abscess is called pyogenic liver abscess (PLA) [2, 3].

The incidence rate of PLA in the western countries has been estimated to range between 3.59 and 2.9 per 100,000 population years based on different population-based studies [4, 5]. In Western countries, 80% of PLA are bacterial, mainly Escherichia coli (E. Coli) and Streptococcus spp. [6], but recent reports reveal that incidence of Klebsiella pneumoniae in the Western hemisphere is increasing [7]. The most common presenting clinical symptoms, right upper quadrant pain, fever, rigors, and jaundice vary from mild to severe. Clinically occult abscess may simply present with malaise, weight loss and vague abdominal pain [2, 5].

PLA can occur in the course of intra-abdominal biliary infections that contaminate the biliary tract at the same time or can be secondary to seeding via the portal venous system of non-biliary infections (appendicitis or diverticulitis) [8, 9]. It has been suggested that PLA might be an indicator of gastrointestinal cancer, particularly occult colon cancer [10]. PLA can also complicate surgical procedures (pancreatoduodenectomy, or liver transplantation) or hepatobiliary procedures (thermal ablation and/or intra-arterial chemoembolization). More rarely, PLA develops after liver trauma or arterial embolization for trauma [8, 9]. Nevertheless, it is estimated that up to 40% of liver abscesses have no recognizable route to the liver and are labeled cryptogenic [9].

Diagnostic imaging has a vital role in suspected cases of liver abscess. Contrast-enhanced computed tomography (CECT) and ultrasound (US) are the primary tools used to image the liver. The US and CECT sensitivities for diagnosis of PLA are 85% and 97%, respectively [11, 12]. At US, microabscesses (< 2 cm) appear as hypoechoic nodules or ill-defined areas of distorted hepatic echogenicity [13]. Large abscesses range from hypoechoic to hyperechoic masses, depending on the presence of internal echoes due to thickened septa and debris [13, 14]. More recently, the introduction of US contrast agent has improved characterization of focal liver lesions [15] and contrast-enhanced ultrasound (CEUS) features of PLA have been described [15, 16]. The most typical CEUS findings, i.e., the non-enhancement of the liquefied portions combined with arterial rim enhancement, refer to the mature stage of PLA, but additional findings such as presence of hyperenhanced septa and transient segmental enhancement (TSE) at the periphery of PLA in the arterial phase have been reported [15–17]. CEUS has been shown to improve PLA diagnosis in comparison with US B-mode [18], achieving the same diagnostic accuracy rates as CECT and magnetic resonance imaging (MRI) [19].

In combination with targeted antimicrobial therapy, percutaneous drainage techniques [through catheter positioning and/or needle aspiration (NA)] are considered the mainstay of treatment for PLA [1, 20, 21], whereas surgery (simple drainage or even hepatectomy) may be done if other forms of treatment have failed or in special circumstances when PLAs are multiple or very large and multiloculated [21–23].

The aim of this study is to propose a CEUS-based morphologic classification of PLA reflecting different evolutive stages and to assess the added value of CEUS in the management of PLA.

PLAs were mainly located in the right lobe (n = 28, 63.6%) with a mean diameter of 6.8 cm (SD ± 3.2, range 1.7–15 cm). As far as etiology was concerned, benign (n = 10) and malignant (n = 8) obstruction accounted for 34% of the cases whereas PLA synchronous with acute cholecystitis was found in 18% of the cases. Main presenting symptoms were fever > 38°C, abdominal pain, and jaundice.

Causative organisms were detected in 60% of the drained material and in 42% of blood culture (mostly E. Coli, Streptococcus spp., Klebsiella Pneumoniae and mixed).

In this series, a morpho-evolutive classification of PLA based on CEUS findings was built up and CEUS impact on personalized management of patients with PLA was assessed. Although typical CEUS findings (i.e., hyperenhanced rim and non-enhanced cavity in the arterial phase) have been described [15, 16], they mostly refer to fully developed PLA when the entire abscess cavity is filled with necrotic debris and purulent fluid and a thick inflammatory capsule is present. Hyperenhancing internal septa and TSE have been reported as ancillary findings [17–19].

In the present experience, the early arterial phase turned out to be the most diagnostic for PLA since the CEUS hallmarks [TSE, non-enhancing area(s), PHR, hyperenhancing septa] were all appreciated during that phase. Indeed, during the portal and late venous phases, the vascular behavior turned out to be quite variable (a clear wash-out is not often observed for peripheral rim and septa), the only notable exception being TSE which invariably disappeared in portal phase merging with the adjacent parenchyma without true wash-out.

The combination of these signs enabled us to recognize four main CEUS patterns representing different evolutive stages of PLA: from tumor-like aspect (type I) when there is no or little liquified component to mature abscess cavity filled with pus and encircled by an inflammatory capsule (type IV). The latter was the single most frequent pattern (36.5%), the type II (“honeycomb” appearance) ranking second (27.3%). The intermediate stages (type II and III) reflect the progressive liquefaction of the inflamed residual parenchyma inside the abscessed cavity. Recently, Kunze et al. [24] recognized four PLA stages based on correlation between CEUS imaging and pathological findings. Our classification differs for several reasons: 1) multiplicity was considered as an expression of etiology (e.g., in the course of cholangitis) rather than of an evolutive stage; 2) subtypes were avoided to simplify classification: by considering all subtypes, CEUS patterns were as many as 7 in Kunze’s classification, with very subtle differences among some of them; 3) the extent and morphology of tissue liquefaction were main criteria of PLA stage identification.

In addition, the role of CEUS in the decision-making process for PLA percutaneous treatment was assessed: in 67.4% of the patients treated by CD and/or NA, CEUS results enabled the operator to choose the most appropriate therapeutic modality by clarifying the internal echostructure of the index PLA. These results support the recent report by Morita et al. [25] that CEUS may be useful for making decisions in the treatment of PLA. These authors concluded that only PLA showing a prevalent solid component (enhanced in the arterial phase) benefited from conservative treatment alone whereas drainage was required when non-enhanced areas were prevalent.

It is worth mentioning that, even if in a minority of patients, CEUS was necessary as guidance modality to reach the largest cavity inside multiloculated PLAs as it occurs in case of percutaneous biopsy or ablation whenever the target is inconspicuous or not visible at US B-mode [26, 27].

In the management of PLA, CEUS offers additional advantages since it does affect the kidney function, and has a high tolerability and safety profile, which can be used in real time without a risk of radiation exposure and hence is repeated to check abscess evolution under treatment.

Moreover, a particular application of CEUS, the so-called intracavitary CEUS (i.e., injection of diluted SonoVue through needle and catheters), allows an interventional clinician to immediately assess needle/catheter placement success, make therapeutic decisions and optimize catheter removal timing [28, 29].

Limitations of this paper should be highlighted: 1) this study is based on a single operator experience whose skills and clinical results may be not reproducible elsewhere; 2) lack of randomization to a control group (either managed by computed tomography or without CEUS) to verify the added value of CEUS in terms of choice of therapeutic approach; however, it is very difficult if not impossible at all to run such a randomized study in the real life.

In conclusion, CEUS depicts the internal structure of PLA so allowing a morpho-evolutive classification of PLA and provides invaluable information to immediately tailor management to the single case.