LAUSR

Metastases are one of the most common malignant neoplasms of the liver. In patients with known primary malignancies, early detection of liver metastasis will significantly impact the course of further management. Sonography is an important tool in the initial screening and imaging followup of such cases. Detection of a ‘hypoechoic halo’ (Fig. 1) bordering the lesion along with lesion multiplicity is highly suggestive of metastasis [1]. It resembles the halo of light sometimes visible around the sun or moon (Fig. 2). The halo is discernible only if the lesion centre is isoor hyperechoic. The difference in histological composition and hence acoustic impedance of the central and peripheral components gives rise to this sonographic appearance [2]. When present, the halo sign is a useful indicator of malignancy, particularly in distinguishing metastases from typical echogenic hemangiomas, with a positive predictive value of up to 95% [3]. The incidence of atypical sonographic appearance of hemangiomas with central area of reduced echogenicity and echogenic rim (i.e. hyper rather than hypoechoic margin) has been estimated at ~ 15% [4]. Observing a hypoechoic halo with hemangioma although reported in the past is quite unusual [5]. The halo has been attributed most commonly to the peripheral tumoural component with a high proliferative index or to a rim of compressed hepatic parenchyma. In a study by Wernecke et al., radio-pathological correlation revealed the halo composition as proliferating tumour cells in majority of the lesions. A single outlier had the halo constituted by a parenchymal pseudocapsule. Another observation was that, with a tumoural rim, the halo showed decreasing echogenicity towards the periphery corresponding histologically to the advancing tumour edge with high cellularity. The contrary was true for a pseudocapsule with lesser echogenicity towards the centre of the lesion having more parenchymal compaction [6]. Although the halo sign has been classically associated with metastases, it is not a pathognomonic feature. It has also been described in mycotic abscesses, localized lymphomatosis and hepatocellular carcinoma [7]. Hence, this sign should be interpreted in appropriate clinical context, being particularly useful in the setting of a preexisting malignancy