Tuesday, November 23, 2010

Ultrasound of Amoebic Liver Abscess

Amoebic liver abscess is usually single but can be multiple. Typically located in the right lobe of liver subcapsular close to the diaphragm and posterolateral, though it can be situated in any location. It`s size may vary from few centimeters to a large size occupying almost entire right lobe of liver.

Very early stage :
In the initial stage, cell death occurs but entire dissolution and liquefaction is not complete as the contents are not liquid. This may be termed as solid abscess. On ultrasound these lesions are usually small and probably are the most challenging as compared to the other stages of the liver abscesses. The margins of the abscess may be ill defined, the abscess is hypoechoic as compared to the surrounding liver. However, there is no true liquefaction at this stage and therefore there is poor or no posterior acoustic enhancement. The demarcation between the abscess and the surrounding liver is also poor.

At this stage, the differential diagnosis of fat spared area in a fatty liver or an early neoplastic lesion have to be considered.

Recently formed amoebic abscesses :
An abscess of recent onset has a distinct central liquified area. This is seen on ultrasound as a sonolucent or an hypoechoic area usually with fine internal echoes. Because of the liquefaction, there is associated posterior acoustic enhancement. The cavity may be round, oval or branching. The walls of the abscess at this stage are usually not very thick and sometimes the demarcation between the wall and the surrounding tissue can be poor. Sometimes the walls may be thicker and these may be seen as shaggy, ill-defined echogenic areas along the walls (see Picture). It is at this stage of the abscess that aspiration may be required. Small amount of air in the abscess because of secondary infection or following an aspiration is seen as highly reflective dots.

It is at this stage of the abscess that differential diagnosis of a cyst in the liver, a cyst with haemorrhage, cystic metastatic deposit or sometimes a hydatid cyst and haematoma are to be considered.

Abscesses of some duration :
The basic difference between an acute abscess and an abscess of some duration is that; in the latter the body has had time to wall up the lesion by producing a layer of fibrous tissue around it. On sonography an abscess shows thick walls which may vary from a few mm to 1.5 cm in thickness. The echogenicity of the abscess also varies, abscesses generally become more sonolucent at this stage, some abscesses become more echogenic because of organisation of fluid.

Healing Stage :
The abscess heals, the liquid contents dry up, which has been described as putty appearance. On ultrasound it is seen again as a lesion with thick walls fairly echogenic as compared to surrounding organs. This shadow can be seen on ultrasound for a long time, even years. It is usually at this stage that the differential diagnosis of a neoplasm, haemangioma or granuloma in liver come into picture.