The double target sign

Clinical History

A 29-year-old male patient, immigrant in Angola for 3 years, presented to the hospital with right upper quadrant pain, high grade fever (up to 39ºC) and weight loss for the past month. In the last 15 days he was in Portugal on vacation.

Imaging Findings

The patient presented with high fever, abdominal pain in the right upper quadrant and weight loss. Blood analysis revealed leukocytosis and elevation of ESR and transaminases. He was referred for ultrasound examination of the liver which detected two hypoechoic round lesions with homogeneous appearance - unilocular cystic appearence. For further characterisation of the hepatic lesions, he underwent an enhanced CT, which confirmed the presence of two well-defined hepatic lesions with a round morphology. They appeared hypodense, with complex fluid content (18HU), and exhibited a thick enhancing wall (8mm in thickness) and a peripheral zone of oedema - the double target sign.
This sign is somewhat characteristic of an abscess and together with epidemiological (the patient was emigrant in an endemic area) and clinical data (young male) the diagnosis of amebic abscess was suggested. The positive amebic serology confirmed the diagnosis.

Discussion

Amebic liver abscesses (ALA) are caused by the protozoan Entamoeba histolytica and are the most common extraintestinal manifestation of Amebiasis. This abscesses arise from hematogenous spread (via the portal circulation) of amebic colonic trophozoites[1]. ALA has a significant male preponderance and the patients are younger than those with pyogenic abscesses[2].
As a general rule, bacterial and fungal abscesses are often multiple, whereas ALA are more frequently single.
Sonographic features of ALA include a round lesion, absence of significant wall echoes, hypoechogenicity compared to normal liver with fine, homogeneous low level internal echoes, distal sonic enhancement and continuity with the liver capsule[3].
The most typical presentation at contrast-enhanced CT (CE-CT) of ALA is that of a rounded, well-defined lesion with an unilocular cystic appearance (hypodense mass), in which necrosis and liquefaction predominate[4]. Frequently, it shows a peripheral rim or capsule (3-15mm in thickness) that is clearly enhanced on CE-CT (target sign) and consists of granulation tissue[5, 6]. The double-target sign consists of a hypodense central abscess cavity surrounded by an inner hyperdense ring and an outer hypodense zone on CE-CT, which is secondary to increased capillary permeability in the surrounding liver parenchyma[5, 6]. This target appearance (double or single) of the rim sign is considered a characteristic sign of hepatic abscess and is more common in this etiology of abscesses. Others features of ALA that may aid in distinguishing it is its tendency to be a solitary right lobe lesion and to extend beyond the surface of the liver[3].
Although the diagnosis of the microbial etiology of an hepatic abscess is difficult only by its imagiologic appearance, the combination of the clinical data and appropriate exposure history (residency or travel in an endemic area) should remind the radiologist of this diagnosis and if a there is a high index of suspicion for ALA, prompt treatment with metronidazole should be initiated without waiting for the positive amebic serological test. Most uncomplicated ALA can be treated successfully with metronidazole alone and most patients show a response within 72–96 hours without the need of surgical or percutaneous drainage[1].
In this case, the residency history in an endemic area raised a red flag and suggested the diagnosis of an amebic abscess that was posteriorly confirmed with serologic tests.
Aspiration should be reserved for individuals in whom diagnosis is uncertain, non-responders to metronidazole, large left-lobe abscesses (risk of rupture into the pericardium) and severely ill patients with large abscesses that make rupture seem imminent[7].

Differential Diagnosis List

Final Diagnosis

Amebic liver abscesses

URL:	https://www.eurorad.org/case/10428
DOI:	10.1594/EURORAD/CASE.10428
ISSN:	1563-4086